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Assessment of the effects of greywater reuse on gross solids movement in sewer systems R. Penn, M. Schütze and E. Friedler
ABSTRACT Onsite greywater reuse (GWR) and installation of water-efficient toilets (WETs) reduce urban freshwater demand and thus enhance urban water use sustainability. Research on GWR and WETs has generally overlooked their potential effects on municipal sewer systems: GWR and WETs affect the flow regime in sewers, and consequently also influence gross solids transport. To asses these impacts, a gross solids transport model was developed. The model is based on approaches found in the literature. Hydrodynamic calculations of sewage flow were performed using the SIMBA6 simulator and then used for the gross solid movement models. Flow characteristics in the up- and downstream sections of the sewer network differ. Therefore different approaches were
R. Penn (corresponding author) E. Friedler Faculty of Civil and Environmental Engineering, Technion – Israel Institute of Technology, Haifa 32000, Israel E-mail: [email protected] M. Schütze ifak- Institut für Automation und Kommunikation e.V. Magdeburg, Werner-Heisenberg-Str. 1, 39106 Magdeburg, Germany
used to model solids movement in each of these two parts. Each model determines whether a solid moves as a result of a momentary sewage flow, and if it moves, calculation of its velocity is possible. The paper shows the adoption and implementation of two gross solids transport models using SIMBA6 and depicts the results of the effects of various GWR and WET scenarios on gross solids movement in sewers for a real case study in Israel. Key words
| greywater reuse, gross solids movement, modeling, municipal sewers, simulation, water-efficient toilets
INTRODUCTION Greywater (GW) is generally defined as domestic sewage excluding the stream generated by toilets (blackwater). Kitchen wastewater (kitchen sink and dishwasher) is defined as dark GW and is often combined with the blackwater stream. GW streams generated by the bath, shower and washbasin are defined as light GW. On-site GW reuse (GWR) for toilet flushing and landscape irrigation is thought to be an efficient tool for reducing potable water demand, decreasing the need for developing new (and usually costly) water sources, and for enhancing sustainable urban water use. Onsite light GWR for toilet flushing can reduce daily household water consumption by 26% (Penn et al. ). Campisano & Modica () investigated reuse of washbasin GW for toilet flushing and found that ∼10–30% of the in-house water usage can be saved. Using the excess amount of the light GW (the remaining GW that is not reused for toilet flushing) for garden irrigation can further reduce the domestic water demand up to 41% of the baseline scenario, where no GW is reused (Penn et al. ). Nevertheless, in order to prevent health risks and to minimize negative aesthetic and doi: 10.2166/wst.2013.555
environmental effects, treatment of GW is necessary prior to reuse (Dixon et al. ; Diaper et al. ; Maimon et al. and others). Friedler () has shown that, as the demand for GW within the urban environment (i.e. for toilet flushing and garden irrigation) is lower than its production, it is not necessary to recycle all GW streams, but rather to focus on the less polluted light GW, and to discharge the more polluted dark GW stream together with the blackwater stream to the existing urban sewer system. Research on GWR to date has generally overlooked the effects that GWR may have on the municipal sewer sector. GW treatment and reuse changes the quantity and quality characteristics of domestic wastewater released to the sewer and eventually conveyed to the wastewater treatment plant (WWTP). This may lead to positive and negative effects that may influence sewer systems and WWTPs. Accordingly, one can ask what could be the effects on urban wastewater collection systems and on WWTPs? Are they positive or negative? How will they change with increasing implementation of on-site GWR systems?
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This paper aims to answer some of these questions by providing a quantitative assessment of the effects of GWR and water-efficient toilets (WETs) on gross solids transport in sewers, as their movement may play a role in creation of sewer blockages. A simulation model for gross solids movement in sewers was developed and used for analyzing the effects of six future GWR and WET scenarios on gross solids movement in a representative separate sewer system. Since in-sewer flow characteristics have a pronounced effect on gross solids movement, selected results from a previous study conducted by the authors on the effects of GWR on flow characteristics in urban sewer systems are briefly quoted.
METHODOLOGY Effects of GWR on the urban wastewater network The positive and negative effects of GWR and WETs were studied by modeling a representative, separate, urban sewer system of a densely populated neighborhood located in a coastal city in Israel, with an area of ∼1.3 km2 and some 15,000 residents living in flats. The sewer system is composed of 154 nodes and 153 links, having diameters ranging from 0.2 m (in upstream links) to 0.4 m (in downstream ones) and an overall length of about 6 km (further information on the case study site can be found in Penn et al. ()). The urban water system simulator used was SIMBA6 (Simulation Biologischer Abwassersysteme), developed by ifak (). SIMBA6 is an integrated simulator for sewer systems, WWTPs and river water quality. The hydrodynamic modeling modules of SIMBA, which are based on an extended version of the US-EPA SWMM (Rossman ; US Environmental Protection Agency website) were applied for simulating wastewater flow. These yield the full dynamic solution of the Saint Venant differential equations. It should be stressed that in this study, as stated above, a separate sewer system was modeled as this is the prevailing type of sewer systems in Israel. In a separate sewer system, pipe diameters are smaller than those found in a combined system, since they conveys only wastewater, compared with the latter which convey stormwater as well. Three combinations of GWR types (in single houses/ flats) were used as an input to the model:
•
Type 1 (No GWR) – No GW recycling is practised and the combined domestic wastewater stream (incorporating discharges from all domestic sources) is discharged into the sewage system.
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Type 2 (GWR WC) – GW generated from the bath, shower and wash basin (i.e. light GW) is treated and used for toilet flushing. Excess light GW that is not used for toilet flushing is discharged to the sewage system as overflow, prior to treatment (as raw GW). Type 3 (GWR WC þ IRR) – Same as type 2, but the excess GW flow (after treatment) is used for irrigation.
The quantity of domestic wastewater discharge input to the model was defined by the appropriate diurnal hydrographs of wastewater discharged to the sewer by individual persons during weekdays (GWR type 1 or 2 or 3, as described above). Weekends exhibit different domestic wastewater generation behavior and were not modeled. Derivation of these hydrographs is described by Penn et al. (). Five GWR scenarios were modeled, simulated and analyzed. In each scenario, a different combination of GWR type households (as described above) was used: 1. 100% of the households implement GWR type 1 (no GWR) – the current situation in Israel. 2. 100% of the households implement GWR type 2 (GWR WC) – extreme implementation scenario. 3. 100% of the households implement GWR type 3 (GWR WC þ IRR) – extreme implementation scenario. 4. 70% of the households implement GWR type 1 and 30% of the households implement GWR type 2 – typical uptake to be expected with in two decades (Friedler ). 5. 70% of the households implement GWR type 1, 15% of the households implement GWR type 2 and 15% of the households implement GWR type 3 – typical uptake to be expected with in two decades. Toilet flush volumes in the above five scenarios were 9 and 6 L for full and half-flush, respectively. These flush volumes are mandatory for all toilets in Israel. For the examination of the effects of reduced toilet flush volumes, toilet flush volumes in scenarios 1, 2 and 3 were reduced to 6 and 3 L for full and half-flushes respectively, and the simulation was carried out again. These lower flush volumes are mandatory for all toilets installed in new buildings in Israel as of 2012. The scenarios were compared using the following parameters: flow, velocity, capacity (proportional depth) and Froude number. Flow and velocity have the largest effect on gross solids movement (as will be described later), therefore only their results are presented in this paper. Additional results regarding the other two parameters can be found in Penn et al. ().
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Gross solid movement We present the way that GWR and WETs affect the flow regime in sewers, which in turn has the potential to influence gross solids transport. Hence there is a need to assess the effects of reduced flows on gross solids transport in sewers. To do so, a gross-solids transport model has to be developed. Flow characteristics in the up- and downstream sections of the separate sewer network differ. In upstream segments, flow is intermittent and large un-submerged solids are more likely to be found. In the downstream part of the sewer, flow is continuous and smaller submerged solids can be found. Therefore, different approaches were used to model solids movement in each of the two sections of the sewer system. Various approaches for modeling solids movement in separate sewers were reviewed (Butler et al. a, b, ; Digman et al. ; Enfinger & Mitchell ; Walski et al. , ) and the most appropriate approaches were selected based on Walski et al. (, ). The selection was based on the following criteria: applicability, minimum requirement for coefficient values, simplicity and compatibility between conditions of the research reported for the development of the model and the characteristics of our case study. For the upstream part a model based on ‘critical’ flow/volume developed by Walski et al. () appeared to be most suitable, while for the downstream sections a model based on tractive force (shear stress) (Walski et al. ) was used.
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distinguishing between full and partial movement and 0.25 for distinguishing between partial movement and no movement (e.g. a solid will fully move if the wastewater flow is higher than the calculated flow in Equation (1) where a equals 0.45). Values of a were determined by trial and error (Walski et al. ). The movement of standard solids as a result of flows from short events like toilet flushing and hand washing was also examined (Walski et al. ). For runs of short duration (less than 20 s) with an attenuation section, the volume of the pulse proved to be the controlling parameter, and the following equation was derived: V ¼ aSG=S0:2
(2)
where V is the volume of pulse (L) and a equals 18 for distinguishing between full and partial movement and 10 for distinguishing between partial movement and no movement. Values of a were determined by trial and error (Walski et al. ). Six scenarios were simulated: 1, 2 and 3 with toilet flush volumes of 9 and 6 L for full and half-flushes respectively (NO GWR96, GWR WC96 and GWR WC IRR96, respectively), and three additional scenarios, which are the same as 1, 2 and 3 above but with reduced toilet flush volumes of 6 and 3 L for full and half-flushes respectively (NO GWR63, GWR WC63 and GWR WC IRR63, respectively). Downstream reaches
Upstream reaches Walski et al. (), examined the transport of large solids in sewers functioning under unsteady flow conditions. They tested the movement of standard solids created from different materials, and of several household solids, which included disposable diapers, cotton rags and feminine hygiene pads. They found that the specific gravity of the solid, the slope of the pipe and the flow rate were the most important parameters in determining whether a solid moves or not. They derived Equation (1) to distinguish between three types of movement (full movement, creeping movement and no movement): Q ¼ aSG=S0:2
(1)
where SG is the specific gravity of the solid; S is the slope of pipe; Q is the ‘critical’ flow (L/s) and a equals 0.45 for
As mentioned above, modeling solids movement in the downstream sections of the sewer network was based on the shear stress (tractive force) model by Walski et al. (). This approach is based on the average shear stress acting on the sewer pipe walls and the critical shear stress needed to move dense discrete particles that lie on the bottom of the pipe. Movement of a solid will occur if the average shear stress is higher than the critical one. A baseline assumption in this approach is that the solid is fully submerged. The calculation is based on a design discrete grit particle (specific gravity of 2.7) that is transported often enough so it does not accumulate to form a cohesive layer along the pipe invert. All other solids are considered to be more easily transported. Average boundary shear stress In open channel flow, the average boundary shear stress exerted by flowing fluid on the channel (sewer pipe in our
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RESULTS AND DISCUSSION
case) can be expressed as Equation (3): (3)
Effects of GWR on the urban wastewater sewer system
where τ is the shear stress (Pa), ρ is the density of liquid (kg/m3), R is the hydraulic radius (m) and S is the slope of pipe. An alternative approach would be to use the slope of the energy instead of the slope of the pipe, which is more suitable for unsteady flow conditions. This will be investigated in future publications.
Figure 1 presents the diurnal patterns of the flow and velocity. Each graph presents an individual pipe segment: one in the middle (link 97) and one downstream (link 154) of the sewer network (other links in the sewer showed similar trends). In each graph, the results of five simulated scenarios are shown (all with toilet flush volumes of 9 and 6 L). Moving down the sewer system, the flow increases as expected (more households contribute wastewater to the system), and attenuation (i.e. reduction of the peak flow by redistributing the same volume of flow over a longer period of time) and a time shift of the peak flows occur. The maximum flow occurs at 8:04 a.m. and at 9:01 a.m. in links 97 and 154 respectively (Table 1). The velocity in the pipes is influenced by the flow and geometry of the pipes; hence moving down the sewer system, no unambiguous pattern of velocity increase/decrease was observed. As GWR increases, the flow (discharge) and velocity decreases. This decrease is not evenly distributed through the day, but occurs mainly during the morning peak discharge period. As shown in
τ ¼ ρgRðS=100Þ
Critical shear stress The critical shear stress needed to move dense discrete particles (quartzite sand) as a function of particle size may be expressed as follows (Walski et al. ): τ c ¼ kd0:277
(4)
where τc is the critical shear stress (Pa), k equals 0.867 and d is the nominal diameter (mm) for a discrete sand particle of specific gravity 2.7 (the design particle).
Figure 1
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Diurnal flow and velocity patterns in two selected sewer pipes for the five GWR scenarios examined. Each column represents a pipe segment (link) along the sewer network (97, middle section; 154, downstream section). NO RU – no GWR; WC – GWR for toilet flushing; WC IRR – GWR for toilet flushing and garden irrigation; 70 30 – 70% NO RU, 30% WC; 70 15 15 – 70% NO RU, 15% WC, 15% WC IRR. Adopted from Penn et al. (2013).
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Links 97 and 154 – geometry, and timing and value of maximum flow (scenario 1) Pipe diameter (mm)
Slope (%)
Time of maximum flow (h)
Maximum flow (m3/min)
Link 97
315
1.11
8:04
2.6
Link 154
400
1.23
9:01
4.5
Figure 1 a significant reduction occurs mostly in scenario 3 (all households GWR WC þ IRR), an extreme situation that is unlikely to happen. The simulation further revealed that the maximum velocity in most scenarios was higher than the minimum velocity required for solids movement, which lies in the range of 0.6–1.0 m/s (Walski et al. ). Exceptions were found in scenario 3 which, as previously mentioned, is a situation not likely to occur. Examination of the effects of reduced toilet flush volumes (results can be found in Penn et al. ()) revealed that in some of the GWR scenarios flows and velocities in the sewer were reduced, while in others they did not change. The above results indicate that as a result of GWR and installation of WETs, sewer blockage rates are not expected to increase significantly. However, this should be further examined as explained below.
Gross solid movement Upstream reaches Figure 2 presents three pipe segments, namely: link 36 (located upstream); link 42 (located some distance downstream from link 36, with no extra wastewater being discharged along the way); and link 48 (located in the middle of the sewer system,
Figure 2
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where additional wastewater is discharged into the sewer along the way. Diurnal flow patterns in these three pipe segments are presented, and the dividing lines for full, partial and no movement, for a solid with a specific gravity of 1.06 are shown by horizontal lines. 1.06 is the specific gravity of a typical NBS solid, which is the US National Bureau of Standard artificial fecal solid (Brown et al. .). One can see that, in link 36 (upstream) for the scenario having the highest GWR and the lowest toilet flush volumes (GWR WC IRR63), the conditions of no movement of the solid occur for most of the day (76%, 18.2 h), partial movement occurs for 22% of the day (5.3 h) and the smallest duration is of conditions enabling full movement (2% of the day, 0.5 h). Whereas in scenario 1 (no GWR and flush volumes of 6 and 9 L), the situation is reversed with 28, 39 and 33% of the day with conditions enabling full, partial and no movement, respectively. Going downstream, as long as no additional wastewater is discharged to the sewer (link 42), similar patterns and proportions are observed (minor differences depicted result from differences in pipes’ slopes and diameters). It is worth mentioning that even in this ‘sewage ample’ situation, for as much as 33 and 35% of the day (links 36 and 42, respectively), gross solids will not move in the upstream segments of the sewer system. When additional households discharge their wastewater to the sewer (link 48), the duration of full movement increases, and consequently, the duration of partial and no movement conditions decreases. This difference is especially significant for the lowest wastewater discharge scenario (GWR WC IRR63); where in link 48, for 78% of the day conditions for full movement prevail, while in links 36 and 42 they prevail only for 2% of the day. As mentioned previously, this method is suitable only for upstream
Diurnal flow pattern in three sewer links (link 36: upstream, link 48: downstream) as a result of the simulation. The horizontal lines represent the boundary between full and partial movement (upper) and partial and no movement (lower). Values in the rectangles represent the proportion of the day for each type of movement (upper, full movement; middle, partial movement; lower, no movement). All relate to a solid having a specific gravity of 1.06.
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Diurnal shear stress patterns in three sewer links (link 97: upstream, link 154: downstream) as result of the simulation. Horizontal lines: critical shear stress for a 6 and 1 mm (diam.) gross solids (upstream and downstream respectively). Rectangles: portions of the day in which the average shear stress is higher/lower than the critical (upper: τ > τc for d ¼ 6 mm; middle: τc for d ¼ 1 mm < τ < τc for d ¼ 6 mm; lower: τ < τc for d ¼ 1 mm).
links where the flow is intermittent and thus not suitable for link 154 which is located downstream. These results reveal that under extreme GWR and WETs, sewer sections located in the uppermost sections of the network may suffer from higher blockage rates, as the transport of gross solids will be slower than under the current situation (no GWR). Nevertheless, Littlewood et al. () have shown that for ultra-low flush toilets (and thus decreased wastewater flows) decreasing the sewer diameter would enhance solids movement. Their suggestion falls in line with Penn et al. () who demonstrated that under extreme GWR and WET scenarios, the diameters of sewer pipes could be decreased by 20%. Downstream reaches The diurnal pattern of the average boundary shear stress and the constant critical shear stress (constant since it is a function of the design solid diameter and not the wastewater flow characteristics) in links 97, 71 and 154 are depicted in Figure 3. These links are located in the downstream section of the sewer system studied, where link 154 is the furthest downstream and links 71 and 97 are located above it. As solids move along the sewer system they undergo disintegration. Hence, solids diameters simulated in this section were smaller than the diameter of the NBS solid. The diameters considered were 6 mm (a typical gross solid diameter, taken from Butler & Davies ()) and 1 mm (expected diameter for solids found downstream). Wastewater density was taken to be equal to the density of water (1,000 kg/m3). Further shown in Figure 3 are the portions of the day during which the average shear stress is greater than the critical stress required for the movement of 6 mm diameter solids; the proportion of the day during which the average shear stress is between the critical shear stress required for
the movement of 6 mm solids and that required to move 1 mm diameter solids; and the portion of the day during which the average shear stress is lower than the critical stress required to induce movement of solids having a diameter of 1 mm (numbers in rectangles from top to bottom respectively). These portions are shown for the two extreme scenarios simulated: scenario 1 having the largest wastewater discharges, i.e. no GWR at all and toilet flush volumes of 9 and 6 L; and scenario 6 with the lowest wastewater discharges, i.e. all households reuse GW for toilet flushing and garden irrigation and toilet flush volumes are 6 and 3 L for full and half-flush respectively. It can be seen that downstream (in link 154) full movement should be achieved throughout the day, in all scenarios simulated, for a reasonable sized gross solid. This is in contrast to the findings for the upstream sections. Also in links located upstream from this link but downstream in the network (links 97 and 71), except during small portions of the day, full movement for a reasonable sized solid, will be achieved. For example, in link 97 in scenario 1 (NO GWR 9/6), during 93% of the day solids of a 6 mm diameter (or smaller) will move, while during 7% of the day solids with a diameter of 6 mm (or higher) will not move. In scenario 6 (GWR WC IRR 6/3), which is the most extreme scenario, the portion of the day in which 6 mm (or smaller) solids will move is reduced to 83% and during 17% of the day solids with diameter of 6 mm (or higher) will not move. In all scenarios full movement of 1 mm (or smaller) solids was achieved throughout the day.
CONCLUSIONS AND OUTLOOK This research quantified the expected effects of changes in urban wastewater quantity as a result of GWR and
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implementation of WETs on gross solid movement in sewers. It was shown that in upstream links, with small amounts of wastewater discharged when there is no GWR, for most of the day full or partial movement of the solids occurs (67%), whereas when GW is being reused the situation reverses, with longer duration of no movement of solids (∼76% of the day). However, down the sewer system, when additional households are connected, and hence additional wastewater is discharged to the sewer, an increase of the proportion of the day for full movement occurs, while in downstream links, full movement of the solids in all the scenarios can be expected, since the average boundary shear stress was higher than the critical one throughout the day. This implies that the blockages rate in the uppermost sections of the sewer system may increase if there is very high uptake of GWR and WETs. However, the problem could be averted by constructing narrower pipes in these reaches. Intelligent implementation of the results can support the planning of future sewer systems in which GW would be reused on a large scale, and thus contribute to sustainable reuse of precious water resources. Currently, work is underway to create and integrate a ‘solids generator’ module, ‘discharging’ solids into the sewer with the simulator developed here.
ACKNOWLEDGEMENTS This research was supported by a grant from the Ministry of Science & Technology of the State of Israel and the Federal Ministry of Education and Research (BMBF) of the Federal Republic of Germany (Grant reference No. 02WA1263). The authors further wish to thank the Netanya Water Corporation for their cooperation.
REFERENCES Brown, D. M., Butler, D., Orman, N. R. & Davies, J. W. Gross solids transport in small diameter sewers. Water Sci. Technol. 33 (9), 25–30. Butler, D. & Davies, J. W. Urban Drainage. 3rd edn, T & F Books, UK. Butler, D., Davies, J. W., Jefferies, C. & Schutze, M. a Gross solids transport in sewers. Water Marit. Eng. 156 (WM2), 175–183. Butler, D., May, R. & Ackers, J. b Self-cleansing sewer design based on sediment transport principles. J. Hydraul. Eng. 129 (4), 276–282.
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Butler, D., Littlewood, K. & Orman, N. A model for the movement of large solids in small sewers. Water Sci. Technol. 52 (5), 69–76. Campisano, A. & Modica, C. Experimental investigation on water saving by the reuse of washbasin greywater for toilet flushing. Urban Water J. 7 (1), 17–24. Diaper, C., Dixon, A., Butler, D., Fewkes, A., Parsons, S. A., Stephenson, T., Strathern, M. & Strutt, J. Small scale water recycling systems – risk assessment and modelling. Water Sci. Technol. 43 (10), 83–90. Digman, C. J., Littlewood, K., Butler, D., Spence, K., Balmforth, D. J., Davies, J. & Schuetze, M. A model to predict the temporal distribution of gross solids loading in combined sewerage systems. In Ninth International Conference on Urban Drainage, Portland/Oregon, USA. Dixon, A. M., Butler, D. & Fewkes, A. Guidelines for greywater re-use: health issues. Water Environ. Manag. 13 (5), 322–326. Enfinger, K. L., P. E. Mitchell, P. S., P. E. Scattergraph principle and practice. Evaluating self-cleansing in existing sewers using the tractive force method. ADS Environ. Services 1–10. Friedler, E. Quality of individual domestic greywater streams and its implication on on-site treatment and reuse possibilities. Environ. Technol. 25 (9), 997–1008. Friedler, E. The water saving potential and the socio-economic feasibility of greywater reuse within the urban sector – Israel as a case study. Int. J. Environ. Studies 65 (1), 57–69. ifak SIMBA6 – Simulation of Wastewater systems, Reference and Tutorial. ifak – Institut für Automation und Kommunikation e. V. Magdeburg, Germany. Littlewood, K., Memon, F. A. & Butler, D. Downstream implications of ultra-low flush WCs. Water Pract. Technol. 2 (2). Maimon, A., Tal, A., Friedler, E. & Gross, A. Safe on-site reuse of greywater for irrigation – a critical review of current guidelines. Environ. Sci. Technol. 44, 3213–3220. Penn, R., Hadari, M. & Friedler, E. Evaluation of the effects of greywater reuse on domestic wastewater quality and quantity. Urban Water J. 9 (3), 137–148. Penn, R., Schütze, M. & Friedler, E. Modelling the effects of on-site greywater reuse and water-efficient toilets on municipal sewer systems. J. Environ. Manag. 114, 72–83. Rossman, L. A. Stormwater Management Model. User’s Manual. SWMM5.0. Environment Protection Agency, November 2004. Wat. Supp. & Wat. Resourc. Div., Nat. Risk Manage. Res. Lab., Cincinnati, OH, USA. US Environmental protection agency website: http://www.epa. gov/nrmrl/wswrd/wq/models/swmm/. Walski, T. M., Barnard, T. E., Harold, E., Merritt, L. B., Walker, N. & Whitman, B. E. Wastewater Collection System Modeling and Design. Haestad, Waterbury, Conn. Walski, T., Edwards, B., Helfer, E. & Whitman, B. E. Transport of large solids in sewer pipes. Water Environ. Res. 81 (7), 709–714. Walski, T., Falco, J., McAloon, M. & Whitman, B. Transport of large solids in unsteady flow in sewers. Urban Water J. 8 (3), 179–187.
First received 1 July 2013; accepted in revised form 10 September 2013. Available online 25 October 2013
© IWA Publishing 2014 Water Science & Technology
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Assessment of the effects of greywater reuse on gross solids movement in sewer systems R. Penn, M. Schütze and E. Friedler
ABSTRACT Onsite greywater reuse (GWR) and installation of water-efficient toilets (WETs) reduce urban freshwater demand and thus enhance urban water use sustainability. Research on GWR and WETs has generally overlooked their potential effects on municipal sewer systems: GWR and WETs affect the flow regime in sewers, and consequently also influence gross solids transport. To asses these impacts, a gross solids transport model was developed. The model is based on approaches found in the literature. Hydrodynamic calculations of sewage flow were performed using the SIMBA6 simulator and then used for the gross solid movement models. Flow characteristics in the up- and downstream sections of the sewer network differ. Therefore different approaches were
R. Penn (corresponding author) E. Friedler Faculty of Civil and Environmental Engineering, Technion – Israel Institute of Technology, Haifa 32000, Israel E-mail: [email protected] M. Schütze ifak- Institut für Automation und Kommunikation e.V. Magdeburg, Werner-Heisenberg-Str. 1, 39106 Magdeburg, Germany
used to model solids movement in each of these two parts. Each model determines whether a solid moves as a result of a momentary sewage flow, and if it moves, calculation of its velocity is possible. The paper shows the adoption and implementation of two gross solids transport models using SIMBA6 and depicts the results of the effects of various GWR and WET scenarios on gross solids movement in sewers for a real case study in Israel. Key words
| greywater reuse, gross solids movement, modeling, municipal sewers, simulation, water-efficient toilets
INTRODUCTION Greywater (GW) is generally defined as domestic sewage excluding the stream generated by toilets (blackwater). Kitchen wastewater (kitchen sink and dishwasher) is defined as dark GW and is often combined with the blackwater stream. GW streams generated by the bath, shower and washbasin are defined as light GW. On-site GW reuse (GWR) for toilet flushing and landscape irrigation is thought to be an efficient tool for reducing potable water demand, decreasing the need for developing new (and usually costly) water sources, and for enhancing sustainable urban water use. Onsite light GWR for toilet flushing can reduce daily household water consumption by 26% (Penn et al. ). Campisano & Modica () investigated reuse of washbasin GW for toilet flushing and found that ∼10–30% of the in-house water usage can be saved. Using the excess amount of the light GW (the remaining GW that is not reused for toilet flushing) for garden irrigation can further reduce the domestic water demand up to 41% of the baseline scenario, where no GW is reused (Penn et al. ). Nevertheless, in order to prevent health risks and to minimize negative aesthetic and doi: 10.2166/wst.2013.555
environmental effects, treatment of GW is necessary prior to reuse (Dixon et al. ; Diaper et al. ; Maimon et al. and others). Friedler () has shown that, as the demand for GW within the urban environment (i.e. for toilet flushing and garden irrigation) is lower than its production, it is not necessary to recycle all GW streams, but rather to focus on the less polluted light GW, and to discharge the more polluted dark GW stream together with the blackwater stream to the existing urban sewer system. Research on GWR to date has generally overlooked the effects that GWR may have on the municipal sewer sector. GW treatment and reuse changes the quantity and quality characteristics of domestic wastewater released to the sewer and eventually conveyed to the wastewater treatment plant (WWTP). This may lead to positive and negative effects that may influence sewer systems and WWTPs. Accordingly, one can ask what could be the effects on urban wastewater collection systems and on WWTPs? Are they positive or negative? How will they change with increasing implementation of on-site GWR systems?
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Assessment of the effects of GWR on gross solids movements
This paper aims to answer some of these questions by providing a quantitative assessment of the effects of GWR and water-efficient toilets (WETs) on gross solids transport in sewers, as their movement may play a role in creation of sewer blockages. A simulation model for gross solids movement in sewers was developed and used for analyzing the effects of six future GWR and WET scenarios on gross solids movement in a representative separate sewer system. Since in-sewer flow characteristics have a pronounced effect on gross solids movement, selected results from a previous study conducted by the authors on the effects of GWR on flow characteristics in urban sewer systems are briefly quoted.
METHODOLOGY Effects of GWR on the urban wastewater network The positive and negative effects of GWR and WETs were studied by modeling a representative, separate, urban sewer system of a densely populated neighborhood located in a coastal city in Israel, with an area of ∼1.3 km2 and some 15,000 residents living in flats. The sewer system is composed of 154 nodes and 153 links, having diameters ranging from 0.2 m (in upstream links) to 0.4 m (in downstream ones) and an overall length of about 6 km (further information on the case study site can be found in Penn et al. ()). The urban water system simulator used was SIMBA6 (Simulation Biologischer Abwassersysteme), developed by ifak (). SIMBA6 is an integrated simulator for sewer systems, WWTPs and river water quality. The hydrodynamic modeling modules of SIMBA, which are based on an extended version of the US-EPA SWMM (Rossman ; US Environmental Protection Agency website) were applied for simulating wastewater flow. These yield the full dynamic solution of the Saint Venant differential equations. It should be stressed that in this study, as stated above, a separate sewer system was modeled as this is the prevailing type of sewer systems in Israel. In a separate sewer system, pipe diameters are smaller than those found in a combined system, since they conveys only wastewater, compared with the latter which convey stormwater as well. Three combinations of GWR types (in single houses/ flats) were used as an input to the model:
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Type 1 (No GWR) – No GW recycling is practised and the combined domestic wastewater stream (incorporating discharges from all domestic sources) is discharged into the sewage system.
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Type 2 (GWR WC) – GW generated from the bath, shower and wash basin (i.e. light GW) is treated and used for toilet flushing. Excess light GW that is not used for toilet flushing is discharged to the sewage system as overflow, prior to treatment (as raw GW). Type 3 (GWR WC þ IRR) – Same as type 2, but the excess GW flow (after treatment) is used for irrigation.
The quantity of domestic wastewater discharge input to the model was defined by the appropriate diurnal hydrographs of wastewater discharged to the sewer by individual persons during weekdays (GWR type 1 or 2 or 3, as described above). Weekends exhibit different domestic wastewater generation behavior and were not modeled. Derivation of these hydrographs is described by Penn et al. (). Five GWR scenarios were modeled, simulated and analyzed. In each scenario, a different combination of GWR type households (as described above) was used: 1. 100% of the households implement GWR type 1 (no GWR) – the current situation in Israel. 2. 100% of the households implement GWR type 2 (GWR WC) – extreme implementation scenario. 3. 100% of the households implement GWR type 3 (GWR WC þ IRR) – extreme implementation scenario. 4. 70% of the households implement GWR type 1 and 30% of the households implement GWR type 2 – typical uptake to be expected with in two decades (Friedler ). 5. 70% of the households implement GWR type 1, 15% of the households implement GWR type 2 and 15% of the households implement GWR type 3 – typical uptake to be expected with in two decades. Toilet flush volumes in the above five scenarios were 9 and 6 L for full and half-flush, respectively. These flush volumes are mandatory for all toilets in Israel. For the examination of the effects of reduced toilet flush volumes, toilet flush volumes in scenarios 1, 2 and 3 were reduced to 6 and 3 L for full and half-flushes respectively, and the simulation was carried out again. These lower flush volumes are mandatory for all toilets installed in new buildings in Israel as of 2012. The scenarios were compared using the following parameters: flow, velocity, capacity (proportional depth) and Froude number. Flow and velocity have the largest effect on gross solids movement (as will be described later), therefore only their results are presented in this paper. Additional results regarding the other two parameters can be found in Penn et al. ().
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Gross solid movement We present the way that GWR and WETs affect the flow regime in sewers, which in turn has the potential to influence gross solids transport. Hence there is a need to assess the effects of reduced flows on gross solids transport in sewers. To do so, a gross-solids transport model has to be developed. Flow characteristics in the up- and downstream sections of the separate sewer network differ. In upstream segments, flow is intermittent and large un-submerged solids are more likely to be found. In the downstream part of the sewer, flow is continuous and smaller submerged solids can be found. Therefore, different approaches were used to model solids movement in each of the two sections of the sewer system. Various approaches for modeling solids movement in separate sewers were reviewed (Butler et al. a, b, ; Digman et al. ; Enfinger & Mitchell ; Walski et al. , ) and the most appropriate approaches were selected based on Walski et al. (, ). The selection was based on the following criteria: applicability, minimum requirement for coefficient values, simplicity and compatibility between conditions of the research reported for the development of the model and the characteristics of our case study. For the upstream part a model based on ‘critical’ flow/volume developed by Walski et al. () appeared to be most suitable, while for the downstream sections a model based on tractive force (shear stress) (Walski et al. ) was used.
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distinguishing between full and partial movement and 0.25 for distinguishing between partial movement and no movement (e.g. a solid will fully move if the wastewater flow is higher than the calculated flow in Equation (1) where a equals 0.45). Values of a were determined by trial and error (Walski et al. ). The movement of standard solids as a result of flows from short events like toilet flushing and hand washing was also examined (Walski et al. ). For runs of short duration (less than 20 s) with an attenuation section, the volume of the pulse proved to be the controlling parameter, and the following equation was derived: V ¼ aSG=S0:2
(2)
where V is the volume of pulse (L) and a equals 18 for distinguishing between full and partial movement and 10 for distinguishing between partial movement and no movement. Values of a were determined by trial and error (Walski et al. ). Six scenarios were simulated: 1, 2 and 3 with toilet flush volumes of 9 and 6 L for full and half-flushes respectively (NO GWR96, GWR WC96 and GWR WC IRR96, respectively), and three additional scenarios, which are the same as 1, 2 and 3 above but with reduced toilet flush volumes of 6 and 3 L for full and half-flushes respectively (NO GWR63, GWR WC63 and GWR WC IRR63, respectively). Downstream reaches
Upstream reaches Walski et al. (), examined the transport of large solids in sewers functioning under unsteady flow conditions. They tested the movement of standard solids created from different materials, and of several household solids, which included disposable diapers, cotton rags and feminine hygiene pads. They found that the specific gravity of the solid, the slope of the pipe and the flow rate were the most important parameters in determining whether a solid moves or not. They derived Equation (1) to distinguish between three types of movement (full movement, creeping movement and no movement): Q ¼ aSG=S0:2
(1)
where SG is the specific gravity of the solid; S is the slope of pipe; Q is the ‘critical’ flow (L/s) and a equals 0.45 for
As mentioned above, modeling solids movement in the downstream sections of the sewer network was based on the shear stress (tractive force) model by Walski et al. (). This approach is based on the average shear stress acting on the sewer pipe walls and the critical shear stress needed to move dense discrete particles that lie on the bottom of the pipe. Movement of a solid will occur if the average shear stress is higher than the critical one. A baseline assumption in this approach is that the solid is fully submerged. The calculation is based on a design discrete grit particle (specific gravity of 2.7) that is transported often enough so it does not accumulate to form a cohesive layer along the pipe invert. All other solids are considered to be more easily transported. Average boundary shear stress In open channel flow, the average boundary shear stress exerted by flowing fluid on the channel (sewer pipe in our
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RESULTS AND DISCUSSION
case) can be expressed as Equation (3): (3)
Effects of GWR on the urban wastewater sewer system
where τ is the shear stress (Pa), ρ is the density of liquid (kg/m3), R is the hydraulic radius (m) and S is the slope of pipe. An alternative approach would be to use the slope of the energy instead of the slope of the pipe, which is more suitable for unsteady flow conditions. This will be investigated in future publications.
Figure 1 presents the diurnal patterns of the flow and velocity. Each graph presents an individual pipe segment: one in the middle (link 97) and one downstream (link 154) of the sewer network (other links in the sewer showed similar trends). In each graph, the results of five simulated scenarios are shown (all with toilet flush volumes of 9 and 6 L). Moving down the sewer system, the flow increases as expected (more households contribute wastewater to the system), and attenuation (i.e. reduction of the peak flow by redistributing the same volume of flow over a longer period of time) and a time shift of the peak flows occur. The maximum flow occurs at 8:04 a.m. and at 9:01 a.m. in links 97 and 154 respectively (Table 1). The velocity in the pipes is influenced by the flow and geometry of the pipes; hence moving down the sewer system, no unambiguous pattern of velocity increase/decrease was observed. As GWR increases, the flow (discharge) and velocity decreases. This decrease is not evenly distributed through the day, but occurs mainly during the morning peak discharge period. As shown in
τ ¼ ρgRðS=100Þ
Critical shear stress The critical shear stress needed to move dense discrete particles (quartzite sand) as a function of particle size may be expressed as follows (Walski et al. ): τ c ¼ kd0:277
(4)
where τc is the critical shear stress (Pa), k equals 0.867 and d is the nominal diameter (mm) for a discrete sand particle of specific gravity 2.7 (the design particle).
Figure 1
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Diurnal flow and velocity patterns in two selected sewer pipes for the five GWR scenarios examined. Each column represents a pipe segment (link) along the sewer network (97, middle section; 154, downstream section). NO RU – no GWR; WC – GWR for toilet flushing; WC IRR – GWR for toilet flushing and garden irrigation; 70 30 – 70% NO RU, 30% WC; 70 15 15 – 70% NO RU, 15% WC, 15% WC IRR. Adopted from Penn et al. (2013).
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Table 1
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Links 97 and 154 – geometry, and timing and value of maximum flow (scenario 1) Pipe diameter (mm)
Slope (%)
Time of maximum flow (h)
Maximum flow (m3/min)
Link 97
315
1.11
8:04
2.6
Link 154
400
1.23
9:01
4.5
Figure 1 a significant reduction occurs mostly in scenario 3 (all households GWR WC þ IRR), an extreme situation that is unlikely to happen. The simulation further revealed that the maximum velocity in most scenarios was higher than the minimum velocity required for solids movement, which lies in the range of 0.6–1.0 m/s (Walski et al. ). Exceptions were found in scenario 3 which, as previously mentioned, is a situation not likely to occur. Examination of the effects of reduced toilet flush volumes (results can be found in Penn et al. ()) revealed that in some of the GWR scenarios flows and velocities in the sewer were reduced, while in others they did not change. The above results indicate that as a result of GWR and installation of WETs, sewer blockage rates are not expected to increase significantly. However, this should be further examined as explained below.
Gross solid movement Upstream reaches Figure 2 presents three pipe segments, namely: link 36 (located upstream); link 42 (located some distance downstream from link 36, with no extra wastewater being discharged along the way); and link 48 (located in the middle of the sewer system,
Figure 2
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where additional wastewater is discharged into the sewer along the way. Diurnal flow patterns in these three pipe segments are presented, and the dividing lines for full, partial and no movement, for a solid with a specific gravity of 1.06 are shown by horizontal lines. 1.06 is the specific gravity of a typical NBS solid, which is the US National Bureau of Standard artificial fecal solid (Brown et al. .). One can see that, in link 36 (upstream) for the scenario having the highest GWR and the lowest toilet flush volumes (GWR WC IRR63), the conditions of no movement of the solid occur for most of the day (76%, 18.2 h), partial movement occurs for 22% of the day (5.3 h) and the smallest duration is of conditions enabling full movement (2% of the day, 0.5 h). Whereas in scenario 1 (no GWR and flush volumes of 6 and 9 L), the situation is reversed with 28, 39 and 33% of the day with conditions enabling full, partial and no movement, respectively. Going downstream, as long as no additional wastewater is discharged to the sewer (link 42), similar patterns and proportions are observed (minor differences depicted result from differences in pipes’ slopes and diameters). It is worth mentioning that even in this ‘sewage ample’ situation, for as much as 33 and 35% of the day (links 36 and 42, respectively), gross solids will not move in the upstream segments of the sewer system. When additional households discharge their wastewater to the sewer (link 48), the duration of full movement increases, and consequently, the duration of partial and no movement conditions decreases. This difference is especially significant for the lowest wastewater discharge scenario (GWR WC IRR63); where in link 48, for 78% of the day conditions for full movement prevail, while in links 36 and 42 they prevail only for 2% of the day. As mentioned previously, this method is suitable only for upstream
Diurnal flow pattern in three sewer links (link 36: upstream, link 48: downstream) as a result of the simulation. The horizontal lines represent the boundary between full and partial movement (upper) and partial and no movement (lower). Values in the rectangles represent the proportion of the day for each type of movement (upper, full movement; middle, partial movement; lower, no movement). All relate to a solid having a specific gravity of 1.06.
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Diurnal shear stress patterns in three sewer links (link 97: upstream, link 154: downstream) as result of the simulation. Horizontal lines: critical shear stress for a 6 and 1 mm (diam.) gross solids (upstream and downstream respectively). Rectangles: portions of the day in which the average shear stress is higher/lower than the critical (upper: τ > τc for d ¼ 6 mm; middle: τc for d ¼ 1 mm < τ < τc for d ¼ 6 mm; lower: τ < τc for d ¼ 1 mm).
links where the flow is intermittent and thus not suitable for link 154 which is located downstream. These results reveal that under extreme GWR and WETs, sewer sections located in the uppermost sections of the network may suffer from higher blockage rates, as the transport of gross solids will be slower than under the current situation (no GWR). Nevertheless, Littlewood et al. () have shown that for ultra-low flush toilets (and thus decreased wastewater flows) decreasing the sewer diameter would enhance solids movement. Their suggestion falls in line with Penn et al. () who demonstrated that under extreme GWR and WET scenarios, the diameters of sewer pipes could be decreased by 20%. Downstream reaches The diurnal pattern of the average boundary shear stress and the constant critical shear stress (constant since it is a function of the design solid diameter and not the wastewater flow characteristics) in links 97, 71 and 154 are depicted in Figure 3. These links are located in the downstream section of the sewer system studied, where link 154 is the furthest downstream and links 71 and 97 are located above it. As solids move along the sewer system they undergo disintegration. Hence, solids diameters simulated in this section were smaller than the diameter of the NBS solid. The diameters considered were 6 mm (a typical gross solid diameter, taken from Butler & Davies ()) and 1 mm (expected diameter for solids found downstream). Wastewater density was taken to be equal to the density of water (1,000 kg/m3). Further shown in Figure 3 are the portions of the day during which the average shear stress is greater than the critical stress required for the movement of 6 mm diameter solids; the proportion of the day during which the average shear stress is between the critical shear stress required for
the movement of 6 mm solids and that required to move 1 mm diameter solids; and the portion of the day during which the average shear stress is lower than the critical stress required to induce movement of solids having a diameter of 1 mm (numbers in rectangles from top to bottom respectively). These portions are shown for the two extreme scenarios simulated: scenario 1 having the largest wastewater discharges, i.e. no GWR at all and toilet flush volumes of 9 and 6 L; and scenario 6 with the lowest wastewater discharges, i.e. all households reuse GW for toilet flushing and garden irrigation and toilet flush volumes are 6 and 3 L for full and half-flush respectively. It can be seen that downstream (in link 154) full movement should be achieved throughout the day, in all scenarios simulated, for a reasonable sized gross solid. This is in contrast to the findings for the upstream sections. Also in links located upstream from this link but downstream in the network (links 97 and 71), except during small portions of the day, full movement for a reasonable sized solid, will be achieved. For example, in link 97 in scenario 1 (NO GWR 9/6), during 93% of the day solids of a 6 mm diameter (or smaller) will move, while during 7% of the day solids with a diameter of 6 mm (or higher) will not move. In scenario 6 (GWR WC IRR 6/3), which is the most extreme scenario, the portion of the day in which 6 mm (or smaller) solids will move is reduced to 83% and during 17% of the day solids with diameter of 6 mm (or higher) will not move. In all scenarios full movement of 1 mm (or smaller) solids was achieved throughout the day.
CONCLUSIONS AND OUTLOOK This research quantified the expected effects of changes in urban wastewater quantity as a result of GWR and
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implementation of WETs on gross solid movement in sewers. It was shown that in upstream links, with small amounts of wastewater discharged when there is no GWR, for most of the day full or partial movement of the solids occurs (67%), whereas when GW is being reused the situation reverses, with longer duration of no movement of solids (∼76% of the day). However, down the sewer system, when additional households are connected, and hence additional wastewater is discharged to the sewer, an increase of the proportion of the day for full movement occurs, while in downstream links, full movement of the solids in all the scenarios can be expected, since the average boundary shear stress was higher than the critical one throughout the day. This implies that the blockages rate in the uppermost sections of the sewer system may increase if there is very high uptake of GWR and WETs. However, the problem could be averted by constructing narrower pipes in these reaches. Intelligent implementation of the results can support the planning of future sewer systems in which GW would be reused on a large scale, and thus contribute to sustainable reuse of precious water resources. Currently, work is underway to create and integrate a ‘solids generator’ module, ‘discharging’ solids into the sewer with the simulator developed here.
ACKNOWLEDGEMENTS This research was supported by a grant from the Ministry of Science & Technology of the State of Israel and the Federal Ministry of Education and Research (BMBF) of the Federal Republic of Germany (Grant reference No. 02WA1263). The authors further wish to thank the Netanya Water Corporation for their cooperation.
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Butler, D., Littlewood, K. & Orman, N. A model for the movement of large solids in small sewers. Water Sci. Technol. 52 (5), 69–76. Campisano, A. & Modica, C. Experimental investigation on water saving by the reuse of washbasin greywater for toilet flushing. Urban Water J. 7 (1), 17–24. Diaper, C., Dixon, A., Butler, D., Fewkes, A., Parsons, S. A., Stephenson, T., Strathern, M. & Strutt, J. Small scale water recycling systems – risk assessment and modelling. Water Sci. Technol. 43 (10), 83–90. Digman, C. J., Littlewood, K., Butler, D., Spence, K., Balmforth, D. J., Davies, J. & Schuetze, M. A model to predict the temporal distribution of gross solids loading in combined sewerage systems. In Ninth International Conference on Urban Drainage, Portland/Oregon, USA. Dixon, A. M., Butler, D. & Fewkes, A. Guidelines for greywater re-use: health issues. Water Environ. Manag. 13 (5), 322–326. Enfinger, K. L., P. E. Mitchell, P. S., P. E. Scattergraph principle and practice. Evaluating self-cleansing in existing sewers using the tractive force method. ADS Environ. Services 1–10. Friedler, E. Quality of individual domestic greywater streams and its implication on on-site treatment and reuse possibilities. Environ. Technol. 25 (9), 997–1008. Friedler, E. The water saving potential and the socio-economic feasibility of greywater reuse within the urban sector – Israel as a case study. Int. J. Environ. Studies 65 (1), 57–69. ifak SIMBA6 – Simulation of Wastewater systems, Reference and Tutorial. ifak – Institut für Automation und Kommunikation e. V. Magdeburg, Germany. Littlewood, K., Memon, F. A. & Butler, D. Downstream implications of ultra-low flush WCs. Water Pract. Technol. 2 (2). Maimon, A., Tal, A., Friedler, E. & Gross, A. Safe on-site reuse of greywater for irrigation – a critical review of current guidelines. Environ. Sci. Technol. 44, 3213–3220. Penn, R., Hadari, M. & Friedler, E. Evaluation of the effects of greywater reuse on domestic wastewater quality and quantity. Urban Water J. 9 (3), 137–148. Penn, R., Schütze, M. & Friedler, E. Modelling the effects of on-site greywater reuse and water-efficient toilets on municipal sewer systems. J. Environ. Manag. 114, 72–83. Rossman, L. A. Stormwater Management Model. User’s Manual. SWMM5.0. Environment Protection Agency, November 2004. Wat. Supp. & Wat. Resourc. Div., Nat. Risk Manage. Res. Lab., Cincinnati, OH, USA. US Environmental protection agency website: http://www.epa. gov/nrmrl/wswrd/wq/models/swmm/. Walski, T. M., Barnard, T. E., Harold, E., Merritt, L. B., Walker, N. & Whitman, B. E. Wastewater Collection System Modeling and Design. Haestad, Waterbury, Conn. Walski, T., Edwards, B., Helfer, E. & Whitman, B. E. Transport of large solids in sewer pipes. Water Environ. Res. 81 (7), 709–714. Walski, T., Falco, J., McAloon, M. & Whitman, B. Transport of large solids in unsteady flow in sewers. Urban Water J. 8 (3), 179–187.
First received 1 July 2013; accepted in revised form 10 September 2013. Available online 25 October 2013
