6

CHAPTER 2

REVIEW OF LITERATURE

Various treatment methods are available for the treatment of PIWW. Many

chemical, physical and biological methods are generally used to remove color,

chemical oxygen demand (COD), turbidity from the PIWW. Various chemical

methods are Fenton-oxidation process, Photo Fenton oxidation process, electro

chemical oxidation, sequential batch reactor and physical treatment methods such as

adsorption, electro coagulation, etc.

2.1 Treatment of PIWW using various methods

The critical literature review of all these physical, chemical and biological

methods for the treatment of PIWW is discussed in this chapter.

2.1.1 Biodegradation

Activated sludge system based on pure oxygen has been developed to

increase the biodegradation of wastewater and this system was used for the treatment

of combined wastewater from the PIWW and sewage. The results of continuous test

showed that it is possible to achieve 87.8-93.6 % COD and 97.7-99.2 % of BOD

removal resulting in an effluent quality of 55-90 mg/L of COD and less than 10.0

mg/L of BOD [12].

2.1.2 Fenton oxidation

Fenton oxidation process was principally used to treat the water-based PIWW

which is formed during the coating step of metal surfaces. Treatability runs were

carried out by using rust (oxidized iron) particles obtained as a waste of the metal

rolling process, as the Fenton‟s catalyst. In order to change the variables such as

dimensions of the column, packing material size, reactive dosage, pH value and

reaction time, the experiments on the basis of packed columns and mixed reactors

were carried out to increase the COD removal about 80 % [26].

7

2.1.3 Fenton and photo-Fenton processes

The effectiveness of the Fenton and photo-Fenton processes was compared in

the treatment of PIWW by means of chemical oxygen demand (COD), total organic

carbon (TOC) and phenolic compounds removal, generated during alkydic resins

manufacture. The best results were obtained with photo-Fenton process assisted with

solar radiation, with reductions of 99.5 and 99.1 % of COD and TOC levels,

respectively [27].

2.1.4 Electrochemical oxidation

The electrochemical oxidation of water-based PIWW was investigated batch-

wise in the presence of NaCl electrolyte with carbon electrode. The optimum

conditions were satisfied at 35 g/L external electrolyte concentration, 30 º C reaction

temperature and 8 V potential difference (64.37 mA/cm2 current density) realizing

51.8% COD and complete color and turbidity removals and 3010.74 mg/L h initial

COD removal rate [28].

2.1.5 Sequential batch reactor

In this research a mixture of Industrial wastewater from chemical industry

(varnish, paint and pigments production) and municipal wastewater was treated in

pilot sequencing batch reactor (SBR). Results of the pilot experiments show that the

foaming problem has great influence on the behavior of SBR, especially when the

ratio between industrial and municipal wastewater is very high [29].

2.1.6 Electrochemical

The electrochemical treatment of industrial water-based PIWW was

investigated in a continuous tubular reactor constructed from a stainless steel tube

with a cylindrical carbon anode. The optimum residence time in the reactor was

determined 6 h for a cost driven approach, enabling COD, color and turbidity

removal as 44.3 %, 86.2 % and 87.1 %, respectively and a discharge pH value of

7.33 [17].

8

2.1.7 Fenton process combined with coagulation

Attempts were made in this study to examine the efficiency of Fenton process

combined with coagulation for treatment of water-based printing ink wastewater.

86.4 % of color and 92.4 % of chemical oxygen demand (COD) could be removed at

pH 4, 50 mg/L H2O2, 25 mg/L FeSO4 and 30 minutes settling time. The coagulation

using polyaluminium chloride (PAC) and ferrous sulfate (FeSO4) was beneficial to

improve the Fenton process treated effluent in reducing the flocs settling time,

enhancing color (100%) and COD (93.4%) removal [15].

2.1.8 Electro coagulation

Treatability of PIWW by electro coagulation (EC) process was investigated.

The highest removal efficiencies for COD and TOC in PIWW were obtained with

93% and 88% for Fe and 94% and 89% for Al electrodes at the optimum conditions

(35 A/m2, 15 min and pH 6.95). Operating costs for removal of PIWW at the

optimum conditions were calculated for Fe and Al electrodes as 0.187 € /m3 and

0.129 €/m3

[8].

2.1.9 Bio sorption

Zinc biosorption characteristic of locally isolated Aspergillus flavus NA9

were examined as a function of pH, temperature, pulp density, contact time and

initial metal ion concentration. The maximum zinc uptake was found to be

287.8 ± 11.1 mg/g with initial metal concentration 600 mg/ L at initial pH 5.0 and

temperature 30 °C. The biosorbent was regenerated using 0.01 M HCl with 83.3%

elution efficiency and was reused for five sorption–desorption cycles with 23.5%

loss in biosorption capacity. The biosorption assays conducted with actual PIWWs

revealed efficiency of 88.7 % for Zn (II) removal by candidate biomass [30].

9

2.1.10 Small scale laboratory reactor

In this study Pseudomonas aeruginosa a metal tolerant strain was not only

applied for heavy metal removal but also to the solublization performance of the

precipitated metal ions during effluent treatment. The synergistic effect of the isolate

and FeO enhanced the metal removal potential to 72.97 % and 87.63% for Cr (VI)

and cadmium, respectively. The decrease in cadmium ion removal to 43.65%

(aeration + stirring reactors), 21.33 % (aerated reactors) and 18.95 % (without

aerated + without stirring) with an increase in incubation period was observed [31].

2.2 Treatment of PIWW using coagulation

A perusal of relevant literature reveals that although PIWW can be treated

successfully using various methods, a majority of the research work is being carried

out on the coagulation technique (Fig. 2.1), which is discussed below.

Figure 2.1 Pie chart for methods involved in the treatment of PIWW

In the present study, the coagulation of PIWW was examined using ferrous

and aluminum sulphate and polyaluminum chloride (PAC). Optimum pH for FeSO4

addition was near 9.7, the required coagulant dose was about 2g/L and average

process efficiency varied between 30 and 80% in COD and between 70 and 99% in

Coagulation cum

flocculation

Electrochemical

Fenton oxidation process

Microfiltration

Biological

Oxidation

10

turbidity terms, for a wide spectrum of wastewater batches. In the case of Al2(SO4)3,

no pH adjustment was needed and process efficiency varied between 70 and 95% in

COD and between 90 and 99% in turbidity terms, for an effective dose of 2.5 g/L

[32].

Characteristics of PIWW varied widely according to the production rate.

Average values of COD and BOD were 1950 mg /L and 683 mg/ L. Oil and grease

ranged from 63 to 1624 mg/L. Chemical treatment using ferric chloride in

combination with lime at the optimum operating conditions achieved good results.

Residual values after treatment of COD, BOD and oil and grease reached 120, 36

and 8.6 mg /L, respectively [33].

This work researched the effect of treating oil PIWW using modified

rectorite-amylose composite flocculant. The best prescription of this composite

flocculant was rectorite: amylose = 20: 1 (w/w) and its best dosage was 80 mg/L

modified rectorite and 4 mg/L amylose. Treating oil PIWW using this composite

flocculant, the removal rates of COD, SS and coloring matter were 43.1 %, 6.4 %

and 22.6 % higher than those of PAC and were 19.1 %, 3.4 % and 21.8 % higher

than PAM; on the other hand its cost to treating one ton wastewater were 16.85 %

and 30.7 % less than that of PAC and PAM, respectively [34].

This study reviewed the treatment of PIWW using, coagulation cum

flocculation with the help of different coagulants including alum, ferric chloride and

poly-electrolyte. Using physico-chemical processes, removal of 90% or greater has

been achieved for chemical oxygen demand (COD) [35].

The coagulant iron chloride and the flocculants Polysep 3000 (PO), Superfloc

A-1820 (SU) and Praestol 2515 TR (PR) have been used in this study to show the

efficiency of coagulation flocculation process in the chemical precipitation method

for the removal of organic and coloring matters from the PIWW. The results

indicated that FeCl3 is efficient at pH range 8–9 and at optimal dose of 650 mg /L.

Iron chloride allows the removal of 82% of chemical oxygen demand (COD) and

94% of color [14].

11

In this study, treatability of wastewater generated from a water-based paint

and allied products industry has been investigated through coagulation-flocculation

unit followed by an activated sludge process. In this context the COD removal

efficiencies and operating costs of different coagulants i.e., sodium bentonite, alum,

FeCl3 and FeSO4 have been evaluated [36].

Wastewater samples from battery, paint and textile industries were treated in

the pH range of 5.9-7.5 with different doses of locally available alum, aluminum

sulphate and ferric chloride, in order to determine and compare their effectiveness in

removing heavy metal contents of the wastewater increased with mg/L dosage of the

coagulants used with optimal performance generally at a slightly alkaline pH [37].

This study was aimed to comparatively evaluate the effect of replacement of

the conventional binder PVA with latex. Wastewater characterization studies

indicated that latex-based production wastewater were several fold diluted with

respect to PVA-based PIWWs while they were still strong having COD values in the

order of 16 g/L. Coagulation of PVA based wastewater with alum and FeCl3

provided 80% COD removal at 2000 mg/L doses. Lime was also effective at high pH

as it was combined with FeCl3 [38].

The proposed complete wastewater treatment of paint industry process

includes physical/chemical treatment followed by filtration process through palm

hems as filtration media with possible aeration for 1 hr and Dissolved Air Floatation

(DAF) unit when needed. The chemical agents used were NaOH, Alum, ferric salt

and polymer. The average percentages of removals for COD, TSS and BOD5 after

applying the proposed treatment process were 85%, 91% and 90%, respectively

which are considered accepted as secondary treated effluent when compared with

traditional treatment processes [5].

The wash water from water based paint industry is currently treated in a

flocculation process using ferric chloride as a coagulant. This research presented the

findings of the effect of concentration of the coagulant on the coagulation process.

Both the solids content of the wash water and the concentration of the coagulant have

12

been found to be important variables to ensure efficient flocculation and coagulant

utilization [39].

The performance of generalized minimum variance (GMV) was examined

when applied to the wastewater pH control of waterborne paint production process.

The control strategy was tested under batch coagulation condition on the wastewater,

using Al2 (SO4)3 as coagulant. The constant offset problem was experienced with the

GMV control. This offset was reduced by employing a reduction of coagulant

amount [40].

Various types of coagulants used, for the treatment of PIWW is given in

following Table 2.1.

Table 2.1 Treatment of PIWW using various coagulants

Coagulants Wastewater Reference

Ferrous sulphate, Aluminum sulphate, PAC Paint industry [32]

FeCl3 with lime Paint industry [33]

Modified rectorite amylase Oil paint [34]

Alum, FeCl3, polyeletrolyte Water based paint [35]

Iron chloride, Polysep 3000, superfloc A 1820,

Praestol 2515 TR

Paint industry [14]

Sodium bentonite, alum, FeCl3, Ferrous

sulphate

Water based paint [36]

Alum, aluminum sulphate, FeCl3 Paint industry [37]

FeCl3, lime Latex based paint [38]

Sodium hydroxide, alum, ferric salt and

polymer

Paint industry [5]

FeCl3 Water based paint [39]

Alum Water borne paint [40]

2.3 Treatment of PIWW using adsorption

Many scientists have treated the PIWW by adsorption. All the works were

based on the heavy metal removal from PIWW using various adsorbents in batch

operations.

Powdered waste sludge (PWS) from the PIWW treatment plant was used for

recovery of Cu (II) ions from aqueous solution by biosorption after pre-treatment

13

with 1% H2O2. The maximum biosorption capacity (116 mg/ g) of the pretreated

powdered waste sludge for Cu (II) ions was found to be superior as compared to the

other biosorbents reported in literature [41].

In this study, the removal of heavy metals (Pb and Co) from Binalood paint

industry (Kerman, Iran) effluent was investigated using wood ash as natural low

price adsorbent in batch condition. The maximum Pb removal efficiency was 96.1%

at pH 2 with a contact time of 3 h and 100 g/L wood ash and the maximum Co

removal efficiency was 99 % at pH 2 with a contact time of 3 h and 100 g/L wood

ash [42].

The hydrotalcite-like compounds and the heated solids were used as

adsorbents for Cr (VI) in aqueous solutions and also presented in paint and pigment

manufacturing, leather tanning, chrome plating and textile processing unit. The

maximum Cr (VI) uptake by hydrotalcite and the heated solids was determined using

the Langmuir equation and was found to range between 26 and 29 mg Cr (VI)/g

adsorbent [43].

The ability of light expanded clay aggregate (Leca) to remove lead and

cadmium from paint industry‟s effluents was studied at different levels of adsorbent,

contact time and pH in batch reactors. The amount of adsorbed lead and cadmium

exposure to Leca increased from 1.41 to 3 mg/g and 0.22 to 0.75 mg/g, respectively.

The maximum removal efficiency for Pb was 93.75 % at pH = 7 and exposure to 10

g/L of Leca, while for cadmium, it was nearly 89.7 % at the same condition [44].

2.4 Treatment of various industrial wastewater using coagulation

2.4.1 Pharmaceutical wastewater

The selection of a coagulant-flocculant agent which, based on the maximum

chemical oxygen demand removal, warrants the best performance of the removal

system for a very complex high-load chemical-pharmaceutical industry wastewater,

was described. A total of 23 coagulants/flocculants was tested, including salts, poly-

hidroxy aluminates, synthetic polymers as well as natural gums. It was demonstrated

that the appropriate coagulation-flocculation system is capable of diminishing the

14

COD, the apparent color and the dissolved solids up to 40.6, 25.6 and 39.4%,

respectively [45].

2.4.2 Paper and pulp wastewater

The efficiency of alum and polyaluminum chloride when used alone and in

coupled with cationic polyacrylamide (C-PAM) and anionic polyacrylamide (A-

PAM) on the treatment of pulp and paper mill wastewater were studied. At the

optimum alum dosage of 1000 mg/L and optimum pH of 6.0, turbidity reduction is

found to be 99.8 %, TSS removal is 99.4 % and COD reduction is 91%. The

optimum dosage and pH for PAC are 500 mg/L and 6.0, respectively, at which it

gives 99.9 % reduction of turbidity, 99.5 % of TSS removal and 91.3 % of COD

reduction [46].

2.4.3 Tannery wastewater

Alum was used as coagulant with cationic and anionic polymers as coagulant

aid conducted to treat the tannery wastewater through coagulation–flocculation–

sedimentation in jar test apparatus. The results of the study revealed that the

combination of alum with cationic polymer C-492 resulted in effluent turbidity

removal of 97 %, total suspended solids (TSS) removal of 93.5 %, total chemical

oxygen demand (TCOD) removal of 36.2 % and chromium removal of 98.4 [47].

2.4.4 Distillery wastewater

The effects of dosage, pH and concentration of salts were investigated for an

optimized condition of color removal from the distillery spent wash. The design was

employed to derive a statistical model for the effect of parameters studied on removal

of color using M. oleifera coagulant (MOC). The actual color removal at optimal

conditions was found to be 53 % and 64 % respectively for NaCl and KCl salts

which confirms close to RSM results [48].

2.4.5 Dairy wastewater

It was reported that the treatment of simulated dairy wastewater (SDW) by

inorganic coagulants such as poly aluminum chloride (PAC), ferrous sulphate

15

(FeSO4) and potash alum (KAl(SO4)2·12H2O) were effective. Optimum coagulant

dose (mop) was found to be 300, 800 and 500 mg/L for PAC, FeSO4 and

KAl (SO4)2·12H2O, respectively, giving 69.2, 66.5 and 63.8% COD removal

efficiency in 30 minutes [49].

2.4.6 Municipal wastewater

The potentials of using the hydraulic technique in combined unit for

municipal wastewater treatment were studied. A combined unit in which processes of

coagulation, flocculation and sedimentation, has been designed utilizing hydraulic

mixing instead of mechanical mixing. Alum, ferrous sulfate, ferric sulfate, a mixture

of ferric and ferrous sulfates and mixture of lime and ferrous sulfate were all tested.

The optimum dose of coagulants used in the combined unit gives removal

efficiencies for COD, BOD and total phosphorous as 65%, 55% and 83%,

respectively [50].

2.4.7 Food industry wastewater

In the present investigation thorough treatment studies were carried out on

diary, sweet-snacks and ice-cream industrial effluents using alum, electro

coagulation and powdered activated charcoal as adsorbent. The electro coagulation

was performed with aluminum electrodes at different time intervals in order to check

the variations in effluent parameters. Present studies revealed that electro coagulation

and adsorption have better ability to reduce the water parameters [51].

2.4.8 Laundry wastewater

In this work, a combined chemical coagulation–flocculation/ultraviolet

photolysis process was used to separate and oxidative degrade the linear alkyl

benzene sulfonate (LAS), an anionic surfactant in laundry wastewater, aiming at

making the effluent dischargeable with suitable characteristics. Mineral ash, ZnCl2

and Praestol-650 (P-650) were chosen as the coagulant-sorbent, the complex former

and the cationic high-molecular flocculants, respectively. Results showed that the

maximum LAS removal efficiency of 71.26 % and 74.58 % were achieved for the

16

self-made LAS wastewater and the actual laundry wastewater when the dosages of

ZnCl2, ash and P-650 was 29.54, 1936.35 and 196.38 mg/L, respectively [52].

2.4.9 Textile wastewater

In this study, Ocimum basilicum L. (basil) has been evaluated as an active

natural coagulant for the removal of dye from a model textile wastewater containing

Congo red. A high color (68.5 %) and COD (61.6 %) removal efficiency was

obtained by using a low amount of the coagulant, 1.6 mg/L. The mucilage of

O. basilicum was also found to be highly effective in treating real textile wastewater

as a sole coagulant and in combination with alum [53].

2.4.10 Winery wastewater

This review, presented the state-of-the-art of the processes currently applied

and/or tested for the treatment of winery wastewater, which were divided into five

categories: i.e., physicochemical, biological, membrane filtration and separation,

advanced oxidation processes and combined biological and advanced oxidation

processes. Both bench- and pilot/industrial-scale processes have been considered for

this review [54].

Table 2.2 Treatment of industrial wastewater using coagulation

Industry Pollutant removed Coagulants Reference

Pharmaceutical COD, color, TDS Salts, polyhidroxy

aluminate, synthetic

polymer, natural gums.

[45]

Paper and pulp COD,TSS, SVI PAC [46]

Tannery TSS, Total COD, Cr,

Sludge production

Alum with ionic polymer [47]

Distillery Color M. oleifera [48]

Dairy COD, Sludge generation PAC, FeSO4, Potash alum [49]

Municipal COD, BOD, Total

phosporous

Alum, Ferrous sulphate,

Ferric sulphate, mixtures

[50]

Food industry BOD, COD Alum [51]

Laundry Linear alkyl benzene

sulfonate

Mineral ash, ZnCl2,

Praestol 650

[52]

Textile Color, COD Ocimum basilicum [53]

Winery Color, BOD Alum [54]

17

The above Table 2.2 consolidated the removal of COD, BOD, color, etc.,

from various industrial effluent treatment using different coagulants. Coagulation

was concluded as an efficient physical treatment method for the treatment of various

industrial wastewaters.

2.5 Treatment of various industrial wastewater using adsorption

Though commercially available activated carbon with a high surface area,

microporous character and a high adsorption capacity has proven its potential as an

adsorbent, it is expensive. Hence, there is a growing demand to find low-cost,

efficient and locally available adsorbents for the adsorption of heavy metals from

wastewater.

Columns packed with calcium alginate (CA) beads with or without humic

acid (HA) have been used as an adsorbent and tannery effluent was passed through it

for the removal of chromium (Cr). Data showed that the CA beads along with HA

could be effectively utilized in removal of 54% Cr and also in reducing the toxicity

(EC50 (%) in 5 min = >100 in fractions collected after 72 h) [55].

A continuous adsorption study in a fixed-bed column was carried out by

using phoenix tree leaf powder as an adsorbent for the removal of methylene blue

(MB) from aqueous solution. The effect of flow rate, influent MB concentration and

bed depth on the adsorption characteristics of adsorbent was investigated at pH 7.4

[56].

In this study, the ability of surfactant-modified zeolite (SMZ) to remove color

from real textile wastewater was investigated. Tests were performed in a fixed-bed

column reactor and the surface of natural zeolite was modified with a quaternary

amine surfactant hexadecyltrimethyl ammonium bromide (HTAB). Effects of

wastewater color intensity, flow rates and bed heights were also studied [57].

In this study, the potential of activated carbon derived from sugarcane

bagasse was studied for the removal of aqueous phenol in a fixed column.

Accordingly the ideal breakthrough curves (IBC) were prepared and bed capacity

(BC), length of the unused bed (LUB), the time required for full bed exhaustion at

18

infinite rapid adsorption TS and the breakthrough times Tb were calculated for each

scenarios [58].

A commercial powdered organoclay in a mixture with inert sand particles

was used to study its capability for removal of a crude oil from salty water in a fixed

bed adsorption column. A dispersed plug flow model with an overall mass transfer

resistance through column was proposed and solved numerically. The axial

dispersion coefficient (DL) and the overall mass transfer coefficient (KF) were

estimated using the experimental data and Nelder–Mead simplex optimization

method [59].

Adsorption experiments were carried out in a fixed bed column for the

decolorization of palm oil mill effluent using anion base resin. It was found that the

highest uptake capacity was obtained at pH 3. The exhaustion time appeared to

increase with increase in bed length and decrease in flow rate [60].

The process consisted of an electrocoagulation cell (EC), a spouted bed

bioreactor (SBBR) with P. putida immobilized in polyvinyl alcohol gel and an

adsorption column packed with granular activated carbon produced from agricultural

waste, specifically date pits. At optimum conditions and unit arrangement, the

process was able to reduce the concentration of COD, phenol and cresols by 97 %,

100 % and 100 %, respectively [61].

This study evaluated the performance of fixed-bed columns with activated

carbon as the adsorbent for the removal of benzaldehyde present in an aqueous

solution. The results showed that the bed capacity, total bed capacity and saturation

time decreased as the feed flow rate was increased. The opposite effect was observed

with an increase in bed depth. Increasing the inlet concentration resulted in higher

aroma adsorption. An increase in the inner diameter without changing the feed flow

rate resulted in better aroma recovery [62].

It is difficult to eliminate phosphate from large volumes of water in batch

mode using an adsorbent such as andosol. In a fixed-bed column andosol has a very

low permeability. In this study andosol was mixed with bagasse to increase

19

permeability. The mixture was then applied for the adsorption of phosphate in a

fixed-bed column. Optimum and stable permeability was obtained with a 50-50

mixture of andosol and bagasse. The maximum adsorption capacity obtained was

4.18 mg/g for a column with a bed depth of 1.8 cm and a flow rate of 4 mL/min [63].

Table 2.3 Treatment of industrial wastewater using adsorption

Adsorbate Adsorbent Reference

Chromium from tannery

wastewater

Calcium alginate beads with or

without humic acid

[55]

Methlyene blue from dye effluent Phoenix tree leaf powder [56]

Color from real textile

wastewater

Surfactant modified zeolite [57]

Phenol from aqueous solution Activated carbon derived from

sugarcane baggase

[58]

Crude oil from salty water Organo clay [59]

Color from Palm oil mill effluent Anion base resin [60]

Refinery wastewater Activated carbon derived from date

pits

[61]

Benzaldehyde from aqueous

solution

Activated carbon [62]

Phosphate Andosol mixed [63]

The various combinations of adsorbents and adsorbates were discussed in the

above segment and consolidated in the Table 2.3. Almost all the wastewater could be

treated using adsorption technique, development of innovative and low cost

adsorbents are the urging criteria.

2.6 Treatment of various industrial wastewater using natural coagulants

The growing scenario of natural materials serving as coagulant takes root in

the promise of removal efficiency. This section discusses the various natural

coagulants applied to industrial effluents

2.6.1 Cassia obtusifolia

The present study investigated the potential use of natural Cassia obtusifolia

seed gum in treatment of raw and undiluted pulp and paper mill (PPME) through

coagulation process. Recommended conditions (initial pH 5, 0.75 g/L dosage, 10 rpm

20

and 10 min slow-mixing and 1 min settling time) allowed C. obtusifolia gum

removed high total suspended solids and chemical oxygen demand up to 86.9 and

36.2%, respectively [64].

In this study, the removal of TSS and COD from palm oil mill effluent

(POME) was studied in relation to wastewater strength, coagulant dosage and initial

pH of wastewater, settling time, slow stirring speed and temperature using Cassia

obtusifolia and alum. Optimized treatment conditions when using C. obtusifolia seed

gum for the treatment of POME (7500 mg/L) were determined to include a natural

coagulant dosage of 1.0 g/L, initial pH of 3 and a settling time of 45 min [65].

2.6.2 Plantago major L

In this study, Plantago major L. has been evaluated as an active natural

coagulant for the removal of dye from a model textile wastewater containing neutral

red. A high color (92.4 %) and COD (81.6 %) reduction efficiency was obtained

using P. major L. at the optimal conditions of 49.6 min, pH 6.5 and 297.6 mg/L

coagulant dose [66].

2.6.3 Unmodified rice starch

In this study, treatment performance of unmodified rice starch and alum was

tested on agro-industrial wastewater produced from crude palm oil extraction,

namely palm oil mill effluent (POME). The treatment enabled TSS and COD

removals up to 86.65 and 49.23 %, respectively under the optimum conditions of

0.38 g/L alum, 0.28 g/L unmodified rice starch, pH 4.45 and settling time of

5.54 min [67].

2.6.4 Jatropha curcas

In the new method, the Jatropha curcas seeds were extracted using different

solvents in different concentrations, using NaCl (JCSC-NaCl) and NaOH (JCSC-

NaOH) to extract the active coagulant agent from the Jatropha. JCSC-NaCl at 0.5 M

was found to provide a high turbidity removal of > 99% compared to JCSC-DW and

JCSC-NaOH at pH 3 using 120 mg/L of the coagulant agent [68].

21

2.6.5 Moringa oleifera

This study proposed a complementary treatment for wastewater from a

concrete plant, which has a conventional treatment system composed by

sedimentation tanks. The proposed process used Al2(SO4)3 and Moringa oleifera

(MO) as coagulants. With this combination of coagulants, more than 90% of the

turbidity was removed and a ratio of 20:80 (w/w) was obtained for MO and Al2

(SO4)3. After treatment, the wastewater was suitable for reuse in washing vehicles or

flushing toilets [69].

2.6.6 Tannin

Four types of water sample were treated using tannin-based coagulant-

flocculant (Tanfloc): surface water (collected from a river) and municipal, textile

industry (simulated by a 100 mg /L aqueous solution of an acid dye) and laundry

(simulated by a 50 mg /L aqueous solution of an anionic surfactant) wastewater. The

efficacy of the water purification was notable in every case: total turbidity removal in

the surface water and municipal wastewater, about 95% dye removal in the case of

the textile industry wastewater and about 80% surfactant removal in the laundry

wastewater [70].

2.6.8 Ipomoea dasysperma and guar gum

An investigation of dye decolorization from synthetic dye solutions using the

non-ionic, water-soluble, high molecular weight seed gums Ipomoea dasysperma and

guar gum as coagulants was undertaken for textile effluent treatment. The seed gums

alone were found to be effective for decolorization of direct dye and in combination

with PAC their coagulation efficiency was well extended even for reactive and acid

dyes [71].

2.6.9 Surjana seed powder (SSP), Maize seed powder

The low-cost, easily available naturally prepared coagulants like Surjana seed

powder (SSP), Maize seed powder (MSP) and Chitosan as an ideal alternative to

recent expensive coagulant methods for Congo Red (CR) dye removal has been

22

investigated in this study. The maximum percentage CR removal was found to be

98.0%, 94.5% and 89.4% for SSP, Chitosan and MSP, respectively, at pH 4.0,

coagulant dose of 25 mg/L, flocculation time 60 min and temperature of 340 K [72].

2.6.10 Chestnut and acorn

The ability of seed extracts of several species of chestnut and acorn to act as

natural coagulants was tested using synthetic turbid water. Active components were

extracted from ground seeds of Horse chestnut and acorns of some species of family

Fagaceae: Common oak, Turkey oak, Northern red oak and European chestnut. The

seed extracts from European chestnut and Common oak acorn were the most

efficient expressing the highest coagulation activities, about 80% and 70%,

respectively, in both low and medium investigated water turbidities at the lowest

coagulant dose 0.5 mL/L [73].

2.6.11 Polyelectrolytes

Based on the present review, some novel pre-hydrolysed coagulants such as

Polyaluminium chloride (PACl), Polyaluminium ferric chloride (PAFCl),

Polyferroussulphate (PFS) and Polyferric chloride (PFCl) have been found to be

more effective and suggested for decolorisation of the textile wastewater. Moreover,

use of natural coagulants for textile wastewater treatment has also been emphasized

and encouraged as the viable alternative because of their eco-friendly nature [24].

2.6.12 Phaseolus vulgaris

The ability of coagulation active proteins from common bean (Phaseolus

vulgaris) seed for the removal of water turbidity was studied. Results revealed that

the highest values of the adsorbed protein were achieved in 50 mmol/L phosphate

buffer at pH 7.5 and the maximum adsorption capacity was calculated to be 0.51 mg

protein/mL matrix. Partially purified coagulant at initial turbidity 35 NTU expressed

the highest value of coagulation activity, 72.3 %, which was almost 22 times higher

than those obtained by crude extract considering applied dosages [74].

23

Table 2.4 Treatment of industrial wastewater using natural coagulants

Coagulant Industrial wastewater Pollutants

removed

Reference

Cassia obtusifolia Pulp and paper mill TSS, COD [64]

Palm oil mill TSS, COD [65]

Plantago major L Textile Color, COD [66]

Unmodified rice statch Palm oil mill TSS, COD [67]

Jatropha curcas Synthetic Turbidity [68]

Moringa oleifera Concrete plant Turbidity [69]

Tannin Surface water and industrial

wastewater

Turbidity,

color

[70]

Schinopsis balansae Drinking water Turbidity [71]

Ipomoea daysperma

and guargum

Textile Color [72]

Suranjana and maize seed

powder

Congo red synthetic

solution

Color [73]

Chestnut and acorn Synthetic turbid water Turbidity [24]

Polyelectrolyte Textile Color [74]

By looking into an above discussion (Table 2.4), it was confirmed that PIWW

could also be treated using the natural coagulants.

2.7 Application of Strychnos potatorum in wastewater treatment

S.potatorum (Nirmali), known as a clearing nut tree, is a moderate – sized

tree found in Southern and central parts of India, Sri Lanka and Burma, used

predominantly as a traditional medicinal extract. Sanskrit writings from India

reported that the seeds were used to clarify turbid surface water over 4,000 years ago

which indicated that they were the first reported plant –based coagulant used for

water treatment [23]. The ability of the S.potatorum seeds as coagulant and adsorbent

are reported by many environmental scientists, which are discussed below.

2.7.1 Strychnos potatorum seeds as a coagulant

The agro based materials evaluated are Surjana seed (M.oleifera), Nirmali

seed (S.potatorum) and maize (Zeemays) for the treatment of synthetic turbid water.

The filter performance was defined by water quality and head loss development

across the filter bed. When Nirmali seed or maize was used as a coagulant aid, the

24

alum dose required was 25 and 15 mg/L, respectively and the filtrate turbidity

achieved was less than 0.2 NTU, whereas alum alone with a dose of 45 mg/L

achieved filtrate turbidity levels higher than 1 NTU. Thus, the use of ABM improved

the filtrate quality [75].

In the research study the plant based coagulants namely, the seeds of

S.potatorum, pods of C.opuntia and mucilage extracted from the fruits of Coccinia

indica in powder forms were applied to a water treatment sequence comprising

coagulation-flocculation-sedimentation sand filtration on synthetic turbid water

created by kaolin. The optimum dose obtained from batch coagulation-sedimentation

test conducted for S. potatorum, Cactus and Coccinia indica were 1.5 mg/L, 30

mg/L, 0.5 mg/L and the filtrate turbidities obtained after filtration were 0 NTU, 1

NTU and 0 NTU respectively, which are lesser than 5 NTU [76].

Seeds of the plant species S.potatorum and M.oleifera contain natural

polyelectrolytes which can be used as coagulants to clarify turbid waters. In

laboratory tests, direct filtration of a turbid surface water (turbidity 15–25 NTU,

heterotrophic bacteria 280–500 cfu mL/L and fecal coliforms 280–500 MPN 100

mL/L), with seeds of S. potatorum or M. oleifera as coagulant, produced a substantial

improvement in its aesthetic and microbiological quality (turbidity 0.3–1.5 NTU,

heterotrophic bacteria 5–20 cfumL/L and fecal coliforms 5–10 MPN 100 mL/L) [77].

In the method the S. potatorum seeds were treated with different solvents of

NaCl and NaOH to extract the active coagulant agent has been investigated to

evaluate the turbidity removal of the synthetic and real turbidity water has been

investigated. The jar test was conducted on kaolin as a model wastewater. 0.5M

Sodium chloride extract was found to provide a high turbidity removal of > 99%

compared to NaOH and distilled water extract. The optimum turbidity removal at

different values of initial synthetic wastewater turbidity from 100 to 500 NTU was

investigated [78].

The present study, discussed about the usage of natural coagulants extracted

from M.oleifera and S. potatorum for the removal of color in the synthetically

25

prepared textile wastewater. The maximum percentage of color removal using alum,

M.oleifera and S. potatorum was found to be 83%, 89% and 93%, respectively [79].

2.7.2 Strychnos potatorum seeds as an adsorbent

In the present study, Pb(II) removal efficiency of S. potatorum seed powder

(SPSP) from aqueous solution has been investigated. Batch mode adsorption

experiments have been conducted by varying pH, contact time, adsorbent dose and

Pb (II) concentration. Pb(II) removal was pH dependent and found to be maximum at

pH 5.0. The maximum removal of Pb(II) was achieved within 360 min. The

monolayer adsorption capacities of SPSP as obtained from Langmuir isotherm was

found to be 16.420 mg/g [80].

In the present study the ability of S. potatorum seed proteins to bind aqueous

cadmium has been investigated. The Cd (II) biosorption efficiency by the proteins

has been investigated. Different experiments have been conducted (i) over a range of

pH (2.0–7.0), (ii) contact time (5–600 min), (iii) temperatures (4–40 ◦C) and (iv)

metal ion concentrations (80–110 mg /L). The results showed that the optimum

conditions for Cd (II) adsorption are almost same for the three proteins used in the

study [81].

Kinetic, mechanism, equilibrium and thermodynamic behavior of adsorption

of methylene blue (MB) dye onto surface modified S. potatorum seeds (SMSP), in an

aqueous solution were studied. Batch adsorption experiments were carried out to

analyze the effect of initial solution pH, adsorbent dose, contact time, initial MB dye

concentration and temperature on the removal of MB dye. The adsorption of MB dye

onto SMSP was found to be controlled by both surface diffusion and pore diffusion

[82].

The removal of Pb(II) ions from aqueous solution by chemically surface

modified S. potatorum seeds (SMSP) was investigated. The maximum adsorption

capacity of SMSP for Pb(II) ions was found to be 166.67 mg/g at optimum

conditions of pH 5.0, contact time of 30min, SMSP dosage of 2 g/L and temperature

of 30◦C [83].

26

In this study, the unmodified S. potatorum seeds were examined as an

adsorbent to remove the metal ions such as Cu(II), Cd(II) and Ni(II) ions from their

aqueous solutions. From the Langmuir adsorption isotherm model, the maximum

monolayer adsorption capacity of the adsorbent for Cu(II), Cd(II) and Ni(II) ions was

found to be: 8.649, 7.023 and 5.140 mg/g, respectively [84].

S. potatorum seeds have been utilized for the preparation of adsorbent,

surface modified S. potatorum seeds (SMSP), by sulfuric acid treatment with 1:2

ratios of precursor to sulfuric acid. The maximum removal of Ni(II) ions was

observed at an optimum conditions: pH of 5.0, adsorbent dose of 5 g/L, contact time

of 30 min and at temperature of 30°C for an initial Ni(II) ions concentration of 100

mg/L. The Freundlich constant “n” was found to be of 3.888 g/L which indicates that

the adsorption of Ni(II) ions onto the SMSP followed the physical process [85].

The above discussion clearly indicated that the S.potatorum was mainly used

as coagulant to remove the turbidity from synthetic solution. As an adsorbent it was

mainly utilized in the removal of heavy metals such as Pb (II), Cu (II), Cd (II) and

Ni (II) from aqueous solution.

2.8 Application of Cactus species in wastewater treatment

The most commonly studied cactus genus for water treatment is opuntia

which is colloquially known as “nopal” in Mexico or „prickly pear‟ in North

America. The high coagulation capability of Opuntia is most likely attributed to the

presence of mucilage which is a viscous and complex carbohydrate stored in cactus

inner and outer pods that has great water retention capacity [23]. C.opuntia is a cheap

and abundantly available plant. The main constituent of the cactus cladode is a hetero

polysaccharide with a molecular weight of 2.3 – 300 × 104 g/mol. Cactus has been

used as a food thickener, food emulsifier, as a water purifier and for purposes of

cosmetic application [86]. It has long been associated with its medicinal properties

and dietary food sources. Besides, it has also been successfully used as a natural

coagulant. It has a high possibility of galacturonic acid and exists predominantly in

polymeric form that provides a bridge for particle to adsorb on [87]. The

applicability of cactus species on water and wastewater treatment as a coagulant and

an adsorbent was shorted below.

27

2.8.1 Cactus species as a coagulant

In this study, the effectiveness of a natural macromolecular coagulant derived

from a cactus species for turbidity removal from estuarine and river waters were

evaluated using jar test. Turbidity values were reduced by as much as 98%

(estuarine) and 70% (river). High turbidity removal determined in this study

indicated that C.opuntia has the potential to be utilized for surface water treatment

applications [87].

The ability of two plant materials, C. latifaria and the seeds of P. juliflora, to

act as natural coagulants was tested using synthetic water. Both materials produced

comparable turbidity removals and were able to produce final water whose turbidity

was close to the required standard of 5 NTU with both high (100–200 NTU) and low

(30–40 NTU) initial turbidities. The optimum coagulant dose was found to be lower

than that for aluminum sulphate [88].

The cactus coagulation attained comparatively high turbidity removal

efficiency and water with turbidity less than 5 NTU could be obtained with initial

turbidities from 20 to 200. When used to treat the same water sample, the optimum

dosage of cactus coagulant was found similar to that of AlCl3.6H2O. High removal

efficiency of turbidity and COD could be obtained when cactus solids were used to

treat sewage water, potable water source and high turbidity seawater [89].

Cactaceae nopalea cochenillifera cell cultures and intact plants (cladodes)

transform various toxic textile dyes, including Red HE7B into less phytotoxic, non-

hazardous metabolites. Present foundation work could add another plant candidate

for phytoremediation of undesirable products from industry wastes and harmful

chemicals [90].

A mixture of aluminum salts and natural polyelectrolytes, extracted from the

C.opuntia ficus índica, has been used for cleaning of wastewater from poultry

slaughterhouse. A mixture of aluminum salt in a concentration range of 300 to 600

mg/L and natural polyelectrolytes of 0.6 to 0.8 mg/L was used for flocculation and

28

coagulation. The combination of coagulant and natural polyelectrolytes was able to

remove chemical oxygen demand (86%), oil and grease (93%), turbidity (89%) and

suspended solids (93%) [91].

This work was evaluated the role of three biopolymers used as coagulant –

flocculant aids in the treatment of a high load cosmetic industry wastewater. When

guar, locust bean gum and opuntia mucilage was used, conductivity and turbidity

removals as high as 20.1% and 67.8% were found, respectively. Chemical oxygen

demand (COD) removals as high as 38.6% were observed. The maximum removal

efficiency was found for mucilage, with 21.1 mg COD/ mg polymer [92].

2.8.2 Cactus species as an adsorbent

The biosorption of cadmium (II) and lead (II) ions onto a natural, plentiful

and low-cost biosorbent developed from cactus cladodes was investigated in batch

mode. The experimental results indicate that, the percentage of biosorption increases

with an increase in the biosorbent dosage and the decrease of particle size. The

equilibrium data fitted very well to the Langmuir model with a maximum monolayer

biosorption capacity of 30.42 and 98.62 mg/g, respectively for cadmium (II) and lead

(II) ions [86].

The biosorption of Methylene Blue (MB), Eriochrome Black T (EBT) and

Alizarin S (AS) from aqueous solutions by dried prickly pear cactus cladodes as a

low-cost, natural and eco-friendly biosorbent was investigated. The experimental

results show that, the percentage of biosorption increases with an increase in the

biosorbent dosage and the decrease of particle size. Langmuir maximum monolayer

biosorption capacity was 189.83 mg/g for Methylene Blue, 200.22 mg/g for

Eriochrome Black T and 118.35 mg/g for Alizarin S [93].

Binary oxidized cactus fruit peel (CFP) was used as adsorbent for the

removal of brilliant green (BG). The equilibrium adsorption data was found to follow

the Langmuir isotherm model and maximum monolayer capacity was found to be

166.66 mg/ g at 20 °C [94].

29

Cactus species were made use to remove the metal ions and dye from

synthetic solution through adsorption. From the results of previous work it was

confirmed its potentiality as coagulant and adsorbent in wastewater treatment. It

could also be tested in PIWW.

2.9 Application of crab shells in wastewater treatment

The successful treatment process, not only depends on the pollutant removal

ability of the coagulant, but also in abundance of the material for the treatment

processes everywhere. So the coagulant should either be an industrial waste or

available plenty in nature [25]. Crab shells are the huge quantity of the natural waste

product from the seafood processing industries. Millions of tons of crab shells are

being generated annually all over the world. Due to high costs and strict

environmental regulations, landfills are becoming less popular for waste disposal.

Proper reuse of this material can be the better solution and also generates possible

revenue to the industries [95].

The chemical composition of crab shell was CaCO3 40-66%, MgCO3 3-5%,

protein 11-29%, chitin 20-27%, lipid 1.35% and less than 2% others on dry basis.

Chitosan in crab shell waste has the advantage of low cost and high biocompatibility

[96]. 100 g of crab shell powder yielded 6.83 g raw chitin after dimeneralization,

deproteinization and 4.65g of chitosan [97].

Chitosan a linear cationic polymer of high molecular weight obtained from

the outer shells of crustaceans particularly crabs and shrimp, has recently been

proposed for applications of heavy metal sorption, drinking water treatment and

industrial effluent treatment. Chitosan not only acts as an adsorbent, but also

spontaneously coagulates to agglomerate the pollutants [98]. The following section

consolidated the results of previous work.

2.9.1 Crab shell as a coagulant

Shrimp and crabs are becoming a source of raw materials for the chemical

process industries because the crustaceans' shells contain chitin. Deacetylation of

chitin yields chitosan, a cationic electrolyte. It may serve as a flocculant, coagulant

30

and food thickener or extender and also may find use in making fibers and moisture

proof films and coatings. Some technical and economic aspects of shellfish waste

utilization are discussed including a sketch of chemical plant for chitin route [99].

This experiment discussed the productive condition of chitosan with crab

shell as material and the effect of absorption, as absorbent, to wastewater with heavy

metal ions under static condition and the effect of coagulation, as coagulant, to

dyeing wastewater. The results showed that pH of the system and absorb time effect

significantly on absorption properties of chitosan. Using chitosan with the optimum

condition, the removal efficiency of Pb2+

, Cr6+

, Cu2+

could reach above 98 % [100].

Chitosan, a natural cationic polyelectrolyte and other similar coagulants were

used in the treatment of an olive oil water suspension as a model for the processing

wastewater. The effect of chitosan, starch, alum and ferric chloride on the

coagulation of oil droplets were determined by the jar test apparatus and

turbidometric measurements. In the air flotation experiments, a concentration of 100

ppm of chitosan, an air flow rate of 3 L /min, aeration time of 45 s, temperature of

20 ºC and pH 6 produced optimum levels [101].

In this study the wastewater from the system of cleaning in place (CIP)

containing high content of fat and protein was coagulated using chitosan and the fat

and the protein can be recycled. The result shows that the optimal result was reached

under the condition of pH 7 with the coagulant dosage of 25 mg/L [102].

In the present work the efficiency of chitosan and conventional coagulants

(aluminum sulphate and ferric chloride) was compared in terms of turbidity and

natural organic matter (NOM) removal, as well as acute toxicity on Daphnia magna

of coagulated and coagulated/chlorinated surface water. All coagulants decreased

toxicity on D. magna from 100% to 0% immobilization. Moreover, the chlorination

step after coagulation increased toxicity too according to the coagulant type as

follows: chitosan > ferric chloride > aluminum sulphate [98].

The application of chitosan as the coagulant/flocculant in a microfiltration

process of natural water has been evaluated. At the permeation velocity of 45 L m2/h,

31

microfiltration led to reductions up to 33 % of UV254 compounds, 9 % of TOC and

65 % of iron, while the coupled coagulation–microfiltration process resulted in a

reduction up to 70 %, 47 % and 100 % for UV254 compounds, TOC and iron,

respectively. Although the coagulation process caused an increase in fouling, the

treated water quality was higher comparatively to the simple microfiltration process

[103].

In this study, the jar-test method was used to identify the best chitosan

conditions for harvesting the Chlorella sp. from their cultures. Chitosan not only acts

as an adsorbent, but also spontaneously coagulates to agglomerate the microalgae

cells. This two-in-one process makes the chitosan a good coagulant, allowing

removal of microalgae cells, even at low concentrations. Chitosan successfully

removed 99.0 ± 0.4 % of the microalgae cells at the following optimal parameters:

chitosan concentration of 10 ppm, mixing time of 20 min, mixing rate of 150 ppm

and sedimentation time of 20 min [104].

Results of laboratory experiments into the removal of humic substances by

cationic biopolymer chitosan are presented. Chitosan is partially soluble in dilute

mineral acids such as HNO3, HCl, H3PO4. We have used 1 % solutions of chitosan

diluted in 0.05M; 0.1M and 0.15M HCl. Aggregates of humic substances after

inorganic coagulant or chitosan addition were separated by centrifugation. Residual

concentration of coagulant (Fe and Al) and absorbance at 387 nm and 254 nm were

evaluated [105].

2.9.2 Crab shell as an adsorbent

This work discussed the possible application of a biosorption system with

acid-washed crab shells in a packed bed up-flow column for the removal of nickel

from electroplating industrial effluents. Between two nickel-bearing effluents,

effluent-1 was characterized by considerable amount of light metals along with trace

amounts of lead and copper. Effluent-2 was characterized by relatively low

conductivity, total dissolved solids and total hardness compared to effluent-1. Crab

shells exhibited uptakes of 15.08 and 20.04 mg Ni/g from effluent-1 and effluent-2,

32

respectively. The data from regeneration efficiencies for seven cycles provided

evidence that the reusability of crab shell [106].

Biosorption of each of the heavy metals, copper (II) and cobalt (II) by crab

shell was investigated in this study. At optimum particle size (0.767 mm), biosorbent

dosage (5 g/L) and initial solution pH (pH 6); crab shell recorded maximum copper

and cobalt uptakes of 243.9 and 322.6 mg/g, respectively, according to Langmuir

model. The biosorbent was successfully regenerated and reused using EDTA for five

cycles [107].

The carapace of the crab (C. pagurus), a waste material disposed of by the

seafood industry, has recently been shown to have potential as a biosorbent for the

removal of metals from aqueous media. In sequential-batch process Zn (II)uptakes of

105.6 and 67.6 mg/g were recorded for 0.25–0.8mm and 0.8–1.5mm particles,

respectively, while values of 141.3 and 76.9 mg/g were recorded in fixed-bed column

studies. Binary-metal studies showed that the presence of Cu (II) or Pb (II)

significantly suppressed Zn (II) uptake [108].

Sorption potential of pretreated crab and arca shell biomass for lead and

copper from aqueous media was explored. Effects of common ions like sodium,

potassium, calcium and magnesium on the sorption capacity of pretreated crab and

arca biomasses were also studied. At equilibrium, the maximum uptake by crab shell

biomass was 19.83 ± 0.29 and 38.62 ± 1.27 mg/g for lead and copper, respectively.

In case of arca shell biomass the maximum uptake capacity was 18.33 ± 0.44 mg/g

and 17.64 ± 0.31 mg/g for lead and copper, respectively [25].

In this study, crab shells were recycled as an adsorbent for the removal of

phosphate. Although removal efficiency was highest at pH 2.0, the efficiency

remained 50–60 % at pH of 4.0–10.0. The maximum removal capacity was

calculated as 108.9 mg/g through Langmuir isotherm plotting, which was 17.0 and

4.7 times higher than those of coal fly ash and scallop shells, respectively [109].

The ability of crab shell to biosorb two rare earth elements (REE), namely Ce

(III) and Eu (III) from single and binary systems has been studied. At optimum pH of

6, in single component system, crab shell exhibited maximum Ce(III) and Eu(III)

uptakes of 144.9 and 49.5 mg/g, respectively, according to the Langmuir model [96].

33

This work explored the potential of crab (P. sanguinolentus) shell particles

for the removal of Mn (II) and Zn (II) ions from aqueous solutions. The process of

metal biosorption was rapid (90% removal in 120 min for Mn (II) and 90% removal

in 90 min for Zn (II)) at an initial metal concentration of 500 mg/L. Furthermore,

isotherm experiments revealed that crab shell possesses high uptake capacities of

69.9 and 123.7 mg/g for Mn (II) and Zn (II), respectively, according to the Langmuir

model [95].

It was affirmed that the crab shells as well as the chitosan extracted from crab

shells is applied in the wastewater treatment as a coagulant and an adsorbent. Variety

of metal ions from synthetic and industrial wastewater could be removed using crab

shells.

2.10 Fundamentals and mechanism of coagulation and flocculation process

All waters, especially surface waters, contain both dissolved and suspended

particles. Coagulation and flocculation processes are used to separate the suspended

solids portion from the water. The suspended particles vary considerably in source,

composition charge, particle size, shape and density. Correct application of

coagulation and flocculation processes and selection of the coagulants depend upon

understanding the interaction between these factors. The small particles are stabilized

(kept in suspension) by the action of physical forces on the particles themselves. One

of the forces playing a dominant role in stabilization results from the surface charge

present on the particles. Most solids suspended in water possess a negative charge

and, since they have the same type of surface charge, repel each other when they

come close together. Therefore, they will remain in suspension rather than clump

together and settle out of the water (Fig. 2.2).

Processes

Coagulation

Flocculation ( species being bound by coagulant aids)

Sedimentation (Aggregation of flocs and consequent settling)

34

Figure 2.2 Mechanism of coagulation –flocculation and sedimentation

2.10.1 Coagulation

The first step destabilizes the particle‟s charges. Coagulants with charges

opposite those of the suspended solids are added to the water to neutralize the

negative charges on dispersed non-settlable solids such as clay and color-producing

organic substances. Once the charge is neutralized, the small suspended particles are

capable of sticking together. The slightly larger particles formed through this process

and called microflocs, are not visible to the naked eye. The water surrounding the

newly formed microflocs should be clear. If it is not, all the particle charges have not

been neutralized and coagulation has not been carried to completion. More coagulant

may need to be added.

A high-energy, rapid-mix to properly disperse the coagulant and promote

particle collisions is needed to achieve good coagulation. Over-mixing does not

affect coagulation, but insufficient mixing will leave this step incomplete.

Coagulants should be added where sufficient mixing will occur. Proper contact time

in the rapid-mix chamber is typically 1 to 3 minutes [110].

Fundamentals and mechanism of coagulation

Aggregation of particulates in a solution can occur via four classic

coagulation mechanisms:

35

(a) Double layer compression

(b) Sweep flocculation

(c) Adsorption and charge neutralization and

(d) Adsorption and interparticle bridging. [23]

Mechanisms

2.10.1. a A double layer compression

The negative colloid and its positively charged atmosphere produce an

electrical potential across the diffuse layer. This is highest at the surface and drops

off progressively with distance, approaching zero at the outside of the diffuse layer.

The potential curve indicates the strength of the repulsive force between colloids and

the distance at which these forces come into play. A particular point of interest on the

curve is the potential at the junction of the Stern layer and the diffuse layer. This is

known as the zeta potential.

It is an important concept because zeta potential is the potential at the surface

of shear, i.e. the boundary surface between the fixed ion layer and the solution. This

layer acts as a shear plane when the particle undergoes movement in the solution.

Zeta potential is an effective tool for coagulation control because changes in zeta

potential indicate changes in the repulsive force between colloids.

A coagulant is added to help destabilize the particles. A coagulant can do this

in three ways. A cationic coagulant reduces the zeta potential of the particles by

adding positive charge. This is usually accomplished by adding a metal salt to the

water. The metal forms strong bonds with the oxygen of the water molecules

weakening them and releasing hydrogen ions into solution. The hydrogen ions are

attracted to the negative surface charge of the particles and neutralize it. Double layer

compression involves adding salts to the system. As the ionic concentration

increases, the double layer and the repulsion energy curves are compressed until

there is no longer an energy barrier. Particle agglomeration occurs rapidly under

these conditions because the colloids can just about fall into the van der Waals “trap”

without having to surmount an energy barrier.

36

The thickness of the double layer depends upon the concentration of ions in

solution. A higher level of ions means more positive ions are available to neutralize

the colloid. The result is a thinner double layer. Decreasing the ionic concentration

(by dilution, for example) reduces the number of positive ions and a thicker double

layer results. The type of counter-ion will also influence double layer thickness. Type

refers to the valence of the positive counter-ion and its effect is explained in a

previous section. Increasing the concentration of ions or their valence or both

referred to as double layer compression.

The quantity of ions in the water surrounding a colloid has an effect on the

decay function of the electrostatic potential. The high ionic concentration compresses

the layers composed predominantly of counter ions toward the surface of the colloid.

If this layer is sufficiently compressed, then the Van der Waals force will be

dominant across the entire area of influence, so that the net force will be the

attractive force. In general, double layer compression is not a practical coagulation

technique for water treatment but it can have application in industrial wastewater

treatment if waste streams with divalent or trivalent counter-ions happen to be

available [111].

2.10.1. b Adsorption and charge neutralization

Inorganic coagulants (such as alum) and cationic polymers often work

through charge neutralization. It is a practical way to lower the energy barrier and

form stable flocs. Charge neutralization involves adsorption of a positively charged

coagulant on the surface of the colloid. This charged surface coating neutralizes the

negative charge of the colloid, resulting in a near zero net charge. Neutralization is

the key to optimizing treatment before sedimentation, granular media filtration or air

flotation. Adsorption of the counter ions on the colloid surface causes charge

neutralization, which brings about van der Walls forces become dominant.

Charge neutralization alone will not necessarily produce dramatic macroflocs

(flocs that can be seen with the naked eye). Microflocs (which are too small to be

seen) may form but will not aggregate quickly into visible flocs. Charge

neutralization is easily monitored and controlled using zeta potential. This is

37

important because overdosing can reverse the charge on the colloid and redisperse it

as a positive colloid. The result is a poorly flocculated system. When a coagulant salt

is added to water, it dissociates and the metallic ion the metallic ion goes hydrolysis

and creates positively charged hydroxometalic ion complexes. The

hydroxometallicions are polyvalent, possess high positive charges and adsorbed to

the surface of the negative colloids. This results in a reduction of the zeta potential to

a level where the colloids are destabilized. The destabilized particles, along with

their adsorbed hydro-metallic hydroxometallic complexes, aggregate by

interparticulate Van der Waals forces. These forces are aided by the gentle mixing in

water [111].

2.10.1. c Enmeshment in a precipitate

Colloid entrapment involves adding relatively large doses of coagulants,

usually aluminum or iron salts which precipitate as hydrous metal oxides. The

amount of coagulant used is far in excess of the amount needed to neutralize the

charge on the colloid. Some charge neutralization may occur but most of the colloids

are literally swept from the bulk of the water by becoming enmeshed in the settling

hydrous oxide floc. This mechanism is often called sweep floc. Sweep floc is

achieved by adding so much coagulant to the water that the water becomes saturated

and the coagulant precipitates out. Then the particles get trapped in the precipitant as

it settles downward [111].

2.10.1.d Interparticular bridging

Bridging occurs when a coagulant forms threads or fibers which attach to

several colloids, capturing and binding them together. Inorganic primary coagulants

and organic polyelectrolytes both have the capability of bridging. Higher molecular

weights mean longer molecules and more effective bridging. Bridging is often used

in conjunction with charge neutralization to grow fast settling and/or shear resistant

flocs. For instance, alum or a low molecular weight cationic polymer is first added

under rapid mixing conditions to lower the charge and allow microflocs to form.

Then a slight amount of high molecular weight polymer, often an anionic, can be

added to bridge between the microflocs.

38

The fact that the bridging polymer is negatively charged is not significant

because the small colloids have already been captured as microflocs. In recent years

the coagulation and flocculation of colloidal suspensions by organic polyelectrolytes

has become increasingly important, since both laboratory and plant scale work have

demonstrated their effectiveness in extremely low concentrations. The polymeric

substances started to be used as a coagulant have a specific site, which can be

absorbed by the colloidal particles possessing long chain structure. These polymers

are highly surface reactive. Thus, several colloids may become attached to one

polymer and several of the polymer-colloid groups may become enmeshed resulting

in a settleable mass. In order to assist interparticle bridging, some synthetic polymers

may be used in addition to, organic polyelectrolytes instead of metallic salts.

Adsorption sites on the colloidal particles can adsorb a polymer molecule. A

bridge is formed when one or more particles become adsorbed along the length of the

polymer. Bridge particles become intertwined with other bridged particles during the

flocculation process [111].

2.10.2 Flocculation

Following the first step of coagulation, a second process called flocculation

occurs. Flocculation, a gentle mixing stage, increases the particle size from

submicroscopic microfloc to visible suspended particles. Once particles have stuck

together they are called a floc and the process of encouraging the formation of flocs

is called flocculation.

The microflocs are brought into contact with each other through the process

of slow mixing. Collisions of the microfloc particles cause them to bond to produce

larger, visible flocs called pinflocs. The floc size continues to build through

additional collisions and interaction with inorganic polymers formed by the

coagulant or with organic polymers added. Macroflocs are formed. High molecular

weight polymers, called coagulant aids, may be added during this step to help bridge,

bind and strengthen the floc, add weight and increase settling rate. Once the floc has

reached it optimum size and strength, the water is ready for the sedimentation

39

process. Design contact times for flocculation range from 15 or 20 minutes to an

hour or more [112].

Types

The flocculation process can be broadly classified into two types,

Perikinetic and

Orthokinetic.

Perikinetic flocculation refers to flocculation (contact or collisions of

colloidal particles) due to Brownian motion of colloidal particles. The random

motion of colloidal particles results from their rapid and random bombardment by

the molecules of the fluid.

Orthokinetic flocculation refers to contacts or collisions of colloidal particles

resulting from bulk fluid motion, such as stirring. In systems of stirring, the velocity

of the fluid varies both spatially (from point to point) and temporally (from time to

time).

The spatial changes in velocity are identified by a velocity gradient, G. G is

estimated as G = (P/hV)1/2

, where P=Power, V=channel volume and h= Absolute

viscosity.

2.10.3 Sedimentation

The last process to the first barrier against water contamination is

sedimentation. During sedimentation, the flow of the water is slowed to resemble a

calm environment. As the water is calmed, the large flocs that have been formed

settle to the bottom of the sedimentation basin, sometimes called a clarifier. As the

flocs are settling to the bottom, the relatively particle free water passes over a system

of weirs and moves to the filtration process. Sedimentation basins are designed to be

rectangles or circles, but in both cases the water is commonly introduced at the

bottom of the basin to give the flocs the best chance at completely settling out. A

mechanical rack collects the flocs that have reached the bottom and remove them

40

onto what is called sludge treatment. However, not all of the flocs are large enough

to settle out and can continue to stay in the water. Stoke‟s law describes the velocity

at which the flocs settle,

𝑣𝑠 =2

9

𝜌𝑓−𝜌𝐿

𝜇𝑔𝑅2 (2.1)

Where,

vs is the settling velocity (cm/s)

ρf is the density of the floc (g/cm3)

ρL is the density of the liquid (g/cm3)

μ is the liquid viscosity (g/cm. s)

g is the force of gravity (g/cm.s2) and

R is the radius of the floc (cm).

From Stokes‟ equation, the two parameters that determine whether or not the

flocs successfully settle to the bottom are the floc‟s density and radius. It is the job of

coagulation and flocculation to make the flocs dense and large, but engineers can

design the sedimentation basin so that the water spends long enough in the basin to

settle out a maximum amount of the flocs formed. Water typically spends a couple of

hours in the sedimentation before the top water flows over to be filtered [113].

2.10.4 Factors affecting coagulation

For all raw water types, there are several water quality parameters that affect

coagulation performance, including

the amount of particulate material

NOM properties (such as size, functionality, charge and hydrophobicity)

the bulk chemical and physical properties of the water

coagulant type

41

dose

mixing conditions

pH

temperature

The pH at which coagulation occurs is the most important parameter for

proper coagulation performance, as it affects the:

Surface charge of colloids.

Charge of NOM functional group.

Charge of the dissolved-phase coagulant species.

Surface charge of floc particles.

Coagulant solubility. [114]

2.10.5 Types of coagulation

Coagulation mechanisms destabilization of turbidity and color-causing

substances can be induced by different mechanisms (Fig. 2.3). The following

subdivisions can be made [112]:

Electrostatic coagulation

Adsorptive coagulation

Precipitation coagulation

2.10.5. a Electrostatic coagulation

In electrostatic coagulation, positively charged ions approach the negatively

charged colloids. In the diffusive layer around the colloid, the positively charged ions

accumulate, destabilizing the colloid [112].

2.10.5. b Adsorptive coagulation

In adsorptive coagulation, particles are adsorbed to the positively charged

hydrolyses products. Characteristics of adsorptive coagulation are that dosing is

42

proportional to the removal of organic matter and that restabilization can occur after

an overdose of coagulant. After an overdose, the colloids will be positively charged

and repulsion of the particles will take place. It is a rapid process. Within a second,

positively charged hydrolyses products are formed and are adsorbed to the negatively

charged particles [112].

2.10.5.c Precipitation coagulation

In precipitation coagulation, or sweep coagulation, colloids are incorporated

into neutral (iron) hydroxide flocs. This mechanism occurs mainly in waters with low

suspended solids content (10 mg/L). In order to form hydroxide flocs, more

coagulant must be dosed than is necessary for adsorptive coagulation [112].

Figure 2.3 Mechanism of coagulation processes

2.10.6 Coagulants

Generally, coagulation is the destabilisation of pollutants using coagulant(s),

which can be classified into two main categories i.e. metal coagulants and polymers.

Coagulation tends to overcome the factors that promote pollutant stability and form

agglomerates or flocs. Flocculation in other words is the process of whereby

destabilised particles, or particles formed as a consequence of destabilisation, are

induced to come together, make contact and thereby form large(r) agglomerates.

Coagulation of water-soluble pollutant is challenging because of their high solubility.

In addition to this and due to the development of synthesis technology, new varieties

43

of dyes with different structures appear continuously, which provides difficulties for

the selection of an appropriate coagulant [24].

Chemical precipitation is a proven technology with regard to removal of

metals and other inorganics from wastewater. Precipitations of contaminants that are

dissolved or suspended in the wastewater are settled out when a coagulant is added.

It then forms a precipitate that can be settled out either by filtration or centrifugation.

Coagulants that are commonly used consist of long-chained polymers that are

cationic, anionic or neutral in charge, allowing the coagulant to interact with other

charged ions in the solution causing a „„bridge‟‟ that is responsible for binding

molecules together. Coagulation–flocculation is similar to chemical precipitation in

that charged particles in suspension are neutralized when they collide. Sedimentation

can occur if the density of the combined particles becomes more than that of the

aqueous phase and is considered a form of chemical precipitation [115].

Polymeric coagulants can be cationic, anionic or nonionic, in which the

former two are collectively termed as polyelectrolytes. Many studies concerning

natural coagulants referred to them as „polyelectrolytes‟ even though many of these

studies did not actually conduct in-depth chemical characterization to determine their

ionic activity. Natural coagulants are mostly either polysaccharides or proteins. In

many cases, even though polymers labeled as non-ionic are not necessarily absent of

charged interactions, as there may be interactions between the polymer and a solvent

within a solution environment as the polymer may contain partially charged groups

including –OH along its chain. It is imperative to fully grasp the underlying

coagulation mechanisms associated with these natural coagulants so that complete

understanding of their usage can be realized [23].

2.11 Fundamentals and mechanism of adsorption process

2.11.1 Adsorption phenomenon

Adsorption is a surface phenomenon with common mechanism for organic

and inorganic pollutants removal. Adsorption is a process that occurs when a gas or

liquid solute accumulates on the surface of a solid or a liquid (adsorbent), forming a

44

molecular or atomic film (the adsorbate). The term desorption is the reverse process.

Adsorption is operative in most natural physical, biological and chemical systems

and is widely used in industrial applications such as activated charcoal, synthetic

resins and water purification.

When a solution containing adsorbable solute comes into contact with a solid

with a highly porous surface structure, liquid–solid intermolecular forces of

attraction cause some of the solute molecules from the solution to be concentrated or

deposited at the solid surface. The solute retained (on the solid surface) in adsorption

processes is called adsorbate, whereas, the solid on which it is retained is called as an

adsorbent. This surface accumulation of adsorbate on adsorbent is called adsorption.

This creation of an adsorbed phase having a composition different from that

of the bulk fluid phase forms the basis of separation by adsorption technology. In a

bulk material, all the bonding requirements (be they ionic, covalent, or metallic) of

the constituent atoms of the material are filled by other atoms in the material.

However, atoms on the surface of the adsorbent are not wholly surrounded by other

adsorbent atoms and therefore can attract adsorbates [116-118].

2.11.2 Driving force for adsorption

Particularly for the liquids the adsorption may occur due to electrostatic

attraction. As the adsorption progresses, an equilibrium is attained between the

solution and adsorbent. The adsorption amount (qe, mg/g) of the molecules at the

equilibrium step was determined according to the following equation:

𝑞𝑒 =

𝑉

𝑀

𝐶𝑜 − 𝐶𝑒 (2.2)

Where,

V is the solution volume (L)

M is the mass of monolithic adsorbents (g) and

Co and Ce are the initial and equilibrium adsorbate concentrations (mg/L),

respectively.

45

The driving force for adsorption is the reduction in interfacial

(surface) tension between the fluid and the solid adsorbent as a result

of the adsorption of the adsorbate on the surface of the solid.

The surface or interfacial tension, σ, is the change in free energy, G,

resulting when the

Area between two phases, A, is increased. The definition of σ is [119]:

𝜎 = 𝜕𝐺

𝜕𝐴 𝑇 ,𝑃,𝑛𝑗

(2.3)

2.11.3 Factors affecting adsorption

The most important factors affecting adsorption are:

Surface area of adsorbent

Particle size of adsorbent

Contact time or residence time

Solubility of solute (adsorbate) in liquid (wastewater)

Affinity of the solute for the adsorbent (carbon)

Number of carbon atoms

Size of the molecule with respect to size of the pores

Degree of ionization of the adsorbate molecule

pH

2.11.4 Adsorption mechanism-diffusion of adsorbate

There are essentially four stages in the adsorption of an organic/inorganic

species by a porous adsorbent [120] (Fig. 2.4):

1. Transport of adsorbate from the bulk of the solution to the exterior film

surrounding the adsorbent particle;

46

2. Movement of adsorbate across the external liquid film to the external surface

sites on the adsorbent particle (film diffusion);

3. Migration of adsorbate within the pores of the adsorbent by intraparticle

diffusion (pore diffusion);

4. Adsorption of adsorbate at internal surface sites.

All these processes play a role in the overall sorption within the pores of the

adsorbent. In a rapidly stirred, well mixed batch adsorption, mass transport from the

bulk solution to the external surface of the adsorbent is usually fast. Therefore, the

resistance for the transport of the adsorbate from the bulk of the solution to the

exterior film surrounding the adsorbent may be small and can be neglected. In

addition, the adsorption of adsorbate at surface sites (step 4) is usually very rapid and

thus offering negligible resistance in comparison to other steps, i.e. steps 2 and 3.

Thus, these processes usually are not considered to be the rate-limiting steps in the

sorption process [120].

In most cases, steps (2) and (3) may control the sorption phenomena. For the

remaining two steps in the overall adsorbate transport, three distinct cases may occur:

Case I: external transport > internal transport.

Case II: external transport < internal transport.

Case III: external transport ≈ internal transport.

In cases I and II, the rate is governed by film and pore diffusion, respectively.

In case III, the transport of ions to the boundary may not be possible at a significant

rate, thereby, leading to the formation of a liquid film with a concentration gradient

surrounding the adsorbent particles [120].

Usually, external transport is the rate-limiting step in systems which have

(a) Poor phase mixing,

(b) Dilute concentration of adsorbate

47

(c) Small particle size and

(d) High affinity of the adsorbate for the adsorbent.

In contrast, the intra-particle step limits the overall transfer for those systems

that have (a) a high concentration of adsorbate, (b) a good phase mixing, (c) large

particle size of the adsorbents and (d) low affinity of the adsorbate for adsorbent.

Figure 2.4 Mechanism of adsorption processes [121]

2.11.5 Modes of adsorption

In all these processes adsorbent is added to the wastewater for a given period

of time to allow pollutants to be adsorbed. This process can be carried out as [120]

1) Batch adsorption

2) Continuous fixed bed column adsorption

Batch adsorption

Batch adsorption consists of contacting finely divided adsorbent or

immobilized adsorbent with the wastewater for a given period of time in

a mixing vessel.

Surface area of the adsorbent is maximized for mass transfer and

agitation is minimized for suspension.

48

After the process is complete (typical contact time: 10 - 60 minutes) the

spent adsorbent is separated from the wastewater (e.g., by filtration) and

regenerated or disposed off (Fig. 2.5).

Continuous fixed bed column adsorption

As the wastewater moves through a fixed bed of adsorbent the pollutant to be

adsorbed will move from the wastewater to the adsorbent bed. Several steps are

involved in the overall adsorption process of a single molecule of pollutant:

Mass transfer step:

Mass transfer from the bulk of the wastewater to the surface of the adsorbent

particle, through the boundary layer around the particle.

Diffusion step:

Internal diffusion through a pore.

Adsorption step:

Adsorption on to the surface of the particle relative magnitude of the rates

involved in adsorption process.

In most wastewater treatment applications the overall adsorption process is

dominated by mass transfer, especially intraparticle mass transfer. A qualitative

ranking of the magnitude of the resistances is:

External interparticle mass transfer step → slow to not-so-slow, depending on

the operation

Intraparticle diffusion step → typically slow

Adsorption step → typically fast

2.11.6 Breakpoint and breakthrough curve

Eventually the forward part of the adsorption wave reaches the end of the

bed. When this happens the bed begins to release wastewater having a concentration

49

of pollutant higher than the desired value (typically 5-10% of the influent

concentration). This point is called the breakpoint. The corresponding curve of

pollutant concentration in the effluent vs. time is called the breakthrough curve

exhaustion point and bed saturation (Fig. 2.6). Past the breakpoint the pollutant

concentration in the effluent rises rapidly (i.e., the breakthrough curve is typically

steep), until it reaches an arbitrarily defined exhaustion point where the column

approaches saturation. If more wastewater is passed through the bed the entire

adsorbent content of the bed becomes saturated. Then, the wastewater leaving the

bed has the same concentration of pollutant as the incoming wastewater [122].

Analysis of fixed-bed adsorption columns

The primary objectives of such an analysis are:

Determination of the

Total (maximum) column adsorption capacity.

Depth of the adsorption zone and the shape of the breakthrough curve.

Breakpoint, including the volume of wastewater that can be treated before the

breakpoint is reached, the time at which this happens and the degree of

column saturation at breakpoint.

Figure 2.5 Unsteady state mass balance for batch adsorption

50

Figure 2.6 Breakpoint and breakthrough curve of continuous adsorption

51

Scope of the present study

The above literature review indicates that natural materials of different

origins can be used for treating PIWW. The systematic investigations of researchers

aimed at evaluating the competence of natural materials in pollutant removal show

that these biodegradable, low cost materials offer great promise for commercial use

in the future.

S.potatorum, C.opuntia and P.sanguinolentus(crab) shells were selected for a

detailed study in batch coagulation, for the evaluation of their performance towards

removal of the color, chemical oxygen demand (COD) and turbidity from water

based PIWW and to compare with conventional coagulants, alum and ferric chloride.

The potentiality of these materials as an adsorbent was checked using a fixed bed

adsorption column (FBC).