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Chemosphere 71 (2008) 147–155

Technical Note

Chemical solutions for greywater recycling

Marc Pidou a, Lisa Avery a, Tom Stephenson b, Paul Jeffrey a, Simon A. Parsons a,Shuming Liu c, Fayyaz A. Memon c, Bruce Jefferson a,*

a Centre for Water Science, School of Applied Sciences, Cranfield University, Cranfield MK43 0AL, United Kingdomb School of Applied Sciences, Cranfield University, Cranfield MK43 0AL, United Kingdom

c School of Engineering, Computer Science and Mathematics, University of Exeter, Exeter EX4 4QF, United Kingdom

Received 15 August 2006; received in revised form 24 October 2007; accepted 24 October 2007Available online 21 December 2007

Abstract

Greywater recycling is now accepted as a sustainable solution to the general increase of the fresh water demand, water shortages andfor environment protection. However, the majority of the suggested treatments are biological and such technologies can be affected, espe-cially at small scale, by the variability in strength and flow of the greywater and potential shock loading. This investigation presents thestudy of alternative processes, coagulation and magnetic ion exchange resin, for the treatment of greywater for reuse. The potential ofthese processes as well as the influence of parameters such as coagulant or resin dose, pH or contact time were investigated for the treat-ment of two greywaters of low and high organic strengths. The results obtained revealed that magnetic ion exchange resin and coagu-lation were suitable treatment solutions for low strength greywater sources. However, they were unable to achieve the required level oftreatment for the reuse of medium to high strength greywaters. Consequently, these processes could only be considered as an option forgreywater recycling in specific conditions that is to say in case of low organic strength greywater or less stringent standards for reuse.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Coagulation; Greywater; Magnetic ion exchange resin; Recycling

1. Introduction

Interest in wastewater recycling has been raised by theincrease of water demand, water shortage due to low rain-fall, economic and environmental issues (Eriksson et al.,2002). Among the different options for water reuse suchas industrial, irrigation, and ground water recharge, waterrecycling within urban environments is the least developed.Urban recycling usually integrates the reuse of black, greyor rain waters. Greywater is defined as domestic wastewa-ter excluding water from the toilet, and generally includeswastewaters from baths, showers, hand basins, washingmachines, dishwashers and kitchen sinks. However, atsmall scale the heavily polluted sources such as washing

0045-6535/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.chemosphere.2007.10.046

* Corresponding author. Tel.: +44 0 1234 750111; fax: +44 0 123475167.

E-mail address: b.jefferson@cranfield.ac.uk (B. Jefferson).

machines, dishwashers and kitchen sinks tend to beexcluded whereas at larger scale all sources are used tomaximize water savings. The most common applicationfor greywater reuse is toilet flushing which can reducewater demand within dwellings by up to 30% (Karpiscaket al., 1990). However, other applications such as irrigationof parks, school yards, cemeteries and golf courses, vehiclewashing, fire protection and air conditioning are practiced(Lu and Leung, 2003). The water quality standards forwastewater recycling depend on location and applicationbut generally include parameters based on organic, solidsand microbiological contents of the water. The most strin-gent criteria require a biochemical oxygen demand (BOD)of less than 10 mg l�1, a turbidity below 2 NTU and a non-detectable level of either total or faecal coliforms (USEPA,2004; Tajima, 2005). However, other standards which areless restrictive allow higher concentrations of the differentparameters or do not include some of the parameters atall (USEPA, 2004; Gross et al., 2007).

148 M. Pidou et al. / Chemosphere 71 (2008) 147–155

A large range of technologies has been used for grey-water recycling from simple 2-stage processes (coarse filtra-tion and disinfection) to physical, physicochemical andbiological processes (Jefferson et al., 2000). The latter,widely used in large building (Santala et al., 1998; Suren-dran and Wheatley, 1998; Nolde, 1999; Friedler et al.,2004) suffer from feed source variability and potentialshock loading at smaller scale. Such problems are avoidedwith simple physical processes such as cartridge filters ordepth filtration beds. However, whilst these are effectiveat removing the physical pollution within the greywater,they do not significantly alter the organic fraction (Jeffer-son et al., 2000). As such chemical processes such as coag-ulation and adsorption provide great potential for theremoval of the dissolved organic fraction within greywater.Indeed, coagulation with metal salts remains the main pro-cess utilised in the potable water treatment field for theremoval of high concentrations of dissolved organic carbon(DOC) (Parsons and Jefferson, 2006). In more recent timesa novel magnetic ion exchange resin (MIEX�) has been tri-alled and is being used to reduce organic loads onto somewater treatment works, to reduce coagulant demand orimprove the structural properties of the flocs produced(Jefferson et al., 2004a). In opposition to traditional ionexchange resins, MIEX� has a magnetic component in itsstructure which facilitates agglomeration and settling.Moreover, with an average particle size of 180 lm, 2–5times smaller than traditional ion exchange resins, MIEX�

has a high surface area for adsorption. And finally, it isdesigned to be added to the water as slurry in a mixed reac-tor. The particles dispersed in the water maximise the con-tact with the organics reducing the contact time neededcompare to a fixed-bed set up (Boyer and Singer, 2005).The aim of the present work is to assess the potential forutilise these chemical processes for greywater recycling.

2. Material and methods

2.1. Sampling

Greywater was collected from a purpose built facilitywhich diverts water from the bath, shower and hand basinof 18 flats within a student hall of residence located atCranfield University. This source water was defined asthe mixed source in the subsequent experiments. As analternative, shower water was also collected from a singlelocation. Care was taken to standardise the products used,with respect to their concentration and the duration of theshower between samples. Samples were taken on the morn-ing that the shower was used with all the analyses per-formed on the same day.

2.2. Experiments

MIEX� (Orica, Australia) resin was prepared by mea-suring the required dose in measuring cylinders andallowed to settle for approximately 1 h. If needed, the dose

was then adjusted with a plastic pipette and allowed to set-tle for another half hour. 1 l of the greywater to be treatedwas placed on a PB900 jar tester (Phipps and Bird, Vir-ginia, USA) which was set to 150 rpm. The resin was sha-ken in the measuring cylinder and added to the watersample, and the residual was rinsed into the jar with deion-ised water. The test was carried out for various contacttimes varying between 10 and 90 min. At the end of eachtest, the treated water was filtered through 0.45 lm glassfibre filter papers.

For the coagulation test, 1 L of the water to be treatedwas placed on the jar tester. Two speeds were used, a rapidmix at 200 rpm for 90 s, time during which the coagulant,either ferric sulphate (solution of FeSO4 (13%), FeripolXL, EA West, UK) or aluminium sulphate (solution ofAl2(SO4)3, 14H2O (48%), Kemira Chemicals, UK) wasdosed in the jar and the pH adjusted to the chosen value(4.5, 6 and 7). The sample was then flocculated for 15 minat 30 rpm and allowed to settle for an additional 15 min.

Finally, both tests were coupled with the MIEX� resinprepared as explained above and added to the water, atthe optimum conditions found under the previous tests.The jar tester was set up at 150 rpm for 10 min, after a set-tling period the treated water was filtered. The filtrate thenunderwent the coagulation experiments as described beforefor a range of concentrations and pH.

All tests reported here were duplicated and carried outat room temperature.

2.3. Fractionation

The raw water samples were fractionated into theirhydrophobic and hydrophilic components with a methodadapted from Malcolm and McCarthy (1992). The rawwater was first filtered through a 0.45 lm filter and acidi-fied to pH 2 using HCl (1 M). The acidified sample wasthen put through the XAD-8/XAD-4 column pair. Theeffluent from both columns contained the hydrophilicnon-adsorbed fraction. XAD-8 and XAD-4 columns wereeluted with NaOH (0.1 M) and the eluates were the hydro-phobic acid fraction and the hydrophilic acid fraction,respectively. The organic content of each fraction was thendetermined by measuring the DOC with a recovery of 88%on average.

For comparison, an anionic, a cationic and a non-ionicsurfactants were separated into their hydrophobic andhydrophilic fractions. Synthetic solutions of the anionicsurfactant sodium lauryl sulphate (Fisher Scientific, UK),the cationic surfactant cetyl trimethylammonium bromide(Sigma–Aldrich, UK) and the non-ionic surfactant Triton�

X-100 (Acros Organics, UK) at a concentration of 1 mMwere fractionated using the method previously described.

2.4. Zeta potential and charge density

Zeta potential was measured using a Malvern Zetasizer(Malvern, UK). The charge density of water samples was

M. Pidou et al. / Chemosphere 71 (2008) 147–155 149

determined by using the zeta potential and polydiallyldimethyl ammonium chloride (PolyDADMAC) (Sigma–Aldrich, UK). The samples were placed in a 1-l beakerand stirred. A 0.1% solution of PolyDADMAC was dosedinto the solution, the pH adjusted to 7 and the zeta poten-tial measured until the point of zero charge or iso-electricpoint was reached. The charge density of the samples(meq g�1

DOC) was then deduced from the amount of Poly-DADMAC (charge density: 6.2 meq g�1) used (Sharpet al., 2004).

2.5. Analytical procedures

DOC (mg l�1) was measured using a total organic car-bon analyser Shimadzu TOC-5000A (Shimadzu, MiltonKeynes, UK). Turbidity (NTU) was measured with a turbi-dimeter Hach 2100N. Escherichia coli and total coliforms(MPN 100 ml�1) were measured using the method Colilert18 with quanti-tray 2000 (Idexx, UK) and faecal Entero-

cocci (MPN 100 ml�1) using the Enterolert with quanti-tray 2000 (Idexx, UK). BOD (mg l�1) was measured usingthe procedure 5 day Biochemical Oxygen Demand fromThe Standard Methods for Examination of Water andWastewater (APHA, 1992). Merck cell tests (VWR Inter-national, UK) were used for the following tests: COD,ammonia, nitrate, phosphate and total nitrogen (accordingto Korleff’s method). UV absorbance was measured with a6505 UV/vis. Spectrophotometer (Jenway, UK) at a wave-length of 254 nm.

3. Results and discussion

3.1. Characteristics

The two sources of greywater tested in the current inves-tigation varied considerably in terms of their organic con-centration (Table 1). For instance, the BOD and COD ofthe two sources were 39 ± 17 and 144 ± 63 mg l�1 for the

Table 1Water sources characteristics

Greywaters

Mixed (n = 14) (low strength) Shower (n

BOD (mg l�1) 39 ± 17 166 ± 37COD (mg l�1) 144 ± 63 575 ± 98COD/BOD 3.7 ± 1.2 3.5 ± 0.4DOC (mg l�1) 12 ± 4 56 ± 7Turbidity (NTU) 35 ± 16 42 ± 9TN (mg l�1) 7.6 ± 3.0 16.4 ± 3.0PO3�

4 (mg l�1) 0.5 ± 0.2 1.3 ± 0.1NHþ4 (mg l�1) 0.7 ± 0.7 1.0 ± 0.3NO�3 (mg l�1) 3.9 ± 1.6 7.5 ± 1.2pH 6.6–7.6 7.3–7.8Charge density (meq g�1

DOC) 0.6 ± 0.1 2.4 ± 0.1SUVA (l mg�1 m�1) 2.5–3.5 1.5–2.5Hydrophobic fraction (%) 40 30

a In secondary treated effluent (Jarusutthirak et al., 2002; Hu et al., 2003); n

mixed source and 166 ± 37 and 575 ± 98 mg l�1 for theshower water. Published greywater strengths indicate thatthe organic load exerted by a greywater can vary consider-ably from one scheme to the next but normally fall withinthe range 50–300 mg l�1 BOD (Jefferson et al., 2004b).Interestingly, the organic strength of the mixed sourcewas the lowest level recorded in the literature for a realgreywater source. Water production rates from the facilityare roughly at the expected level and so the cause of the lowstrength is unclear but is probably a combination of prod-uct choice by the students and the residence time in the col-lection system. Comparison of the organic parametersindicates that the COD to BOD ratio is approximately3.5 in both waters studied in the current investigation. Thiscompares to 2.2 for typical domestic sewage and 3–10 forfinal effluent suggesting greywater contains more non-bio-degradable material than sewage (Metcalf and Eddy,2003). This confirms the limitation of biological processesto treat greywater and supports the case for investigationof non-biological treatment options. The higher nutrientconcentrations and charge in the shower water than inthe mixed source were due to the washing products choice.

Characterisation of the greywaters in relation to param-eters commonly used when describing coagulation revealthe DOC of the two sources to be 12 ± 4 and 56 ± 7 mg l�1

for the mixed and shower sources, respectively. The formeris equivalent to that of a potable water source with a highconcentration of natural organic matter (NOM) (Ratnawe-era et al., 1999) whilst the latter represents a very highstrength and as such high coagulant doses are expected.The specific UV absorbance (SUVA) was between 2.5and 3.5 l mg�1 m�1 for the mixed greywater and between1.5 and 2.5 l mg�1 m�1 for the shower water suggestingboth greywater sources contain mainly hydrophilic andlow molecular weight (MW) compounds (Edzwald andTobiason, 1999; Karanfil et al., 2002). Fractionation ofthe water confirmed this with between 60% and 70% ofthe organic matter being associated with the hydrophilic

Wastewater Natural water

= 15) (high strength) Metcalf and Eddy, 2003 Sharp et al., 2006

110–450 nr250–800 nr1–3 nr80–260 7–14nr nr20–70 nrnr nrnr nr0 nrnr nrnr 5–151.5–2.7a 5nr 59–75

r: not reported.

150 M. Pidou et al. / Chemosphere 71 (2008) 147–155

fractions separated during the XAD resin fractionationprocess (Table 1). Similar values are found for river watersources containing a high degree of sewage effluent werethe hydrophilic components can constitute around 40–60% of the total organic strength in the water (Parsonsand Jefferson, 2006). Surfactants, commonly used in house-hold products and proved to be the biggest fraction of theorganic content of greywater (Wiel-Shafran et al., 2006),are known to contain both hydrophilic and hydrophobicsections (Ikehata and El-Din, 2007). However, the fraction-ation of three surfactants individually, sodium lauryl sul-phate, an anionic surfactant commonly used as athickener, cetyl trimethylammonium bromide, a cationicsurfactant used as an antiseptic and Triton� X-100, anon-ionic surfactant used as a detergent, confirmed the ten-dency of greywater to be mainly hydrophilic. Indeed, theresults of the fractionation on XAD resins were similarfor the three surfactants with a hydrophilic fraction of92%, 92% and 88% for the anionic, cationic and non-ionicsurfactants, respectively.

In relation to the electrical character of the greywatersstudied, the colloids were negatively charged in naturalpH environments with a zeta potential of �13.2 ± 0.7 mVand �19.4 ± 3.1 mV for the mixed and shower sources,respectively. The corresponding charge densities of thetwo waters were 0.6 ± 0.1 and 2.4 ± 0.1 meq g�1

DOC indicat-ing that the fresher shower water contained considerablymore charged components compared to the mixed sourcewater. Comparison with other species commonly coagu-lated suggests that the colloids in greywater exert a rela-tively low charge demand to the water per unit oforganic material. For instance, reported values of othersystems are around 5 meq g�1

DOC for NOM (Kam and Greg-ory, 2001; Sharp et al., 2006), 0.1–3.2 meq g�1

DOC for algalorganic matter (Henderson et al., 2006) and 0.1–1 meq g�1

for inorganic colloids such as kaolin (Edzwald, 1993).Interestingly, reported charge densities for the hydrophiliccomponents within NOM are 1 ± 0.6 meq g�1

DOC which is inbetween the values obtained for greywater. Conversion ofthe charge densities to charge concentration reveals thatthe greywater will exert a charge load of 0.0072 meq l�1

and 0.134 meq l�1 for the mixed source and shower water,respectively. Comparative levels for NOM are around0.02–0.05 meq l�1 (Sharp et al., 2006) such that the mixedsource water represents relatively low coagulant demandbut the shower water exerts a very high coagulant demandwhich is around 25–70 times greater than a typical NOMrich water. Calculation of the neutralising charge of coag-ulants based on speciation data (Jiang and Graham,1998) reveals that iron based coagulants provide 35.5,30.7, 12.6, 1.7 meq g�1

Fe at pH values of 4, 5, 6, 7, respec-tively. Similarly, over the same pH value aluminium coag-ulants provide 104.5, 45.8, 12.5, 1.9 meq g�1

Al . Hence thecharge neutralising capacity of both coagulants is greaterunder acidic conditions. To illustrate, the neutralisingcapacity reduces by a factor of 8 and 3 for alum and iron,respectively, as the pH increases from 4 to 6. Interestingly,

at near neutral pH levels as found in greywater the neutral-ising capacity of both systems is very similar. Differences inthe neutralising capacity of each coagulant are based on theweight of the metal ion such that conversion to a meq basisreveals both systems to provide almost identical capacities(Sharp et al., 2006).

3.2. Coagulation

Both greywater sources were trialled over a range ofcoagulant doses and pH with the results presented in termsof BOD as it represents the most common complianceparameter for urban reuse (Fig. 1). In the mixed greywater,the residual BOD concentration remained quite constant ataround 1–5 mg l�1 for alum and 1–7 mg l�1 for Ferric rep-resenting a removal efficiency between 68% and 99%. Theresponse of the system appeared independent of pH overthe ranges tested. In contrast, in the case of the showerwater the residual BOD decreased as a function of doseuntil it reached a plateau value (Fig. 1c and d). To illustrate,the BOD decrease from 166 ± 37 mg l�1 to 23 and30 mg l�1 for alum and ferric, respectively, representingremoval efficiencies of 85% and 79%. In both cases the effec-tiveness of the coagulants was improved under more acidicconditions although this was more pronounced in the caseof alum than ferric. For example, when coagulating with18 mg l�1 of alum the residual BOD was 69, 39 and20 mg l�1 at pH 7, 6 and 4.5, respectively. This had alsoan impact on the dose required to reach the plateau as inthe case of alum the minimum dose necessary was 24, 28and 32 mg l�1 for pH values of 4.5, 6 and 7, respectively.

Comparison of the two coagulants revealed that themaximum level of removal was around 85% for both sys-tems suggesting little difference in performance. Compari-son of the required doses showed that more ferric wasrequired by mass to achieve a set level of removal. Conver-sion to molar concentrations indicates the required mini-mum dose was 0.79 mM for ferric and 0.89 mM for alumindicating that in fact proportionally more alum wasrequired per unit of treatment. Comparison with COD,DOC and UV254 revealed that the level of treatment wasless dose and pH dependant in terms of these parameters.To illustrate, the most apparent case was in terms ofUV254 where the level of removal remained around 74%in case of alum. Information on the types of organics thatare removed is provided by comparing the SUVA beforeand after treatment. In the case of both coagulants theSUVA decreased from 2.5–3.5 in the raw water to 0.6post-coagulation suggesting that the hydrophobic compo-nents have been predominately removed.

3.3. MIEX�

Treatment of both greywater sources with MIEX�

showed a similar pattern to coagulation. For the mixedgreywater (Fig. 2a), the treatment appeared to be indepen-dent of the contact time and dose, with BOD residual con-

pH

0

4

8

12

16

20

0 5 10 15 20

Al dose (mg.l-1)

BO

D (

mg

.l-1)

7 6 4.5 pH

0

10

20

30

40

0 5 10 15 20Fe dose (mg.l-1)

BO

D (

mg

.l-1)

7 6 4.5

pH

0

40

80

120

160

200

0 10 20 30 40 50 60

Al dose (mg.l-1)

BO

D (

mg

.l-1) 7 6 4.5

pH

0

40

80

120

160

200

0 20 40 60 80

Fe dose (mg.l-1)

BO

D (

mg

.l-1)

7 6 4.5

a

c

b

d

Fig. 1. Influence of coagulant dose and pH on biochemical oxygen demand (BOD) removal in the (a and b) mixed and (c and d) shower greywaters.

Contact time (min)

0

20

40

60

80

0 5 10 15 20 25 30 35

MIEX dose (ml.l-1)

BO

D (m

g.l-1

)

10 20 30 45 60 Contact time (min)

0

50

100

150

200

250

MIEX dose (ml.l-1)

BO

D (m

g.l-1

)

10 30 60 90

0 20 40 60

a b

Fig. 2. Influence of MIEX� dose and contact time on biochemical oxygen demand (BOD) removal in the (a) mixed and (b) shower greywaters.

M. Pidou et al. / Chemosphere 71 (2008) 147–155 151

centrations between 1 and 14 mg l�1 over the range of con-ditions tested. Corresponding removal efficiencies variedbetween 80% and 99% which reflects the low level ofBOD in the influent. In contrast, the residual BOD of theshower greywater decreased as both the contact time anddose of MIEX� increased (Fig. 2b). To illustrate, at a10 ml l�1 MIEX� dose, the BOD residual was 60.2, 32.7,27.6 and 17.6 mg l�1 for respective contact times of 10,30, 60 and 90 min. Whereas, for a 10-min contact time,the BOD residual was 112.7, 85.7, 60.2 and 40.2 mg l�1

for a MIEX� dose of 2, 5, 10 and 20 ml l�1, respectively.However, the residual BOD reached a plateau with aremoval efficiency of around 83% once the dose reached20 ml l�1 or above. Similar patterns were observed in termsof COD, DOC and UV254 except considerably less varia-

tion was observed between contact times in terms ofUV254. For example, in the case of the shower water dosedat a MIEX� concentration of 10 ml l�1 the residual UV254

was 0.158, 0.132, 0.134 abs at contact times of 10, 30 and60 min, respectively. Comparison of the different parame-ters reveals that the feed water has a specific UV absor-bance of between 2.5 and 3.5 suggesting the water ismoderately hydrophilic in nature compared to the effluentwhich is always less than 1 suggesting the residual organicsare mostly hydrophilic in nature such that the MIEX� pro-cess appears to be targeting the same organics as the coag-ulation process. This agrees with the work of Fearing et al.(2004) who studied the use of MIEX� for NOM removaland showed that only the very small, most hydrophilicmaterial remains after treatment.

152 M. Pidou et al. / Chemosphere 71 (2008) 147–155

3.4. MIEX� + coagulation

A series of experiments were conducted on a combinedsystem for which the greywater was first treated with theoptimum dose of MIEX� and then coagulated with differ-ent doses of either ferric or alum. As both treatment sys-tems were capable of achieving a sufficient treatmentaccording to the water quality standards for the lowstrength greywater the combined treatment was only testedon the shower water. Residual BOD concentrations variedbetween 20 and 40 mg l�1 under all the conditions tested interms of coagulant choice, dose and pH (Fig. 3). Compar-ison with the previous tests reveals that the combined sys-tem was not able to reduce the BOD concentration belowthe level previously obtained. However it was found thatthis treatment was achieved independently of the coagulantdose or pH. Similar trends to the one presented for BODwere found for COD, DOC and UV254. And removals of64% COD, 53% DOC and 70% UV254 were observed. Sim-ilarly, Singer and Bilyk (2002) reported removals between53 and 96% for UV254 and between 46% and 87% forTOC for the treatment of natural waters with MIEX� incombination with alum. The lower removals were observedfor the raw waters with the lower SUVA. To illustrate,UV254 removals of 53% and 94% were recorded for waterswith a SUVA of 1.4 and 4.5 l mg�1 m�1, respectively. This

pH

0

50

100

150

200

250

0 10 20 30 40

Al dose (mg.l-1)

BO

D (

mg

.l-1)

7 6 4.5a b

Fig. 3. Influence of coagulant dose and pH on biochemical oxygen demand(10 ml l�1, 30 min) and coagulation ((a) alum and (b) ferric).

Table 2Shower greywater characteristics after treatment with the different systems at

Optimum Raw MIEX� Alum10 ml l�1, 30 min 24 mg

Turbidity (NTU) 46.60 8.14 4.28COD (mg l�1) 791 272 287BOD (mg l�1) 205 33 23DOC (mg l�1) 171.4 78.2 93.4TN (mg l�1) 18 15.3 15.7NHþ4 (mg l�1) 1.2 1.1 1.2NO�3 (mg l�1) 6.7 4.7 5.7PO3�

4 (mg l�1) 1.66 0.91 0.09Total coliforms (MPN 100 ml�1) 56500 59 <1Escherichia coli (MPN 100 ml�1) 6490 8 <1Faecal Enterococci (MPN 100 ml�1) 2790 <1 <1

confirms once more the preference of treatment of MIEXand coagulation for hydrophobic and high MW materials.

3.5. Comparison of the systems

With an organic removal from 68% to 100%, both sys-tems, coagulation and ion exchange, proved to be efficientto treat the mixed greywater to the most stringent standardfor reuse with the lowest doses and contact times tested.However, it must be noted that the raw water strengthwas very low and it is perhaps not too surprising that thesystems were capable of removing sufficient materials tomeet the compliance standard. In the case of the showerwater, optimum conditions for MIEX� was shown to be10 ml l�1 at a contact time of 30 min (Table 2) to achievea DOC removal of 54%. Similar dose and contact timewere used for NOM removal in drinking water for a finalDOC removal of 60% (Fearing et al., 2004). The optimumconditions for coagulation were always observed at pH 4.5and a dose of 24 mg l�1 (0.89 mM) alum and 44 mg l�1

(0.79 mM) ferric when used alone and a reduced dose ofonly 5 mg l�1 for both coagulants when used in conjunc-tion with MIEX�. With these optimum doses of both ferricand alum, a general removal of 64% in terms of COD wasthen achieved. Comparable studies presented similarresults. Lin et al. (2005) showed that 25 mg l�1 of alum

pH

0

50

100

150

200

250

0 10 20 30 40

Fe dose (mg.l-1)

BO

D (

mg

.l-1)

7 6 4.5

(BOD) removal in the shower greywater after treatment with MIEX�

optimum conditions

Ferric MIEX� + Al MIEX� + Fel�1, pH 4.5 44 mg l�1, pH 4.5 5 mg l�1, pH 4.5 5 mg l�1, pH 4.5

5.20 3.01 3.30288 247 25430 27 2987.4 78.8 80.717.9 15.3 17.41.2 1.2 1.26.1 4.4 4.80.06 0.11 0.13<1 <1 <11 <1 <1<1 <1 <1

M. Pidou et al. / Chemosphere 71 (2008) 147–155 153

was needed to achieve a COD removal of 60% in a grey-water treated by electro-coagulation. And a dose of only5 mg l�1 alum was needed to achieve a 36% removal in alaundry wastewater with an initial COD of 280 mg l�1

(Sostar-Turk et al., 2005). Effluent characteristics in thecurrent study were similar for all five systems tested witha slight improvement in COD and DOC removal observedfor the combined MIEX� and coagulant systems (Table 2).Residual turbidity was measured as 8.1, 4.2 and 5.2 forMIEX�, alum and ferric, respectively. In comparison thelevels decreased to 3.3 and 3 NTU for the combined sys-tems with ferric and alum, respectively. This is once againsimilar to results found in the literature. El Samrani et al.,2004 reported a reduction of the turbidity from 40 to 5NTU in sewage treated with a dose of 43 mg l�1

(0.77 mM) of ferric. The removal of the total coliformswas excellent at 3 log (99.8%) with MIEX� correspondingto a residual of 59 MPN 100 ml�1. In comparison all theother systems recorded a non-detectable level in theeffluent.

4. Discussion

Comparison with water recycling standards required forurban reuse indicates that coagulation and MIEX� are notalways able to meet the required levels of treatment for allsituations. In the case of the shower water the treatmentsystems failed to comply with both the turbidity andorganic concentration requirements. Comparison of theinfluent water strengths indicates that the maximumstrength of the mixed systems was 72 mg l�1 BOD andthe minimum of the shower water was 110 mg l�1. Conse-quently a threshold organic concentration value wouldappear to exist between these two limits beyond whichcoagulant is unlikely to be able to meet effluent standards.In fact the systems appear unable to remove sufficientorganics even at high doses of coagulant or MIEX� indi-cating that it is likely to be a recalcitrant proportion ofthe greywater to chemical solutions.

MIEX� is an ion exchange process specifically designedto remove NOM from potable water and is believed to beeffective at removing mid and higher MW compounds. Theresults in the current study support this suggestion as thegreatest removal was achieved with MIEX� alone (54%).In comparison in potable water treatment DOC removalswith MIEX� alone are commonly 10–20% lower than withcoagulants although in combined systems the overallremoval is slightly better than either MIEX� or coagula-tion alone (Fearing et al., 2004).

The main coagulation mechanisms at the optimum pHare charge neutralisation for colloidal material and chargecomplexation for soluble material (Sharp et al., 2006). Inboth cases the process is driven by charge interactions suchthat preferentially removal of charged materials is likely tooccur. Consequently low charge or neutral materials arelikely to be poorly removed although some removal is pos-sible due to adsorption mechanisms on to the pre-formed

flocs. Such suggestions are supported as the zeta potentialwas always within the ranged previously reported forcharge dominated processes (Sharp et al., 2006). Examina-tion of standard speciation diagrams suggests that at thehigh doses applied sweep flocculation from the precipitatedhydroxide should be the dominate removal mechanisms.Whilst this is possible the complexation reaction betweenorganics and the metal coagulants are known to be fastsuch that it is likely that the complex precipitates ratherthan the straight hydroxide. Typical dose requirementsfor organic dominated systems are around 1:1 on a massDOC per mass coagulant basis (Jarvis et al., 2005). Inthe current study on the shower water optimum doses wereobserved at ratios of 7 for alum and 3.9 for ferric. Themuch lower levels required here relate to the nature ofthe feed water and its relatively low charge density. Con-version of the data to a mass ratio based just on the hydro-phobic content of the greywater converts the dose ratio toaround 1.2:1 which is around the level reported for coagu-lation of hydrophobic rich waters and indicates the differ-ences between the dose ratios used for NOM removaland greywater are based on there respective hydrophiliccontents.

Comparison to coagulation of organics from other fieldsreveals a low percentage removal in the case of greywater.To illustrate, typical DOC removals during the coagulationof NOM are between 60% and 80% (Fearing et al., 2004)and this compares to 40–50% in the current case. The dif-ference can be attributed to the make up of the organicmolecules in both cases. NOM is commonly rich in anionichydrophobic humic and fulvic acids which are easilyremoved by coagulants. The residual are generally thesmaller, hydrophilic neutrals that are difficult to removeby most treatment methods. Typical MW sizes for NOMare between 2 and 5 kDa for the hydrophobics and<2 kDa for the hydrophilics based on UV absorption(Fearing et al., 2004). Greywater in contrast is mainly madeup of <3 kDa material (Jefferson et al., 2004b) and appearsto be mainly hydrophilic in nature (Table 1). Indeed, a rela-tionship is known to exist between the hydrophilic concen-tration of NOM in the raw water and the residual DOCthat can be achieved under optimum coagulation condi-tions (Sharp et al., 2006). The current data suggest thatgreywater coagulation fits into a similar correlationwhereby the residual DOC after coagulation is approxi-mately 80–90% of the raw water hydrophilic content. Fur-ther support is provided by analysis of the SUVA which isbetween 2 and 3 in the raw water but always 1 or lower inthe treated waters suggesting that all the tested processesare removing the hydrophobic components form the water.

Overall, the findings suggests that whilst chemicals pro-cesses such as coagulation and MIEX� can achieve suffi-cient levels of organic removal to meet some standardsfor reuse, they are not capable of meeting the most strin-gent of reuse standards reported around the world. Thislimit appears to be a fundamental one based on the charac-ter of the raw water and suggest that such processes are

154 M. Pidou et al. / Chemosphere 71 (2008) 147–155

never going to be suitable if very tight standards arerequired without the use of downstream processes such asadsorption to achieve the remaining removal. However,reuse standards are not always so strict especially for appli-cations not based around dwelling such as landscape andgarden irrigation, fire protection, vehicle washing etc. (Luand Leung, 2003). Consequently, chemical processes suchas those discussed above can have a role where less strin-gent standards are required. In particular, they provide apossible alternative to biological systems where such pro-cesses are not preferred or technically difficult to operate.

Ultimately, the economical feasibility of the systems willdetermine their potential application. However, based oncoagulant costs of £0.05–0.20 kg�1 and £0.09–0.42 kg�1

for alum and ferric, respectively (Akhtar et al., 1997; Gebbie,2005; Ahmed, 2007), the costs of coagulants at optimumdose would be £0.0012–0.0048 m�3 and £0.0040–0.0185 m�3 of treated greywater. Although better treatmentwas found for acidic pH, it is not advised to run a full scalesystem in this condition. Indeed, pH adjustment beforetreatment and readjustment after, depending on the reuseapplication, would be required and would then increasethe costs of the system. Considering an average drinkingwater price of £0.9 m�3 (European Environment Agency,2003) and a greywater production of about 100 l d�1

capita�1 (Kujawa-Roeleveld and Zeeman, 2006), annualsavings of £32.2, £321.7 and £1608.7 per year are possiblefor 1, 10 and 50 persons respectively which would need tomeet the capital cost of the equipment and associated run-ning costs. Whilst exact capital costs are difficult to estimateit appears unlikely that adoption of chemical solution will bedriven from a purely economic standpoint at small scale.

5. Conclusion

Although good organic removal, comparable to the dataseen in the literature for NOM removal in potable water ororganics in other wastewater, has been observed for the dif-ferent systems tested, they showed limitations to meet thestandards for reuse. The chemical solutions tested in thecurrent study have revealed that both MIEX� and coagu-lation are suitable treatment solutions for low strengthgreywater sources. However, all the systems tested wereunable to achieve the required level of treatment for thereuse of medium to high strength greywaters. Chemicalsolutions appear to be limited due to the recalcitrant natureof a proportion of the greywater which prevents the neces-sary level of treatment being achieved. Ultimately chemicaltreatment solutions with coagulants and ion exchange resinappear to be limited for the reuse of greywater withinurban environments.

Acknowledgements

This work forms part of the ‘Water Cycle Managementfor New Developments’ (WaND) project funded under theEngineering & Physical Science Research Council’s ‘Sus-

tainable Urban Environment’ Programme by EPSRC,UK government and industrial collaborators (www.wand.uk.net).

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