See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/267633076 Coagulation-flocculation processes in water and wastewater treatment. (II) Fine particles and its removal using electrolytes and polyelectrolytes Chapter · September 2014 CITATIONS 0 READS 1,178 1 author: Some of the authors of this publication are also working on these related projects: Treatment procedure of wastewaters from textile industry. Patent No. RO127284-B1 / 30/09/2016 View project Manufacturing of value-added textiles for aromatherapy and skin care benefits, project COFUND-MANUNET III-AromaTex View project Carmen Zaharia Gheorghe Asachi Technical University of Iasi 236 PUBLICATIONS 1,124 CITATIONS SEE PROFILE All content following this page was uploaded by Carmen Zaharia on 05 April 2016. The user has requested enhancement of the downloaded file.
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See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/267633076
Coagulation-flocculation processes in water and wastewater treatment. (II)
Fine particles and its removal using electrolytes and polyelectrolytes
Chapter · September 2014
CITATIONS
0READS
1,178
1 author:
Some of the authors of this publication are also working on these related projects:
Treatment procedure of wastewaters from textile industry. Patent No. RO127284-B1 / 30/09/2016 View project
Manufacturing of value-added textiles for aromatherapy and skin care benefits, project COFUND-MANUNET III-AromaTex View project
Carmen Zaharia
Gheorghe Asachi Technical University of Iasi
236 PUBLICATIONS 1,124 CITATIONS
SEE PROFILE
All content following this page was uploaded by Carmen Zaharia on 05 April 2016.
The user has requested enhancement of the downloaded file.
Current topics, concepts and research priorities in environmental chemistry (III)
167
Chapter 8
Coagulation-flocculation processes in water and wastewater treatment. (II) Fine particles and its removal using electrolytes
and polyelectrolytes
Carmen Zaharia*
‘Gheorghe Asachi’ Technical University of Iasi, Faculty of Chemical Engineering and Environmental Protection, Department of Environmental Engineering and Management,
The permanent watercourses nearly industrial sites of cities are known to have contaminated water bodies with various pollutants such as organics, minerals and heavy metal ions. Some quality indicators are permanently controlled alongside the length of watercourse (i.e. some general and specific quality indicators for oxygen regime and solid contents, aquatic fauna), or at least few times per year (especially for toxic compounds: pesticides, PCBs, total arsen, lead and chrome ions).
Some physical-chemical-biological processes are usually interacting such as chemical precipitation, colloids’ aggregation by coagulation-flocculation processes, redox processes, complexation, neutralization (acid-basic process), ionic exchange, adsorption processes at water-sediment, or water-solid phase interfaces, or in-deep water mass, aerobic and/or anaerobic biological processes caused by specific metabolism of different aquatic plants or organisms, sedimentation, filtration/biofiltration, etc.), or other hybrid processes. One of these processes that occurs in natural state in all surface waters (e.g., river, lake) with differentiated magnitudes is coagulation-flocculation associated with natural gravity sedimentation, or filtration/biofiltration (or air flotation), which is presented as mechanism, destabilizing agents and different factors influencing the process efficiency and stability of formed aggregates. In addition, some laboratory scale set-up tests of coagulation-flocculation (Jar tests) applied for a Romanian Northern-Eastern river (i.e. Bahlui River, Iasi City area) were performed and the results summarized, especially for coagulation-flocculation results working with inorganic coagulants like ferric sulphate - Fe2(SO4)3, aluminium hydroxochlorosulfate (AHCS), and specific polyelectrolyte (i.e. anionic Ponilit GT-2 polyelectrolyte), respectively, in different environmental conditions. The best treatment degree (i.e. > 90% for colour and turbidity, and > 44% for organics expressed by COD) were performed with a individual dose of around 5-30 mg/L Fe2(SO4)3, 50-70 mg/L AHCS, and/or 10-50 mg/L Ponilit GT-2 polyelectrolyte, much lower (more than 10-100 times lower) if the electrolytes and the polyelectrolyte are acting together for the same reason (separation of fine turbidity solids). Without Fe3+ ions, the highest efficiency (turbidity and colour) was performed with a dose of 75 mg/L AHCS, and 10-30 mg/L anionic polyelectrolyte.
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At least one mechanical cleaning treatment (with special mechanical tools) is annually obvious necessary before the beginning of hot season, more exactly in earlier spring times (March - April) for elimination of in-excess wastes and aquatic plants/organisms, but also each time necessary the application of local simple physical-chemical cleaning treatments of surface water. The sediments must also be removed (i.e. collected, dried, eventually treated), and valorised as soil (in mixture with existing reference soil in different local area) or ingredient for composites, among others.
The fresh water is not pure and contains different constituents in different proportions permitting the classification of fresh water in different types of natural aquatic environment resources. These constituents are consisting of solid particles of inorganic or organic origin, and different dissolved ionic and molecular species in the whole water mass. Only 0.5 % of total existing fresh water of Earth is in easely accessible surface forms (i.e. rivers, ponds and lakes) (van Loon and Duffy, 2005; Zaharia, 2011a; Zaharia and Suteu, 2013).
In quantitative terms, the fresh water is unevenly distributed, and countries that have available less than 2,000 m3/year per capita of fresh water are considered to be in chronic water deficit (van Loon and Duffy, 2005; Zaharia and Suteu, 2013). In qualitative terms, the fresh water is highly variable, and a lot of physical-chemical and microbiological elements must be considered for determination of its acceptability for different purposes (Zaharia and Teslaru, 2012; Zaharia and Suteu, 2013).
The dissolved species present in the surface water can participate to simple or complex homogenous interactions (i.e. reactions of dissolved species with formation of new dissolved species in aquatic environment) or heterogeneous interactions (i.e. reactions of dissolved species with formation of new solid or gaseous species), in the presence or absence of dissolved oxygen or carbon dioxide in water (as dissolved gases in water), favoured or not by action of some catalysts (e.g., metal ions) and organisms (e.g., microorganisms, algae, fungi, etc.), or solar energy. The representative homogenous interactions for the aquatic environment are exchange interactions of protons (acid-basic reactions), of different types of cations and anions (ionic exchanges) and exchange of electrons (redox reactions). In the case of heterogeneous interactions, the representative ones are precipitation and dissolution interactions in the aquatic environment or at water-rock/mineral interfaces, and also coordination/complexation/chelatising, coagulation-floculation, adsorption/sorption/biosorption processes which lead to some gaseous species formation in-deep surface water and/or water-atmosphere interface. Even if the last three ones are limited as number in comparison with the precipitation-dissolution processes, the CO2 and O2 exchanges play an essential role in the superficial processes and also metabolism of aquatic organisms and plant photosyntesis (Zaharia, 2011a,b; Zaharia and Surpateanu, 2006; Zaharia and Suteu, 2013).
In ground water, filtered through layers of permeable soil material, water is usually clear, indicating the presence mostly of dissolved species. In other components of hydrosphere, the situation is different, and considerable fractions of different inorganic or organic polluting species exists as suspended particles that requires separation by simple or
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complex processes and/or operations such as natural sedimentation, filtration, or coagulation-flocculation associated with sedimentation, etc.
A fast-flowing river carrying an abundant sediment load can give a distinct milky appearance (e.g., the ‚White Amazon’ in northern America, or the ‚White Nile’ in north-eastern Africa) (vanLoon and Duffy, 2005). In general, the suspended material consists of typical minerals and organic matters, especially those in fine (nano-, colloidal- and micro-size) or settleable fraction as illustrated in Table 1. Therefore, the elements usually present are consisting of high concentrations of silica, clays, fine polymeric solids, alkali and alkaline earth metals, aluminium, and iron precipitates, along with smaller amounts of other metals associated, more or least, with the adsorbed species on the fine particle surface.
Table 1 - Estimation of contaminants in different aggregation phases present in various raw waters (Odegaard, 1987; Zaharia, 2000, 2006)
Indicator/expression mode of contaminant (indicator)
Classification of contaminants in water Soluble Colloidal Supra-colloidal Settleable
BOD5 [% of total] 17 16 46 21 COD [% of total] 12 15 30 43 TOC [% of total] 22 6 36 36 Total P [% of total] 63 3 12 22 Organic N [% of total] 27 15 38 20 k- Biochemical oxidation rate
[d-1] 0.39 0.22 0.09 0.08
The ratio of suspended to dissolved fine substances in rivers is quite variable, with
typically values ranging from less than 0.03 to much greater than 1. Also, metal in suspended sedimentary material of various origins is considered to be less available for uptake by organisms than are the dissolved forms of the same element. The elements associated with iron or manganese (hydr)oxide particles can be remobilized if the particles participate at reductive dissolution. Suspended organic particles can decompose into smaller, soluble molecules under oxidizing conditions, and can lead to release of soluble forms of complexed/coordinated metals. Some organisms (e.g., filter-feeders like clams and mussels in water bodies) can take up colloid-bound metals and nutrients directly from the suspended material (sedimentary, or in-deep water flowing solids), and interesting is also the fact that their excretions may include more soluble forms of rejected elements, thus contributing to the local alteration/pollution of aquatic environment.
The colloidal particles, considered also as fine turbidity particles, are usual constituents in natural surface water resources, and cause often taste, odor and color problems. Therefore, its possibility to be separated from surface waters for use in different purposes by coagulation-flocculation, or other treatments associated or not with gravity se-dimentation, or natural barrier filtration/biofiltration, is one of the main goal of this work.
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These fine particles (both hydrophilic and hydrophobic colloids) possess electrokinetic property causing double layer formation, and consequently surface charges.
Hydrophilic colloids are in principal of biological origin (such as proteins, bacteria) and usually consist of water-soluble functional groups like –NH2, -COOH, -OH, -SO3H, and –OPO3H2 (e.g., a protein particle: HOOC – R – NH3
+ formed at high pH level, or –OOC – R – NH2 formed at low pH level). In practice, the Zeta potential (determined from the electrophoretic mobility measurements, and expressed by relation: Me =
ηε
⋅⋅
KZ ,
where: Me – electrophoretic mobility, [m2/s. V]; ε - dielectric constant of aquatic medium, [F/m]; η - dynamic viscosity, [Poisse]; K – factor depending on colloid type and dimension (usually it has the value of 4); Z – Zeta potential, [V]) is important being implicated in many environmental applications, but an essential factor it has in the transport of organics, bacteria in porous media as shown in table 2. In general, a zeta potential of 100 mV in water at 25⁰C is equivalent with a field of about 1500 V/cm created by electrodes to produce a velocity of 1 cm/s of air bubbles in the capillary at the return path (Yen, 2007).
Table 2 - Zeta potential of Bacillus subtilis and Pseudonomas putida (Jang and Yen, 1985)
Species Culture age, [h] Suspending medium Zeta potential, [mV]
Bacillus subtilis -
a spore-forming Gram positive facultative anaerobe bacteria
* 1,000 ppm NaCl-containing electrolyte solution; ** 0.0663 M KH2PO4 and 0.0267 M NaOH-containing buffer solution (pH=7.00); *** 0.0127 M KH2PO4 and 0.0054 M NaOH-containing buffer solution (pH=7.00).
The hydrophobic colloids are especial of inorganic or mineral origin (such as clay,
silt, and silica), being charged at face boundaries of particle surfaces as result of different ionic exchanges or isomorphous replacements within the lattice due to lattice imperfections. Most metal oxides and hydroxides are amphoteric, but can adsorb H+ and HO- ions or its complexes, and are changing their surface charges passing through zero (isoelectric point-IEP), fact expressed mainly by pHZPC (pH of zero point of charge). Some values of pHZPC for fine inorganic turbidity particles (i.e. clay minerals - phyllosilicates as kaolinite; montmorillonite - smectite as muscovite, vermiculite, chlorite; feldspar, quartz), present in fresh water are given in table 3 (Yen, 2007; Zaharia, 2013; Zaharia and Suteu, 2013).
The colloidal and near-colloidal-size particles have an ‚apparent’ surface charge, but its magnitude and sign is dependent of pH value. Because fine particle in-deep surface water is generally neutral (no net charge), a double ionic layer (consisting of a Stern layer covering the external colloidal particle surface, and also a diffuse layer at a specific distance from colloidal particle surface, with multiple co-ions and counter ions present) is developed
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(Haller, 1995; Schroeder, 1977; Yen, 2007; Zaharia, 2006, 2013; Zaharia et al., 2007): - ions with the same charge as the particle are rare near the particle surface, but
gradually increase in number as the distance from particle increases (present in the diffuse layer as co-ions).
- unlike ions or counter ions predominate near the particle surface, but gradually decrease in number with increasing distance (present in the Stern layer but in a decreasing number in the diffuse layer).
Table 3. The pHZPC values of different fine turbidity particles (adapted after Yen, 2007; Zaharia, 2011a, 2013; Zaharia and Suteu, 2013)
Fine particles pHZPC Fine particles pHZPC Fine particles pHZPC α-Al2O3 9.1 γ−Fe2O3 6.7 ZrSiO4 5.0
Repulsive (coulombic repulsive forces) and attractive forces (van der Waals attraction forces) analogous to gravitational forces exist between particles, and regulate the distance between colloidal turbidity particles. Increasing ionic strength is produced by adding electrolytes (coagulants), termed as compression of double layer, which is significant in coagulation of colloidal clay particles moving from fresh water rivers into estuaries. Coagulation is induced also by simple electrostatic adsorption of counterions that effectively neutralize the turbidity particles and decrease the surface potential. The effectiveness of a counter ion in coagulation increases with charge; e.g., in the increasing order of Na+, Ca2+ (Fe2+), Al3+ as measured by ionic concentrations necessary to coagulate the fine turbidity particles which are usually of 1: 10-2: 10-3.
In addition, the colloidal turbidity particles can be separated by chemical-mechanical interactions, in principal by coagulation and flocculation processes in natural self-purification systems of fresh surface water, or induced in situ treatments of fresh water bodies that are normally carried out prior to sedimentation and filtration.
In general, the fresh water utilization for different human consumption demands such as drinking purposes is based on ex situ treatments (as production and population supply with potable/drinkable water, preparation of alcoholic or non-alcoholic, aromatic or refreshing drinkings, food conservation, etc.), or irrigation, or sportive activities, among others, which imposes some obligatory steps of water treatment in a plant in order to respect the strict requirements of water quality and safe, such as coagulation-flocculation, gravity sedimentation, rapid filtration, adsorption on activated carbon, disinfection, etc.
2. THEORETICAL PART ON THE IN / EX SITU COAGULATION AND
FLOCCULATION PROCESSES IN NATURAL WATERS AND WASTEWATERS
In general, it is considering that coagulation is the process by which the colloidal systems are destabilized, and particles are allowed to aggregate and flocculate to higher sizes that settle with high, or most satisfactory velocities (Haller, 1995; Yen, 2007). The existing turbidity particles in natural surface waters are within colloidal sizes possessing
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electrokinetic properties which cause double layer formation, and their stabilities are governed by colloidal chemistry charges. The electrokinetic properties are caused by different field effects, such as electricity and so forth, at a different solid surface. The main electrokinetic phenomena for colloidal surface are divided in four types as (Yen, 2007): • electrophoresis, due to electric field on colloidal particles with motion of disperse
phase; • sedimentation potential, due to gravitational field on particles with potential gradient; • electroosmosis, due to electric field on tube wall or packed bed with motion of the
aquatic medium; • streaming potential, due to motion of aquatic medium (e.g., on tube wall or packed
bed) with potential gradient. Some examples of charged colloidal particles (mineral particles) are illustrated in
Fig. 1, and also define below as: - colloidal particles having on its surface groups of silanol, i.e. ≡SiOH acting as
≡Si – OH2+ ↔ (K1
)/-H+ Si – OH ↔ (K2)/-H+ Si – O- (1)
- colloidal particles of metal oxides and hydroxides both amphoteric, with possibility of adsorbing H+, or HO- ions, or its complexes. Surface charges can originate from a mineral particle, or hydrous metal (Fe or Al) oxide layer as solute coordinated to bind to a specific solid mineral surface (Kam and Gregory, 2001; Yen, 2007); e.g.:
Fig. 1. The surface charges of a mineral colloid particle with adsorbed humic substances and surface complexed divalent cations and also counter ions in diffuse double
layer (Yen, 2007)
Fig. 2. Representation of a negative colloidal particle with its electrostatic field
(adapted from Preisting, 1962)
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The charges of colloidal particles are essential to the understanding of colloidal agglomeration, or stabilization by coagulation-flocculation, but also the formed flocs destabilization (break-up). The stages of a coagulation process are summarized in table 4.
A colloidal system, like a lake or river containing suspended clay particles, is stable because the small charged particles repeal each other. As they move in the aquatic environment due to thermal motion, these are able to come suficiently close in order to overcome the repulsive forces of the surface charge and aggregate into larger, settleable units. The result is a long-lasting suspension of colloids (vanLoon and Duffy, 2005).
Table 4 - The stages of a coagulation-flocculation process (Zaharia, 2006; Zaharia et al., 2006b)
Stage Phenomenon Defined term Addition of coagulant
Processes of ionization, hydrolysis, polymerization in water or aquatic medium
Hydrolysis
Destabilization Compression of double electric layer. Specific adsorption of coagulant ions at the surface of colloidal particles. Specific binding of ions sau species at particle surface. Colloid inclussion in a precipitate of hydroxide. Bridging between particles due to polymeric species.
Coagulation (or flocculation)
Transport Brownian motion Dissipated energy (velocity gradient)
There are known four major coagulation mechanisms explaining the agglomeration
of colloidal particles at higher particle-size dimensions, and consequently ease separation by gravity sedimentation or filtration (Odegaard, 1987;Walldal, 2001;Zaharia, 2006, 2013): (i) double-layer compression. The DLVO (Derjaguin-Lindau-Verney-Overbeek) model is
considering that the space charge density of ions in water decreases rapidly with distance from the surface, and potential declines exponentially as function of distance (Figs. 2-3) (Yen, 2007). For water and monovalent electrolyte (i.e. ordinary salt, NaCl), the reciprocal double layer thickness is about 11 Å at 0.1 M, and 101 Å for 1.10-3 M. Some salt concentrations may be small enough for two particles to approach each other and result in aggregation. Also, the higher valence of counter ions is shorten the double layer thickness (i.e. Hardy rule: coagulation efficiency of counter ions increases mark-edly with valency). The interaction energy is a simple additive term (ET), defines as:
ET = ER + EA ≤ 0 (4) where: EA is van der Waals attraction energy, and ER is coulombic repulsive energy of double layer. Some agglomeration behaviours for different destabilizing agents (coagu-lants or flocculants) are illustrated in Fig. 4 (Yen, 2007; Zaharia, 2000, 2006, 2013).
EA = - 212 H
A⋅⋅π
= -u
A⋅12
(5) ER = 2
)exp(2
+⋅−⋅⋅⋅
uua χψε δ (6)
where: A – Hamaker constant; H – distance between colloid particles, [m]; u =ax , x – distance
between the separation surfaces, [m]; a – radium of spheric particle, [m]; ε - water dielectric constant, [F/m]; ψδ - superficial potential, equal to Zeta potential (ψδ=Z), [V]; χ - reversal of thickness of double electric layer (H), [m].
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The exact concentration of electrolyte (monovalent, divalent, or trivalent metal salt) required to coagulate the colloidal particles depends on the co-ions rather than counter ions (phenomenon known as salting-out). In addition, some anions can be added to complete for charged particles for bound water formation. The effectiveness for coagulation of added anions respects the Hofmeister series rule which considers the following order: SO4
2- > Cl- > NO3- > I-.
(ii) charge neutralization and adsorption. The coulombic electrostatic energy involved across the diffusion layer (potential of about 100 mV) of a typical colloidal particle and a monovalent counter ion is only around 2 kcal/mole, being quite weak in comparison with covalent, coordinative, or even hydrogen bonds (Yen, 2007). The colloid-coagulant, coagulant-solvent (water), and colloid-solvent (water) interactions are important when compared with the coulombic energy (i.e. colloids - surfactant-like molecules such as dodecylammonium chloride). When a sufficient number of counter ions is adsorbed, charge reversal occurs, and restabilization is acting.
Fig. 3. Representation of a double electric layer (mainly the diffuse layer)
Fig. 4. Particles’ agglomeration behaviour for different electrolytes (inorganic cations – Al3+, Ca2+, Na+ species, organic species) and polyelectrolytes (hydrolyzed polyacrylamide)
If long-chain counter ions are attached to the colloid within the Stern layer, the effective charge outside the shear layer is reduced, in contrast to the double layer repression (double layer compression), which alters the charge distributions within diffuse layer.
(iii) entrapment in a precipitate (co-precipitation) and adsorption. The addition of coagu-lants (i.e. Fe3+ and Al3+ salts) in colloids-containing water leads to formation of polynuclear hydrolysis products – M(OH)n
z+ which are adsorbed at particle-water interfaces (e.g., hydrous metal oxide inferfaces). Moreover, the adsorption of cations and anions at hydrous colloid interfaces can lead to surface complex formation and ligand exchange equilibrium; e.g., amphoteric hydrous metal oxides reacting with cations and anions in coodination reaction (Fig. 5; Yen, 2007) such as:
The adsorption of cations by hydrous metal oxides will lower the pHZPC, and adsorption of anions will increase the pHZPC. The adsorption at the hydrous metal oxides sites will modify the net electric charges. The maximum settling velocities of particles are predicted at zero charge (at point which is at pH of isoelectric point - IEP). Other charged particles are similarly enmeshed in colloids.
Fig. 5. Interactions of hydrous oxides with cations and anions in terms of surface complex formation
and ligand exchange equlibria (Stumm, 1978)
Fig. 6. Representation of turbidity and color removal by mineral coagulants based on
magnetite (Yen, 2007)
(i) interparticles bridging. The polyelectrolytes (i.e. polymer or macromolecular compounds with multiple ionizable functional groups on its chain, soluble in water, cationic, anionic, or ampholytic in nature, and having macromolecular structures with flexible chains and charges spreading over extensive areas) can modify the surface of mineral particles leading to floc formation (Fig. 6). Restabilization can occur, not throught charge inversal, but with excess of polymer, or prolonged agitation, or no particle available and intramolecular adsorption by folding (Figs. 4, 6).
In practice, all four mechanisms are involved but latter two ones are implicated in particle-size growth and/or aggregation known as flocculation process. Consequently, coagulation-flocculation can be produced by (Zaharia, 2000, 2006, 2011, 2013): - the attraction forces which will decrease the surface charge for permitting the
aggregation of particles in distinct flocs of higher sizes easely separable by sedimentation (coagulation process);
- the simple electrostatic adsorption of counter ions that effectively neutralize the particles, and decrease the surface potential (depending of particular simple cation or anion; e.g., efectiveness of Na+, Ca2+ and Al3+ in coagulation process measured by ionic concentrations which is of 1:10-2:10-3, or depending of large, complex molecules used as coagulants or flocculants, and implicitly ordinary adsorption). Overdosage with coagulant can result in particle-charge reversal due to adsorption of excess ions
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176
(resulting stable colloids) (coagulation-flocculation process) (Zaharia, 2006; Zaharia and Suteu, 2013; Zaharia et al., 2006 a,b, 2007);
- formation of precipitates (hydrated-metal hydroxides with low solubility limits) which can adsorb on its particle surface other existing colloids, and act through charge neutralization (pH effect is important; especially, the value of isoelectric point of metal hydroxide) (coagulation-flocculation process) (Haller, 1995; Yen, 2007);
- enmeshment in a agglomerate-precipitate and adsorption, mainly when are used as coagulant aids/flocculants the organic polymers (cationic, anionic and nonionic ones). The ions presented in water interact with the polymer, and particle-aggregates are formed, even in suprising situation when an anionic polymer is the most suitable coagulant-flocculant for a negatively charged colloid (flocculation) (Schroeder, 1977; Haller, 1995; Zaharia, 2000; Zaharia and Suteu, 2013; Zaharia et al., 2006 a,b, 2007).
Flocculation usually refers to the postdestabilization process in which aggregations and large flocs are formed, as result of particle or small floc collision due to a rapid stirring either by Brownian motion (perikinetic flocculation) or by velocity gradients (orthokinetic flocculation). Both rapid and slow stirring are needed for a good and complete flocculation. A schematic representation of the bridging model for the destabilization of colloids by polymers is illustrated in Fig. 7 (O’Melia, 1972; Leu and Ghosh, 1988; Kawamura, 1991; Tanaka and Pirbazari, 1986; Yen, 2007). As general agreed definition, flocculation is the actual accumulation of the particles into settleable mass.
Fig. 7. Flocculation by bridging model of colloids with polymers (O’Melia, 1972; Zaharia, 2006) The separation of dense formed agglomerates or flocs from surface water is
achieved by sedimentation or filtration/biofiltration. Sedimentation usually refers to settling of the formed flocs without stirring for quiescence settling. Filtration usually refers to separation of the formed flocs by free, vacuum or under pressure passing through a granular solid layer of varying porosity and density (graded sand, garnet, coal, resins) supported by
Bridging between polymers (polelectrolytes) and colloid particles
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gravel layers and/or porous underdrains (depth filters) or precoat filters. In natural aquatic environment, the separation of fine particles is in majority of cases achieved by sedimentation with or without coagulation-flocculation processes, but for specific water bodies different filtration/biofiltration also occur by passing throught different solid layers (rocks, aquatic trees, or other aquatic flora etc.).
Some particularities of orthokinetic flocculation related to perikinetic flocculation are summarized in table 5, but both leads to reduction of potential for the double layer of colloidal particle.
The collision efficiency for different colloidal particles-coagulant/flocculant systems are presented in table 6, and some comparisons between coagulation-sedimentation and filtration (as final separation after coagulation, flocculation and/or sedimentation) in tables 7-9, and Figs. 8 and 8 (Strumm, 1978; Yen, 2007; Zaharia, 2006).
Table 5 - Orthokinetic flocculation versus perikinetic flocculation
Characteristics Orthokinetic flocculation Perikinetic flocculation Interparticle process involved Hydrodynamic fluid motion or agitation
(velocity gradient - G) Brownian interparticle contact (Boltman constant - k, and temperature - T)
Energy regulation, i.e. Maxwell-Boltzman distri-bution, associated with sedimentation (Zaharia, 2006)
23
32 NGd
dtdN
ok
⋅
−=
α
, NG
dtdN
ok
⋅
−=
π
φα4
, ln N/N0 = -4afGt / π,
VPk
VPG ⋅=⋅
=η
tF = 2 / k1N0, or tA= -ln(1-α)k12N0, where: tF - flocculation time; tA - adsorption time; α- fraction of colli-sion leading to agglomeration; G - velocity gradient; P – real dissipated power, [m2 Kg/s3, or W]; V – volume occupied of water, [m3]; η - dynamic viscosity, [Kg/m.s]; φ - volume fraction of colloidal particles (φ = πd3N/6); d - particle diameter; k1, k12 - appropiate rate const.
2
34 NkT
dtdN
pk
⋅
−=
µα
, or
N = N0 / [1 + (4αkTN0 / 3μ)t] , or t1/2 = 3μ/(4αkTN0) = 1.6.1011/(αN0)
where: α - fraction of collision leading to agglomeration; D - Brownian diffusion coefficient (D= kT/(3πμd)); d - particle diameter; k - Boltzman constant; T - temperature; μ –absolute viscosity.
Energy regulation, i.e. Maxwell-Boltzman distribution, associated with filtration
For packed-bed filtration: N
df
dldN αη
−=
−1
23
where: f - porosity; 1-f - the volume of filter media per unit volume of filter bed; l - bed depth, η - a single collector efficiency, reflecting the rate at which particle contacts occur between suspended particles and filter bed.
The overall rate of diminution of particles is expressed as:
+=−
πφ
µα GNkTN
dtdN
34
, usually G = 10/s, φ = 10-4, α = 10-1, and t = 103 s.
Overall flocculation process (Gregory, 1987; Gregory and Lee, 1990)
Step: Effect of: Typical time scale: Particle concentration Mixing rate
Seconds 0.5 s to minutes 1 s to hours (?) 0.1 s to minutes
Seconds where: ‚-‚ sign indicates little or no effect expected; ‚+’ sign - estimated importance of the step effect; ‚++’ - both important adsorption and flocculation steps; ‚?’ - some incertainties; ‚+++’ - three important steps.
Example of different values of coagulation-flocculation
For a 0.1 µm particle, G is 104/s, whereas for a particle of 10 µm, G is 102/s. Small
For a poor destabilized system (low α), t1/2 becomes large.
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178
parameters or measures particles are not sufficient for separation, but they should be aggregated into larger ones (of >1 µm). In general, G values varies as 400 – 1000 s-1 (in coagulation), and around 100 s-1 (in flocculation).
If a virus of 104/mL is completely destabilized (α=1), t1/2 still requires 200 days.
Table 6 - Collision efficiency for some colloidal systems treated by coagulation-flocculation (O’Melia, 1972)
Colloid Coagulant / Flocculant Type of flocculation
Table 7 - Comparison between coagulation/flocculation-sedimentation, flotation and filtration
Design and operational
variables
Chemicals Coagulation aids, media size, sludge recirculation
Energy input, mass transport
Residence time, filter lengths,
media diameter Coagulation-Flocculation / Sedimentation
α Collision efficiency
φ Volumetric concentration of
suspended particles
G Velocity gradient
t Time
Flocculation / Flotation
α Collision efficiency
φ Volumetric concentration of
suspended particles
G Velocity gradient /
Fair - air disperse flow, d- air bubble diameter
t Time
Coagulation-Flocculation /
Filtration
α 1-f Volumetric concentration
of filter medium
η Single collector efficiency (ν, d)
contact
L/d Number of collectors
opportunities
REMARKS for separation of formed aggregates by coagulation-flocculation at daily design flow (Jiang et al., 1994): Separation process Chemicals G, [s-1] Tcoag/flocc, [min] Overflow rate,
vf, [m/h] 1st
comp.* Intermed. Comps.**
Last comp.
Coagulation- Flocculation/ Sedimentation
Al3+ / Fe3+ Ca(OH)2
Fe2+ + Ca(OH)2 Al3+/Fe3+ + polymer
40-50 30-40 25-35 50-70
15-20 15-20 10-20 30-40
≤ 10 ≤ 10 ≤ 10 ≤ 10
25−35 15−20 15−20 10−15
0.8-1.0 1.0-1.2 0.8-1.0 1.5-2.0
Flocculation/ Flotation
Al3+ / Fe3+ 70-90 70-90 70-90 15−25 5.0-7.5
Flocculation / direct Filtration
Al3+ / Fe3+
+ polymer > 100 >100 >100 5−15 5.0-10.0
* 1st comp. - means the first settlement compartment; ** Intermed. Comps - means the intermediate settlement compartment.
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.Tabel 8 – Characteristics of destabilization processes by coagulation-flocculation
Parameters Coagulation Coagulation by adsorption Flocculation Destabilization agent Non-hydrolysing conter
ions - Hydrolysing salts (of iron or aluminium); - Superficial active conter ions, soluble polynuclear compounds
Polymers and/or poyelectrolytes
Electrostatic effects Predominant Important Subordonated Chemical and adsorption effects
Do not take place usually Important Predominant
Zeta aggregation potential
Almost zero Almost zero In general different of zero
Addition of agent Without effect Restabilization because of charge exchange
Restabilization because of complete covering of surface
Relative covering of surface for optimal destabilization
Not observable 0 < φ < 1 φ = 0.5
Necessary quantity of destabilization and restabilization agent
Independent of disperse phase concentration. It follows the Schultze –Hardy rule.
Stoichiometric towards superficial concentration of disperse phase
Stoichiometric
Fig. 9. Typical variables in coagulation-flocculation in natural waters and wastewaters (Yen, 2007)
Fig. 10. Most suitable ranges for various applications for particle removal in water and wastewater treatments
(Yen, 2007) The efficiency of coagulation-floculation process in sedimentation of natural surface water is important, the removal of suspended solids must be higher than 70%, ideally of 100%.
In practice, the effect of coagulation-flocculation on sedimentation performance is beneficial, being improved the suspended solids and turbidity removal (28-82 %) as presented in tables 9 and 10 (Bolto, 1994; Zaharia, 2006).
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Tabel 9 – Effect of coagulation-flocculation in primary sedimentation of a surface water (Zaharia, 2006)
Tabel 10 – Application (in Australia) of organic polyelectrolytes in wastewater treatments (Barkacs et al., 2000; Bolto, 1994; Bolto et al., 2001; Zaharia, 2006)
Stage Aplications Suspended load, [mg/L]
Polyelectrolyte characteristics
Polyelectro-lyte dose, [mg/L]
Observa- tions
Preliminary Initial cleaning High solids removal; Pre-aeration
125 – 400 - - -
Primary Removal of organic and inorganic matters by physical operations
100 – 400 - Moderate and high charges; - HYPAM* with with high macromolecular mass used with/without inorganic /organic coagulant - AMPAMS** cationic with high-medium charge
0.1 – 6 Cationic polyelec-trolytes are used when separated solid is recycling.
Secondary Reduction of CBO leading to biological attack
0.1 – 1.5 - moderate and high charge - cationic polymers
0.2 –10 Dense flocs for settlement
Sludge concentra-tion
Improvement of water elimination process
0.1 – 5 - High and moderate charge - Cationic polymers
2 –10 -
Sludge deshydra-tation
Reduction of sludge humidity content
2 – 10 % - high charge - cationic polymers with mode-rate-high molecular mass
3. COMMON ELECTROLYTES AND POLYELECTROLYTES ACTING AS EFFICIENT COAGULANTS AND FLOCCULANTS
Turbidity in water is due mainly to different collection of solids having a range of particle sizes (nano-, colloidal-, micro-size), shapes (sphere, tube, rod), and densities. The coagulants and flocculants serve mainly to assist and aggregate particles in coagulation and flocculation processes, in order to maximize removal of very small solid particles of
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various compositions, but also to react with and remove some potential chemical polluting species such as phosphorus species from the water (vanLoon and Duffy, 2005).
Direct addition of metal salts as iron salts to the water to be treated may cause extremely rapid and uncontrolled hydrolysis process, and frequently, very rapid precipitation (Jiang et al., 1994). In contrast, a controlled hydrolysis process may produce a range of ferric hydrolysis products which may adsorb to colloidal surfaces to neutralize their charge or may chemically interact with dissolved components in the raw water. Therefore, the rate of coagulant interaction with colloidal and dissolved substances must be faster than the rate of hydrolysis to low charge species and hydroxyde precipitate.
There are many organic and inorganic compounds that possess the ability to retard the hydrolysis and polymerization processes of Fe(III), and the main criteria mostly considered are: (i) appropriate affinity to Fe(III) comparable to OH; (ii) non-toxic and un-treablesome/unproblematic in water and wastewater, and (iii) cheap and capable of being produced industrially, or existing in sufficient amount in natural water (at interface rocks/minerals-water, or in-deep water mass). One example of coagulation process using ferric salts studied by many authors and useful in removal of polluting species from water as phosphates (important role in promoting water eutrophication) is illustrated below. The following known information is mentioned considering the affinity of Fe(III) for phosphate that is inferior to that for the hydroxy ion but much stronger than that for other anions such as: HO- > PO4
3- > F- > SO42-
> Cl- > NO3- > ClO4
-. The interactions of phosphate and hydroxy ion with Fe(III) species can lead also to formation of fresh precipitates, composite complexes, or polymers following some possible known mechanisms as (Gillberg, 1994; Tang et al., 1994; Zaharia, 2006): - The precipitation reaction as the formation of ferric sulphate (Fe2(SO4)3), or inert part
of fresh precipitates like FexOz(OH)y(3z-2z-y)+ and high hydroxy ferric polymers
xFeOOH, xFe(OH)3 (mixtures) or Fex(OH)y(3x-y)+, where x is an order equal with 103
and/or 104:
Fe3+ (aq) + PO43- (aq) → FePO4 (s) (14)
Fe3+ (aq) + 3HO- (aq) → Fe(OH)3 (s) (15)
This reaction is completed sometimes with formation of complex precipitate if it was present or added lime (calcium hydroxyde) in the water as:
All these mechanisms lead to inclusion of phosphate groups in composite structures of Fe(III), and also substitution of a part of hydroxyl groups. Important contribution of
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phosphate stabilizer is improving of Fe3+ ion presence together with the active and effective action for coagulation and flocculation.
Coagulation agents. The most used coagulants in current practice are still considering aluminium salts (aluminium sulfate or chloride, sodium aluminate), iron salts (ferric chloride or sulfate, ferrous sulfate, ferric chloro-sulfate), lime, mixed coagulants (based on Al3+/Fe3+, polyaluminium chloride, polyferric sulphate etc.). A schematic classification of some well-known coagulants and flocculants based on origin of the compounds and their ionic characters is given in tables 11 (for coagulants) and 12 (for flocculants), additionally being claimed in different patents (Jiang et al., 1994; Kitchener, 1972; Lee et al., 2000; Morlay et al., 2000; Tang et al., 1994; Walker and Bob, 2001; Zaharia and Macoveanu, 1992, 1994; Zaharia, 2006). In addition to the sign of charge (i.e. polyanion, or polycation), the relative charge density has an important influence in the coagulation and/or flocculation performance together with the molecular weight of the polymer (i.e. polyelectrolytes), or macromolecule and water pH. Table 11 - Classification of some known coagulants used in destabilization/stabilization of colloidal systems
- Monomeric, dimeric, and polymeric hydroxo iron complexes as dimer Fe2(OH)2
4+, Fex(OH)y(3x-y)+
, FexOx(OH)y(3x-3z-y)+
(used doses of 1-120 mg/L)
Fe2(SO4)3 + 6HCO3- ↔ 2Fe(OH)3+ 3SO4
2- + 6CO2
FeCl3 + 3H2O ↔ Fe(OH)3 + 3H+ + 3Cl-
2FeClSO4 + 6HCO3- ↔ 2Fe(OH)3 + 2Cl- +
2SO42-+ 6CO2
Fe2(SO4)3+ 3Ca(OH)2 ↔ 2Fe(OH)3+ 3SO42- +
3Ca2+ FeSO4 + 2HCO3
- ↔ Fe(OH)2 + SO42- + 2CO2
FeSO4 + Ca(OH)2 ↔ Fe(OH)2 + Ca2+ + SO42-
Lime CaO,Ca(OH)2,Ca(H2O)62+
(used doses of 5-40 mg/L) CaO + H2O → Ca(OH)2
Acids (HA) H+, or (H3O)+
(used doses of 5-50 mg/L) Anionic Colloid(-) + H3O+ → hydrated and relatively neutralized colloid + (H2O)
CuSO4. 5H2O Cu2+, Cu(H2O)6
2+ (used doses of 25-130 mg/L)
CuSO4 + 2HCO3- ↔ Cu(OH)2 + SO4
2- + 2CO2
(II ) Effects of surface charge and initial suspension stability on inorganic coagulant performance (vanLoon and Duffy, 2005, 2010; Levine et al., 1991)
Colloidal particle
CEC* range, [cmol(+)/kg]
Colloid CEC range, [cmol(+)/kg]
Colloid CEC range, [cmol(+)/kg]
Current topics, concepts and research priorities in environmental chemistry (III)
Montmorillonite 80-150 (100) Hydrous iron and aluminium oxides
~ 4 Organic matter
150-500 (200)
* CEC – Cation Exchange Capacity of various materials found in colloidal size fraction.
(III) Effects of basicity, OH/Coagulant, ion charge in destabilization of colloidal particles from high turbidity water with aluminium salts (Gillberg, 1994; Jiang et al., 1994; Tang et al., 1994)
Basicity, [%] Ion formula OH/Al * Average charge per aluminium atom
Ion charge
0 33.3 33.3 82.1
Al(H2O)63+
AlOH(H2O)52+,
Al2(OH)2(H2O)84+,
Al13O4(OH)24(H2O)127+
0 1 1
2.46
+3.00 +2.00 +2.00 +0.53
+3 +2 +4 +7
* The OH/Al ratio of the coagulant and the other most efficiently precipitation agents for phosphates in the raw waters are: (i) 1.9-2.1 (for 2.45 mg P/L) in Norweige (the best OH/Al=1.9), and (ii) 0-1.5 (for 9.03 mg P/L) in Sweden (the best OH/Al=1.1), respectively. (IV) Water treatment by coagulation (e.g., coagulants as ferric sulphate-FS, polyferric sulphate-PFS, aluminium
sulphate-AS (Jiang et al., 1994) and sedimentation leading to polluting compound removals of: Coagulant
Temperature,
[°C] Algae Removal, [%] *
Total cell, [No./mL]
Color (A436 nm)
UV (A 254 nm)
Turbidity, [NTU]
COD, [mg O2/L]
Ferric sulphate (FS - FeSO4) (8 mg/L Fe,
pH= 4.0)
4 18
Microcystis 53 62
81.2 85.3
69.2 76.8
50 60.5
12.5 15.5
4 18
Asterionella
72 77
89.5 91.1
80.5 84.8
56 68
18 24
Polyferric sulphate (PFS)
(6 mg/L Fe, pH= 4.0)
4 18
Microcystis 75 81.6
91.3 92
82.2 83.2
68 69.5
22.4 24.5
4 18
Asterionella
78.5 84
89.6 90.3
80.6 82.4
70 72.5
27.5 30.5
Aluminium (AS) sulphate (pH5.0, 5 mg/L Al)
4 18
Microcystis 60 78
91.4 92.0
75.7 82.5
59 81
20 24
Raw water (pH=8.1)
Fe=0.005 mg/L Mn=0.14 mg/L
Microcystis Asterionella
5.8 x104 6.2 x 104
4.86 / 5.82 [m-1]
38.8 /32.5 [m-1]
6.1 / 8.4 [NTU]
8.2 / 9.4 [mg O2/L]
* After water treatment by coagulation-sedimentation the concencentration of residual Fe(III) and Al(III) was no more than 0.098 [mg Fe/L] (4°C) and 0.09 mg Fe/L (18°C] for PFS; 0.18 [mg Fe/L] (4°C) and 0.165 mg Fe/L (18°C] for PS, and 0.074 [mg AL/L] (4°C) and 0.064 mg Al/L (18°C] for AS.
Table 12 - Classification of some known flocculants used in destabilization of colloidal systems
Origin of flocculants
Non-ionic Anionic Cationic Amphoteric
Natural products (Divakaran and Pillai, 2001; Renault et al., 2009)
- Chitosan (polymer of D-glucose) - Cationic starches
- Coss-linked gelatine: aminated tannin
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Synthetic polymers (e.g., polyelectrolytes) - the used polyelectro-lyte dosage of 0.1-10 mg/L for organic, and 5-100 (mainly 5-30) mg/L for inorganic one.
- Organics-Polyacrylamide (AAm) as commercial products: Superfloc 992, Polyfloc 365P, Nalfloc N671, BTI, or Teprofloc TE75, Mahnafloc LT27
(III) Water treatment by using flocculants (MW varying from 10-3 to 10-6 - 10-7 [g/mol]) (Bolto, 1994; Barkacs et al., 2000; ), having different unitary processes/operations such as:
Process Polymer type CD (charge density)
MW (molecular weight)
Example
Dissolved air anionic L-M H AAm/AA Flotation cationic L H AAm/Q Foam flotation cationic L H AAm/Q Enhanced primary treatment stage with:
L L
H H
AAm/AA AAm/Q
Alum/Fe3+ Lime
anionic cationic
Sludge thickening anionic cationic
M M
H H
AAm/AA AAm/Q
Red mud ex bauxite anionic H H AAm/AA * DADMAC - poly(diallyl dimethylammonium chloride) (medium-high MW); ** DMA/ECH - polymer from dimethylamine and epichlorohydrin (medium-high MW); *** AAm/Q - copolymer of acrylamide and quaternized dimethylaminoethylacrylate (R=H; high MW, or low to high CD). (IV) Water treatment by flocculation (e.g., flocculants as PDADMAC (100%), PEI (95%, or 6%), Chitosan (98%,
or 17%) (Bolto et al., 2001) and sedimentation leading to natural organic matter, color or clay removals of: pH
Polyelectrolyte
Origin
Optimal doses, [mg/L]
Color removal, [%]
UV absorbersd
removal, [%] No clay Clayc No clay Clay No clay Clay
a PDADMAC – polydiallyldimethylammonium chloride of high MW (CD 100%); b PEI – polyethyleneimine with low CD at neutral pH and low MW; c Clay added (20 mg/L) as types of palygorskite, illite, bentonite, etc; d UV absorbers means UV absorbing species.
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Natural organic matter (NOM) together with fine turbidity particles is removing from natural water resources for drinking purposes by primary coagulation-flocculation with polyelectrolytes (especially cationic ones) followed by settling and filtration. The target is to covert soluble organics into an insoluble form. The use of polymers as primary coagulants avoids the production of extra solids like metal hydroxides formed by hydrolysis when alum or metal salts are used as the coagulant, but the NOM reduction is only partial and fine divided solids where present in water after filtration. The combination of metal salt coagulants with long-chain polyeletrolytes of high charge density (CD) is more useful than only metal coagulants which produce voluminous sludges than that resulted in shorter filter runs. Mixes of coagulant and polymer are a convenient way to have suitably reactive particles present, waters being more easier to treat.
The interactions involve polyvalent metals at the clay surface and adsorption of humic acids or generally NOM (i.e. > M – OH + RCOO- → > M – OOCR + HO-, where NOM is considered as ligand in exchange between carboxylate groups of organics and hydroxyl groups bound to a metal on the surface of the solid) (Bolto et al., 2001), together with H-bonding, hydrophobic interaction between polymer molecules adsorbed onto adjacent particles, other ionic exchanges mainly because of water salinity, ionic strenght and amount of divalent cations in water.
4. EXPERIMENTAL PART: A CASE STUDY OF
BAHLUI RIVER IN IASI CITY AREA (SPRING, 2007)
This work continues author research studies in which are presented some results performed in laboratory scale set-up experiments applied on samples of Bahlui River (Romania) based mainly on coagulation-flocculation tests followed by sedimentation, and analysis of the most important influencing factors (pH, temperature, coagulant/flocculant dose, stirring regime at almost neutral pH, operating time, or sedimentation) for separation of fine turbidity particles from fresh water and its performance (in spring season, 2007). 4.1 Bahlui River – location and important characteristics The Bahlui River is originated in Botosani County, at the Western part of Suceava Plateau, and Northern part of Dealu Mare hill, 500 m far from Tudora site. The Bahlui River mouth is in Jijia River, not far away from Chipiresti. The most important transitory cities are Hârlău and Iasi (Zaharia and Suteu, 2013).
The total length of Bahlui River is 104 km, having an annual average flow of 2.8 m3/s. The total hydrographic basin surface is of ca 1,915 km2, being situated in a water deficitary area, with a lot of industrial and agricultural units alongside that influence the river quality and health status, mainly by ecological implications (Benchea et al., 2011; Robu et al., 2008; Zaharia and Teslaru, 2012; Zaharia and Suteu, 2011 a,b).
The Bahlui River has a length of 14 km in Iasi city, and passes through it separating the industrial site area by the important residential city areas. The principal pollutants of Bahlui River are the organics in nature (biodegradable and non-biodegradable compounds), nitrogen- and phosphorus-containing compounds, heavy metals, micro-organisms or biotas. The Bahlui River was monitoring in spring time season (2007) in seven control sections for rapid weekly control of some quality indicators (i.e. more than 10
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physical-chemical indicators; e.g., pH, turbidity, suspended solids; oxygen regime: dissolved O2, COD, BOD5; mineralization or salinity level; specific toxics: some heavy metals, extractible substances in solvents, petroleum products, phenols), and in three rapid daily control sections where Bahlui water corresponds or is obvious necessary to be of the Ist quality category (i.e. Letcani, Tudor Vladimirescu, and Holboca).
Data from annual statistic report indicated globally the IInd and IIIrd water quality categories of Bahlui River in Iasi city (from physical-chemical, microbiological and biological quality indicators), with small exceptions. 4.2 Analysis methods of water quality Some physical-chemical quality indicators were analysed based on Romanian standard methods (RS) internationally approved (ISO or EN), together with its chemical reagents (kits), stock reference solutions and operating parameters for the gravimetric, titrimetric and spectrophotometer-based method used (Zaharia and Teslaru, 2012; Zaharia and Suteu, 2013; Zaharia and Radu, 2013; Zaharia, 2014 a,b,c), such as: - General indicators as pH (HACH OneLine laboratory pH-meter, with specific
electrode, SR ISO 10523-97); colour (SR ISO 7887/1997, absorbance at 436, 525 and 620 nm, or Hazen colour unit); turbidity (SR EN ISO 7027: 2001, or test with Drell DR 2000 spectrophotometer, Hach Co.), suspended solids (STAS 6963-81), total hardness (titrimetric method with EDTA); oxygen regime indicators as dissolved oxygen (SR EN 25814/1999), chemical oxigen demand (COD) (SR EN 1484/2001, or SR ISO 6060/1996), biochemical oxygen demand (BOD5) (SR EN 1899/2002); salinity indicators as fixed residue (STAS 6963-81);
- Specific indicators as some contents of anions and cations such as nitrites (STAS 3048/1-96), nitrates (STAS 8900/1,2-96), ammonium ions (STAS 6328-85), phosphates (SR EN 1189/2000), sulphates (STAS 8601-70);
- Toxic indicators as phenol and phenolic derivates (SR ISO 6439/2001 or SR ISO 8165-1/2000), extractible substances in organic solvents (SR 7587/1996), and heavy metals (STAS 7884-91, STAS 8637-79; SR ISO 11083:1998).
All the chemical reagents were of analytical purity (p.a.) necessary for advanced instrumental laboratory analysis. The pH adjustment was done with HCl 0.1M or NaOH 0.1M. There were used for water quality analysis some digital analysers (Hach One Laboratory pH-meter; Drell DR/2000 spectrophotometer, Hach Co., with specific reagent kits; SP Plus 830 spectrophotometer, Metertech Inc.), but also conventional methods (gravimetric or titrimetric) for: chemical oxygen demand (COD), biochemical oxygen demand (BOD5), total hardness, chlorides, sulphates, extractible substances, fixed residues, and/or spectrophotometer-based methods for: nitrate, nitrite, phosphate, ammonium ion, and phenol index (Surpateanu and Zaharia, 1999, 2002, 2006; Zaharia, 2014c).
Water sampling is performed in recipients (of plastic and glass, capacity of 2 L) from the second daily control section of Iasi city (Tudor Vladimirescu) in spring season (March-April). Some analysis were immediately done but for others the collected samples were individually conserved and storaged (by refrigeration/cooling) for more than seven days for later on contradictory data validation. The method sensibility is depending of type, reference material and apparatus utilised, all analysis accuracy and precission being good enough for evaluation of water quality (0.01-0.0001 M, deviation error of max. ± 10%).
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4.3 Working methodology Surface water samples (300 mL) from Bahlui River were treated at laboratory scale set-up scale by coagulation-flocculation processes using Jar tests mainly for removal of fine turbidity solids, suspended solids and dissolved organics in as much as possible high proportion; the solid separation is followed by sedimentation and/or filtration. All the coagulation-flocculation tests were performed in the same operating mode (i.e. 2 min-rapidly stirring at 300 rpm, and 20 min-slowly stirring at 50 rpm, followed by min. 30 min-settling) for comparative performance study of different coagulants/flocculants (mainly dose and type). pH adjustment was not performed, and the studied coagulants and polymeric flocculants in coagulation/flocculation-sedimentation tests were as follows: - aluminium hydroxychlorosulphate – AHCS (CAS No. 17 927-65-0; EC No. 233-
1350; solubility in water of 600 g/L at 20°C, pH= 3.5 for 50 g/L-aqueous AHCS solution; LD50 (on rats)= 5000 mg/lg) ,
- ferric sulphate (stock solution of 34 mmol/L, Fe2(SO4)3.18H2O, purchased from SC
Nordic Invest SRL Cluj Napoca), and - bentonite (fine powder solid of indigen origin, Iasi city area); - anionic Ponilit GT-2 polylectrolyte (Zaharia, 2000, 2006, 2013; Zaharia et al., 2006 a,
b, 2007), the sodium salt of a copolymer based on maleic acid and vinyl acetate prepared at the “Petru Poni” Institute of Macromolecular Chemistry, Iasi (Patent, 1981), stock concentration of active compounds of ≅ 33-36% (w/w), density of 1.18-1.21 g/cm3, pH range (6.5-8), water soluble, viscosity (20⁰C) of 1500-3000 cP, with high macromolecular weight of 2x105 - 3x106 [g/mol]. The working polyelectrolyte concentration was of 0.5 % (w/v).
Supernatant samples (25 mL) were analysed for determination of the important studied quality indicators, especially turbidity, content of suspended solids, colour, COD, and total iron content. The coagulation-flocculation performance for Bahlui River treatment is appreciated by the treatment degrees or removals of turbididy, suspended solids, and colour.
5. RESULTS AND DISCUSSION
The quality analysis of Bahlui River from annual report (spring season, 2007) is indicated the IInd and/or IIIrd quality category, for majority of dissolved substances (Fig. 11) as follows (Zaharia, 2008b; Zaharia and Teslaru, 2012; Zaharia and Suteu, 2013):
- in control section – Letcani-Iasi: COD of 5.34-10.23 mg O2/L (IIIrd category), ammonium ions of 0.59-1.04 mg/L (IInd category), phenol of 0.002-0.033 mg/L (IIIrd category), and total iron ions of 0.02-0.45 mg/L (IInd category),
- in control section - Tudor Vladimirescu-Iasi: COD of 10.71-16.17 mg O2/L (D-degraded category), ammonium ions of 3.20-4.91 mg/L (IIIrd category), phenol of 0.003-0.10 mg/L (IIIrd-D-degraded category), and total iron ions of 0.20-0.75 mg/L (IIIrd category),
- in control section - Holboca-Iasi, downstream of Dancu wastewater treatment plant: COD of 8.86-12.13 mg O2/L (IIIrd category), ammonium ions of 3.92-4.39 mg/L (IInd category), phenols of 0.001-0.022 mg/L (IIIrd - D-degraded category), and total iron ions of
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0.9-1.20 mg/L (IInd category). The average values of some heavy metal concentrations, dissolved oxygen and
total phosphorus (total P) content are illustrated in Fig. 12 (Benchea et al., 2011; Robu et al., 2008; Zaharia and Suteu, 2013). It seems that these values are in the admissible limits, exceeding sometimes a little bit the limits as consequence of periodic climate change (seasonal change), thermal stratification of in-deep water layers, and soil errosions.
The relative high content of suspended solids, turbidity and coloured water bodies in this period of year (spring season) was always a serious concern for environmental protection regulators. Some in situ induced water treatments can be considered for improving the water and sediment quality in some control sections, especially for Tudor Vladimirescu-Iasi control section of Bahlui River (for suspended solids, dissolved organics and also heavy metals). Therefore, some laboratory scale set-up coagulation-flocculation/sedimentation tests were performed for some collected water samples.
Fig. 11 - Average values [mg/L] of some quality indicators (COD, BOD, ammonia, nitrate, nitrite, phenols) of Bahlui River in Tudor Vladimirescu-Iasi control section area (spring season, 2007)
Fig. 12 - Average values [mg/L] of some quality indicators (pH, dissolved oxygen, total P, some heavy metals) of Bahlui River in Tudor Vladimirescu-Iasi control section (spring 2007)
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Some coagulation-floculation – sedimentation laboratory scale set-up tests (Jar test: 2 min-rapidly stirring at 300 rpm, and 20 min-slowly stirring at 50 rpm, followed by 30 min-settling) were organized for study the efficiency of two types of coagulants (AHCS, ferric sulfate), a coagulation aid (bentonite), an anionic polyelectrolyte (Ponilit GT-2), and some combinations of these, together with at least once annually mechanical sediment minimization and cleaning (sediment evacuation with special devices and vaccum tools). All tests were performed on the same collected samples of Bahlui River with the main characteristics presented in table 13, in order to obtain high removals using iron-based coagulants, coagulation aids and/or convenient polyelectrolyte.
Table 13. Some physical-chemical quality indicators of Bahlui River (Tudor Vladimirescu control
section-Iasi, spring 2007)
Indicator Measured value Indicator Measured value Colour, absorbance 456 nm (HU) 1.797 (1604.5) CODCr, [mg O2/L] 20.12
pH 7.80 Phenol and its derivates, [mg/L] 0.96 Chlorides, [mg/L] 70.7 Sulfates, [mg/L] 117.8
Total hardness, [°G] 7.86 Fixed residues, [mg/L] 895
The results of coagulation-sedimentation experiments (Jar tests) are summarized in table 14 (Zaharia and Suteu, 2013) using all tested coagulants individually or in mixture, and the removal efficiency for turbidity, suspended solids (SS) and colour are illustrated in Figs. 13-14 working with inorganic coagulants, Fig. 15 using anionic polyelectrolyte, and comparatively working with all types of tested chemicals in Fig. 16.
Table 14. Results of Jar tests applied for Bahlui River samples (Tudor Vladimirescu-Iasi control section, spring 2007) using the tested chemicals (AHCS, ferric sulphate, bentonite) (Zaharia and Suteu, 2013)
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There were performed relative good removals higher than 70 % for AHCS coagulant doses higher than 25 mg/L, but maximal doses of 75 mg/L are enough in these Jar tests, because the removals were decreased for colour and turbidity at higher coagulant doses. The best removals were performed with 50 mg/L AHCS (i.e. 88.59% for colour, 89.49% for turbidity, and 90.34% for suspended solids).
Fig. 13 - Removals by coagulation-sedimentation with Fe3+ ions-based coagulant dose (3-75 mg/L)
Fig. 14 - Removals by coagulation-sedimentation with different AHCS coagulant dose (3-75 mg/L)
and ferric sulphate (3.35 and 10.35 mg/L) It is observed that the most high removals are performed with a mixture of coagulants,
AHCS + Fe2(SO4)3, but with individual AHCS coagulant, the removals of all three quality indicators are closed enough.
Fig. 15 - Removals by coagulation-sedimentation with individual and mixed
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The use of organic polyelectrolyte (Ponilit GT-2) increases with only 12.1 – 38.6 % the removals in all Jar tests, the highest removals (colour – 91.87%, turbidity – 80.59%, and suspended solids – 87.99%) being performed with combination of relative small doses of polylectrolyte (0.3 mg/L), trivalent iron salt (5 mg/L ferric sulphate) and coagulation aid – bentonite (0.3 g/L). Without bentonite, the best removals (colour – 93.66%, turbidity – 95.11%, and suspended solids – 95.4%) where obtained with relative small concentration of coagulant (15 mg/L) and flocculant (10 mg/L) being closed enough to the above mentioned ones, and usable mainly in cost-restrictive times.
Fig. 16 - Removals by coagulation/flocculation-sedimentation with individual or mixed
mg/L Ponilit GT-2, 6- 5 mg/L Fe2(SO4)3+0.30 mg/L Ponilit GT-2+0.3 mg/L bentonite It seems that the highest removal for all three quality indicators (colour – 95.66%,
turbidity – 95.11%, and suspended solids – 95.4%) is performed with the combination of inorganic coagulants together with the coagulation adjuvant (i.e. 50 mg/L AHCS + 3.35 mg/L Fe2(SO4)3 + 3.35 g/L bentonite) but the removal value is closed enough to the removal performed with combination of only iron-based coagulants (colour – 92.38%, turbidity – 93.12% and suspended solids – 93.56%).
The lowest removal had AHCS alone (colour - 51.67%, turbidity - 80.59%, suspended solids – 81.99%) closely followed by that with ferric sulphate as coagulant. The utilization of organic anionic flocculant (Ponilit GT-2 polyelectrolyte) in combination with ferric sulphate and also bentonite was beneficial but not exceeded the removal results performed with only inorganic coagulants used (colour – 91.67%, turbidity – 80.59% and suspended solids – 87.99%).
For obtaining water treatment degrees higher than 70-75% for all quality indicators can be used combination of twice (i.e. AHCS + Fe2(SO4)3, or Fe2(SO4)3 + Ponilit GT2) or three (i.e. AHCS + Fe2(SO4)3 + bentonite, or Fe2(SO4)3 + Ponilit GT-2 + bentonite) stabilizing/destabilizing chemicals.
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The residual concentrations of total iron (Fe2+ + Fe3+) in the clear supernatant after 45 minutes of Jar tests at the normal pH of Bahlui River samples directly collected (no addition of pH regulators) are indicated in table 15 (Zaharia and Suteu, 2013), together with the volume of sludge/sediment produced during coagulation-sedimentation treatment.
The increasing of ferric ions dose decreases the residual iron concentration in the supernatant, mainly because of coagulation-flocculation-sedimentation process. The higher dose of ferric sulfate increased supersaturation of Fe(OH)3, and consequently increased nucleation rate of Fe(OH)3, and removed the dissolved iron in solution by adsorption and/or retaining by the precipitate. Increasing in the supersaturation of Fe(OH)3 nuclei favors their union and hence effective coagulation/flocculation (Zaharia, 2000, 2013; Zaharia and Suteu, 2013; Zaharia et al., 2006 a,b, 2007).
Table 15. Residual concentration of iron (Fe2++ Fe3+) (mg/L) and sludge/sediment volume vs. coagulant
dose (Zaharia, 2000, 2013; Zaharia and Suteu, 2013)
Coagulation process using 50 mg/L AHCS efficiently reduced colour, turbidity
and total suspended solids by more than 90 %. Increasing the coagulant dose above 75 mg/L caused total suspended solids (TSS) to increase in a diminishing fashion. The pH values of 7.10-7.30, required higher dose of metallic cations. This is the reason of addition of ferric ions as coagulant, or higher increasing of coagulant dose. This fact might pose a health hazard as a result of residual quantities of excess chemical additives (aluminium and iron ionic species), which can also interfere with fish survival and growth in the receiving surface water.
The data from table 15 indicate that the total iron (Fe2+ + Fe3+) ions are still in admissible limits in the aquatic environment treated by coagulation-sedimentation (legislative reference data; Zaharia, 2008a).
The sediment separated at the bottom of basin must be periodically evacuated from Bahlui River, dried and treated in order to be efficiently valorized (e.g., as soil in
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mixture with the reference soil from covering area or ingredient in preparation of composite materials).
6. CONCLUSIONS
This chapter is written firstly to support the statement of theoretical knowlege in
stabilization/destabilization field of fine turbidity solids in natural waters and final effluents discharged in its, as the different principles and mechanisms that govern the various particle separation unit processes, and also by demonstations of water characteristics (e.g., a Romanian watercourse case study of Iasi city) and different destabilization process performances mainly by coagulation-flocculation-settling (Jar tests) using different destabilizing inorganic and/or polymeric chemicals, in laboratory scale set-up practice.
An important part of contaminants in raw water is associated with particles, and therefore the direct particle separation is the most effective way of lowering contaminants levels in the water. In addition, coagulation-flocculation / precipitation / formed floc separation is probably the most cost effective practical way to remove contaminants in raw waters and discharging effluents in its.
Improved particle removal in physical-chemical and biological treatments may be performed by improving coagulation-flocculation prior to settling (e.g., by implementing the sludge recirculation in biological flocculation, and chemical coagulation-flocculation in the total phosphorus post-precipitation).
In order to evaluate the particle separation efficiency of different treatment processes, water samples should always be analyzed for total and soluble matter, but also of turbidity and suspended solids content. From all the presented information, it is concluding the strong opinion that the only safe methods of fine turbidity particles separation are firstly by chemical precipitation, then the coagulation-flocculation and land (or other) filtration or air flotation, and afterwards river or other watercourse oxidation.
Some practice results of various water and wastewater treatments by coagulation-flocculation processes followed by settling, filtration or air flotation are illustrated in terms of efficiency, cost, and load changes. Also, some general remarks on the optimisation of coagulation/flocculation within respect to the floc separation process used are discussed together with the factors influencing the separation of newly formed agglomerates like particles type and content, organic loading and coagulation-flocculation conditions during treatment on subsequent particle separation. Must be mentioned that the removal of organic matter in many of these treatments is suprisingly high, and higher than what is normally experienced in the pre-precipitation step of some continental water treatments.
In addition, a study of coagulation-flocculation-sedimentation using the ferric sulphate, AHCS, synthetic PONILIT GT-2 anionic polyelectrolyte with/without bentonite was applied at laboratory scale set-up for Bahlui River samples collected in spring season of 2007, for removing of water turbidity and color. Therefore, the values of some physical-chemical quality indicators were analyzed, and permitted the estimation of Bahlui River quality of IInd and IIIrd category in the Tudor Vadimirescu-Iasi control section. The most indicated values for coagulant dose are of 25-75 mg/L AHCS, 3.35-10.35 mg/L ferric sufate, and 0-3.33 g/L bentonite, and permitted to get removals closed to 68.45-89.49 % for turbidity, 28.21-88.59 % for colour, and 27.15-90.34 % for suspended solids. The combination of iron-based coagulants has as direct result the production of
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sludge, but in a reduced volume with more than 30-40 % in comparison with that formed with individual (solely) use of coagulant for water treatment. The use of organic flocculants as the anionic polyelectrolyte (Ponilit GT-2) was beneficial for use in combination with ferric salts, but did not surpassed the removals performed with only inorganic iron-based coagulants in mixture. These laboratory results are useful as direct, ease and simple remediation actions, in case of serious and severe local pollution episodes of Bahlui River by fine turbidity particles loading.
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