FIRST APPLICATION OF THE BABE PROCESS AT S-HERTOGENBOSCH WWTP. Pettelaarpark 70, PO Box GA, s-hertogenbosch, The Netherlands

Transcription

1 FIRST APPLICATION OF THE BABE PROCESS AT S-HERTOGENBOSCH WWTP Bastiaan Hommel 1, Eric van der Zandt 2, Debby Berends 2, Victor Claessen 1 1 Dutch Water Authority Aa en Maas Pettelaarpark 70, PO Box GA, s-hertogenbosch, The Netherlands 2 DHV B.V., Amersfoort, The Netherlands ABSTRACT Problems with meeting nitrogen standards at the s-hertogenbosch WWTP in The Netherlands led to an examination of a number of technologies to improve the performance. A new side stream process, called BABE, was selected after an feasibility study. In September 2005, the BABE process went into operation. The first-ever application of this innovative process has proved successful. The augmentation of nitrifying bacteria within the activated sludge process increased the nitrification capacity at the plant and thus improves nitrogen removal. KEYWORDS BABE, Sludge Water Treatment, Augmentation, Nitrification, First Application, s- Hertogenbosch WWTP INTRODUCTION The completion of Phase A of the extension of the s-hertogenbosch WWTP early 2000 was the first piece of an effort to upgrade three local WWTPs. The goal of these upgrades was to comply with European and Dutch effluent standards regarding nitrogen and phosphorus removal. Three phases were defined beforehand: Phase A comprised the introduction of anaerobic and anoxic zones in the existing aeration volume and various improvements in sludge treatment and dewatering. Phase B consisted of additional measures, in the form of sludge water treatment, to further improve nitrogen removal. Phase C (conventional extension of the secondary treatment) would come into force as a result of increased wastewater loads or new effluent standards, in which case Phase B would be skipped. Since the first phase of the Dutch s-hertogenbosch wastewater treatment plant was commissioned, nitrogen removal had proved disappointing. As a result the yearly average effluent standard of 12 mg total N per litre was regularly exceeded. Various small measures to improve nitrogen removal have had little success. A risk analysis showed that once in every five years the effluent standards would be exceeded. 5227

2 FUTURE DEVELOPMENTS An up-front choice had to be made between implementing either sludge water treatment or a conventional extension of the activated sludge system. The outcome largely depended on the expected increase of wastewater loads and future effluent standards. Until 2018, an increase of 27,000 p.e. is expected, of which 80% would be designated to industrial wastewater. More stringent effluent standards are not expected until then and present hydraulic capacity of the WWTP will be sufficient to handle dry weather flow increase. Stormwater flow will remain the same. SLUDGE WATER TREATMENT An extensive analysis of the WWTP s performance showed that the disappointing nitrogen removal was a consequence of the critical minimum aerobic sludge age, causing a decreased nitrification capacity at low temperatures. Of the various possibilities to resolve the problem, sludge water treatment seemed most feasible, because it has the potential to improve nitrogen removal in a compact and cost efficient way. A wide range of innovative and proven technologies is available, which calls for a system evaluation and selection process. Due to its poor cost effectiveness, conventional extension of the secondary treatment was not considered. The effect of sludge water treatment on the effluent quality had been calculated prior to system selection [modelling by Delft University of Technology], distinguishing between techniques with and without augmentation of nitrifiers to the activated sludge system. Calculations were made for both present and future loads. The results show that augmentation of nitrifiers has far more effect on the effluent quality than just the removal of nitrogen from the sludge water, as shown in Table 1. This is remarkable, especially since the effect of augmentation on the nitrification capacity of the activated sludge was assessed rather conservatively. Table 1 - Yearly average effluent nitrogen concentration [mg/l] Nitrogen removal efficiency in Yearly average N concentration [mg/l] sludge water treatment Present load Present load + 15% Present WWTP w/o sludge water treatment % nitrogen removal efficiency % nitrogen removal efficiency % nitrogen removal efficiency % removal + augmentation of nitrifiers % removal + augmentation of nitrifiers Numbers 2 to 4 represent sludge water treatment without augmentation of nitrifiers. Numbers 5 and 6 represent the BABE-process. Table 1 shows that sludge water treatment could bring the effluent within the standard of 12 mg TN/L at the present load conditions, regardless of the applied technique. With 15% increased 5228

3 loads, only techniques with augmentation of nitrifiers remained able to comply with effluent standards. The prognosis shows an increase of 8% until Sludge water treatment techniques without augmentation have just enough capacity to deal with this increase, but the risk of loss of nitrification capacity at low temperatures would not have been resolved. Only sludge water treatment with augmentation of nitrifiers to the secondary treatment system was shown to be suitable to meet effluent standards, even with a significant future increase of wastewater loads. SYSTEM SELECTION Four Dutch sludge water treatment techniques were assessed in the system selection process: Sharon, Sharon+Anammox, Canon and BABE. Apart from the investment and operational costs, all techniques were judged on criteria that were critical for stable nitrogen removal and durability. Primary criteria were: the allowable increase of wastewater loads the effect on the nitrifying capacity of the secondary treatment system (considering present minimum aerobic sludge age at low temperatures) the sustainability of the processes (sum of energy and chemical consumption) the ease of retrofitting of new components into the existing layout. Operational costs (including capital costs) of all techniques were comparable. In terms of improvement to the effluent quality, the BABE process required 1.75 per kg TN removed. For techniques without augmentation, this figure would rise to approx. 4.40, based on equal operational costs, but they would have less effect on effluent quality as shown in Table 1. The combined Sharon+Anammox process required a slightly higher investment whereas the Canon process was slightly less expensive. The BABE process not only improves effluent quality, but also lengthens the sludge age of nitrifying bacteria due to the constant augmentation of nitrifiers, thus reducing the risk of noncompliance with effluent standards. Furthermore, augmentation boosts nitrification capacity, enabling a smoother handling of peak loads. Other techniques without augmentation of nitrifiers also reduce the nitrogen load within secondary treatment. At average loading conditions, this has a favorable effect on effluent quality. However, the minimum aerobic sludge age remains unchanged, and the fraction of nitrifiers in the activated sludge would diminish through the reduced nitrogen load. As a consequence, the risk of loss of nitrification capacity in the secondary treatment is not decreased, but has grown instead. Also the capacity to handle peak loads would be reduced. The energy consumption of the four techniques is more or less equal. The combined Sharon+Anammox and Canon processes have no need for carbon dosing, and are therefore considered more sustainable. Retrofitting the BABE process is a bit more complicated. The effluent must be discharged to all four aeration tanks and return sludge must be fed towards the reactor, requiring more piping then the other techniques. As a result of the system selection the BABE technology was selected as the only process that can address the basic problem of the s-hertogenbosch WWTP, i.e. the critical sludge age at low temperatures. The constant augmentation of nitrifying bacteria boosts the nitrification process in 5229

4 the secondary treatment system, creating enough extra capacity to handle a 15% increase of (future) wastewater loads, without exceeding effluent standards. BABE PROCESS The BABE process is a technique for treatment of sludge water, developed by DHV in cooperation with Delft University of Technology. BABE is the acronym of Bio Augmentation Batch Enhanced. The BABE process has a two-fold effect on an activated sludge system. Firstly, it reduces the nitrogen load from the sludge treatment recycle streams, and secondly - and most importantly - it increases (augments) the amount of nitrifying bacteria within the activated sludge treatment process. Augmentation of nitrifiers can be a solution for wastewater treatment plants that experience elevated effluent ammonia levels due to a critically low sludge age, as was the case at the s- Hertogenbosch works. In addition, augmentation can enhance nitrification to the extent that it allows more of the aeration volume to be devoted to denitrification, resulting in improved overall nitrogen removal. With respect to the s-hertogenboschden Bosch facility, the extra growth of nitrifiers is the result of the BABE side stream reactor fed with centrate at elevated ammonium concentrations and centrate temperature. The nitrogen load in the centrate equals 15% to 25% of the total load to the s-hertogenbosch plant. The centrate temperature is about 30 C and its flow is determined by the operation of the dewatering centrifuge. At the s-hertogenbosch plant, the centrifuge is typically operated during six to eighteen hours a day and four to six days a week. The centrate flow varies between 300 and 900 m 3 on days when the centrifuge is in operation. The ammonium concentration is about 1,000 mg N/L at the max, resulting in loads varying roughly between 300 and 600 kg/d. Figure 1 Process WWTP s-hertogenbosch including BABE process 5230

5 The BABE reactor at the s-hertogenbosch plant has been designed to handle 700m 3 of centrate a day at an average ammonium concentration of 700 mg N/l, resulting in an average design load of 490 kg/day. The BABE reactor has a volume of 1,350m 3 and the design temperature is 25 C. The treated sludge water from the BABE reactor, together with activated sludge containing an elevated number of nitrifiers is divided between all four aeration tanks of the mainstream activated sludge process. Since these are strictly separated, a small part of the return activated sludge from each of the four tanks is pumped to the reactor. The BABE reactor is operated as a discontinuous process. Each run consists of five steps: filling, aeration, mixing, settling and discharging and takes about three hours. Filling continues while the centrifuge is in operation. Aeration and mixing are alternated in order to achieve controlled nitrification and denitrification within the BABE reactor. As some of the activated sludge in the reactor is discharged with the BABE effluent, a small portion of return activated sludge (RAS) is added during denitrification. The carbon in the RAS also reduces methanol requirements. Figure 2 BABE Process 5231

6 PERFORMANCE GUARANTEE TESTING To demonstrate both the augmentation of nitrifying bacteria and the increased nitrogen removal capacity of the plant, the BABE process was connected to just two of the four aeration tanks, with the other two serving as a reference. The control tanks were not augmented with nitrifiers but were spared the extra nitrogen load from the centrate. In fact, these aeration tanks represented a situation in which it looks as if a side stream treatment system with 100% nitrogen removal was installed. In the BABE reactor the target nitrification efficiency was 70%, with 80% denitrification efficiency. The BABE reactor effluent nitrogen load, which has now been more than halved, was released to the two aeration tanks connected to the BABE reactor. Despite the increased nitrogen load, nitrogen removal in these tanks was expected to outperform the control aeration tanks. The performance was assessed through nitrogen balance measurements, nitrification activity tests and FISH analyses. The test period concluded with performance guarantee tests at the end of February NITROGEN BALANCE Although the nitrogen loads in influent, effluent and sidestream fluctuate significantly, the nitrogen balances shown in Table 2 reflect the general picture throughout the entire testing period. Table 2 - Nitrogen loads (kg N/d, AT1+2: BABE ATs; AT3+4: Control ATs) Centrate Effluent Influent Influent Effluent Effluent Removed Removed BABE AT1+2 2) AT3+4 AT1+2 AT3+4 AT1+2 AT3+4 average ,036 1, minimum 186 1) 76 1) maximum ,754 1, ,367 1,158 1) only counting the operational days of the centrifuge 2) excluding effluent BABE The ammonia load in the centrate averaged 381kg N/d. About 75% of that load was nitrified in the BABE reactor and the denitrification efficiency was about 80%. Consequently, after sidestream treatment in the BABE reactor, some 150kg N/d remained. Of that portion that was oxidized, two thirds was in the form of nitrite. The nitrogen load in the raw influent wastewater to the plant was 2,040 kg N/d on average, evenly distributed across the two primary clarifiers, so the nitrogen load in the sidestream was 19% of the influent load. To examine the effect of the BABE process on the effluent quality, the effluent from the BABE reactor was divided across aeration tanks (ATs) 1 and 2 only. As a result, these ATs received a 5232

7 15% higher nitrogen load compared to ATs 3 and 4, but also the waste sludge from the BABE, which contained the enhanced concentration of nitrifiers. Figure 3 Nitrogen balance Only 20 kg of the extra 150 kg of nitrogen escaped into the plant effluent, resulting in a 1 mg/l increase in the effluent nitrogen concentration. The BABE ATs removed 910 kg nitrogen daily, 16% more than the control ATs. Considering the average effluent nitrogen concentration of 12.7 mg/l in the control ATs, the availability of nitrogen could not be the limiting factor for nitrification. The extra nitrogen removal in the BABE ATs must have been the result of the augmentation of the nitrifiers, a theory that is supported by the results of the nitrification activity tests. The average effluent quality in January and February 2006 was 13.8 and 12.7 mg/l. Since these are the coldest months of the year, it is likely that the target effluent quality of 12 mg/l as a yearly average is well within reach. Even though the BABE ATs received the extra nitrogen load, and the control ATs were underloaded, the BABE ATs performed about as well as the control basins. In normal practice, the BABE process will be connected to all ATs, allowing all aeration tanks to profit from the augmentation while the nitrogen load from the sidestream will be evenly distributed. 5233

8 AUGMENTATION Nitrification activity tests and Fluorescence In Situ Hybridization (FISH) analyses quantified the effect of augmenting nitrifiers to the activated sludge system. Table 3 summarizes the results of the nitrification activity tests. Table 3 - Summarized results of nitrification activity tests Nitrification activity (g N / kg TSS.h) Relative to control (%) Activated sludge from control tanks Activated sludge from BABE tanks Activated sludge in the BABE reactor The BABE ATs show a 23% improvement in nitrification activity compared to the reference ATs. The activated sludge in the BABE reactor itself more than doubles that activity. These results are confirmed by the outcome of the FISH analyses. The amount of ammonia oxidizing bacteria in the augmented activated sludge in the BABE reactor shows a 50% increase over the activated sludge in the reference ATs. The activated sludge in the BABE reactor contains 2.5 times as many nitrifiers. Counting the amount of nitrite oxidizing bacteria produced a rather different view. While the numbers do not differ much across the four activated sludge systems, the sludge in the BABE reactor contains hardly any nitrite oxidizing bacteria. NITROGEN REMOVAL OVER NITRITE The FISH analyses and the nitrate and nitrite concentrations in the BABE effluent indicate that in the BABE reactor nitrite oxidation is much slower than ammonia oxidation. This means that most of the nitrification only goes as far as forming nitrite. Nitrogen removal, while skipping much of the oxidation of nitrite to nitrate, is considered very beneficial since it saves considerably on energy input (for aeration) and supplemental carbon requirements. Stoichiometrically, denitrification of nitrite requires 1.14 kg of methanol per kilogram of nitrogen, compared to 1.90 when denitrifying nitrate. According to Dutch research, and based on experience, higher methanol dosing is required, that is, 1.8 kg and 2.6 kg of methanol per kilogram of NO x -N, respectively. Figure 4 clearly shows that the methanol consumption in the BABE reactor is below 2 kg of methanol per kg N, providing clear evidence that most of the denitrification was from nitrite rather than nitrate. During the performance guarantee tests, methanol consumption was as low as 1.27 to 1.45 kg of methanol per kilogram of NO x -N, approximating to the stoichiometrical rate for denitrification of nitrite. Carbon in the RAS may contribute to the favorable methanol consumption. 5234

9 Figure 4 Methanol consumption In the authors opinion, inhibition by free ammonia is the most likely explanation for the depressed rate of nitrite oxidation in the BABE reactor. As a result, nitrite- oxidizing bacteria cannot compete in the activated sludge in the BABE reactor, a fact confirmed by the FISH analyses. The result is extremely beneficial. BABE cultivates and augments those nitrifiers that grow slowest at low temperatures and removes nitrogen in a sustainable way, needing a minimum of energy and chemicals. CONCLUSION The BABE process has proved itself in a full-scale application as a technology that is able to remove nitrogen from side streams, minimizing the usual negative consequences of nitrogen overload in the main stream aeration basins that are typically experienced in conventional processes. Because of the augmentation of nitrifiers in the activated sludge systems the nitrification capacity is increased, as demonstrated in the nitrification activity tests. The augmented activated sludge systems showed a 16% greater total Nitrogen removal capacity. The augmentation of nitrifiers was also demonstrated through the FISH analyses and confirmed by the nitrogen balances. Augmentation proved to be limited to ammonia oxidizing bacteria. Nitrite oxidizing bacteria are inhibited in the BABE reactor, and are therefore barely found in the reactor s activated sludge. As a result, the nitrification process essentially stops once nitrite is formed, providing savings for aeration energy and carbon for denitrification. This is confirmed by the low consumption of 5235

10 methanol in the BABE reactor. The BABE process thereby ranks among the most sustainable side stream treatment systems. REFERENCES This article is an adaptation of two articles previously published in Water21 magazine: Zandt, van der, E, Hommel, B, Claessen, V, Berends, D (2005) First outing for the BABE process. Water 21, April 2005, pp 36-37, IWA-Publishing Zandt, van der, E, Hommel, B, Claessen, V, Berends, D (2006) BABE proves its worth. Water 21, October 2006, IWA-Publishing 5236