NITROGEN REDUCTION IN RECIRCULATING BIOFILTER TREATMENT CONCEPTS. David Venhuizen 1

Transcription

1 NITROGEN REDUCTION IN RECIRCULATING BIOFILTER TREATMENT CONCEPTS David Venhuizen 1 Since the recirculation concept was first proposed by Hines and Favreau in the 1970 s, recirculating biofilters utilizing sand, gravel and newer synthetic filtration media have proliferated and become recognized as one of the most consistent and reliable treatment concepts for small-scale wastewater systems. Further work on the concept in the 1980 s most notably by Roland Mote at Tennessee, Dee Mitchell at Arkansas, Rich Piluk at Maryland, and A. T. Sandy at West Virginia illuminated how this concept could be harnessed to achieve considerable nitrogen reduction by clever manipulation of the nitrogen cycle within the process. Recirculation is the key, introducing effluent that was nitrified in the filter bed to the anaerobic environment of a septic tank, which serves as the preclarification treatment stage in front of the biofiltration process. Because recirculation is practiced in any case, this modification provides essentially free nitrogen removal. This paper reviews this concept and the potential nitrogen removal rate attainable from it. Detailed data from the Washington Island project the most extensive field trial of this treatment concept are examined to elucidate the impact of design decisions and operating conditions on the level of reduction attainable. DESCRIPTION OF THE PROCESS Formulation of an enhanced version of the recirculating biofilter process was guided by the experiences of the Washington Island project. The author has termed this process the high performance biofiltration concept. This process is illustrated in Figure 1. The heart of the treatment process is of course the biofilter bed. The balance of the system is designed to allow the biofiltration process in the filter bed to be as stable thus as consistent and reliable as practical, and to enhance the ease of system operations and maintenance. This process is reviewed in detail in another paper in these proceedings. Fig. 1. High Performance Biofiltration Treatment System Concept 1 Principal, David Venhuizen, P.E., 5803 Gateshead Drive, Austin, Texas 78745, waterguy@ix.netcom.com

2 Here we consider the nitrogen removal capability of this process. As Figure 1 illustrates, in the high performance biofiltration concept, the flow out of the filter bed is split into a recirculation flow and an effluent flow. The nitrogen cycle in this process is illustrated in Figure 2. As shown, nitrification occurs in the filter bed, and then nitrate in the recirculation flow is denitrified during the passage of the recirculation flow through the septic/recirculation tank. With this being the case, the theoretical limit on nitrogen removal rate would be the ratio of the recirculation side flow to the effluent side flow. Fig. 2. Nitrogen Cycle in the High Performance Biofiltration Concept For example, if the flow split were 2:1 that is, two gallons flows onto the recirculation side of the filter bed for every one gallon that flows onto the effluent side then the removal rate limit would be 2/3, or 67%, since 2/3 of the total flow onto the filter bed would flow through the recirculation loop, where the nitrate it carries could be denitrified. Likewise, if the flow ratio were 3:1, then 3/4, or 75% removal, would be the limit. This capability could be further limited due to incomplete nitrification in the filter bed and/or incomplete denitrification in the septic/recirculation tank. The overall nitrogen removal rate might be increased above this limit, however, due to denitrification occurring within the filter bed. Recognizing that gravity recirculation schemes employed at the time which did not include an effluent bypass valve, so when there was low, or no, forward flow, there would be gaps in the filter bed loading cycle represented a major compromise of the uniform dosing regime demanded by coarse media filters receiving high hydraulic loading rates, the Washington Island project employed pumped recirculation systems. That system concept is illustrated in Figure 3. This, however, proved to impart its own hazards when one of the recirculation pumps failed and was not noticed, thus not repaired, for over a month and critically compromised one of the systems. There is no straightforward way to trigger an alarm when the recirculation pump fails, so it was determined to correct the gravity recirculation scheme, which was accomplished with the effluent bypass valve. While the operation of the effluent bypass is not belabored here see again the paper in these proceedings detailing the high performance biofiltration concept it can be seen from Figure 1 that, when it opens, flow from the effluent side drops into the filter bed dosing tank instead of flowing to the final effluent holding tank. When this occurs, further denitrification of nitrates in this flow may occur in the filter bed dosing tank, if sufficient anoxia 2

3 prevails in it. This could provide some additional nitrogen removal that was not attainable in the system concept used in the Washington Island systems. Fig. 3. Washington Island Treatment System Concept This difference in the treatment concept may call to question the validity of accepting the results of the Washington Island project as an indication of the nitrogen removal capability of the high performance biofiltration concept. However, other than the factor just noted, the two concepts are essentially equivalent in regard to nitrogen removal capability. In the pumped recirculation concept shown on the left side of Figure 4 the total recirculation flow remained the same regardless of the amount of flow into the system, since it was provided by a pump that was run by a timer. The actual recirculation rate would vary over any given time period, depending on the actual amount of forward flow into the system. In the high performance biofiltration concept shown on the right side of Figure 4 the total recirculation flow also remains the same regardless of the amount of forward flow since the filter bed dosing pump is run by a timer and the split between the recirculation side and the effluent side is always the same. Thus, in either concept, the amount of flow back through the septic/recirculation tank is constant, based on the timer setup, so that the actual recirculation rate over any time period would vary with the actual amount of forward flow in exactly the same way. Fig. 4. Pumped Recirculation Concept vs. High Performance Biofiltration Concept 3

4 When using the pumped recirculation concept, however, flow rate onto the filter bed would vary with the actual forward flow, and the timing of the filter bed doses would also be affected. This is because the filter bed dosing pump was started and stopped by float switches that is, run under volumetric control. With no forward flow, a maximum dosing interval would be determined by the recirculation flows. When forward flow entered the system, this would add to the flow entering the filter bed dosing tank, so the dose volume would build up more quickly and a shorter dosing interval would result. In the high performance biofiltration concept, those variations are eliminated so that true steady-state hydraulic loading of the filter bed is obtained. This being the case, performance of the high performance biofiltration concept may be superior to that obtained using the pumped recirculation concept. The other major difference between the two concepts is the presence of the upflow filter in the Washington Island systems. Based on the work of the previously noted researchers, it was expected at that time that this sort of attached-growth bed would be required to obtain a high denitrification rate. It was observed, however, that when recirculation flow was routed through even a small secondary chamber of a two-chamber septic tank prior to entering the attachedgrowth filter, as in the Washington Island systems, most of the nitrate entering this tank was denitrified there, leaving little for the upflow filter to do. For the five systems, the observations showed the following average nitrate concentrations over the approximately 5-month period this data was collected in both second chamber septic tank effluent and upflow filter effluent: System No. 1 (36 samples): Second chamber septic tank effluent 0.5 mg/l NO 3 Upflow filter effluent 0.1 mg/l NO 3 System No. 2 (20 samples): Second chamber septic tank effluent 1.5 mg/l NO 3 Upflow filter effluent 0.5 mg/l NO 3 System No. 3 (19 samples): Second chamber septic tank effluent 7.1 mg/l NO 3 Upflow filter effluent 6.3 mg/l NO 3 System No. 4 (22 samples): Second chamber septic tank effluent 2.2 mg/l NO 3 Upflow filter effluent 0.6 mg/l NO 3 System No. 5 (12 samples): Second chamber septic tank effluent 4.5 mg/l NO 3 Upflow filter effluent 2.3 mg/l NO 3 Other projects most notably the work of Rich Piluk in Anne Arundel County, Maryland also demonstrated that eliminating the attached-growth filter would not significantly degrade system performance, provided that the enhanced clarification provided by the upflow filter was imparted by other means. In the high performance biofiltration concept, this is accomplished with an effluent filter in the septic tank and an in-line filter on the filter bed dosing pump discharge. Therefore, the Washington Island project data can be used as a valid indication of the performance to be expected from the high performance biofiltration treatment concept. 4

5 NITROGEN REMOVAL PERFORMANCE Observations of treatment system performance for the five systems that were operated yearround in the Washington Island project are displayed in Tables 1-5. The important physical characteristics of these systems are outlined below: System S.T. 1 st Chamber S.T. 2 nd Chamber Filter Bed Area No gal. 500 gal sq. ft. No gal. 343 gal sq. ft. No gal. 559 gal sq. ft. No gal. 559 gal sq. ft. No. 5 unknown 464 gal sq. ft. Filter Bed Design: System No. 1 startup through December 1993, stratified bed design Top layer 12 fine gravel (~1/4-3/8, mm) Bottom layer 12 coarse sand (~1.5 mm E.S.) System No. 1 December 1993 and after, single media layer 24 fine gravel (~1/4, 6 mm) System No coarse sand (~1.5 mm E.S.) System No fine gravel (~1/4-3/8, mm) System No. 4 stratified bed design Top layer 12 fine gravel (~1/4-3/8, mm) Bottom layer 12 coarse sand (~1.5 mm E.S.) System No coarse sand (~1.5 mm E.S.) Paired observations of influent and effluent quality were obtained about once per week. The nitrogen data in Tables 1-5 include influent Total N almost always entirely TKN and effluent TKN, nitrate, and Total N. Also shown are influent and effluent BOD 5, effluent temperature, and forward flow and recirculation flow meter readings, allowing calculations of interval average daily flows for each. From the interval average daily forward flow rate, the forward flow hydraulic loading rate (HLR) onto the filter bed and the organic loading rate (OLR) on the system were calculated. Note that influent values were taken at the discharge from the first chamber of the septic tank, since it was not practical to obtain a representative influent sample from the building drain. This is not expected to have any impact on systemic nitrogen removal data, as it is not expected that nitrogen removal would occur in the first chambers of the septic tanks (unless there were nitrates in the raw wastewater this was not evaluated, but is typically not to be expected). They did not typically accomplish complete mineralization typically 1/4 1/3 of TKN was in the organic form, indicating perhaps that considerable TKN was passing through the tank with solids. The actual influent BOD s, however, may have been higher than indicated in the tables, so the organic loading rates may be understated. These chambers were rather small and not very optimally configured, so considerable solids indeed passed through. In any case, the tables show that influent BOD s were generally higher than typically expected in first chamber septic tank 5

6 effluent. The result is that even though the hydraulic loading rates onto the filter beds were often modest, the organic loading rate remained fairly high in most cases, and was extremely high >0.02 lb/s.f./day through some intervals, in particular through peak periods in System No. 4. As in any such investigation, the Washington Island project experienced some glitches, so the data was selected to represent well behaved periods. The conditions limiting each system are: In System No. 1 (a residence), the recirculation pump failure mentioned previously was a prominent glitch. It was one in a series of events that rendered the data from this system of questionable value from when that pump failed until nitrification was established in the filter bed after the media was changed, a gap of over a year, as Table 1 indicates. System No. 2 (a residence) was operated as a single-pass system for about 8 months, for which very poor performance was observed. It was then converted into a recirculating system, which is the point at which the data in Table 2 begins. Thereafter, problems with meters and with operation of the recirculation system caused a data gap of several months. System No. 3 (a residence) was operated with recirculation flow directly into the upflow filter for about 8 months before it was modified to route it through the second chamber of the septic tank. The data reported in Table 3 is only for the period that this system operated in that mode. In System No. 4 (the island s grocery store, with meat cutting, vegetable washing and toilet flushing being the main wastewater generators), the effluent line froze up for a short period each winter, and there appeared to be a prolonged period after startup during which denitrification was incomplete. Data from the full 2-year evaluation period is shown in Table 4, but that prolonged startup period is neglected when calculating overall averages. For System No. 5 (a residence and guest house), it was determined that recirculation flow was causing dilution of the first chamber septic tank effluent samples, so the removal rates were not valid. Data for that system is reported in Table 5 only for the period after a new port was installed to obtain undiluted first chamber septic tank effluent samples. The results show that a high degree of total nitrogen reduction can be delivered consistently and reliably by this treatment system concept. For the data periods shown in Tables 1-5, total N removal rate ranged from 63.9% to 91.7%. The overall averages for each of the five systems over their respective periods of observation are listed below. The anomalous startup period for System No. 4 is not included in these averages. System No. 1 Influent = 54.4 mg/l, effluent = 14.1 mg/l, 74% removal System No. 2 Influent = 43.2 mg/l, effluent = 15.3 mg/l, 65% removal System No. 3 Influent = 85.8 mg/l, effluent = 16.8 mg/l, 80% removal System No. 4 Influent = mg/l, effluent = 13.8 mg/l, 89% removal System No. 5 Influent = 42.4 mg/l, effluent = 11.6 mg/l, 73% removal Performance was maintained at these high levels without regard to influent strength, flow fluctuations, organic loading rate, and all the vagaries of operation in the field environment. An effluent total N concentration of about 15 mg/l or less appears typically attainable. 6

7 In the spirit of full disclosure, more data points for effluent quality were obtained than are reported in Tables 1-5. Reported there are only the samples for which paired influent-effluent data was taken, as this allowed calculation of removal rates. For part of the observation period, sampling alternated between first chamber septic tank effluent samples and upflow filter effluent samples, so paired observations were not available for the latter sampling events. Reviewed below are the means, medians, and standard deviations for the full data set of effluent samples, compared to those measures for the data sets shown in Tables 1-5. From these comparisons, it is concluded that the data in Tables 1-5 is a fair representation of the overall performance of each system. Note that for System No. 3 and System No. 5 the data sets were identical, and for System No. 4, the anomalous startup period is again eliminated in both data sets. System No. 1 effluent Mean Total N Median Total N Std. Dev. Total N Table 1 data set Full data set System No. 2 effluent Mean Total N Median Total N Std. Dev. Total N Table 2 data set Full data set System No. 3 effluent Mean Total N Median Total N Std. Dev. Total N Table 3 data set Full data set System No. 4 effluent Mean Total N Median Total N Std. Dev. Total N Table 4 data set Full data set System No. 5 effluent Mean Total N Median Total N Std. Dev. Total N Table 5 data set Full data set From the data in Tables 1-5, it appears as if the N removal rate increases with increasing influent total N concentration. However, this may be an artifact of the recirculation rates having been higher in systems where influent total N is higher. Recall that the theoretical limit of removal rate is the recirculation rate, expressed as a percent of total flow onto the filter bed that recirculates. Examination of Tables 1-5 show how well the total N removal rate matches with the percent of total flow that is recirculated. The comparisons for each of the 15 data periods (omitting the System No. 4 startup period, prior to Jan. 25, 1993) are shown below. Overall averages would not be very meaningful due to the varying conditions among the data periods. System No. 1 1 st period: Influent TKN = 65.2 % Recirc. = 67.8 % Denit. = 71.7 Diff. = -3.9% 2 nd period: Influent TKN = 54.4 % Recirc. = 78.0 % Denit. = 76.8 Diff. = +1.2% 3 rd period: Influent TKN = 38.6 % Recirc. = 90.3 % Denit. = 73.1 Diff. = +17.1% 7

8 System No. 2 1 st period: Influent TKN = 42.8 % Recirc. = 63.5 % Denit. = 63.9 Diff. = -0.4% 2 nd period: Influent TKN = 46.7 % Recirc. = 77.8 % Denit. = 70.3 Diff. = +7.4% System No. 3 1 st period: Influent TKN = % Recirc. = 87.0 % Denit. = 87.3 Diff. = -0.2% 2 nd period: Influent TKN = 56.7 % Recirc. = 87.7 % Denit. = 68.8 Diff. = +18.9% 3 rd period: Influent TKN = 52.5 % Recirc. = 87.0 % Denit. = 65.7 Diff. = +21.3% System No. 4 1 st period: Influent TKN = % Recirc. = 93.1 % Denit. = 87.4 Diff. = +5.7% 2 nd period: Influent TKN = % Recirc. = 86.9 % Denit. = 87.8 Diff. = -1.0% 3 rd period: Influent TKN = % Recirc. = 92.1 % Denit. = 90.7 Diff. = +1.4% 4 th period: Influent TKN = % Recirc. = 91.0 % Denit. = 91.7 Diff. = -0.7% System No. 5 1 st period: Influent TKN = 50.2 % Recirc. = 79.5 % Denit. = 75.9 Diff. = +3.7% 2 nd period: Influent TKN = 36.8 % Recirc. = 83.1 % Denit. = 71.4 Diff. = +11.7% 3 rd period: Influent TKN = 46.9 % Recirc. = 81.1 % Denit. = 72.3 Diff. = +8.9% The individual interval removal rates shown in the tables vary widely not surprising given that this is all based on grab samples but the above averages do indicate that the total N removal rate in this treatment process can be roughly predicted relative to the recirculation rate employed in the system, with some caveats: In System No. 1, it was seen that when average recirculation rate was greatly increased while average influent total N decreased significantly, the removal rate did not elevate above what it had been in previous periods with lower average recirculation rates. In System No. 2, where fairly low influent total N concentrations prevailed throughout the data periods, a jump in the recirculation rate induced an increase in the removal rate that was smaller than the increase in average recirculation rate. In System No. 3, while the average recirculation rate remained stable across all the observation periods, the average influent total N concentration drastically decreased in the last two observation periods (there is no explanation for why this happened it may have been a drastic change in diet, as one of the occupants had been diagnosed with cancer), and this resulted in significantly lower removal rates. In System No. 4, where average conditions remained stable from period to period, the difference scores were fairly low and consistent over all periods (noting that for the first period, with a higher difference score, the period over which recirculation rate was calculated is different than the period over which removal rate was calculated). Both influent total N and recirculation rate were high and fairly uniform over all the periods, however, so the influence of changing recirculation rate on removal rate when total N is high could not be determined. 8

9 In System No. 5, where the average recirculation rate remained fairly stable throughout, the difference score rose and fell in inverse relationship with average influent total N concentration. This may indicate this relationship is not linear, rather is a second order relationship. Understanding that the observations are all based on grab samples and that the averages are not flow-weighted, no doubt imparting some errors, the results seem to suggest: With a recirculation rate of about 2:1 (67%) or more, a minimum removal rate of 60% appears fairly well assured when influent total N concentration is in the typical range for normal domestic wastewater of mg/l. At recirculation rates in the range of 2:1 (67%) to 3:1 (75%) and influent total N concentration in the typical range, removal rate rises with increasing recirculation rate. At recirculation rates in the range of 2:1 (67%) to 3:1 (75%) and influent total N concentration in the typical range, an effluent total N concentration of less than 20 mg/l would be assured, consistently and reliably. An average of <15 mg/l is quite likely. When influent total N concentration increases, recirculation rate should also increase in order to achieve an effluent total N concentration in the 15 mg/l range. When influent total N concentration is very high e.g., above 100 mg/l then average removal rate may rather closely track average recirculation rate even at rather high recirculation rates, on the order of 9:1 (90%). There is a point of diminishing returns, where increasing the recirculation rate further appears not to provide a commensurate increase in removal rate, and may even be counterproductive. This appears to be more so the lower influent total N concentration is. All this depends as well on the hydraulic loading rate on the filter bed. When influent strength is high, the forward flow HLR should be de-rated in concert with the increase in recirculation rate so that total HLR onto the filter bed remains moderate. Though that data is not reviewed here, it was observed that there appeared to be some nitrogen loss within the filter bed. It is presumed that this is due to in-bed denitrification, imparted in anaerobic microsites within the filter bed. In-bed losses occurred even in the very coarse gravel media (6 9.5 mm) filter beds. This may be the reason that negative average difference scores that is, removal rates above the expected limit were observed. A confounding factor is the degree of nitrification attained, as only the nitrate portion of total N in filter bed effluent could be denitrified. System No. 1 and System No. 2 in particular experienced periods of inconsistent nitrification, and System No. 4 had a lower nitrification rate during the summer peak period, when organic loading rates were extremely high. There is quite likely some correlation to organic loading rate, as this would tend to inhibit nitrification due to competition from the heterotrophic microbes that digest BOD. The treatment concept, the Washington Island experiences, the data, and these observations will be presented to and discussed with the participants in the NOWRA Pre-Conference Symposium on Nitrogen and Decentralized Systems. 9

10 TABLE 1 SYSTEM No. 1 FLOW AND NITROGEN REMOVAL DATA 2-Jul-92 wastewater flow into system starte 25-Aug recirculation started 27-Aug % % 12.5% 8-Sep % % -13.7% 14-Sep % % -15.9% 21-Sep % % 4.5% 28-Sep % % -4.0% 5-Oct % % 37.4% 12-Oct % % -2.1% 19-Oct % % 22.3% 2-Nov % % -3.4% 9-Nov % % -9.2% 16-Nov % % -6.5% 23-Nov % % -3.3% Average over data period % % -3.8% 28-Feb Mar % % 20.9% 10-Mar % % 14.0% 14-Mar % % 12.4% 17-Mar % % -0.7% 21-Mar % % 8.6% 24-Mar % % 0.6% 28-Mar % % 5.8% 31-Mar % % 10.9% 4-Apr % % -12.1% 11-Apr % % -3.2% 14-Apr % % -3.5% 18-Apr % % 5.6% 21-Apr % % -7.4% 25-Apr % % 1.7% 28-Apr % % -5.2% 2-May % % 2.9% Average over data period % % 1.2% New recirculation meter May % % 2.6% 9-May % % 36.8% 12-May % % 39.5% 16-May % % 28.1% 19-May % % 27.6% 23-May % % 27.3% 31-May % % 27.1% 6-Jun % % 11.5% 13-Jun % % 6.6% Average over data period % % 17.1%

11 TABLE 2 SYSTEM No. 2 FLOW AND NITROGEN REMOVAL DATA 20-May May-93 recirculation started May % % -7.8% 3-Jun % % 2.3% 7-Jun % % 19.9% 10-Jun % % 8.1% 17-Jun % % 14.4% 21-Jun % % -4.5% 24-Jun % % -24.3% 28-Jun % % 19.9% 1-Jul % % 125.5% 6-Jul % % -26.5% 8-Jul % % -7.4% 12-Jul % % 0.9% 15-Jul % % 1.0% 19-Jul % % 14.8% 22-Jul % % 24.6% 26-Jul % % -2.3% 29-Jul % % -15.2% 2-Aug % % 1.2% 5-Aug % % -23.6% 9-Aug % % -16.0% 12-Aug % % 5.9% 16-Aug % % 14.0% 19-Aug % % -13.8% 26-Aug % N/A % -14.3% 30-Aug % N/A % -25.9% 2-Sep % N/A % -10.9% 9-Sep % N/A % 3.7% 13-Sep % N/A % 36.1% 16-Sep % % 18.2% 20-Sep % N/A % 4.7% 23-Sep % N/A % 14.3% 27-Sep % N/A % 30.3% 30-Sep % N/A % 14.1% 4-Oct % N/A % 1.6% 7-Oct % N/A % 9.0% 11-Oct % N/A % 6.0% 14-Oct % N/A % 10.6% 18-Oct % N/A % 6.0% 21-Oct % N/A % -14.1%

12 TABLE 2 SYSTEM No. 2 FLOW AND NITROGEN REMOVAL DATA 25-Oct % N/A % 31.5% 28-Oct % N/A % 20.0% Average over data period % % -0.4% New meters installed 26-May Jun % % 1.8% 9-Jun % % 7.3% 16-Jun % % 3.8% 23-Jun % N/A % 9.5% 30-Jun % % 22.4% Average over data period % % 7.4%

13 TABLE 3 SYSTEM No. 3 FLOW AND NITROGEN REMOVAL DATA 6-May May % N/A % 2.4% 20-May % % 5.1% 27-May % % 9.9% 3-Jun % % -1.0% 10-Jun % % 27.7% 17-Jun % % -3.2% 24-Jun % % 4.4% 1-Jul % % -6.5% 8-Jul % % 0.1% 15-Jul % % -3.4% 22-Jul % % -3.5% 29-Jul % % 3.6% 5-Aug % % -2.7% 12-Aug % % 2.9% 19-Aug % % 13% 1.3% 26-Aug % N/A % -1.9% 2-Sep % N/A % -6.6% 9-Sep % N/A % -6.2% 16-Sep % % 6.0% 23-Sep % N/A % -2.8% 30-Sep % N/A % 7.5% 7-Oct % N/A % 0.2% 14-Oct % N/A % 1.9% 21-Oct % N/A % -5.0% 28-Oct % N/A % -6.2% Average over data period % % -0.2% spring/summer/fall 4-Nov % N/A % -3.2% 11-Nov % N/A % -3.1% 18-Nov % % 21.4% 2-Dec % N/A N/A N/A % -2.7% 9-Dec % % 19.1% 16-Dec-93 N/A N/A N/A N/A N/A N/A N/A 7.0 N/A 106 N/A % N/A 23-Dec-93 N/A N/A N/A N/A N/A N/A N/A N/A N/A 110 N/A % N/A 30-Dec % N/A % 26.0% 13-Jan % % 21.0% 27-Jan % % 17.5% 10-Feb % N/A N/A N/A N/A N/A 17-Feb % % 13.9% 24-Feb-94 N/A N/A N/A N/A N/A N/A N/A N/A N/A 124 N/A % N/A 3-Mar % N/A % 40.5%

14 TABLE 3 SYSTEM No. 3 FLOW AND NITROGEN REMOVAL DATA 10-Mar % % 41.7% 17-Mar % % 44.3% 24-Mar % % 24.7% 31-Mar % % 21.3% Average over data period winter % % 18.9% 14-Apr % % 8.8% 21-Apr % % 60.4% 28-Apr % % 12.8% 5-May % % 31.4% 12-May % % 19.4% 19-May % % 30.5% 26-May % % 24.7% 2-Jun % % 15.3% 9J 9-Jun-94 n % % 2% 14.9% 16-Jun % % 50.6% 23-Jun % N/A % 19.7% 30-Jun % % 13.8% Average over data period % % 21.3% spring/summer

15 TABLE 4 SYSTEM No. 4 FLOW AND NITROGEN REMOVAL DATA 20-Aug-92 wastewater flow into system started 25-Aug recirculation started N/A 3-Sep % N/A % 89.9% 8-Sep % % 47.6% 14-Sep % % 78.6% 21-Sep % % 92.0% 28-Sep % % 61.2% 5-Oct % % 48.5% 12-Oct % % 29.7% 19-Oct % % 22.3% Average over data period % % 58.0% summer/fall since recirc. startup 2-Nov % % 44.6% 9-Nov % % 37.2% 16-Nov % 6% % 58.7% 23-Nov % % 45.0% 3-Dec % % 49.2% 7-Dec % % 44.5% 14-Dec % % 45.9% 4-Jan-93 N/A N/A N/A N/A N/A 3.0 N/A 242 N/A % N/A 11-Jan-93 N/A N/A N/A N/A N/A 2.0 N/A 501 N/A % N/A 18-Jan % % 35.7% 25-Jan % % 12.4% 1-Feb % % 14.7% 8-Feb-93 N/A N/A N/A N/A N/A 2.0 N/A 416 N/A % N/A 15-Feb % % -2.9% 25-Feb-93 effluent line frozen N/A N/A N/A N/A N/A 901 N/A % N/A 1-Mar-93 N/A N/A N/A N/A N/A 2.0 N/A 1186 N/A % N/A 8-Mar-93 N/A N/A N/A N/A N/A 2.0 N/A 1517 N/A % N/A 15-Mar-93 N/A N/A N/A N/A N/A 1.0 N/A 133 N/A % N/A 22-Mar-93 N/A N/A N/A N/A N/A 0.1 N/A 541 N/A % N/A 29-Mar N/A N/A N/A N/A 2.0 N/A 778 N/A % N/A Average over data period % % 25.1% winter 5-Apr % % -6.6% 12-Apr % % 1.0% 19-Apr % % -1.1% 26-Apr % % 3.0% 3-May % N/A % -1.2% 10-May % N/A % 0.8% 17-May % N/A % 3.7%

16 TABLE 4 SYSTEM No. 4 FLOW AND NITROGEN REMOVAL DATA 24-May % % -0.8% 7-Jun % % 1.8% 14-Jun-93 N/A N/A N/A N/A N/A N/A N/A N/A N/A 758 N/A % N/A 21-Jun % % 6.4% 28-Jun % % 5.5% 6-Jul % % 0.0% 12-Jul % % -8.1% 19-Jul % % 3.4% 26-Jul % % -3.7% 2-Aug % % -5.4% 9-Aug % % -6.7% 16-Aug % % 50.1% 23-Aug-93 N/A N/A N/A N/A N/A N/A N/A N/A N/A 1434 N/A % N/A 30-Aug % N/A % -8.4% 7-Sep % N/A % -4.1% 13-Sep-93 N/A N/A N/A N/A N/A N/A N/A N/A N/A 1300 N/A % N/A 20-Sep % N/A % -1.0% 27-Sep % N/A % -0.3% 4-Oct % N/A % 1.7% 11-Oct % N/A % 4.9% 18-Oct % N/A % 1.7% 25-Oct % N/A % 4.2% Average over data period spring/summer/fall % % -1.0% 1-Nov % N/A % 4.9% 8-Nov % N/A % -0.4% 15-Nov % N/A % 7.4% 22-Nov-93 N/A N/A N/A N/A N/A N/A N/A 8.0 N/A 372 N/A % N/A 29-Nov % % 6.8% 6-Dec % % 3.6% 13-Dec % N/A % 7.5% 20-Dec % % 2.2% 27-Dec % N/A % -1.8% 3-Jan-94 N/A N/A N/A N/A N/A N/A N/A N/A N/A 277 N/A % N/A 10-Jan-94 N/A N/A N/A N/A N/A N/A N/A N/A N/A 546 N/A % N/A 24-Jan % % -1.9% 7-Feb % % 0.9% 14-Feb % N/A % -0.1% 21-Feb % % 3.4% 28-Feb-94 effluent line frozen N/A N/A N/A N/A N/A 6.0 N/A 107 N/A % N/A 7-Mar-94 N/A N/A N/A N/A N/A N/A N/A 11.0 N/A 916 N/A % N/A 14-Mar-94 N/A N/A N/A N/A N/A 7.0 N/A 534 N/A % N/A

17 TABLE 4 SYSTEM No. 4 FLOW AND NITROGEN REMOVAL DATA 21-Mar N/A N/A N/A N/A 11.0 N/A 878 N/A % N/A 28-Mar % % -1.7% Average over data period winter % % 1.4% 4-Apr % % 1.1% 11-Apr % % -1.5% 18-Apr % % -4.8% 25-Apr % % 0.9% 2-May % % -3.5% 9-May % % 0.7% 16-May % % 3.3% 23-May % % 2.0% 31-May % % -7.2% 6-Jun % % 2.9% 13-Jun % % 61% 6.1% 20-Jun % % -6.7% 27-Jun % % -1.4% Average over data period % % -0.7% spring

18 TABLE 5 SYSTEM No. 5 FLOW AND NITROGEN REMOVAL DATA 13-Sep Sep % N/A % 10.5% 27-Sep % N/A % -3.0% 4-Oct % N/A % 1.1% 11-Oct % N/A % 11.8% 18-Oct % N/A % 0.4% 25-Oct % N/A % 2.8% Average over data period summer/fall % N/A % 3.7% 1-Nov % N/A % 30.8% 8-Nov % N/A % -6.2% 15-Nov % N/A % 4.1% 22-Nov % % 0.2% 29-Nov % % -10.0% 6Dec93 6-Dec % % 7% 33% 3.3% 13-Dec % N/A % 6.5% 20-Dec % % -3.5% 27-Dec % N/A % 1.4% 10-Jan-94 N/A N/A N/A N/A N/A N/A N/A N/A N/A 263 N/A % N/A 24-Jan % % 2.2% 7-Feb % % 24.3% 14-Feb % N/A % 54.3% 21-Feb % % 91.5% 28-Feb % % 71.3% 7-Mar % % 66.0% 14-Mar % % 59.5% 21-Mar % % 90.8% 28-Mar % % 39.0% Average over data period % % 11.7% winter 4-Apr % % 16.3% 11-Apr % % 11.9% 18-Apr % % 40.3% 25-Apr % N/A % 10.0% 2-May % % 9.2% 9-May % % 5.9% 16-May % % 13.6% 23-May % % -6.6% 31-May % % 5.9% 6-Jun % N/A % 1.7% 13-Jun % N/A % -1.2%