C. Diaper, BEng, MSc, PhD, AMIChemE, B. Jefferson, BEng, PhD, AMIChemE, S. A. Parsons, BSc, PhD, CChem, FRSC, and

S. J. Judd, BSc, MSc, PhD, CChem, FRSC*

Grey water Rainwater Combined greyhainwater Blackwater/sewage effluent

Abstract

Single house

16 75

2 1

Urban water recycling in the form of rain, grey or black water is becoming an important element of demand- management practice in the UK, and appropriate tech- nology allows the use of three different water sources for similar applications. The level of pollution in the water source and the application to which it is being recycled, dictate the level of technology which is used. This paper describes current water re-use practices in the UK (in terms of water source, technology scale, type and efficiency) and examines five case studies. Of more than 150 schemes identified, most are rainwater-collection systems and these are generally sited in rural areas. There are various single-house greywater recycling sites, many of which are part of water company or Environment Agency monitoring programmes. Larger-scale schemes employ both natural and hi-tech treatment options. Operationally successful schemes utilise appropriate technology for the end-use, although the economic benefits of many of the schemes are poor.

Key words: Black water; urey water; rain water; urban water recycling; water re-use.

'Research Officer, Research Officer, Lecturer, and Reader respectively, School of Water Sciences, Cranfield University, Cranfield, UK.

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I n t r o d u c t i o n

An important concept behind water recycling is the quality of water used, commensurate with its applicationll). Current UK practice endorses the use of the highest quality water for all applications. However, the proportion of water which needs to be of potable quality is small -typically below 5% for domestic consumption(*). This suggests that the use of non-potable grade water for lesser applications, such as toilet flushing and garden irrigation, is feasible.

Urban water recycling constitutes three specific types of water source, i.e. rain water, grey water and black water. These feed waters are usually treated as discrete waste streams, there- fore different technologies are applicable. Of more than 150 water re-use schemes which have been identified in the UK, over two-thirds use rainwater collection and treatment (Table 1); however, there are some schemes where water sources are combined for treatment. Performance characteristics of specific treatment processes are described el~ewhere(3~~~5).

Table 1. Water recycling schemes in UK

R a i n w a t e r

Groups of houses i occupancy

Rainwater collection is an established urban water-recycling option throughout the world. However, of more than 100 schemes which have been identified in England and Wales, most small-scale systems are found in rural areas (Fig. 1).

The microbial concentrations in rainwater systems have been shown not to decrease significantly over a period of time, and there is some evidence that storage might induce microbial growth@). Concentrations of total coliforms in large storage tanks have been shown to be high (>24 000 CFUV100 m V ) . However, poor design of the storage tank and collection facility for this system allowed easy ingress of pollutants and might explain the high levels and subsequent build-up of micro- organisms in the initial storage vessel. When there is a risk of direct human contact or accidental consumption, such as for toilet flushing, it is necessary to treat the water to a higher standard before use. Ultra-violet (UV) disinfection is often used as a final process for larger-scale rainwater-collection schemes.

A number of proprietary systems are available in the UK, although four systems seem to dominate. All the systems contain the following generic components:

(i) Storage tank; (ii) Pump; (iii) Treatment system; (iv) Mains water back-up; and (v) An unrestricted overflow to prevent mains' water

contamination.

tCFU: colony-forming units.

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Water-Recycl ing Technologies i n the UK

Key: W- WISY, E- Ecovat, R- Rainharvester, K- KSB HyaRain

0- other or DIY systems,

Underlined sites larger than single domestic systems ~

Fig. 1. Current rainwater recycling schemes

In terms of treatment, a coarse filter is usually included either as a suction filter to the pump or as a direct installation in the vertical downpipe. Downpipe filters are efficient, with reported values of 90% of the flow entering the rainwater storage tank for re-use.@).

In addition to the basic components listed above, some of the systems include either a diverter and/or a disinfection unit. Diverters are included to bypass the initial flow from the collection area because this has been shown to contain most of the pollution(g). The major concern with rainwater systems is the risk to public health through the transfer of pathogenic micro- organisms in the water. Larger schemes (such as the Eden project in Cornwall or the RSPB centre at Sandy) take this into account by including a disinfection stage. Traditionally, this is achieved using either chemical dosing or UV light. Both techniques are used to provide biological kill but require regular maintenance to guarantee their effectiveness.

Case Study 1: Office Building Rainwater Collection and Re-Use

The rainwater collection and re-use facility at the RSPB headquarters in Sandy, Bedfordshire, has undergone intensive study of both water quality(7) and operational performance(lO). Water quality at different process stages was monitored over an eighteen-month period. Despite detection of high E. colicounts in the main external storage tank, these concentrations were not detected in the header-tank located inside the building, suggesting that the UV disinfection system was functioning satisfactorily. However, a HAZOPt study identified a number of

tHAZOP = Hazard and Operability.

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design faults that could lead to either gross contamination of the collected rainwater or disinfection system failure(lO).

Case Study 2: Five Autonomous Homes The Hockerton Housing Project consists of five earth-

sheltered autonomous homes. All water-supply and treatment facilities are the responsibility of the home-owners, and the specification for the treatment facilities was agreed within the group. Rainwater is collected in copper guttering from the glazed roofs of the conservatories and then stored in a series of underground tanks until it is required for use. The treatment process comprises a string filter and carbon filter followed by UV treatment, and this water provides all potable supply (Fig. 2). Water for all other requirements is a collected from other hard surfaces and stored in an open reservoir. This water is treated

Fig. 2. Rainwater-treatment system at Hockerton

C . D iaper , 6. J e f f e r s o n , S . A . P a r s o n s a n d S. J . J u d d o n

through two sand filters, prior to use, for all washing, bathing and toilet-flushing requirements. Water consumption per person on the development was approximately 30% of the average 150 I/person/day, which can be attributed to the fact that the home-owners were more aware of the limits to their water supply and therefore they use water carefully. Home-owners were satisfied with the quality of the water, despite the detection of some bacterial activity in both potable and non-potable water supplies. Because the treatment system provides no residual disinfection, there is the potential for growth of bacteria in the distribution pipework.

Grey W a t e r

Grey water arises from all domestic washing operations. Sources include waste from hand basins, kitchen sinks and washing machines, but specifically exclude foul or black-water sources (toilets, bidets and urinals). A further sub-division is common, restricting sources to baths, showers and hand basins, excluding sources that normally have the highest level of pollution. The cumulative flow balance between the grey water generated and toilet-flushing requirements shows a natural affinity at about 30% of the total water use. However, storage is required to balance out the variations between generation and use.

A detailed examination of the benefits of storage shows that a 1 m3 tank is suitable for a wide range of occupancy scales(ll). Increasing storage capacity above this figure provides only marginal increases in water savings and increases problems associated with grey-water degradation, due to prolonged storage. At present, various technologies, which vary in com- plexity and performance, are being developed or installed at a number of sites around the UK (Fig. 3), ranging from simple systems in single houses to advanced treatment systems for large-scale re-use.

The level of technology approximately matches the scale of the recycling scheme. Cost implications have meant that single- house systems are mainly restricted to coarse filtration devices with downstream disinfection. Present emphasis is on simple, reliable 'fit and forget' technologies as described. However, although these systems have been shown to provide water

South TynesideEcocentre

AUertonPark, Leeds

Emhouse, Leeds

Hockerton Housing Pr,oject, Nottingftam

Blackheath

Millennium / Southern Water, V i r g k Water, \Abbotts MU, Dome, Greenwich West Sussex Surrey Ayleshury

Fig. 3. Grey and grey/rainwater recycling schemes

204

savings of up to 30-35% when operating correctly(l*), the reliability of the components has meant that actual savings are significantly reduced(lO). The exact number of installations which are fully operational is unknown, but more than twenty units have been tested around the UK.

Installations in colleges and small offices have focused on physical or simple biological systems, and these have been shown to be more economically viable than in the case of single- house systems. A scheme in Loughborough has reported a pay- back period of 5-1 0 years(13), compared with paybacks in excess of 18 years for single-house systems(l2). More complex schemes arise in large-scale systems such as in the Millennium Dome, where grey-water treatment is combined with treated rainwater and groundwater to supply water for toilet flushing.

Case Study 3: Millennium Dome The Millennium Dome water-recycling scheme is the

largest of its kind in the UK. Grey-water is collected from hand basins in all the public toilet facilities, rainwater is collected from the roof of the Dome, and groundwater is used to supplement these sources during peak flows(14). There are treatment systems for the three different source waters (Fig. 4). Because of the public prominence and high media attention which has been given to the scheme, the water is treated to potable-water standards and is returned to the public WCs and u

BAFs

Membranes

Reed-beds

L

nals.

Returned to Dome for toilet flushing

Fig. 4. Diagrammatic representation of Millennium Dome water recycling

Case Study 4: Linacre College The water-recycling system at Linacre College, Oxford, has

been developed over a number of years, and the current treat- ment process is the third technology to be utilised. Grey water is collected from baths and showers of a student accommo- dation block, and collected water is transferred to the treatment process which is housed in a separate building. This configur- ation resulted in a long retention period for transfer of grey water from the accommodation block to the treatment plant, and degradation of the grey water occurred prior to treatment. This caused odour problems within the accommodation block when the system was switched on(l5), and complaints from the occupants resulted in the system being switched off.

The second system, comprising a sand filter and micro- filtration membrane, was selected because membrane pro- cesses will generally provide a high level of solids removal,. However, excessive fouling of the membrane occurred, which required cleaning with aggressive chemicals. Degradation of the grey water within the pipework, prior to the treatment process, was also the cause of membrane fouling. The organic constitu- ents of the grey water degraded to lower molecular weight compounds, which were not rejected by the membrane but caused fouling. Due to (a) the intermittent operation of the membrane system, (b) increased costs of operation, and (c) quantities of cleaning chemicals required, this process was abandoned(15).

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Water-Recycli’ng Technologies in the UK

Blackburn

Cheshire

Essex

The design of the system which is in operation at Linacre was approached from a different perspective. A moving-bed bioreactor process provides the first stage of treatment. This process stage capitalises and improves on the natural biological activity of the grey water by providing conditions which are suitable for bacterial degradation of organic matter. The treated water is then passed through two polishing filters (a sand filter and granular activated-carbon filter) and dosed with disinfectant to ensure that there is no biological regrowth in the distribution network (Fig. 5). This treatment system ensures adequate treatment, reduced operating costs, no cleaning chemical requirement, and is viewed as a more environmentally friendly process. However, the cost-benefit of the process is negative, and operational costs outweigh the savings which are made on water and sewage-treatment chargeso.

Black-box technology treating entire sewage stream from housing development

Tertiary-treated sewage effluent for golf-course irrigation(’8)

Direct pumping of UV treated- tertiary sewage effluent to drinking-water r e ~ e r v o i r ( ~ ~ ~ ~ ~ )

Sand Moving Bed filter

bioreactor a

Grey water

I I 1

Carbon filter . Product

water

Fig. 5. Flow d iagra~ of grey water treatment at Linacre College

Air

B l a c k W a t e r

Black-water recycling is defined as recycling either all the wastewater which is produced from a house, or the re-use of domestic sewage. The operational schemes in England and Wales are listed in Table 2. Schemes in Cheshire and Essex are examples of the re-use of sewage ef f l~ent(~6,~~,~8) . In all cases, sewage is treated at an existing sewage-treatment works. Effluent from the works is then further processed in a disinfection stage, rendering the water suitable for re-use. UV light is used as disinfection for both schemes.

Table 2. Black-water recycling schemes in England and Wales

Scheme I Location I Treatment process

Waterwise

High Leigh Golf Club

Hanningfield reservoir

‘Living machine’ for sewage- effluent treatment(*’)

The Earth Centre

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Schemes at Blackburn and Doncaster represent the alternative type. In these cases, black water is processed locally to source in a similar manner to grey water, and the level of technology depends on the re-use application. Natural systems, such as reed-beds, are used in cases where the water is utilised for irrigation(l9). In situations where the water is used for toilet flushing, more sophisticated technologies are employed. In Japan, where black-water recycling is far more common, membrane bioreactors are used, and this technology is particu- larly appropriate because it provides a high level of treatment in a small footprint area(*O).

Case Study 5: Hanningfield Reservoir The Essex and Suffolk scheme at Hanningfield Reservoir in

south-east England is the only scheme identified where sewage effluent is used indirectly for potable-water supplies. The scheme comprises (i) the pumping of UV-treated sewage effluent to a drinking-water reservoir, and (ii) the indirect re-use of sewage effluent via a river(17$18,22). The possible health and environmental impacts of this scheme were immense, and extensive monitoring was carried out including microbiological, chemical and ecotoxicity tests. During both stages of the scheme, no significant changes in the quality of the reservoir were observed. However, during stage 1, local and national media coverage produced some negative feeling towards the scheme(21z22). Despite extensive consultation with local councils and environmental health officers, the Drinking Water Inspec- torate, RSPB and English Nature, and informing the public of the proposal prior to the project, the adverse media coverage gained most credence due to the ‘shock factor’ approach of ‘the general public drinking their own sewage’. The main reason forthis was that the sewage effluent was now being pumped from the sewage-treatment works, directly to a potable-water reservoir, rather than flowing through a natural watercourse and then being abstracted - a situation which occurs in a number of locations in the UK. This general feeling of ‘natural is good, unnatural is bad’was observed during the BSEt crisis and is also reported to apply to the public perception of biote~hnology(*~). Therefore, the requirement for a ‘natural break’ in the treatment cycle is perhaps vital to the public acceptance of large-scale water-recycling schemes in the UK.

C o n c l u s i o n s

1. Guidelines are required for the design and installation of water-reclamation systems and, more importantly, this information should be disseminated to the relevant con- struction industry and architectural organisations.

2. The importance of stakeholder involvement in the acceptance of water-recycling technologies and public knowledge and understanding of the technologies utilised, is a key issue.

3. A high level of technology and improved water quality does not necessarily provide complete acceptance of water- recycling schemes when the water is to be used for drinking.

4. The ‘natural’ process of abstracting water for drinking from a river appears to be more acceptable than ‘unnatural’ processes, i.e. pumping water from a sewage-treatment works to a drinking-water reservoir.

5. Improved reliability of small-scale systems and improved

tBovine spongiform encephalopathy.

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C. Diaper, B . J e f f e r s o n , S . A. P a r s o n s a n d S. J . J u d d o n

design of medium-scale systems are required to reduce excessive payback periods.

6. Holistic assessment of water-recycling schemes is neces- sary, whereby selection of technology, the suitability of particular sites, stakeholder involvement and the environ- mental and economic benefits all need to be considered.

A c k n o w l e d g e m e n t s

The authors wish to thank the Engineering and Physical Sciences Research Council for funding this research, the Royal Society for the Protection of Birds, the Hockerton Housing Project, Thames Water, Anglian Water, Essex and Suffolk Water, the Building Services Research and Information Association and the Construction Industry Research and Information Associ- ation, for providing information on the case-study schemes.

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