Pergamon 0273-1223(95)00318-5 Wal. SCI Tech Vol. 31. No.7, pp. 13-24, 1995. Copynght@ 1995IAWQ Pnnted in Great Britain. All rights reserved. $9'50 + O{)() CHARACTERISING THE QUANTITY AND QUALITY OF DOMESTIC WASTEWATER INFLOWS David Butler*. Eran Friedler* and Kevin Gatt** * Department of Civil Engineering. Imperial College of Science, Technology and Medicine. Imperial College Road, London SW7 28U. UK ** Drainage Department. Works Division. Ministry oj the Environment. 7 Pineo Wharf. Floriana VLTOI. Malta ABSTRACT A major source of inflow into sewer networks derives from domestic (sanitary) wastewater. The wastewater is made up of contributions from the various household appliances, influencing both flow quantity and quality. The results from two appliance usage surveys are presented to give detailed insight into the sub-daily variations of flow (quantity) and the relative importance of each appliance throughout the day. The most significant wastewater generating appliance is shown to be the WC, with the least contribution coming from the wash-basin. To establish at-source quality and its variation, limited published information was analysed in conjunction with the survey data The contribution of each appliance to a wastewater pollutant concentration is a combination of both the appliance flow rate and pollutant load, the proportions of which vary throughout the day. Pollutagraphs were produced for BOD, ortho-phosphate, ammonia and nitrate. The WC was found to be a major contributor to all four pollutants and in particular to ammonia. The washing machine was a significant contributor to ortho-)lhosphate and nitrate and the wash-basin to ortho-phosphate. Inflow pollutant concentrations were of comparable magnitude to published treatment plant data, although BOD values were found to be higher than expected KEYWORDS Domestic wastewater, flow quality; flow quantity; per capita daily discharges, sanitary appliances INTRODUCTION A prerequisite to the understanding of the physical, chemical and biological processes occurring within sewer systems is a detailed knowledge of the key inflows. One ofthe major inflows into foul and combined sewers, both in terms of quantity and quality, comes from domestic (sanitary) wastewater. The traditional approach to estimating the quantity of domestic wastewater has been to assume a daily flow per capita to give average dry weather flow and then to use multipliers for estimating the peak and low flows or the diurnal variation in flow. Even if this approach is coupled with a non-steady hydraulic model, the main difficulty is in estimating a reliable dry weather hydrograph at the inlets to the sewer network. An alternative approach is to use survey data on domestic appliance usage, formulated within a probabilistic framework, to derive the diurnal inflow pattern and to quantify its variability (Butler & Graham, in press) I3
Pnnted in Great Britain. All rights reserved. 0273-122319~ $9'50 + O{)()
CHARACTERISING THE QUANTITY AND QUALITY OF DOMESTIC WASTEWATER INFLOWS
David Butler*. Eran Friedler* and Kevin Gatt**
* Department of Civil Engineering. Imperial College of Science, Technology and Medicine. Imperial College Road, London SW7 28U. UK ** Drainage Department. Works Division. Ministry oj the Environment. 7 Pineo Wharf. Floriana VLTOI. Malta
ABSTRACT
A major source of inflow into sewer networks derives from domestic ( sanitary) wastewater. The wastewater is made up of contributions from the various household appliances, influencing both flow quantity and quality. The results from two appliance usage surveys are presented to give detailed insight into the sub-daily variations of flow (quantity) and the relative importance of each appliance throughout the day. The most significant wastewater generating appliance is shown to be the WC, with the least contribution coming from the wash-basin. To establish at-source quality and its variation, limited published information was analysed in conjunction with the survey data The contribution of each appliance to a wastewater pollutant concentration is a combination of both the appliance flow rate and pollutant load, the proportions of which vary throughout the day. Pollutagraphs were produced for BOD, ortho-phosphate, ammonia and nitrate. The WC was found to be a major contributor to all four pollutants and in particular to ammonia. The washing machine was a significant contributor to ortho-)lhosphate and nitrate and the wash-basin to ortho-phosphate. Inflow pollutant concentrations were of comparable magnitude to published treatment plant data, although BOD values were found to be higher than expected
A prerequisite to the understanding of the physical, chemical and biological processes occurring within sewer systems is a detailed knowledge of the key inflows. One ofthe major inflows into foul and combined sewers, both in terms of quantity and quality, comes from domestic (sanitary) wastewater.
The traditional approach to estimating the quantity of domestic wastewater has been to assume a daily flow per capita to give average dry weather flow and then to use multipliers for estimating the peak and low flows or the diurnal variation in flow. Even if this approach is coupled with a non-steady hydraulic model, the main difficulty is in estimating a reliable dry weather hydrograph at the inlets to the sewer network. An alternative approach is to use survey data on domestic appliance usage, formulated within a probabilistic framework, to derive the diurnal inflow pattern and to quantify its variability (Butler & Graham, in press)
I3
14 D. BUTLER el al.
The quantification of wastewater quality in sewers is in its infancy, as little regard has been paid until recently to the importance of quality variations on wastewater treatment plant performance or receiving water pollution. Standard values of quality parameters have been available for many years for wastewater entering works but these are not related to the type, configuration or extent of the sewer network A limited amount of data are available on the quality of discharges from specific domestic appliances (Laak, 1974; Ligman el al., 1974, Siegrist el aI., 1976; Inman., 1979 and others) It is proposed to use this information where possible, coupled with the quantity approach mentioned above to derive typical inflow "pollutagraphs" for a range of quality parameters. A number of insights can be obtained from this approach. • The magnitude of inflow pollutant concentrations can be compared with those typically measured downstream.
This may give an indication of the importance of in-sewer effects. • The timing of any peaks apparent in the inflow data can be compared with those measured downstream. • The causes of variation in inflow "strength" can be explored in relation to the mix of appliances typically
discharging at different times of the day. • The influence of future changes in appliances ownership and usage on wastewater characteristics can be
predicted.
FLOW CHARACTERISTICS
Relatively little research has been undertaken on the quantity of wastewater discharges from domestic households. The majority of data available has been collected at the inlet to wastewater treatment plants. This wastewater, however, includes commercial and industrial wastewater, infiltration water, as well as domestic wastewater. In order to overcome this problem, the usual practice is to use water supply flow data for the estimation of the average daily wastewater discharges and it is agreed that 60-90% of the per capita water consumption becomes wastewater (Metcalf & Eddy, 1991). Wastewater flows in sewers have observed characteristic patterns on annual, seasonal, daily, hourly and sub-hourly time scales. The estimation of maximum and minimum flows has been achieved conventionally by using specific multiplies of average dry weather flow (Metcalf & Eddy, 1991).
The analysis presented in this study deals with domestic wastewater (sanitary flows) and is focused on the sub•daily time scale, using survey data from two separate surveys on domestic appliance usage. The first took place in South East England during December 1987. This survey included 28 households ranging in size from I to 5 persons, with a total of78 participants, and lasted for seven consecutive days. The second study was carried out for seven consecutive days in Malta during January, 1993. This survey included 51 households, ranging between single occupancy and six-resident dwellings, with a total of 185 participants.
Both studies consisted of three parts. • Questionnaire: The questionnaire provided background information, such as details about the dwelling (type,
age, tenancy), the occupants (number, age, profession) and appliance ownership. • Survey: The survey was undertaken using diary sheets on which the participants recorded every time an
appliance was used and the mode of use (i.e. emptied or taps run to waste). The period of time that the taps were left on when the appliance was used in a run to waste mode was also recorded
• Experiments: The participants were requested to conduct a number of simple experiments on their appliances, which provided estimation of average volumes and discharges from each appliance type and mode of use.
Comprehensive accounts ofthe scope of both studies, the methodologies used and the results obtained have been reported by Butler (1991, 1993) and Gatt (I 993)
Per capita household total wastewater discharge
Figure I shows the pattern of domestic wastewater discharge over a 24 hour period for both England and Malta. The ordinates represent discharge per 100 occupants per 10 minutes. Data from the English survey are based on a moving hourly average and have been smoothed by a cubic spline procedure. Maltese data are based on an unsmoothed moving hourly average.
Quantity and quality of domestic wastewater inflows
Both curves show a similar general behaviour, namely one morning peak and two evening peaks, late night and midday minima. The differences between the curves can be explained by somewhat different lifestyles in these countries. The morning peak corresponds to residents preparing to go to work. In England, the peak is about 1. S times higher than the one in Malta, and it occurs about halfan hour later (8am VS. 7:30am). The first evening peak correspond to dwellers returning from work / school whilst the latter one corresponds to preparation for bedtime. These two peaks occur at 7pm and 11 pm in England, in Malta they occur earlier at Spm and 9pm, respectively.
The late night minimum flow period in both countries lasts about 4 hours, although the one in Malta starts and ends earlier than the English one (I-Sam VS, 2-6am). This low flow corresponds to dwellers'sleeping hours. The daytime flow shows a small decline corresponding to residents' working hours. The daytime flow in Malta is usually a little higher than the one in England, probably due to higher proportion of women working in the home.
In the English survey, the maximum instantaneous flow (on a 10 minute time basis) was found to be 3.48 times the average instantaneous flow, the minimum flow was found to be 0.008 ofthe average. In the Maltese survey, the maximum and minimum instantaneous flows were 2.43 and 0.02 times the average, respectively (table I). The maxima and minima are important as they enable design flows to be established and pipe diameter / gradients to be c:aIculated. Data supplied by the domestic hydrograph should serve as a useful dry weather flow input to urban dramage models. It should be noted, however, that the timing of the maxima and minima downstream will not necessarily coincide with the timing of the ones observed in the domestic hydrograph, since travel time and size of sewer network tend to delay and attenuate the flows.
Appliance wastewater discharge
The household wastewater discharge hydrograph is derived from its appliance sub-components. In the surveys, the following six appliances were found to be the most significant in a typical household we, kitchen sink, wash basin, bath, shower, and washing machine. An analysis of appliances' respective flow patterns should provide an insight into the observed trends of the total discharge hydrograph, and could prove useful for predicting future flows that might differ from the present due to changes in appliance ownership.
Both figure 2 and table I treat appliance flows on an instantaneous relative basis (i.e. flows from each appliance are compared with the total flow for all appliances at each 10 minute increment over a 24 hour period. Thus, 20% of the flow in peak time is different from 20% of the flow in low time). Bath & shower data were lumped together in order to prepare for the quality analysis which follows in the next section. Figure 2 shows the pattern
16 D. BU1LER el al.
of use of appliances over a 24 hour period Table I shows the relative maximum and minimum instantaneous flows of each appliance.
TABLE I APPLIANCE PROPORTIONAL INSTANTANEOUS DISCHARGE
Appliance TID' we Kitchen Sink Wash Basin Bath+Shower Washi'!JL Machine Survey England Malta England Malta England Malta England Malta England Malta E'!S!and Malta
• TID - Total Instantaneous DIscharge •• Average ofthe 10 monute moving hourly average
Generally it was found that the WC is the most significant wastewater "producing" appliance, especially during night time Figure 2 and table I indicate that the WC discharge consists of 60-90% of the total flow at night, and 20-40% during the day. On average the WC contributes as much as 37% (England) and 43% (Malta) to the total instantaneous discharge.
The importance of the WC is followed by the bath & shower, the most significant contribution of which occurs during the early morning (4-8am) and the evening hours (6-IOpm). In this period the discharge of the bath & shower consists of up to 66% (England) and 55% (Malta) of the total instantaneous discharge. During the early hours of the morning, the flow from the bath & shower reduces to virtually zero, and during working hours it consists of 10-15% of the total discharge.
The next most prevalent appliance is the washing machine peak flows from which occur during the late moming•early afternoon, though its activity continues until midnight. From midnight to 6-7am, there are no significant discharges The maximum contribution of the washing machine represents 42% and 54% of the total flow in England and Malta respectively.
Discharges from the kitchen sink occur, in general, from 5am to 12pm, averaging at 10-20% of the total instantaneous discharge However, in England the kitchen sink was also used during the early morning hours (3-5pm) making up a relatively high fraction of the total discharge (up to 25%).
The wash basin discharges constituted a relatively constant fraction of total instantaneous discharge during the whole day (I 0- 13%). apart from late at night, where they reached a maximum on I % of the total flow both in England and Malta
Comparison with published data
Table 2 presents a comparison of per capita domestic appliance wastewater data found in both surveys and available published water usage data. The table is divided into two parts: one shows the data on a volumetric basis and the other on a percentage basis. The total average daily wastewater generation in England and Malta is very similar. being 101 and 95 [I/headlday] respectively. It should be noted, however, that the total volumes in both surveys stand for the SIX monitored appliances and not for all household appliances Thus, in order to obtain the total water usage, a marginal volume should be added The total daily wastewater figures, found in the surveys, show particularly good agreement with the findings of Hall et al. (1988) for the UK, and to a lesser extent with the value given by Siegrist el al. (I 976) in the USA. The work ofLaak (I 974) and Ligman et al. (1974), both conducted in the USA, present much higher daily consumptions (156 & 180 [I/head/day D, probably due to higher toilet flush, bath & shower volumes and more frequent use of the bath & shower.
Quantity and quality of domestic wastewater mflows 17
The proportion of wastewater generated by each appliance as compared with the total, are similar in both the surveys described here. In fact, all the studies shown in table. 2 indicate that the WC is the most water using (and waste producing) appliance in a household, comprising 30-50% of the total usage. The WC is followec! by the bath & shower, with 17-28% of the total usage, the washing machine with 11-31 %, the kitchen sink with 7-16%, and the wash-basin with 5-13%.
Household wastewater is generated from different sources (appliances) within the residence. As a result, domestic wastewater has many different and varying properties. Examination of the quality of wastewater generated by each appliance enables identification of the main "pollution contributing" appliances, and determination of the factors causing variations in domestic wastewater qUality. The contribution of each appliance to the pollution load
III D. BUlLER et al.
depends not only on the volumes of wastewater released by the specific appliance, but also on the quality of the effluent released. Study on an appliance basis will help to predict future changes in domestic wastewater quality, which will occur due to changes in appliance ownership. An insight into the sub-daily patterns of wastewater quality at source provides better input to, and thus better prediction by, existing sewer quality models. Moreover, knowledge of the quality at both ends of the sewer network might give an indication of the importance of in-sewer effects such as dilution (by groundwater infiltration), sedimentation / erosion and biochemical transformations.
TABLE 2 PER (' APITA DOMESTIC WATER USAGE
Water usage Percentage or total daily usage Unit. [Ithead/day] .1%1 Sourcr I 2 3 4 5 6 I 2 3 4 5 6 CountrY UK Malta UK USA USA USA UK Malta UK USA USA USA
As in the case of sub-daily domestic wastewater discharge patterns, there is a lack of data pertaining to the quality patterns of domestic wastewater at source, especially on an appliance basis, and even recent daily average quality values are scarce. In order to overcome this problem a comparative literature review was conducted, the results of which are presented in table 3. The data (when possible) are presented as loading (pollutantmass/head/day). This form of presentation overcomes the different dilution factors of pollutants (i.e. different wastewater discharges, different volumes of toilet flush, sinks, basins etc) and thus reaches a common baseline for comparison between sources.
Four parameters were selected as representatives of domestic wastewater quality in the following analysis; BODI ,
ortho-phosphate, ammonia and nitrate. These parameters were chosen due to their importance in wastewater treatment and pollution control, and the availability of data for each appliance. Representative values for each appliance / parameter combination were obtained simply by averaging the few available data, while excluding unreasonable values. Table 4-A shows the chosen pollutant loading of each appliance.
Table 4-B shows the average wastewater pollutant concentration for each appliance discharge and the overall household discharge in England and Malta. These values were derived by incorporating appliance pollution loading (table 4-A) with appliance daily wastewater discharge volumes (table 2). The results obtained for both countries were compared with available data, and appear to be within the range found in the literature.
The mean concentrations of the four representative quality parameters for each appliance (table 4-B) were combined with the individual appliance discharge patterns in order to simulate sub-daily quality variations, the results of which are shown in table 5 and figure 3
Table 5 reveals that domestic wastewater quality undergoes significant variations during the day. The maximum to minimum concentration ratio is from I 5 to 2 for BODI , from 3 to 4 for ortho-phosphate and ammonia, and from 2.4 to 2.6 for nitrate. Figure 3 presents the sub-daily patterns of the overall household wastewater quality Comparison of the results for England and Malta reveals that the same general trend appears in both countries fJr each individual appliance.
D Appliane~ WC
Source I • 2 3·
Units USA USA
BOD, (mg/e/d( 23,540 23,556
COD (mg/e/d) 67,890
TOC )mg/e/d)
TS )mg/e/d) 96,942
TSS (mg/e/d) 30,804
Total P (as P) (mg/e/d) 1,500 1,359
Ortho-P (as P) (mglcJd) 6,473··
T.K.N. (as N) Img/e/d) 21,000 14,490 16,761
NH,(as N) )mgle/d) 2,782
NO, (as N) (mglcJdl 16.05
Total Coliforms (MPN/IOOml)
Faeeal Coliforms (MPN/lOOm1)
Applaince
Source 2 3 4
Units USA USA USA
BOD, (mglcJdl 6,180 9,060 3,090
COD (mglcJd) 9,080
TOC (mg/e/d) 1,750
TS (mglcJdl 20,838 4,590
TSS (mglcJdl 5,436 2,260
Total P (as P) ImglcJdl 36
Ortbo-P (as P) ImglcJdl 30 20
T.K.N. (as N) (mglcJdl 232+ 310
NH,(as N) (mgle/d) 43 40
NO,(asN) (mglcJdl 11.6 7.4
Total Coliforms (MPN/l00m11 1.1·10>
Faeeal Coliforms (MPN/loomll 2.2·10>
TABLE 3 PER CAPITA POLLUTION LOAD
Kitchen Sink Wash Basin
4 2 3 4 5 6 7 2 USA USA USA USA Japan USA USA
10,720 9,200 5,889 8,340 15,400 8,340 1,860
18,800 13,000 (Mn) 3,250 29,600 (Cr)
7,780 5,000 5,000
28,500 19,932 13,800
12,520 2,718 4,410 11,200 4,111
550 453 420 175 419
310 173 180 177 386
4,140 420 690 424
1,110 74 32.3 32.3 9
17.4 7.6 1.8 1.8 1.1
3·10'
Bath I Shower Washing Machine
5 7 8++ 9 2 3 4 7 USA USA USA USA USA USA
3,090 7,900 9,513 14,810 14,810
20,300
1,749 2,781 10,310 10,303
39,864 48,400
1,261 8,208 7,248 10,970 10,970
36 27 4,790 2,265 2,150 1,610
21 520 523
306 302 730 725
40 316 30.8 30.8
7.4 35.3 27.3 27.3
6.1·10' 1·10' 1.8·10'(W) 5.3· 10' (R)
9.7·10' 6·10> 1.4·10'(W) 3.2·10' (R)
Legend:
. Human excrets 001,. "'" Value seems unreasonable + Based on 2 observations ++ Estimated values assuming
discbarge of 27 (lie/d) (R) Rinse cycle (W) Wasb cycle
~
I. Inman (1979) 2. Laak", al (1974) 3. Ligman'" aL (1974) 4. Siegrist et al (1974) 5. Olsson et al (1968) 6. Ukita '" al (1986) 7. Witll al (1974) 8. Pancuska '" at (1975) 9. Rose '" at (1991)
9 USA
199 (W) 56(R)
126(W) 25(R)
I
I
.0 § co Q
5 Q.
.0
[ ~ g,
~ &l a. n
~ f>
i s·
~
:c
20 D. BUTLER el al.
TABLE 4 APPLIANCE WASTEWATER QUALITY - VALUES CHOSEN FOR SIMULATION
In order to understand the overall quality patterns, there is a need to look into the relative importance of each appliance, which is determined by the proportion of the product offlow and concentration for each appliance, in relation to the product oftotal discharge and overall concentration. The following discussion concentrates on the English data, the same analysis was conducted on the Maltese data, and similar results were obtained
BOD
The kitchen sink wastewater contains the highest BOD concentration, followed by the washing machine and the WC (table 4-B) However, the differences between these three appliances are limited (about 10-15%) BOD concentratIOns found in the bath & shower and the wash basin wastewater are much lower. Thus, higher overall concentration is expected when discharge from anyone ofthe first three appliances makes up most of the total flow, and lower concentration when most of the flow consists of discharges from anyone of the latter two
As shown in figure 3-A. the BOD high concentration period (600-680 mgll) at 2-4am results from the WC and kitchen sink being the main sources of discharge (about 70% & 20% of the total discharge, respectively) The following decline (350 mgll at Sam), is a result of the bath & shower comprising about 65% of the discharge (fig. 2-A). The minor peak at 6am (about 470 mgll) is caused by a lower proportion of the flow being donated by the
bath & shower (25%) and a higher proportion coming from the we (50%). The high concentration period (9am-6pm) is a result of the washing machine, we and kitchen sink comprising together a high proportion of the flow and the load. The evening reduction results from the bath & shower and wash basin having a more significant part of the flow.
Ortho-phosphate
According to table 4-B, both the washing machine and the wash basin wastewater contains high ortho-phosphate concentration (31 & 30 mWI, respectively), the kitchen sink and the we contribute about half as much (14 & 10 mg/I, respectively), and the bath & shower contribution is negligible. Thus, ortho-phosphate concentration is expected to be high when the washing machine and wash basin comprise a high proportion ofthe discharge, and low whenever the bath & shower proportional discharge is relatively high.
Figure 3-B reveals that ortho-phosphate concentration is low in the early morning hours (3-6am) since in this period the we and the bath & shower are the main sources of wastewater (fig. 2-A). Though the wash basin contributes just 15-20% of the dischJlfge, it is responsible for 30-40% of theload.The daytime peak concentration coincides with the washing machine and the wash basin being together 40-50% of the flow. The high proportion of bath & shower discharge (fig. 2-A) and lower discharge from the washing machine, results in a slight decline in the ortho-phosphate concentration in the evening.
Ammonia
Ammonia concentration in the we wastewater (65 mg/l) is about 1-2 orders of magnitude higher than the concentration in wastewater of other appliances (table 4-B). Thus, the ammonia concentration pattern (fig. 3-C) follows the we proportional discharge (fig. 2-A). During the night, when the we makes up a high proportion of the total discharge and is responsible for almost 100% of the ammonia loading, a relatively high range of ammonia concentration is observed (40-50 mWI) These values are lower than the concentration found in the we discharge alone, due to dilution by wastewater of other appliances. During the day, when the we is responsible for 15-25% of the discharge, its proportion of the load also lowers to 80-85%. This results in much lower ammonia concentrations (5-25 mWI).
Nitrate
From table 4-B, the highest concentration of nitrate is found in the washing machine wastewater (1.93 mgll). This concentration is almost 3 times higher than the ones found in the we and wash basin (0.72 & 07 mgll, respectively), and more than 5 times higher than the one found in the bath & shower wastewater (0.36 mgll). Thus, relatively high concentrations of nitrate are observed during the daytime when the washing machine con~butes20-40%of the discharge (fig. 2-A). The lowest nitrate concentrations are found during the morning hours (5-7am) when a high proportion of the flow (30-60%) comes from the bath & shower (fig. 3-0).
CONCLUSIONS
Following two surveys in the UK and Malta, general conclusions can be drawn concerning the relative magnitude of the contributions of household appliances to domestic wastewater. The most significant appliance is the we, contributing approximately40%ofthetotal instantaneous flow, and up to 90 % at night. The least important appliance overall was found to be the wash basin at 13 %. The proportion of household use of each appliance found in the two surveys compared well with published data
Quality data for individual appliancesarecurrently verylimited.However,data that are available(in the form of pollutant loading) were used to calculate the concentration of four representative parameters for each appliance. The contribution of each appliance to pollutant concentration is a combination of both the appliance wastewater flow rate and the pollutant loading. The BOD concentration was found to vary significantly throughout the day, up to 680 mWI in the early morning hours, being influenced by the high proportion ofWC loading at that time.
Quantity and qUality of domestic wastewater inflows 23
Ortho-phosphate varied to a lesser extent, but reached peak concentrations of about 20 mgll. Ammonia emanated almost exclusively from the WC and, as such, was heavily influenced by WC usage patterns. Maximum concentrations were noted at similar times to the BOD, at about SO mgll. Finally, nitrate was fairly consistent throughout the day with a high of about 1.2 mgll.
Early morning peaks (2-4am) are particularly evident in the BOD and ammonia concentration plots due to the relative proportion of appliance use at that time, but based on a constant pollutant load per appliance use. These early peaks were certainly unexpected and may in fact be eliminated if pollutant load per use is found to differ during the night. Confirmation, or otherwise, of this result must await detailed testing throughout the day.
Comparison of these inflow pollutant concentrations shows them to be generally within the range of reported values of wastewater at treatment plants. The exceptions to this are to a lesser extent, ortho-phosphate and to a greater extent, BOD. The average BOD from this analysis was calculated at 492 mgll, which is high for typical UK wastewater. Could this point to the magnitude of in-sewer wastewater stabilisation (i.e. maybe 1 SO mgll reduction)? The timing of inflow peaks shows more radical departures from assumed variations at treatment plants, and this aspect demands further study.
Present work concentrates on a sampling study in UK households. This will corroborate (or not) US data and should answer questions regarding the variability of appliance pollutant concentration with use. It can reasonably be assumed that pollution load from the washing machine is about the same in each use. However, the load from the WC, kitchen sink and wash basin could vary during the day, depending on the use to which they were put.
The usage of all household appliances has the potential to undergo changes in the near future due to water metering and changed ownership patterns. This, in turn will undoubtedly have implications on domestic wastewater quantity and quality. It is hoped this work will contribute to the understanding of such changes and their impact on the sewer network, treatment plant and receiving water.
ACKNOWLEDGMENTS
The work described in this paper forms part of a project funded by the UK Institution of Civil Engineers' R&D Fund and the ConstruC<tion Directorate of the UK Department of the Environment (Contract PECD 7/6/309). One of us (EF) wishes to acknowledge financial support from the Foreign and Commonwealth Office - Clore Foundation and the Royal Society - Israel Academy of Sciences and Humanities. The authors would like to acknowledge Niamh O'Sullivan's contribution to this work.
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