Technologies for domestic wastewater recycling

Urban Water 1 (1999) 285±292

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Technologies for domestic wastewater recycling B. Je€erson *, A. Laine, S. Parsons, T. Stephenson, S. Judd School of Water Sciences, Cran®eld University, Cran®eld, Bedford MK43 0AL, UK Received 9 December 1999; received in revised form 5 April 2000; accepted 13 April 2000

Abstract Domestic wastewater recycling is still in its infancy and as such, there is a paucity of reliable information relating to both the nature of grey water and the range of recycling technologies available. The lack of water quality standards and the poor understanding of the nature of grey water have led to the development of a plethora of technologies for recycling application. The paper discusses the relative merits of the di€erent options and describes the current situation within the UK. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Grey water; Technology; Performance

1. Introduction Wastewater recycling is emerging as an integral part of water demand management, promoting as it does the preservation of high-quality fresh water supplies as well as potentially reducing the pollutant in the environment and reducing overall costs. Water recycling can be conducted both internally, where water is retained within a local process loop, or externally where water is sourced directly from a sewage treatment works. The preference in the UK currently is for internal systems such as industrial or domestic reuse, although some water utility companies are proactive in pursuing market opportunities in bulk reuse of treated municipal wastewater. The viability of internal recycling is to some extent contingent upon there being substantial di€erences in water quality and quantities demanded for di€erent operations. The proportion of water used required to be of the highest quality is small. This then implies that most of the demands within the process scheme is for lower grade water, permitting reuse of water from one application to another. An example of this is in the domestic environment where the reuse of grey waters such as baths and showers for toilet ¯ushing can be achieved with little or no treatment (Sayers, 1998).

*

Corresponding author. Tel.: +44-1234-750111 ext. 2902; fax: +441234-751671. E-mail address: b.je€erson@cran®eld.ac.uk (B. Je€erson).

Domestic water recycling is an attractive option in the UK due to a relatively high domestic water consumption coupled with an intensive population. Water use for domestic purposes constitutes 40% of the total demand placed on the water companies. Given that over half the population lives in towns of 100,000 population or above (Boschet, Wahliss, & Lack, 1998), water recycling in urban regions may be viable especially through the use of centralised storage and treatment, thereby o€ering economy of scale. Although it has gained higher pro®le only within the last 20 years or so, communal water recycling has been an integral part of society for centuries. One of the earliest reported engineering systems was in the Palace of Knossos, built by King Minos of Crete 5000 years ago (Horan, 1998), where rainwater from a dedicated collection system was used, amongst other things, to ¯ush toilets. Bath water reuse for personal garden irrigation has also been practised for centuries. Engineering systems for internal grey water reuse, on the other hand, have been developed only relatively recently. An early example arises in the mid-1970s in the ®eld of space exploration. Work conducted by the National Aeronautics and Space Administration (NASA) demonstrated how bath and laundry wastewater could be reused for toilet ¯ushing by passing the water through diatomaceous earth ®lter followed by activated carbon (Hypes, Batten, & Wilkins, 1975). Grey water treatment and reuse are, in fact, complicated by issues based not only on technical feasibility but also on human issues. It is arguably the public

1462-0758/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 4 6 2 - 0 7 5 8 ( 0 0 ) 0 0 0 3 0 - 3

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perception of water reuse and the risk associated with it which ultimately dictate the standards of product water quality required and so the appropriateness of candidate treatment technologies. A holistic approach to water recycling is thus crucial, and the ecacy of individual treatment technologies or overall treatment schemes must be precisely evaluated. This paper describes the current understanding of problems associated with the technology of water recycling with speci®c reference to the technologies either currently in place of existing recycling schemes or of development of practical recycling operations. Technical data presented include those arising from a 3-year study at Cran®eld University, along with those from other investigations found in the literature. 2. Grey water characterisation Grey water arises from domestic washing operations. As such sources include waste from hand basins, kitchen sinks and washing machines, but speci®cally exclude foul or black water sources (toilet, bidets and urinals). The cumulative ¯ow balance between the grey water generated and toilet ¯ushing requirements shows a natural anity at about 30% of the total water use. However, the dynamics of the situation are not so ideal. Grey water is produced at a time slightly o€set from toilet ¯ushing and generated over short time periods, whereas toilet ¯ushing takes place more consistently through the day. This generally results in a de®cit in water during the afternoon and late evening as depicted in Fig. 1 (Surendran & Wheatley, 1998). This may be recti®ed by bu€ering using appropriately sized storage tanks, but this substantially increases the overall system size. Detailed examination of the bene®ts of storage

reveal that 1 m3 tank is suitable for a wide range of occupancy scales (Dixon, Butler, & Fewkes, 2000). Increasing storage capacity above this provides only marginal increases in water savings and increases problems associated with grey water degradation and disinfection reliability. Grey water is usually generated by the use of soap or soap products for body washing and as such, varies in quality according to, amongst other things, geographical location, demographics and level of occupancy (Table 1). Grey water has a similar organic strength to domestic wastewater but is relatively low in suspended solids and turbidity, indicating that a greater proportion of the contaminants are dissolved. Moreover, although the concentration of organics is similar to domestic wastewater their chemical nature is quite di€erent. The COD:BOD ratio may be as high as 4:1 ± very much greater than values reported for sewage. This is coupled with a de®ciency of macro-nutrients such as nitrogen and phosphorus: the COD:NH3 :P of grey water has been measured at 1030:2.7:1, and this compares with 100:5:1 for domestic wastewater. Both the relatively low values of biodegradable organic matter and the nutrient imbalance limit the e€ectiveness of biological treatment. The major diculty presented for treatment of grey water is the large variation in its composition. Reported mean COD values vary from 40 to 371 mg/l between sites, with similar variations arising at an individual site. This variation has been attributed to changes arising in the quantity and type of detergent products employed during washing. Grey water quality is also subjected to dynamic variation: signi®cant chemical changes may take place over time periods of only a few hours, though little appears to have been reported on this phenomenon. One example (Fig. 2), which refers to a 20 l grey water sample kept either completely mixed or under

Fig. 1. Typical grey water production and toilet ¯ushing requirements in a college (Surendran & Wheatley, 1998).

B. Je€erson et al. / Urban Water 1 (1999) 285±292

287

Table 1 Typical composition of grey water from various sources

Hand basin Combined Synthetic grey water Single personb Single familyc Block of ¯atsb Collegeb Large colleged

BOD5 (mg/l)

COD (mg/l)

Turb (NTU)

109 121 181

263 371 ±

± 69 25

110 ± 33 80 96

256 ± 40 146 168

14 76.5 20 59 57

NH3 (mg/l)

P (mg/l)

Total coliforms

9.6a 1 0.9

2.58 0.36 ±

± ± 1.5 ´ 106

± 0.74 10 10 0.8

± 9.3 0.4 ± 2.4

± ± 1 ´ 106 ± 5.2 ´ 106

a

Total nitrogen. Holden & Ward (1999). c Sayers (1998). d Surendran & Wheatley (1998). b

Fig. 2. E€ect of storage on the characteristics of grey water.

quiescent conditions, reveals ®rst-order kinetics for decay in BOD5 with a rate constant of around 5 hÿ1 ± corresponding to a 50% reduction in BOD5 over a 4-h period. Similar decay rates arise for both quiescent and mixed systems and for both mechanical mixing and aeration, the exact rate of change being related more to the original grey water composition and system temperature than the hydrodynamic conditions of storage. 3. Water quality standards No standards exist in the UK at present and, with the exception of proposed guidelines suggested by the Building Services Information and Research Association (BSRIA), no standards are under development. On the other hand, standards are either in place

or under development in other parts of the world (Table 2). The water quality determinands used de®ne bacteriological quality, biodegradability, clarity and acidity, though the actual permitted levels vary considerably. Two groups of standards can be identi®ed generated from di€erent ideologies. The ®rst of these is based on the quality of the grey water being commensurate with its application, and in such cases the standards are similar to those for bathing water since the level of risk to the user is about the same. The alternative approach is more conservative and considers grey water treatment in a similar manner to that of municipal or industrial e‚uent. The di€erence in ideologies is manifested in the standard for coliform levels, which for the more pragmatic approach is in the range of a few thousand colony forming units per 100 ml as compared to a non-detectable level for the more conservative approach.

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Table 2 Summary of water quality standards and criteria suitable for domestic water recycling (adapted from Surendran & Wheatley, 1998)

Bathing water standardsa USA, NSF USA, EPA Australia UK (BSIRA) Japan WHOa Germany (g) a

Total coliforms count/100 ml

Faecal coliforms

BOD5 (mg/l)

Turbidity (NTU)

Cl2 Residual (mg/l)

pH

10,000 (m) 500 (g) ± Non-detectable <1 Non-detectable <10 1000 (m) 200 (g) 100

2000 (m) 100 (g) <240 ± <4 ± <10 ± 500

±

±

±

6±9

45 10 20 ± 10 ± 20

90 2 2 ± 5 ± 1±2

± 1 ± ± ± ± ±

± 6±9 ± ± 6±9 ± 6±9

Bathing water standards suggested as appropriate for domestic water recycling; (g) ˆ guideline, (m) ˆ mandatory.

The focus on bacteriological quality re¯ects the potential for human exposure to recovered grey water, such that the major criterion is public health protection. Enteric viruses are known to be the most critical group of pathogens as they can cause illness even at low doses and cannot be detected by routine microbial analysis. They also represent the component that is most dicult to process: it can be assumed that a process e€ective in removing enteric viruses will be similarly e€ective for all other pathogens (Asano, 1998). It is normal, however, to base standards on the more readily quanti®able indicator organisms of faecal or total coliforms. These species demonstrate a potential for disease transmission, rather than an actual risk of illness, but are more familiar bacteriological quality determinands than viruses and are more easily measured. On the other hand, no proven correlation exists between concentrations of indicator species and actual pathogen levels, and some pathogens are known to be more resistant to treatment than the indicator species (Asano, 1998). This has resulted in the more conservative approach being adopted in America, Japan and Australia where grey water recycling is an established operation. In the USA speci®cally the US EPA guideline for water recycling (EPA, 1992) promotes non-detectable concentrations of faecal coliforms for urban reuse combined with a speci®cation for minimum level of treatment required. The guideline has improved the applicability of surrogate measures such as indicator organisms and is currently being adopted in California and Florida. 4. Available technologies The paucity of water recycling standards has contributed to the proposal and development of a plethora of technologies which vary greatly both in complexity and performance. Processes developed range from simple systems in single houses to very advanced treatment trains for large-scale reuse.

4.1. Basic two-stage systems Coarse ®ltration plus disinfection represents the most common technology used for domestic reuse in the UK to date with a number of companies supplying products based on this two-stage process. The generic process employs a short residence time so that the chemical nature of the grey water remains unaltered and only minimal treatment is required. The coarse ®lter usually comprises a metal strainer and disinfection is achieved using either chlorine or bromine, dispensed in slow release blocks or by dosing a liquid solution. Systems recently tested (Sayers, 1998) have shown a variety of water saving levels ranging from 3.4% to 33.4% with mains top up acting as a standby supply during periods of low ¯ow or process failure. Such systems are commercially available at moderate cost (between £500± £1000 installed), o€ering minimum payback periods of around 8 years for a four-person household. Although little or no removal of the chemical and biological pollution was reported for such a system (Table 3), the treated water was free of all indicator organisms making it potentially safe for reuse. However, the system also su€ered periodic failure of the disinfection process, such that coliform levels occasionally exceeded all the currently proposed water quality standards. The basic two-stage system is designed to meet the less stringent reuse standards akin to those for bathing water, rather than the more conservative standards. The water thus remains high in organic load and turbidity, thereby limiting the e€ectiveness of the chemical disinfection process for two reasons. Firstly, grey water contains ¯occulent particles above 40 lm in diameter, and these are known to reduce chemical disinfection capability because the disinfectant cannot di€use far enough into the ¯ocs to provide complete killing of pathogens. Secondly, organic matter in the water imparts a disinfectant demand. In the case of chlorine, disinfectant byproducts such as chloramines and trihalomethanes are generated which have lower disinfectant

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289

Table 3 Average performance of a simple ®ltration grey water recycling system (Sayers, 1998)

E‚uent Standards WHO guideline standard US Epa standards (Table 2)

BOD5

NH3

P

Anionic detergents

Total coliforms (count/100 ml)

>50 mg/l

<3 mg/l

<1 mg/l

10 mg/l

0

±

±

±

±

200

10

±

±

±

ND

capability and adversely a€ect human health. Additional problems occur due to the presence of detergents that are known to produce an odour in water at concentrations above 3 mg/l, well below the mean concentration reported (Table 3). 4.2. Physical and physicochemical systems Physical and physicochemical systems are not subjected to problems of chemical shock loads from bactericidal reagents (i.e., bleach) that may adversely a€ect a biological process whilst still producing water of higher quality than that from the simplest systems. Such physical processes developed for grey water treatment comprise mainly depth ®ltration, invariably based on sand, and/or membranes ± the latter usually being combined with appropriate pre-treatment. More recent physicochemical processes include coagulation and advanced oxidation (Table 4). Physical processes achieve substantial clari®cation of water. As such, they are reasonably e€ective in decreasing the organic pollutant load of grey water prior to reuse, since this is to some extent associated with

turbidity. The aesthetic quality of the product water is thus increased and problems associated with downstream disinfection encountered in the coarse ®ltration systems substantially ameliorated. On the other hand, simple ®ltration based on ®brous (cloth) or granular depth ®lters presents no absolute barrier to suspended matter, resulting in coliform breakthrough and a propensity for solids' unloading whenever hydraulic shocks occur. Membrane systems o€er a permanent barrier to suspended particles greater than the size of membrane material, which can range from 0.5 lm for micro®ltration membranes down to molecular dimensions for reverse osmosis. The treated water is thus generally extremely low in turbidity below the limit of detection for coliforms (Table 4). On the other hand, the energy demand for membrane systems is substantially higher than that for depth ®lters: the data shown in Table 4 refer to tubular MF/UF systems, the most commonly used, which are operated at pressures up to 2.0 bar. The key factor constraining the economic viability of membrane systems is the fouling of the membrane surface by pollutant species. This increases the hydraulic resistance of the membrane, thereby commensurately

Table 4 Performance of sand ®lter and membrane ®ltration processes for grey water treatment

a

BOD5 (mg/l)

COD (mg/l)

Turbidity (NTU)

TC (cfu/100 ml)

In¯uenta Post-sand ®lter Post-membrane

33.3 12.3 4.7

143 35.7 22.2

44.5 32.3 0.34

± ± 0 E. Coli

In¯uent Post-membrane

25±185 1±19

86±410 21±112

12±100 <1

2±310 ´ 103 ND-2419

Pre-advanced oxidationb Post-advanced oxidationb

41 (TOC) 25 (TOC)

± ±

± ±

9 ´ 105 NDc

Pre-coagulationd Post-coagulationd

± ±

100 30

29.4 2.41

± ±

Holden & Ward (1999). Based on a 2g/l concentration of TiO2 activated by an UV irradiation source. c Non-detectable. d Based on a ferric chloride dose of 30 mg/l. b

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increasing the energy demanded for membrane permeation and/or decreasing the permeate ¯ux. Fouling can be suppressed by operation at a lower membrane ¯ux, in which case the membrane area requirement is increased, and substantially removed by cleaning. Both increase the overall process cost and cleaning also imparts an undesirable chemical load on the waste stream. Fouling can be such that the ¯ux declines by up to 90% after just 1 h of operation. Problems of poor-treated water quality have been reported with membrane systems treating grey water, as well as diculties in e€ectively cleaning the membrane. The residence time of the system has been identi®ed as being a major cause for concern. Over extended time periods, the grey water can become anaerobic resulting in the generation of organic components which are less readily rejected by the membrane (Holden & Ward, 1999). Membrane trials have shown that purely physical systems fail to reject all coliforms from the waste stream. This has been explained in terms of protein migration through the membrane pores which aids the transport of coliform species (Judd & Till, 2000). 4.3. Biological Filtration in itself is not considered sucient to guarantee an adequate reduction in organic contamination to prevent biological regrowth in distribution systems. Biological treatment is required to remove biodegradable material especially for systems that include large distribution networks such as hotels or community based recycling schemes. The bene®ts of biological and physical treatments are combined in processes such as membrane bioreactors (MBR) and biologically aerated ®lters (BAF), which are small footprint processes capable of producing high-quality e‚uents. BAFs combine depth ®ltration with a ®xed ®lm biological reactor. As such they present no absolute barrier to suspended material and so do not substantially disinfect the water. MBRs combine an activated sludge reactor with a micro®ltration membrane, and have been successfully employed in Japan for grey water recycling in oce blocks and residential buildings (Kishino, Ishida, Iwabu, & Nakano, 1996). The MBR process is shown in Fig. 3, which demonstrates that the system can be con®gured with the

Fig. 3. Schematic of a membrane bioreactor (submerged and sidestream).

membrane placed within the reactor (submerged MBR, Fig. 3(a)) or by externally to it (sidestream MBR Fig. 3(b)). The two systems show similar biological performance but membrane permeation di€ers between them. The external systems are run at higher trans-membrane pressures, producing higher ¯ow rates exacerbating fouling problems and so necessitating regular cleaning. The internal systems are operated hydraulically, generating much lower ¯ow rates which are however, stable, thereby reducing membrane cleaning requirements once in every six months as opposed to monthly, weekly or even daily for the side stream con®guration operating in cross-¯ow mode. Both MBR and BAFs e€ectively remove organics, meeting even the most stringent water recycling standards (Table 5). The loading rates of the two systems di€er considerably: 0:14  0:07 kgCOD mÿ3 dÿ1 and 1:09  0:73 kgCOD mÿ3 dÿ1 for the MBR and BAF, respectively. This arises from the corresponding di€erences in the hydraulic loading rates at which the two systems can be operated. Turbidity and coliform removal from the two processes, on the other hand, di€er substantially. The submerged MBR meets the more stringent water recycling standards 100% of the time, whereas the BAF failed to meet the water quality standards 40% of the time in terms of both water quality determinands. Coliform rejection is 100% and much more consistent than in the case of physical membrane systems. This is probably because the bioreactor digests protein which would otherwise facilitate transport of microorganisms through the membrane, and also because ¯occulation is promoted whereby microorganisms are retained in the

Table 5 Performance of advanced biological systems for grey water recycling

In¯uent MBR e‚uent BAF e‚uent

BOD5 (mg/l)

COD (mg/l)

Turbidity (NTU)

Total coliforms (cfu/100 ml)

41.2 ‹ 30 1.1 ‹ 1.6 4.3 ‹ 4.1

120 ‹ 74.4 9.6 ‹ 7.4 15.1 ‹ 13.1

± 0.32 ‹ 0.28 3.2 ‹ 8.9

1.5 ´ 106 ‹ 4.3 ´ 106 Non-detectable 2 ´ 104 ‹ 5.5 ´ 104

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291

Table 6 Summary of currently operational grey water recycling schemes in England and Walesa No.

Location

Comments

Technology SF

a

1

London

2

Machylleth

3 4 5 6

Oxford Liverpool Loughborough Nottingham

7 8

Leeds Aylesbury

Millennium Dome, Thames water Centre for alternative technology Linacre college Single-house systems Halls of residence The Hockerton housing project Allerton park Single-house systems

N

F

Bio

M

UV

Br2 /Cl2

X

X

X

X

X X

X X

X X X

X

X

X

X X

X

SF: simple ®ltration, N: natural systems, F: ®ltration, Bio: biological, M: membrane, UV: ultraviolet radiation, Cl2 : chlorine disinfection.

¯occulent material. Submerged MBRs incur a substantial capital cost and, like other biological processes, exhibit a limited tolerance to shock by bactericidal agents such as bleach. Indications are that in medium-scale systems dilution of chemical shock is sucient to avoid system failure.

5. Current recyling schemes Table 6 shows grey water schemes currently operational in England and Wales. A number of di€erent levels of technology and scale are currently being tested, ranging from simple ®ltration to complex process trains for large-scale reuse. The complexity of the technology applied is generally commensurate with the scale of the recycling scheme. Cost implications have meant that single-house systems are largely restricted to coarse ®ltration devices with downstream disinfection. The current emphasis is on simple, ostensibly reliable ``®t and forget''. The exact number of installations fully operational is unknown but more than 20 units have been tested around the UK. Installations in colleges and small oces have focused on physical or simple biological systems, and these have been shown to be far more economically viable than single-house systems. The scheme in Loughborough, for example, has reported a payback time between 5 and 10 years (Surendran & Wheatley, 1999). More complex schemes arise in high pro®le large-scale systems such as in the Millennium Dome, where a grey water recycling scheme is to be operated as part of the toilet ¯ushing supply. When operational this scheme will be the largest of its kind worldwide, involving several discrete stages of treatment and encompassing both biological and membrane technologies. The multi-treatment barriers provide a high level of safety and a high-

quality product water, e€ectively demonstrating the ability to process grey water to any level of purity. The economics are unarguably unfavourable: the grey water supply costs around six times that of the mains water. However, the bene®t in terms of public acceptance of the principle of grey water reuse is considered to be of greater importance than the overall system economics, and this is likely to be contingent upon reliability as well as performance is critical. Success of this very high pro®le scheme together with the others shown is crucial for the development of water recycling in the UK.

6. Conclusions Domestic wastewater recycling is still in its infancy and as such, there is a paucity of reliable information on the subject. Based on the information cited in this paper, a number of conclusions can be drawn: · grey water quality varies considerably but can be considered to be of a similar strength to domestic sewage; · storage is required to balance the imbalance in the dynamics of water demanded by toilet ¯ushing and grey water production from washing; · residence time in systems dramatically a€ects the characteristics of grey water; · although standards have been promulgated elsewhere, no water quality standards for recycled grey water currently exist in the UK; · simple two-stage ®ltration + chemical disinfection systems remove coliforms but remain high in turbidity and organic pollution. · advanced ®ltration systems reduce all components of grey water but do not reliably meet all the water recycling standards; · membrane bioreactors meet all the applicable water quality standards but are currently costly.

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Hypes, W. D., Batten, C. E., & Wilkins, J. R. (1975). Processing of combined domestic bath and laundry wastewater for reuse as comode ¯ushing water. NASA technical note TN D-7937. National Aeronautics and Space Administration. Washington, DC. Judd, S. J., & Till, S. W. (2000). Bacteria breakthrough in cross-¯ow micro®ltration of sewage. Desalination (in press). Kishino, H., Ishida, H., Iwabu, H., & Nakano, I. (1996). Domestic wastewater reuse using a submerged membrane bioreactor. Desalination, 160(1±3), 115±119. Sayers, D. (1998). A study of domestic greywater recycling. Interim report, National Water Demand Management Centre, Environment agency, Worthing, UK. Surendran, S., & Wheatley, A. D. (1998). Grey water reclamination for non-potable reuse. J. CIWEM, 12, 406±413. Surendran, S., & Wheatley, A. D. (1999). Grey and roof water reclamation at large institutions ± Loughborough experiences. Presented at the IQPC conference on water recycling and e‚uent reuse. 26±27 April 1999, Berkley Hotel, London, UK.