Characterization of greywater in an Indian middle-class household and

Separation and Purification Technology 130 (2014) 160–166

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Characterization of greywater in an Indian middle-class household and investigation of physicochemical treatment using electrocoagulation Kuntal A. Vakil, Meena K. Sharma, Akansha Bhatia, Absar A. Kazmi, Sudipta Sarkar ⇑ Department of Civil Engineering, Indian Institute of Technology Roorkee (IITR), Roorkee 247667, India

a r t i c l e

i n f o

Article history: Received 4 September 2013 Received in revised form 4 April 2014 Accepted 6 April 2014 Available online 18 April 2014 Keywords: Electrocoagulation Greywater Domestic wastewater Household wastewater treatment Water recycling Wastewater reuse

a b s t r a c t Wastewater produced from all the domestic uses of water sans toilet flushing is known as greywater. It is often the major component in the domestic wastewater but has fewer pollutant load. Recycling and reuse of treated greywater for non-potable purposes may significantly reduce the stress on the fresh water requirement. This article presents the result of a study undertaken for characterization as well as laboratory-based investigation for treatment of greywater generated from an Indian single household. The greywater constituted at least 80% of the total wastewater with maximum contribution (44%) from the kitchen. The treatment studies, undertaken in an electrochemical reactor where the voltage and current were varied for sacrificial aluminum anodes, revealed that about 70% of the total COD and more than 99.9% pathogens could be removed with an energy consumption of 0.3 kW h/m3 of wastewater. COD removal reached a maximum of 70%, irrespective of the applied voltage and current density, at an aluminum release from the anode at a rate of 15 mg/L as aluminum. The electrochemical reactor aluminum electrodes, operated with maximum potential difference of 12 V, showed potential for scale-up for real-life use in households for removal of pathogens, turbidity and COD contents of greywater. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Domestic wastewater may be categorized into two parts, namely greywater and blackwater. It is universally accepted that wastewater generated as a result of toilet flushing is known as blackwater while wastewater produced as a result of usage of water for all other domestic purposes such as at kitchen, wash basins, showers, washing machines, etc. may be termed as greywater [1–4]. Understandably, greywater is likely to be the major contributor to the total wastewater quantity generated from an average household [5,6]. In terms of quality, greywater is expected to have lesser organic and nutrient content, and lower pathogen load when compared to blackwater. Greywater generally represents up to 70% of total consumed water but contains only 30% of organic fraction and from 9% to 20% of the nutrients [7,8]. Although there remains significant difference in water quality between grey and blackwater, conventional collection and conveyance systems for domestic wastewater do not differentiate between these two types of wastewater. Until now, in most of the countries, the aggregate total of wastewater generated in a household is collected and conveyed through the municipal sewerage system to a centralized wastewater treatment plant. Only ⇑ Corresponding author. Tel.: +91 8954386690. E-mail address: [email protected] (S. Sarkar). http://dx.doi.org/10.1016/j.seppur.2014.04.018 1383-5866/Ó 2014 Elsevier B.V. All rights reserved.

recently, due to the global shortage of freshwater supplies, a general necessity is being felt for treatment of greywater for its reuse and recycling. On-site source-separated treatment and reuse of less polluted greywater is therefore an attractive option, particularly in water stressed nations. The most common application for reuse of treated greywater is for specific applications such as toilet flushing, garden irrigation and other processes where high degree of water quality is not a necessary requirement. Such a cascading reuse of water can have multiple benefits. First, it helps reduce the freshwater consumption in a household and related savings in the cost involved in the treatment and supply of drinking quality water to the whole community. Second, the overall quantity of blackwater to be collected and treated at the municipal sewage treatment plant shall reduce, causing huge cost savings in the treatment process. In many Indian wastewater treatment plants, it has been reported that the plants efficiency was unsatisfactory because of the low organic load in the wastewater [9]. Obviously, such problems may be avoided altogether if greywater is separated from the blackwater, so that the wastewater contains only blackwater having high organic content. Thus, onsite treatment of greywater followed by its recycling and reuse may be considered as the important steps when it comes to the reduction of freshwater requirements at the individual households. The success of recycle and reuse of treated greywater depends on the ease and effectiveness with which it is treated on-site. In order to develop an

K.A. Vakil et al. / Separation and Purification Technology 130 (2014) 160–166

effective on-site treatment process for greywater, it is imperative that detailed characterization of the greywater is done. Several factors such as the quality of freshwater supply, activities in the household, the quality of life, climatic conditions of the area, economic strata of the household, etc. are expected to control the greywater generated, in terms of both quality and quantity. Obviously, the quantity and characteristics of greywater varies from country to country, and also, within a country depending on the geographic and climatic variations as well as on the different incomegroups. In many areas of India, the average annual rainfall is as low as only 0.27–10.16 mm/year [10]. Due to the huge growth in population and related activities to support the population, the surface water availability per capita has reduced from 2300 m3 in 1991 to 1980 m3 in 2001. The water availability situation is going to enter into water scarcity scenario within a decade from now. In fact, in 2013 Western Indian state of Maharashtra faced an unprecedented draught [11]. As the quantity of freshwater is dwindling, there is a dire need to conserve the freshwater sources by increasing the recycling practices of used water. Greywater recycling seems to be one viable option that can be implemented within a reasonably short period of time at individual household level. However, there is insufficient data available for estimation of quantity and quality of greywater produced in a typical Indian middle-class household. Also, not much work has been done on the viable treatment options for recycling and reuse of greywater in Indian situation. Treatment technologies for greywater recycling have been in focus of the researchers since the last four decades. Both physicochemical and biological treatment processes have been studied so far. Physical treatment options such as coarse filtration and/or membrane-based processes have often been coupled with disinfection. The technologies tried for biological treatment included rotating biological contactors, biological aerated filters and aerated bio-reactors. Also, advanced treatment technologies such as MBRs and cheaper extensive technologies such as reed beds have been investigated. Three chemical processes, such as photocatalysis [12], electrocoagulation [13] and chemical coagulation [14] were reported in open literature. Electrocoagulation process offers simpler and more efficient treatment due to its compactness and economic competitiveness over conventional coagulation, particularly with regards to hydraulic retention time, less production of sludge and simplicity in operation and maintenance [13,15]. In this article we report the results of a study where main objectives were to characterize household greywater in India and also, to study its treatment using physico-chemical treatment processes that can potentially be scaled up into user-friendly household devices. For the characterization study we took representative greywater samples from an average Indian middle-class family over a period of six (6) months. We also took hourly spot-samples of greywater produced at various water-consuming points over a whole day. There is apparently not much financial benefit for the individual households to treat and recycle greywater. Therefore, it is expected that people would try to spend minimum possible domestic time for this purpose. Hence, the treatment and recycling technology needs to be such that it would demand the minimum user-intervention both during regular operation as well as for its maintenance. Keeping this in view, we intended to develop a dependable physico-chemical process for the treatment of greywater so that it can be recycled. In this article, we also report the logical development of an electro-coagulation process for treatment of greywater.

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collection drains were modified so that all the wastewater generating points except the toilets are connected and led to a PVC tank of size 1000 L placed under the ground located at the backyard of the house. Thus, the wastewater that got collected in the tank was actually wastewater sans the blackwater and hence, may be termed as greywater. The wastewater enters the tank at the bottom and leaves the tank from the top. The wastewater was allowed to get collected within the tank over 24 h after which a composite daily sample was collected after complete mixing of the content of the tank. The tank was emptied daily before the next collection cycle began. The tank had arrangements to measure the volume of greywater collected. The household had population of four adult persons. The study was undertaken over a period of six (6) months, from December 2012 to May 2013. 2.2. Treatment studies The treatment studies were undertaken with an aim to develop a dependable physicochemical treatment method for greywater. The treatment technology option adopted was electro-coagulation followed by floatation/sedimentation. Electrocoagulation is an electrochemical technique for removing various pollutants from wastewater [16,17]. Efficiency of removal of suspended solids, COD and microbial content was assessed using a stirred tank electro-chemical reactor where different variations and arrangements of aluminum electrodes, combination of aluminum and graphite electrodes were used. The bench-scale reactor used for the laboratory study was made of acrylic sheets. The effective dimensions of the reactor were 50 cm (H)  20 cm (L)  10 cm (W), having volume of 10 L. The content of the reactor was stirred with a stirrer rotating at a speed of 30 rpm. Two types of electrode assemblies were used in the reactor. Four aluminum plates each of effective size 48 cm  8 cm  0.2 mm thickness were used as electrodes. The electrodes were connected to DC power source in such a way that they acted as monopolar electrodes, either cathode or anode. A typical sketch of the reactor and the electrode system is shown in Fig. 1. Samples were taken at regular time intervals for determination of turbidity, COD and FC remaining in the greywater. In both the assemblies the system voltage was varied using step up/step down voltage regulator having range from 3 V to 12 V so that different amounts of current were flowing through the system. The current and voltage through the system were measured using Multimeter. Spacing between the electrodes was 4 cm. 2.3. Characterization studies The quality of greywater as well as treated water was assessed in terms of several water quality parameters such as Chemical

2. Materials and methods 2.1. Greywater collection mechanism A representative middle-income group household in Roorkee, Uttarakhand, India was chosen for the study. The wastewater

Fig. 1. Schematic of the electrochemical reactor used for the treatment of greywater.

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Oxygen Demand (COD), Biochemical Oxygen Demand (BOD), Total Suspended Solids (TSS), Total Dissolved Solids (TDS), pH, ammonical-nitrogen (NH3-N), orthophosphate, concentration of total and fecal coliform (TC and FC, respectively) and alkalinity, according to the methods prescribed in Standard Methods [18]. Chemical Oxygen Demand (COD) was measured using COD digester (Model AL 38SC, Aqualytic, Dortmund, Germany) and an ultraviolet (UV) spectrophotometer (Model DR4000 Hach, Loveland, CO) were used for the COD analysis. 3. Results and discussion 3.1. Greywater generation The average amount of greywater generation over the six month period of study was found to be 140 L per person per day. The average greywater flow rate into the collection tank was about 23 L per hour with maximum rate of generation occurring between during morning 0600 A.M. to 12 noon. About 20% wastewater generated was found to be blackwater, generated from the toiletflushing. Out of the total greywater generated from different water consuming points, kitchen generated majority of the greywater, about 44%. Fig. 2a shows the distribution of greywater generated from different consuming points. 3.2. Greywater characterization The greywater was collected and characterized daily over a period of six (6) months, from December 2012 to May 2013. Table 1 shows the average monthly greywater quality over this period of time. The pH varied between 5.9 and 7.8; COD/BOD ratio remained

in the envelope 2.4–4.7. The total and fecal coliform density was low during winter months, namely December and January; however, there was an increase in the TC and FC count at the onset of summer, in April and May. The average maximum and minimum temperatures at Roorkee during winter are 21 °C and 6 °C, respectively; and during summer are 42 °C and 20 °C, respectively. The reason for low coliform count during winter months may be due to the enhanced death rate of the coliform bacteria in the cold water inside the greywater collection tank from where composite daily samples were taken. There was a steady decrease in the COD and BOD values as the summer set in. Due to the onset of summer, the biodegradation rate increased causing some of the BOD and COD to get exerted by the time the composite sample was collected at the end of the day. Fig. 2b–d shows the relative distribution of COD, BOD and FC loads, respectively, generated from the different greywater generation points. Table 2 shows 24 hourly average characteristics of greywater generated at different generation points.

3.3. Greywater treatment: Electro-coagulation studies 3.3.1. Effect of system voltage on the removal efficiency of turbidity, COD and pathogens Laboratory testing of the electrochemical cell involved determining the effect of applied voltage on the efficiency of removal of suspended solids and colloids, organics and pathogens. Fig. 3 shows the suspended solids and colloids removal efficiency measured in terms of removal of turbidity. It may be observed that applying electric field from 6 to 10 V tended to increase the efficiency of turbidity removal. Also, the removal increases over time. Lin et al. [13] reported that in a synthetic greywater having

Fig. 2. Relative quantity and characteristics of greywater produced from different generating points: (a) quantity, (b) COD, (c) BOD and (d) fecal coliform (FC) load in the greywater.

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K.A. Vakil et al. / Separation and Purification Technology 130 (2014) 160–166 Table 1 Average monthly greywater characteristics from a middle-class Indian household. Parameters

Units

December

January

February

March

April

May

pH Alkalinity Turbidity TSS TDS Sulfate Chloride BOD COD TC FC

– mg/L NTU mg/L mg/L mg/L mg/L mg/L mg/L MPN/100 mL MPN/100 mL

7.2 203 69 712 1442 154 19 265 665 500 Nil

5.9 198 62 788 1337 85 23 220 733 240 Nil

7.8 170 59.8 339 707 74 28 118 359 9.3  103 430

6.6 180 66 422 902 185 22 290 560 4.3  103 240

7.1 187 104 288 1483 120 29 95 320 4.6  104 660

7.4 177 88 355 455 155 19 199 356 9.3  106 2.3  105

Table 2 Average characteristics of greywater generated at different generation points. Parameters

Bath/shower

Washbasin

Kitchen

Laundry

pH TDS (mg/L) COD (mg/L) BOD (mg/L) TSS (mg/L) Ammonia–nitrogen (mg/L) Nitrate–nitrogen (mg/L) Orthophosphorus (mg/L) Fecal coliforms (MPN/100 mL)

7.5 277 461 81 148 2.1 2.6 0.0 930

7.5 237 225 43 48 1.6 2.5 0.0 39

6.2 245 602 293 308 4.7 11.4 5.3 230

9.4 1060 824 269 1852 10.7 79 18.0 430

Fig. 3. Degree of turbidity removal over time during the electrocoagulation of greywater at different voltages.

kaolinite suspensions, the particle size increases with increase of voltage as well as time. Increase in the voltage results in more corrosion of aluminum from the anode. Hence, it may be inferred that increasing voltage and time allows for generation of more destabilized suspension which coagulates and flocculates over time resulting in an increase in the effective particle size in the greywater. Larger particles settle more easily and effectively. Therefore, there is an enhanced removal of turbidity. Fig. 4 shows the variation of COD removal efficiency over time at different voltages carried out on greywater collected on different dates. The trend for COD removal over time was similar as the particulates removal; however, it may be observed that the COD removal efficiency reaches a plateau after about 40 min of operation of the electrochemical reactor. Fig. 5 shows the percentage removal of fecal coliform from the greywater over the period of time at different voltages provided in the electrochemical cell. It may be observed that the increase in the voltage improves the removal of the pathogen indicators. The figure shows that after about 20 min, the rate of removal of the pathogens declines. However, in about an hour almost 97–99.9% of the fecal coliforms are removed from the greywater.

Fig. 4. Dissolved organics removal measured as percentage COD removal at different time intervals during electrocoagulation of greywater at different voltages in an electrochemical reactor.

The electrocoagulation method involves at least three fundamental steps. In the first step, precursor metal ions for the production of coagulants are liberated into the aqueous phase through oxidation of the sacrificial electrode, more specifically, the anode. In the second step, the metal ions generated from the first step hydrolyze to form insoluble metal hydroxides and polyhydroxides. In the third step, destabilization of contaminants, particulate suspension, breaking of emulsion and aggregation of destabilized phases take place to form flocs along with the metal hydroxides and polyhydroxides [19]. In the present study, aluminum plates were used as the anode. The aluminum ions (Al3+) are generated by the dissolution of sacrificial anode upon the application of direct current as per reaction (1). 3þ

Al ! AlðaqÞ þ 3e

ð1Þ

At high anode potential, another secondary reaction may also take place where water is hydrolyzed to produce oxygen at the anode, along with generation of H+ ions.

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In order to have more insight into the electrocoagulation for removal of COD, the data from COD experiment as indicated in Fig. 4 was analyzed further to find out the effect of aluminum release dosage on COD removal efficiency. Eq. (7) expresses the theoretical concentration of aluminum released from the anode at the outlet of the electrochemical cell [13].

C Al ¼

MAl I  t  MAl ¼ V zF V

ð7Þ

where CAl is the theoretical concentration of aluminum released into the electrochemical reactor (mg/L), V the volume of the reactor

Fig. 5. Percentage removal of fecal coliform at different time intervals during electrocoagulation of greywater at different voltages.

2H2 O ! O2ðgÞ þ 4HþðaqÞ þ 4e

ð2Þ

Aluminum and hydrogen ions, under the influence of electrical field tend to migrate toward the cathode. At the cathode, the hydrogen gas is generated. Hydrogen gas may get generated through two different reactions occurring at the cathode. The hydrogen ions in the solution, forms hydrogen gas following reaction (3).

2HþðaqÞ þ 2e ! H2ðgÞ

ð3Þ

Water also may get hydrolyzed at the cathode to produce hydrogen gas as well as hydroxyl ions following reaction (4).

2H2 O þ 2e ! H2ðgÞ þ 2OH

Fig. 6. Effect of released aluminum dosage on residual COD in the greywater solution at different voltages.

ð4Þ

The hydroxyl ions generated at the cathode tend to migrate to the anode under the action of the electric field. In the zone between the cathode and anode, the aluminum ions react to produce insoluble aluminum hydroxide flocs. 3þ

ð5Þ

3þ

ð6Þ

AlðaqÞ þ 3H2 O ! AlðOHÞ3 þ 3Hþðaq:Þ AlðaqÞ þ 3OH ! AlðOHÞ3

Insoluble Al(OH)3 particles agglomerates and form flocs which enmesh the colloidal particles as well as macromolecules present in the greywater. Depending on the pH of the aqueous medium,  other ionic species such as AlOH+2, Al2(OH)4+ 2 , Al(OH)4 may also be present on the surface of the solid aluminum hydroxide particles as well as in the solution [20,21]. Colloids which are usually negatively charged are swept by the electric field near the anode where high concentration of aluminum hydroxide destabilizes the colloids in the greywater suspension. There was no significant change in the pH of the greywater after the treatment, suggesting that all the H+ ions generated in the solution gets converted to hydrogen gas at the cathode. Also, it suggests that the reactions (1), (3), and (5) are predominant. COD is an aggregate measure of chemically oxidizable material present in wastewater. Compounds that contribute to COD are biodegradable organic compounds, non-biodegradable organic compounds and inorganic oxidizable components. Total COD shall include both soluble and suspended and colloidal species in wastewater. The greywater is expected to contain colloids such as microorganisms, emulsions, fat, oil and grease, organic acids, salts, sugars, miscible and immiscible liquids such as alcohols, glycerin, oils, and suspensions such as milk. The large macromolecules and colloids including the microorganism get destabilized and produce flocs which finally get separated from the solution either by floatation or sedimentation.

Fig. 7. Effect of COD removal on current density in the electrochemical reactor.

Fig. 8. Effect of COD removal on specific energy consumption in an electrochemical reactor.

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K.A. Vakil et al. / Separation and Purification Technology 130 (2014) 160–166 Table 3 Comparative performance of various chemical treatment technologies used for greywater treatment. Treatment technologies

COD (mg/L)

BOD (mg/L)

TSS (mg/L)

Turbidity (NTU)

Total Coliform (CFU/100 mL)

In

Out

In

Out

In

Out

In

Out

In

Out

26–139 20 22 272 287 51 160

– 195 23 205 205 – –

– 10 9 33 23 – –

– – 29 – – 78 –

– – 9 – – ND* –

– 35 29 46.6 46.6 133 104

– <5 9 8.1 4.3 4.1 15.6

106 – 5100 – – 43  104 9100

0 – ND* <59 <1 49 <10

[12] [14] [13] [7] [7] [15] Present study

Photo-catalytic oxidation 139–660 Coagulation + sand filter + GAC 280 Electrocoagulation + disinfection 55 Magnetic ion exchange resin 791 Coagulation with aluminium salt 791 Electrocoagulation + submerged membrane bioreactor 463 Electrocoagulation 380 *

References

ND: not detected.

Table 4 Comparative costs of treatment of various types of wastewater using electrocoagulation. Treatment technologies

Operating cost (US$/m3)

Type of wastewater

Ref.

Electrocoagulation Electrocoagulation Electrocoagulation Electrocoagulation Electrocoagulation

0.27 0.25 0.23 0.71 0.18

Greywater Sewage after secondary treatment process Sewage after secondary treatment process Slaughterhouse wastewater Greywater

[13] [25] [25] [26] Present study

and disinfection with Fe (anode)–Al (cathode) electrode with Al (anode)–Fe (cathode) electrode with Al electrode

(m3), I the current (A), MAl is the molecular mass of aluminum (26.98 g/mol), z is the valence of aluminum ion corroded out from the anode (z = 3), F is Faraday’s constant (96487 C/mol) and t is time. The current measured for electric fields 6 V, 8.5 V and 10 V were 0.1 A, 0.2 A and 0.3 A, respectively. Fig. 6 shows the residual COD in the greywater at different voltages with respect to the dosage of aluminum released declines as the dosage of released aluminum was increased to 15 mg/L where an average COD removal of about 70% is achieve. Raising the dosage of released aluminum beyond 15 mg/L did not decrease the residual COD. The above observations lead to two important conclusions. First, the removal of COD in the electrochemical reactor is only through destabilization followed by coagulation with flocs generated by the released aluminum ions. There is no direct electrolysis and mineralization of the organics. Second, not all the component organic molecules contributing the total COD are removed through coagulation with electrically generated aluminum hydroxide species. In an investigation made by Moreno-Casillas et al. [22] it was found out that electrocoagulation was not suitable in removing soluble and miscible species contributing to COD, such as glucose, lactose, isopropyl alcohol, phenol, sucrose and similar molecules that do not react with aluminum. The greywater in our case originated from mostly kitchen sinks, water closets and other washing areas. Hence, it consisted of a range of different organic compounds of which a part was soluble and miscible molecules which did not get removed with the aluminum hydroxide flocs generated in the EC reactor. 3.3.2. COD removal, current density and energy consumption EC process is known to be influenced by the density of the applied current. Fig. 7 shows the removal of COD after 60 min of electrochemical treatment at different current densities. It may be observed that the COD removal efficiency increased as the current density is increased [23]. Higher current density results in higher concentration of aluminum ion in the solution evolving from the decomposition of electrode material of the reactor. It has been reported that current density directly influences the treatment efficiency of electrochemical process [22,24]. The results obtained here also support the previous finding. Specific energy consumption or Energy consumption per unit volume of greywater treated by the process is calculated from Eq. (8).

Specific Energy ðkW h=m3 Þ ¼

V It 1000  v

ð8Þ

where I is the current registered (A), V is the potential difference (V), t is the time for electrochemical reaction in h and v is the reactor volume (m3). Fig. 8 shows the COD removal efficiency against the specific energy consumption for the electro-coagulation process run for 1 h time period under different operating conditions of voltage and current through the electrochemical cell. It is observed that the treatment of greywater in the electrochemical route shall have significantly low energy consumption for modest (70%) degree of COD removal. As it was revealed from aforementioned observations that there is an upper limit of removal of COD in the greywater as the residual COD could not be lowered beyond a certain value by further addition of aluminum because the residual COD is comprised of soluble organics which are not removed by aluminum hydroxide flocs. Raising the voltage and current through the electrochemical cell shall definitely increase the dosage of aluminum released in the solution. This will also increase the energy consumption. However, due to the reason cited above, such increase in the energy consumption may not help in achieving higher COD removal. 3.3.3. Performance and cost: Comparison with other studies Performances of the electrocoagulation system in the present study compared to other chemical treatment and recycling options of greywater are reported in Table 3. An analysis of cost of the present treatment calculated based on the consumption of electrical energy as well as aluminum electrode at optimum operating condition indicated an operating cost requirement of US$/m3. Table 4 shows the present cost of treatment in comparison to that reported by other studies for the treatment of greywater as well as domestic and slaughter-house wastewater. 4. Conclusions There is a need to have a fresh look at the aggregate wastewater produced from domestic sources. In terms of quality, wastewater can be clearly segregated into two different units, grey and blackwater. Greywater constitutes larger part of the total domestic wastewater produced and yet, contributes smaller amount

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towards the total contaminant load of domestic wastewater. In this study, survey of a typical Indian middle-class household has shown that around 75% of total water supplied appears as greywater which contains much less contaminants compared to the blackwater which is the wastewater generated from toilet flushes only. Greywater, if recycled and reused at the generation site for toilet flushing operations, gardening purposes, etc. would save a significant quantity of freshwater, thereby saving significant amount of money as well as energy. Electrocoagulation is one promising technology for treatment of greywater ease of onsite deployment in large buildings. In this study, the bench scale electrochemical process using aluminum electrodes proved successful in significant removal of pathogens, turbidity and COD content. COD removal efficiency was almost 70% and was found to be limited by the dosage of aluminum released from the corrosion of anode and is generally restricted by the presence of soluble organic contaminants in the influent. The pathogen removal efficiency achieved in the laboratory study was as high as 99.9%. All experiments were run at a system voltage less than 12 V which typically can be generated by an inexpensive battery charged by solar photovoltaic cells. References [1] E. Nolde, Grey water recycling systems in Germany-results, experiences and guidelines, Water Sci. Technol. 51 (2005) 203–210. [2] WHO, Guidelines for the safe use of wastewater, excreta and greywater use in agriculture, World Health Organization, vol. 4, 2006. [3] E. Friedler, M. Hadari, Economic feasibility of on-site greywater reuse in muftistorey buildings, Desalination 190 (2006) 221–234. [4] A. Gross, A. Wiel-Shafran, N. Bondarenko, Z. Ronen, Reliability of small scale greywater treatment systems and the impact of its effluent on soil properties, Inter. J. Environ. Stud. 65 (2008) 41–50. [5] E. Donner, E. Eriksson, D.M. Revitt, L. Scholes, H.-C. Holten Lützhøf, A. Ledin, Presence and fate of priority substances in domestic greywater treatment and reuse systems, Sci. Total Environ. 408 (2010) 2444–2451. [6] M. Ahmed, S.A. Prathapar, A. Al-Belushi, A. Al-Busaidi, M. Al-Haddabi, Greywater reuse potential in Oman, in: 28th International Hydrology and Water resources Symposium: About Water: Symposium Proceedings, Australia, 2003, 2.155–2.160. [7] M. Pidou, F.A. Memon, T. Stephenson, B. Jefferson, P. Jeffrey, Greywater recycling: treatment options and applications, Proc. Inst. Civil Eng. 160 (2007) 119–131. [8] K. Kujawa-Roeleveld, G. Zeeman, Anaerobic treatment in decentralised and source-separation-based sanitation concepts, Rev. Environ. Sci. Bio-Technol. 5 (2006) 115–139.

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