Environ. Sci. Technol. 2004, 38, 1208-1215
Effects of Separate Urine Collection on Advanced Nutrient Removal Processes J. A. WILSENACH* AND M. C. M. VAN LOOSDRECHT Kluyver Laboratory for Biotechnology, Delft University of Technology, Julianalaan 67, 2628 BC, Delft, The Netherlands
Municipal wastewater contains a mixture of minerals from different origins. Urine contributes 80% of the nitrogen (N) and 45% of the phosphate (P) load in wastewater. Effects of separate urine collection on BNR processes were evaluated by using a simulation model for an existing stateof-the-art biological nutrient removal process. It was found that increasing urine separation efficiency leads to lower nitrate effluent concentrations, while ammonium and phosphorus concentrations remain more or less the same. The improved nitrate effluent quality is most notable up to 50-60% urine separation. Urine separation allows primary sedimentation without an increase in the nitrate effluent concentration. Furthermore, urine separation increases the potential treatment capacity for raw and settled wastewater by 20% and 60%, respectively. Urine separation provides options for increasing the lifetime of existing treatment works.
Introduction Stormwater sewers were originally constructed to prevent flooding of urban areas. At the same time, human excreta had to be dealt with to ensure urban hygiene and health. Stormwater sewers were subsequently used for the transport of excreta and later became known as combined sewers. In today’s industrialized cities, sewers are still regarded as the most efficient infrastructure for the transport of fecal matter and other wastes (1). Wastewater is therefore a concoction of domestic effluent, rainwater, and some industrial effluent that cause many other problems besides transport. Pathogens cause water borne diseases. Organic carbon, measured as chemical oxygen demand (COD), and ammonium both cause oxygen depletion in surface waters. Ammonium is also toxic, and emissions of nitrous (N) and phosphorus (P) substances lead to eutrofication. Over the years, conventional wastewater treatment works have been adapted and advanced to comply with the demands of increasing effluent quality. Nowadays, suspended solids and COD can be removed easily from wastewater, while waterborne diseases practically do not occur in Northern Europe any more. Although N and P removal are more complex, advanced biological nutrient removal (BNR) plants produce good effluents. Relative to earlier improvements, further improvement of the effluent quality and removal of yet untreated pollutants (such as heavy metals in sludge, hormones, and pharmaceutical residues) would escalate the costs and energy demand of wastewater treatment. If this problem is ignored, the environmental impacts of advanced * Corresponding author phone: +31 (0)15 278 23 63; fax: +31 (0)15 278 23 55; e-mail;
[email protected]. 1208
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wastewater treatment (energy consumption and CO2 production) could annul the benefits of good effluent quality (2). Sewers and central treatment plants are subject to increasing criticism. Objections against the use of water as transport medium are based on aspects of sustainability, and alternative concepts, e.g., “ecological sanitation” (3) are promoted. In 1997, a complete edition of Water Science and Technology (4) was dedicated to innovative concepts of sustainable sanitation and wastewater management. However, “sustainability” and “sustainable wastewater treatment” remain vague and subjective terms. Because innovation in itself is not a guarantee for sustainability, new concepts must be compared with existing technology. Typically, one would expect the comparison to focus on quantities of total emissions, land use, overall resource depletion (minerals and energy), and economic costs. Larsen and Gujer (5) argued that anthropogenic boundary conditions must be defined clearly before new concepts are developed (i.e., first investigate the required service before fixing the mind on the stepwise improvement of some favored technique). Consistent with this approach, one should identify the bottlenecks that hinder improvement of the overall wastewater system. The two most obvious and important bottlenecks have to be solved first: (i) Nitrogen removal determines the size of BNR plants, requires around one-half of the total energy demand (for aeration), and requires organic carbon for denitrification. (ii) Phosphate is a finite resource which is normally not recovered in central treatment plants. Chemical removal with iron fixates phosphates in a form that is not usable. However, different processes exist to recover phosphorus at central wastewater treatment plants from side streams or directly (6). This is normally not done, mainly because of high costs. Another problem with phosphate recycling is the presence of heavy metals. At least 80% of the total nitrogen load and 45% of the total phosphate load in municipal wastewater originate from urine (7-9). This fact suggests that separate collection and treatment of urine would eliminate one of the bottlenecks in wastewater treatment. Experience from ecological villages in Sweden and other pilot projects showed that urine can be collected separately (8, 10). One expects urine separation systems to become more efficient as more of these projects are commissioned and operated. Private and public interest is stimulating manufacturers of sanitary ware to improve the technology for no-mix toilets and waterless urinals. Urine contains no heavy metals, which make the recovery and direct use of nutrients as fertilizer more attractive. However, we believe that the purpose of urine separation is unlikely to be nutrient recovery only. The potential contribution from human urine in Northern Europe is limited in comparison to the recovery potential from animal husbandry. If wastewater containing little or no urine permits small and simple wastewater treatment processes, which at the same time improves the effluent quality (11), then the recovery of minerals from urine would be an added benefit. Another benefit of urine separation could be separate treatment of pharmaceutical residues and hormones, which originate largely from urine (12, 13). We did not investigate this any further at this stage. In a previous study (14) we gave an overview of consequences of urine separation on wastewater treatment. In general, one could expect a decrease in total nitrogen effluent concentration with increasing urine separation efficiency. In this study, we investigated different scenarios of urine 10.1021/es0301018 CCC: $27.50
2004 American Chemical Society Published on Web 01/15/2004
FIGURE 1. Schematic representation of the BCFS process for biological/chemical phosphate and nitrogen removal.
TABLE 1: Actual Influent and Effluent Concentrations at Hardenberg WWTW, Compared to Simulated Effluent Concentrations with Actual Influent as Well as with Average Dutch Influent at Maximized Flow Rate parameter
unit
actual influent, 1998
actual effluent, 1998
simulated effluent
average Dutch influent
simulated effluent
ref flow rate CODtot Ntot NH4+-N Ptot
(m3/day) (g/m3) (g/m3) (g/m3) (g/m3)
20 6 900 625 60 51 9.5
20 6 738 42 5.2 0.3 0.2
20 6 636 40 4.0 0.4 0.4
9 15 250 537 50 40 8
this study 14 759 39 5.7 0.9 0.3
separation and primary sedimentation to increase the performance of an existing BNR plant. To this end, we used a calibrated model for a state-of-the-art BNR process. With regard to urine separation, we discussed the modeling aspects of various scenarios and predicted increases in treatment capacity and effluent quality.
TABLE 2: Composition of Urine and Normal Wastewater Load (including urine) per Person (9)
urine wastewater
nitrogen (g of N/p‚d)
phosphate (g of P/p‚d)
COD (g of COD/p‚d)
vol. (L/p‚d)
12 15
1.0 2.4
12 161
36 300
Method Process and Model Description. The BCFS process (Dutch acronym for Biological Chemical Phosphate and Nitrogen removal) is schematically represented in Figure 1. This process functions according to the basic principles of the modified UCT process for biological nitrogen and phosphate removal. A complete description of the BCFS process is given in ref 15. The Hardenberg wastewater treatment works in The Netherlands is a good example of an operating BCFS process and was used as reference for this study. The total volume of the five compartments is 10 000 m3, and the clarifier volume is 2 800 m3. The BCFS process had already been modeled by combining the metabolically structured bio-P removal model (16) with parts of the activated sludge model ASM2d for COD and N removal (17). This model had previously been calibrated and used to simulate several full-scale processes successfully (18-20). It could be extrapolated to full-scale systems without further calibration (20). We used the computer software package AQUASIM 2.0 (21) to implement the dynamic simulation of the BCFS process. Important Model Parameters. The total suspended solids concentration (TSS) was 5 kg/m3. The dimensions of the existing biological reactor and clarifier at Hardenberg were based on this maximum TSS concentration. Operation at constant TSS is standard practice for existing plants. The ash content of the sludge was assumed constant. Sludge treatment and the effects of supernatant return flows were not considered. The sludge volume index (SVI) of BCFS processes is normally below 120 mL/g. Very low effluent concentrations of suspended solids (TSS < 5 g/m3) can be maintained. Recycle flow rates were defined in terms of the influent flow rate, so that an increase in influent flow rate would automatically increase the recycle flows proportionally. The
ratio recycle:influent flow rate is shown in brackets below each recycle flow in Figure 1. An average wastewater temperature of 12 °C is common for winter in The Netherlands. However, North America, Scandinavia, and Central Europe could expect winter temperatures of 8 °C (22, 23). We compared the treatment performance by doing simulations at temperatures of 12 and 8 °C for all cases. This study was done to obtain reference values on a national level for The Netherlands. We therefore used average Dutch influent concentrations (9). The difference between these concentrations and the actual wastewater in Hardenberg (20) is shown in Table 1. The concentrations used here are typical for somewhat diluted wastewater but do not differ too much from the measurements at Hardenberg. All simulations were done for the steady state after 300 days of operation, without variation in influent flow rate or concentrations. Effects of peak loads were not considered here. Reference Simulation. The maximum influent flow rate at zero urine separation was determined by iteration to be 15 250 m3/day at 12 °C (SRT ) 12 days) and 10 250 m3/day at 8 °C (SRT ) 18 days). This represents the wastewater produced by around 51 000 and 34 000 people, respectively. Up to these flow rates, the ammonium effluent concentration was maintained below 1 g of N/m3 by increasing the dissolved oxygen concentration in the mixed reactor (which can be used as a combined anoxic/aerobic reactor) from 0.5 to 2.0 g of O2/m3. Actual and simulated effluent concentrations are shown in Table 1. Urine Separation. We investigated different scenarios with increasing urine separation percentages from 0% to 100%. Decreases in volume and mass loads were considered. VOL. 38, NO. 4, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3: Influent Fractions of COD and N (g/m3) Associated with Soluble and Particulate Substrate in Raw Wastewater (26)
COD organic N
total
soluble acetate
soluble fermentable
soluble inert
particulate inert
particulate slowly degradable
536 10.4
64 0
70 0.7
32 0.3
134 4.0
236 5.4
Currently, around 35 L/p‚d is used to flush urine, including toilet visits away from home (9, 10). Waterless urinals and no-mix toilets require almost no flushing water. We assumed that wastewater discharge to the treatment works could be reduced by 36.25 L/p‚d (including 1.25 L/p‚d urine) at 100% urine separation. The decrease in wastewater volume from 0% to 100% urine separation would be linear. The relative loads (N, P, and COD) in urine and wastewater (including urine) are shown in Table 2 and described in more detail below. Nutrient loads in urine were based on data from urology and expressed per person. We therefore defined all loads and flow rates per person and not per population equivalent (PE). The relation of nutrient loads in urine (per person) to the total load (per PE) would have been somewhat confusing, because the amount of PEs is normally higher than the actual population. The total nutrient load in separated urine is the product of the daily load in an average person’s urine, the separation percentage, and the total number of people. This load was subtracted from the total load in normal wastewater. The mass of this remaining load was divided by the reduced flow rate at the same separation percentage to determine influent concentrations at different percentages. Nitrogen in urine is mainly present as urea, CO(NH2)2, which rapidly hydrolyzes to NH4+ and HCO3- in wastewater (24). As most of the sewers in The Netherlands are flat and retain wastewater for 8-12 h, we assumed that nitrogen in urine would reach the wastewater treatment works as ammonium (25). It was further assumed that 12 g of N/p‚d (out of the total 15 g of N/p‚d in wastewater) is excreted via urine (9). The soluble ammonium influent concentration is then 40 g of N/m3 at zero urine separation. The additional nitrogen loads from sources other than urine are bound to influent COD and were divided as shown in Table 3. The default value was used for the N content of inert particulate material, while the other fractions were lowered slightly to yield a sum equal to 10 g of N/m3, so that the total influent concentration (50 g of N/m3) is obtained. Phosphates in the influent were all assumed soluble, and variables assigning P contents to the different COD fractions were all set to zero for simulations. Although this is strictly speaking not correct, at normal SRT values hydrolysis of organic matter makes this a reasonable assumption. This leads to an underestimation of phosphate removal, especially at high SRTs. This conservative approach was chosen because fractionation of phosphate bound in COD is not accurate. The inflow concentration of soluble phosphate was assumed 8 g of P/m3 in wastewater with zero urine separation. The phosphate load from urine is 1 g of P/p‚d, out of the total 2.4 g of P/p‚d in wastewater (9). The total COD influent concentration was 537 g/m3 (9). The COD fractions were divided into different model components as shown in Table 3 (26). Separate collection of urine will lead to a small decrease in the soluble COD load discharged to wastewater treatment works. The COD load in urine was assumed to be 12 g of COD/p‚d (27), while the total load in municipal wastewater was 161 g of COD/p‚d. It was further assumed that 88% of the COD in urine is readily available substrate and 12% is inert (28). Primary Sedimentation. The combined effects of urine separation with presettling and preprecipitation were investigated. Presettling of wastewater removes most particles 1210
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with a nominal size of more than 65 µm, i.e., particulate COD (29, 30). With presettling, up to 60% of the particulate COD can potentially be removed (22). Presettling was modeled by multiplying particulate influent COD fractions by a factor of 0.4. Smaller particles (down to approximately 25 µm) have to be entrapped in flocs, before they can be removed. With preprecipitation, 90% of the particulate COD can potentially be removed by addition of cationic organic polymers (29, 30). Preprecipitation was modeled by multiplying particulate influent COD fractions by a factor of 0.1. Nitrogen and phosphate would also be removed from the influent wastewater with primary sedimentation. The nitrogen removal is incorporated in the model, in which nitrogen influent values are directly related to the lowered influent values of particulate influent COD fractions. For phosphate, it was assumed that 15% and 25% of the total P in raw wastewater could be removed by presettling and preprecipitation, respectively. This was modeled by simply subtracting 15% (or 25%) of 8 g of P/m3 from the value of soluble phosphate after urine separation was taken into account. Chemical Phosphate Removal. The effect of urine separation on the use of ferric chloride (FeCl3) to precipitate iron phosphate and inorganic sludge was investigated. For an effluent quality of between 0.3 and 0.5 g of Ptot/m3, a molar ratio of Fe:P ) 2 is required based on the influent phosphate load (23). This results in chemical precipitation of FePO4 and a secondary product Fe(OH)3. The precipitated inorganic sludge mass is then around 19 g/p‚d, based on molar mass. Increased Treatment Capacity. The potential increases in the treatment capacity of an existing plant (i.e., increased number of people connected) due to urine separation were investigated. At the maximized flows for zero urine separation (15 250 m3/day at 12 °C and 10 250 m3/day at 8 °C), ammonium and total nitrogen effluent concentrations were the same for both temperatures. These effluent concentrations were taken as reference values (NH4 ) 1 g of N/m3 and Ntot ) 5.7 g of N/m3). The increase in flow rate was determined through iteration. Total suspended solids in the reactor were kept constant (TSS ) 5 kg/m3), and the reference effluent concentrations were not exceeded. With increasing influent flows, the SRT was decreased to maintain constant total suspended solids. In the case of raw wastewater with urine separation, the volume of the anoxic zone was decreased while the mixed zone was increased with the same volume to add to the nitrification capacity. In the case of presettled wastewater, the capacity was increased through the decrease of SRT only. The increase in treatment capacity (L/person) was compared to the capacity of the reference scenario. This comparison included the volume of primary sedimentation tanks and clarifiers. We assumed constant up flow velocities of 1.5 m/h for primary settling and 0.75 m/h for clarifiers (based on constant TSS and SVI), with vertical wall depths of 4 m (22).
Results Raw Wastewater. In general, the effluent quality of a BNR plant treating raw wastewater would improve with increasing
FIGURE 2. Effect of urine separation on effluent ammonium concentration in a BCFS process at different temperatures. percentages of urine separation (14). This statement holds for total N and P removal. Figure 2 illustrates the effect of increasing urine separation on the ammonium effluent concentration at different temperatures. For these simulations, the maximized influent flow of 15 250 m3/day (for wastewater at 12 °C) and constant sludge age of 12 days was used for 12, 10, and 8 °C. The ammonium oxidation rate increases exponentially with increasing temperature. At a low temperature and low sludge age, washout of ammonia oxidizers could easily occur. This is seen in the big difference between ammonium effluent concentration at 10 and 8 °C, compared to the relatively small difference between 12 and 10 °C. Figure 2 also shows that for 8 °C and above 60% urine separation, the ammonium effluent concentration is governed by the decrease in the ammonium influent concentration rather than nitrification. If the maximized influent flow for the treatment process at 8 °C (10 250 m3/day) is used instead of 15 250 m3/day, the profile of the ammonium effluent concentration with increasing urine separation is identical to that for 12 °C shown in Figure 2. Phosphate removal and denitrification processes are much less sensitive to temperature changes. Except for determining the treatment capacity, temperature differences do not change the effects of urine separation on the BCFS process. It seems paradoxical that lower ammonium influent concentrations led to higher ammonium effluent concentrations. However, the concentration of autotrophic organisms (nitrifiers) in a reactor at constant SRT increases with an increase in their influent substrate load. A linear decrease in the concentration of nitrifiers was observed with increasing urine separation. The decrease in nitrifiers led to the slight increase in ammonium effluent concentration. Around 24% of the total nitrogen influent load is removed with excess sludge at zero urine separation. With increasing urine separation percentages, this removal becomes relatively more important. At 90% separation efficiency, almost all the influent N is removed by heterotrophic growth. Figures 2-4 show the urine separation efficiencies between 0% and 90%. At higher urine separation, too little ammonium is available for heterotrophic growth. This leads to a sudden increase in effluent COD and complete process failure. This is strongly related to the specific model parameters assumed. In reality, this might result in selection of different organisms with lower nutrient requirement. Furthermore, in reality, supernatant with a high ammonium concentration is normally returned to the head of works.
This is of little practical importance because the combination of high urine separation efficiencies and conventional treatment plants is highly unlikely. Presettled Wastewater. The effects of urine separation on effluent quality of a BCFS process treating presettled wastewater were simulated. The flow rates, total suspended solids concentration, aeration capacity, temperature, etc., were equal to that of simulating treatment with raw wastewater. The reduction in the influent COD concentration allowed an increase in the sludge age to 28 days, which increases the nitrification capacity. The concentration of autotrophic organisms increased to 127 g of COD/m3, compared to 86 g of COD/m3 with raw wastewater (both at zero urine separation). With zero urine separation and influent flow of 15 250 m3/day, the effluent ammonium concentration was 0.25 g of N/m3 at 12 °C and 0.40 g of N/m3 at 8 °C. There was a linear decrease in the concentration of autotrophic organisms with increasing urine separation. The ammonium effluent concentrations at 90% urine separation were 0.65 g of N/m3 (12 °C) and 0.88 g of N/m3 (8 °C). The influence of temperature on nitrification is clearly much less than with raw wastewater due to the increased sludge age (28 days). Effluent concentrations alone might not give the true picture with regard to the removal efficiency. Figure 3 illustrates the combined effects of urine separation and presettling on different nitrogen effluent loads. For all scenarios and for different temperatures, the ammonium effluent loads were below 1.5% of the total N production (763 kg of N/day, including urine). A decrease in denitrification capacity could be expected for treatment of presettled wastewater. The total nitrogen effluent load was 84 kg of N/day with raw wastewater and 108 kg of N/day with presettled wastewater at zero urine separation. Figure 3 shows that the main contribution to the total nitrogen effluent load is the nitrate effluent load. Still, compared to the total N production, this is a good effluent quality (86% N removal). At 50% urine separation, the total nitrogen effluent load for presettled wastewater (38 kg of N/day) equaled that of raw wastewater at 50% urine separation. This is 95% N removal, based on the total N production. At urine separation efficiencies of 75% or more, almost all nitrogen is removed. However, if we base the total N removal efficiency on the actual influent N load (i.e., without urine), there is little difference at different urine separation efficiencies. Furthermore, the amount of N2 produced, in comparison with VOL. 38, NO. 4, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Decrease in effluent nitrogen loads in a BCFS process treating presettled wastewater, with total influent load of 763 kg of N/day.
FIGURE 4. Decrease in effluent nitrogen loads in a BCFS process treating preprecipitated wastewater, with total influent load of 763 kg of N/day. the actual influent load, decreases exponentially with increasing percentages of urine separation. Preprecipitated Wastewater. The effects of urine separation on effluent quality of a BCFS process treating preprecipitated wastewater were simulated. Due to the further reduction of COD load with preprecipitation (compared to presettled wastewater), the sludge age was increased to 90 days to maintain a TSS of 5 kg/m3. Simulations at a sludge age of around 50 days (TSS ) 3.5 kg/m3) showed that there is little difference in the ammonium and total nitrogen effluent concentrations between sludge ages of 90 or 50 days. However, with a SRT of 50 days, less N2 is produced and more nitrogen is removed with the excess sludge. With a SRT of 90 days, less nitrogen is removed with excess sludge but through increased hydrolysis of activated sludge more N2 is produced. The main consequence of a very high sludge age is the large increase in the portion of dead cell matter (inert particulate COD) in the sludge. Figure 4 illustrates the combined effects of urine separation and preprecipitation on the N effluent load. The autotrophic population increased to 163 g of COD/m3. Almost all the available ammonium was oxidized. The total nitrogen effluent load with zero urine separation was 146 kg of N/day or 81% N removal. However, with 1212
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increasing urine separation, the total nitrogen effluent load was greatly reduced. At 50% urine separation, the total nitrogen effluent was 43 kg of N/day. Again, as with presettled wastewater, the contribution of N2 to the total nitrogen removal decreased exponentially with increasing urine separation. Improved Effluent Quality with Primary Sedimentation. In cases of presettling and preprecipitation discussed above, the dissolved oxygen concentration in the mixed reactor was maintained at 2 g of O2/m3. It is clear from Figures 3 and 4 that almost all ammonium was nitrified with presettled and preprecipitated wastewater. The dissolved oxygen concentration in the mixed zone (Figure 1) could therefore be lowered to 0.5 g of O2/m3. This increased the ammonium effluent concentration to just below 1 g of N/m3 but at the same time increased the denitrification capacity. Figure 5 shows the effects of urine separation and different dissolved oxygen concentrations (2.0 g of O2/m3 and 0.5 g of O2/m3) in the mixed zone on the total nitrogen effluent concentration. Up to urine separation efficiencies of 50%, there is a drastic decrease in total nitrogen effluent concentration due to improved denitrification. With 60% urine separation or more, effluent concentrations were around 2 g of Ntot/m3, regardless of dissolved oxygen concentrations in the mixed zone or
FIGURE 5. Effect of urine separation on total nitrogen effluent concentration in different treatment scenarios in a BCFS processes.
FIGURE 6. Increase in treatment capacity due to the effects of urine separation with and without presettling, for temperatures 12 and 8 °C, relative to the reference flow rate at 12 °C. whether raw, presettled, or preprecipitated wastewater were treated. Figure 5 also shows the percentage of the actual influent nitrogen load that was removed as N2. This percentage was roughly the same for the three cases of raw, presettled, and preprecipitated wastewater. The contribution of denitrification to total N removal decreases exponentially with increasing urine separation. Phosphate Removal. Virtually all phosphate was removed in the case of zero urine separation. One would therefore expect little change in the effluent phosphate concentration with increasing urine separation. This was true for treatment of raw and presettled wastewater. In the case of preprecipitation, however, biological excess phosphate removal was not effective (Ptot ) 50 kg of P/day compared to the influent load of 108 kg of P/day at zero urine separation). Although the amount of phosphate accumulating organisms in the sludge did not vary much from that in the treatment of raw or presettled wastewater, daily sludge production and withdrawal was much lower. The excess sludge withdrawal rate (with SRT of 90 days) was too low for effective phosphate removal. At higher urine separation percentages, the phosphate removal capacity remained constant but the lower influent load led directly to a lower effluent concentration.
In the case of chemical phosphate removal, the benefits of urine separation seem obvious. This benefit is mostly in reducing the amount of chemicals required. The reduction in ferric chloride dosage and the daily chemical sludge production would be linear with increasing urine separation. Without any urine separation, around 960 kg of chemical sludge is produced for 50 500 people. This could be decreased to around 760 and 660 kg of chemical sludge at 50% and 75% urine separation, respectively. This might seem insignificant in comparison to the organic waste activated sludge loads of 4000 kg/day (raw wastewater). However, other problems related to chemical P removal such as corrosion, poor dewatering, clogging, and increased incinerator ash production could be reduced. Increased Treatment Capacity. Figure 6 shows the potential increase in treatment capacity with increasing urine separation up to 75%. The treatment capacity for raw wastewater at 12 °C was the reference (i.e., 15 250 m3/day ) 0%). The secondary axis shows the relative capacity at 8 °C (i.e., 10 250 m3/day ) -33%). For raw wastewater, with 10% urine separation, the aerobic zone was increased by 800 m3, which made a capacity increase of 25% possible. For 25% urine separation and higher, the complete anoxic zone became an aerobic zone. The maxiVOL. 38, NO. 4, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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mum increase in influent capacity, relative to the reference flow rate, with urine separation and increasing the aerobic zone at the cost of the anoxic zone was 35-40%. At higher urine separation percentages, the capacity increase was actually less. The decrease in nitrifiers (Figure 2) made the maximum allowable ammonium concentration limiting, although the total nitrogen effluent concentration still decreased. Different flow rates and SRTs were simulated at 8 °C, but the profile of capacity increase with increasing urine separation was roughly the same as for 12 °C. Presettled wastewater has a completely different composition. Without any urine separation, the treatment capacity was increased by 39% by decreasing the sludge age from 28 to 18 days. In this case, the anoxic zone was not decreased because nitrate concentrations were also limiting the capacity increase. The maximal increase in influent capacity was 108% above the reference, due to the combined effects of presettling and 30-40% urine separation (SRT ) 11.5 days). Again, as with the case of raw wastewater, at urine separation percentages above 40%, the capacity increase would be less. Any further increase requires a sludge age below 11 days to accommodate additional COD load. However, this leads to a decrease in nitrifying bacteria and process failure in terms of ammonium effluent.
Discussion Conventional wastewater treatment plants would experience a few clear and quantifiable advantages if part of the human urine production were collected and treated separately. Effluent Quality. Urine separation had little effect on ammonium and phosphate effluent concentrations in this model study. Nitrate concentrations decreased with increasing urine separation. At 50-60% urine separation, the total effluent nitrogen load would practically be the same for raw, presettled, and preprecipitated wastewater. However, the decrease in effluent loads would be more significant where wastewater with a lower COD:N:P ratio has to be treated, which is less favorable for N and biological P removal. Furthermore, practically all ammonium could be removed due to high sludge ages with primary sedimentation and higher dissolved oxygen concentrations in the mixed zone. Again, with a less favorable influent COD:N ratio, the oxygen concentration in the mixed zone would have to be lowered to increase denitrification. In general, ammonium effluent concentrations would be around 1g of N/m3, regardless of primary sedimentation or urine separation percentages. Improvement in effluent quality is mainly due to lower nitrate concentrations. The same total nitrogen effluent concentration at 50-60% urine separation could also be achieved by addition of a carbon source, such as methanol, to increase the denitrification. A typical ratio for COD/N, required for denitrification, is around 3.5 (23). The difference in total nitrogen removal between scenarios without urine separation and with 50% separation is 50-100 kg of N/day. Denitrification of this extra nitrate would therefore require addition of 200-350 kg of COD/day. Although costly, this technique is common practice and would most probably be preferred to urine separation. It seems clear that the improvement in effluent quality with increasing urine separation cannot be the only motive for implementing urine separation. With urine separation, the lower need for dentrification allows primary sedimentation without affecting the effluent quality. Primary sedimentation has the potential for a great reduction in energy requirement because oxygen demand is removed and methane could be produced. Last, the contribution of dentrification to N removal in a BCFS process becomes almost insignificant at high urine separation efficiencies. Under such conditions, the same 1214
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effluent quality would be possible with much less complex activated sludge processes (14) Capacity Increase. Urine separation allows for higher COD loads to existing BNR processes, without further process modification and without increasing the effluent nutrient concentrations. If urine from an increasing part of the population were collected separately, additional wastewater treatment plants or extensions to existing plants could be avoided. However, it would probably require improvement of the hydraulic capacity of the treatment works. The greatest increase in treatment capacity would be in combination with primary sedimentation. However, the volume required for the primary sedimentation tanks needs to be compared with increase in treatment capacity. In the reference scenario (no urine separation and no primary sedimentation) a treatment volume of around 250 L/p is required. With 40% urine separation (optimum in Figure 6), this is reduced to 210 L/p. With presettling, but without urine separation, the required volume (which included all sedimentation tanks) is also around 210 L/p. If 40% urine is collected separately with presettling, the required volume is only 160 L/p. This represents a capacity increase, based on total volumes of 20% and 60% respectively. Technology Transition. Separate collection and treatment of urine could be implemented over a few decades. Existing wastewater treatment processes would benefit from the gradual decrease of nutrient loads (urine separation) in the influent. Furthermore, only 50% of all urine is produced in residential areas (8). By targeting either residential areas or public buildings for separate collection of urine, the benefit to treatment plants would be similar. Sustainable wastewater treatment necessitates thoughtful use of resources. The existing infrastructure can be seen as one of the resources. Many countries have to upgrade or replace existing wastewater treatment works, due to stricter effluent standards or increases in population. Partial urine separation could prolong the lifetime of existing treatment works. This study did not consider the treatment of separately collected urine. Typical problems related to urine separation include replacement of toilets, struvite scaling in urine drains, and transport. The resources saved with primary sedimentation and lower nitrification (with increasing urine separation) should be compared to resources required to collect and treat urine separately. These boundary conditions, within which urine has to be treated separately and in a sustainable manner, must still be quantified.
Acknowledgments We want to thank the Dutch Foundation for Applied Water Research (STOWA) for their financial support and members of the steering committee for their contributions; Paul Roeleveld, Grietje Zeeman, Ruud Schemen, Arjen van Nieuwenhuijzen, and Paul Versteeg.
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Received for review July 25, 2003. Revised manuscript received November 19, 2003. Accepted December 1, 2003. ES0301018
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