NITROGEN AND PHOSPHORUS-RICH SIDESTREAMS: MANAGING THE NUTRIENT MERRY-GO-ROUND

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1 NITROGEN AND PHOSPHORUS-RICH SIDESTREAMS: MANAGING THE NUTRIENT MERRY-GO-ROUND Heather M. Phillips, P.E.; Ed Kobylinski, P.E.; James Barnard, Pr.Eng.; Cindy Wallis-Lage, P.E. Black & Veatch Corporation 8400 Ward Parkway Kansas City, MO ABSTRACT Many wastewater treatment plants (WWTPs) in the Midwest are facing nutrient removal for the first time, while plants in other parts of the US and the world are struggling with permits that become more stringent with each cycle. Depending on the permit, in-plant sidestreams generated during biosolids processing can greatly affect plant operations and performance, particularly when these sidestreams are recycled intermittently to the liquid treatment facility. Many facilities dewater digested solids eight hours per day, five days per week, which sends slug loads of nutrients back to the liquid treatment facilities. This introduces intermittent demands on the blowers (for nitrification) and chemical feed systems (alkalinity for nitrification, carbon for denitrification). Plants that have biological phosphorus removal and digestion (particularly anaerobic digestion) must carefully monitor sidestream phosphorus loads. Plants that import biosolids from other plants for regional processing are also importing nutrients within these biosolids which can become a problem. If sidestreams are an issue, WWTPs have three choices: (1) export the sidestream, (2) manage the sidestream at the liquid treatment facility, or (3) implement sidestream treatment. This paper presents two sidestream case studies: one WWTP that chose sidestream treatment, and another that chose sidestream management. Both examples involve biological phosphorus removal with anaerobic digestion and imported biosolids. Several sidestream management strategies (including equalization), and sidestream treatment processes are discussed. KEYWORDS Sidestreams, Biological Nutrient Removal, Nitrogen, Phosphorus, Biosolids Processing INTRODUCTION The impact of sidestreams on liquid treatment processes varies with the source of the sidestream. One process that can result in nutrient-rich sidestreams is anaerobic digestion (or similar anaerobic processes). Anaerobic digestion releases nutrients that are returned to the liquid stream in the filtrate/centrate from dewatering processes. For example, ammonia concentrations in dewatering sidestreams can range from 900 to 1,500 mg/l as nitrogen (N) or more, which can increase the ammonia concentration in the plant influent by 3 to 5 mg/l on an average day basis. If such a sidestream is returned over one shift (8 hours), it can have a profound impact on oxygen uptake and aeration system design. It can also result in poorly settling mixed liquor suspended solids (MLSS). 5282

2 At many plants, phosphorus returned in the sidestream has not been an issue. However, in plants using biological phosphorus removal (Bio-P), the return of phosphorus released in the anaerobic digester can stress the Bio-P process. Biological phosphorus removal concentrates the majority of the plant influent phosphorus in the biosolids, unlike the biosolids generated at carbon or nitrogen removal plants. During anaerobic digestion, approximately 60 percent of the stored total phosphorus in the feed sludge is released as ortho-phosphorus. If a regional plant processes biosolids from multiple Bio-P plants, the phosphorus that is released and returned to the liquid treatment facility may double or even triple the phosphorus load entering in the raw influent. In such situations, sidestream treatment may be required. While nutrient-rich sidestreams are normally a concern only with anaerobic digestion, nitrates generated during aerobic digestion can also have an impact on plants for which total nitrogen (TN) limits have been established in their National Pollutant Discharge and Elimination System (NPDES) permits. Normally, the nitrates are not a concern unless the load is unusually high (from a regional biosolids facility), or the dewatering schedule is limited to only a few shifts per week. Unless the flow is equalized, the centrate/filtrate generated by intermittent dewatering can shock load the liquid treatment facility. If nitrogen and phosphorus in sidestreams are a concern, there are a number of ways to treat the sidestreams before returning them to the liquid treatment facility. If phosphorus is the only concern, metal salts can be added directly to the digester, to the digested solids prior to dewatering, or to a separate sidestream treatment flocculation basin followed by a small scale clarifier to remove the precipitated solids. If nitrogen is the primary concern, small-scale activated sludge facilities can be designed to nitrify and/or denitrify the sidestreams. Carbon and alkalinity supplementation is typically required for total nitrogen removal. Several proprietary processes are available to remove nitrogen, including the SHARON, InNitri, BABE, and ANAMMOX processes. REGIONAL BIOSOLIDS PROCESSING Because of the high cost of many of the advanced biosolids treatment processes, neighboring communities may be able to save significant capital and O&M costs by merging their biosolids processing into one regional facility. Regionalization is one method of taking advantage of the economy of scale of these systems. In addition to reducing the overall capital cost, operation and maintenance costs can be reduced through consolidating staff. However, there is a tradeoff to these economic advantages sidestream treatment. Plants that treat biosolids from multiple plants end up treating (or managing) much higher sidestream nutrient loads than plants that process only their own biosolids. Importing biosolids imports nutrients, and the impacts of sidestream loads can be overlooked when designing a new regional biosolids processing facility. Selecting the best site for a regional biosolids facility is critical, but unfortunately it is often based primarily on land availability, accessibility, and other factors related to biosolids processing. It is crucial to look beyond these factors and to consider also the relative capacities of both the liquid and the biosolids treatment facilities. If the regional facility is going to process biosolids from several different communities, it is important that the liquid treatment plant have sufficient capacity to handle the sidestream loads generated. If the host plant does not have 5283

3 enough capacity, provisions need to be included to either treat the sidestreams or to export them to another plant. Wastewater flows and loads must be carefully examined. While a plant may have a large flow capacity, this rating may be based on small nutrient loads. Consequently, small sidestream loads could cause big problems. Accurate mass balances are critical, since sidestream loads can govern the design and operation of a regional biosolids facility. PERMIT LIMITATIONS The plant s discharge permit is another key factor in determining whether sidestreams will be an issue. Plants subject to stringent nutrient limits will have to worry about sidestreams more than plants with relatively relaxed permits. It is also important to work closely with local regulators to predict future permit limits, and to include this information in the planning efforts. There are many things to consider, depending upon the type of discharge limits: No nitrogen limits: o Will new sidestreams cause your effluent ammonia to increase, catching the regulators attention and causing a permit change? Ammonia limits, but no TN limits: o Is your aeration system adequate? o Do you have enough alkalinity? o Will you still meet permit? Moderate TN limits (~ 8 mg/l TN): o Is your aeration system adequate? o Do you have enough alkalinity? Carbon? o Will you still meet permit? o Is it cheaper and easier to add equalization or sidestream treatment? Stringent TN or TIN limits (< 5 mg/l). o Everything above applies, but more is on the line; sidestream control is the key. o A process upgrade may be possible (4-stage BNR, or Denite filters). Stringent TP limits (< 1 mg/l). o Molasses or acetic acid supplementation may be required for biological phosphorus removal. o Fermenter may be required. o Tertiary filtration may be required. OPTION 1: EXPORT THE SIDESTREAM At regional biosolids processing facilities, pipelines are often constructed to convey biosolids to the regional plant. If managing the dewatering sidestreams at the regional facility is costprohibitive, it may be advantageous to include separate pipelines to return part or all of the sidestreams back to the contributing plants. This option depends highly upon the ability of the WWTP to process the additional load. The contributing WWTPs must also be evaluated for their ability to handle nutrients in the sidestream return load. If one plant has additional capacity, but another would require costly upgrading, this all must be considered early in the project. It is necessary to look at all aspects/combinations of treatment upgrading to find the lowest cost option for regional biosolids processing and sidestream management. Another consideration is how to allocate the capital and O&M costs among the participating WWTPs. 5284

4 OPTION 2: MANAGE THE SIDESTREAM AT THE LIQUID TREATMENT FACILITY At WWTPs that are very large, it is often more cost effective to process the sidestreams at the liquid treatment facility, rather than to implement separate sidestream treatment at the biosolids facility. This section presents several factors that must be considered when processing high strength sidestreams at a WWTP. Sidestream Flow Equalization Sidestream flow equalization is often the easiest and most cost-effective way to manage sidestreams. While flow equalization does not necessarily break the nutrient cycle, it enables operators to manage the additional nutrient loads by returning sidestreams in a controlled manner. Equalization can be accomplished either operationally with a continuous dewatering schedule (24 hours per day, 7 days per week), or by design by incorporating a flow equalization basin. Some plants have even devised operational strategies whereby sidestreams are recycled during low-load periods of the day. Without flow equalization, high-strength ammonia loads from sidestreams can place a significant demand on a nitrifying plant s aeration system. Figure 1 shows the effects of sidestream return on the oxygen uptake rate in a nitrifying activated sludge basin. When the facility dewaters during only one shift on weekdays (7 hours per day, 5 days per week), the aeration system has a wide range of uptake rates to support: 20 to 50 mg O 2 /L. However, if the sidestream is equalized, the range of oxygen uptake is much smaller (28 to 35 mg O 2 /L), resulting in less frequent need for blower adjustments. 5285

5 Dewatering 7 hours/day, 5 days/week The aeration system must support a wide range of oxygen uptake: 20 to 50 mg O 2 /L/hr. Equalized Sidestream Return The aeration system has a much narrower range of oxygen uptake to support: 28 to 35 mg O 2 /L/hr. Figure 1: Effects of Filtrate Recycle on Oxygen Uptake Rate in Nitrifying Activated Sludge Slug loads of ammonia will also impact effluent quality. Depending on the relative strength of the sidestream load, ammonia bleed-through can occur and the plant effluent nitrate concentration may also rise. With more nitrates in the system, the mixed liquor recycle for denitrification will return a greater mass of nitrate to the anoxic zone. Plants operating with limited available carbon may require carbon supplementation to maintain low effluent nitrate and TN concentrations. During periods of high oxygen uptake, the dissolved oxygen (DO) concentration may drop to near zero, creating an environment that promotes filament formation. 5286

6 Repeated events of low DO concentrations can produce a MLSS with poor settling characteristics which can lead to permit violations for biochemical oxygen demand (BOD) and total suspended solids (TSS). The ability of the liquid treatment process to handle sidestream return loads will vary depending upon the current influent load to the WWTP. Plants that have been recently upgraded or expanded are typically operating at less than 50 to 60 percent of design capacity, and should therefore have adequate aeration capacity for sidestream slug loads. However, plants that are being loaded at 70 to 90 percent of design capacity may not have adequate aeration capacity. Flow equalization is an effective way to manage sidestreams, and can be important at plants that do not have regional biosolids processing, or anaerobic digestion. In the following example, a small plant with a two-stage BNR process was having difficulty meeting its 10 mg/l limit for effluent TN. As it turned out, the problem was partially linked to the dewatering schedule, since the facility uses aerobic digestion which produces nitrates. To investigate the problem, a dynamic mass balance of the plant was developed that included the liquid treatment and biosolids processing facilities. The model was set up with the capability of operating the centrifuges either continuously or on specified schedules. Several solids handling options were investigated including: (1) a single centrifuge operating for 20 hours per day, 7 days per week; and (2) two centrifuges operating 20 hours per day, 5 days per week. When a centrifuge was operated daily, the plant effluent TN concentration was much more consistent compared to intermittent operation, as indicated in Figure 2. When the centrifuges were operated Monday through Friday only, the effluent TN concentration showed an erratic trend. On Monday, when dewatering began, the effluent TN concentration increased sharply, and continued to increase each day until Friday, when the work week ended and dewatering stopped. The small dips in the curve correspond to the four hours per day during the week when the centrifuges were shut down. It is clear for this plant that a continuous dewatering schedule is better than intermittent dewatering. However, even with a continuous dewatering schedule, the average effluent TN concentration was approximately 11 mg/l which is higher than the permit limit of 10 mg/l. Modeling confirmed that the raw influent at the plant contained insufficient readily biodegradable COD (rbcod) for denitrification. A methanol feed system was added which enabled the plant effluent to routinely meet the permit limit. This example shows that sidestreams can be a problem for plants that do not have anaerobic digestion or imported biosolids, but that influent characteristics and permit limits can also dictate whether sidestream management is necessary. 5287

7 Total N (mg/l) /2/2001 9/3/2001 9/4/2001 Supernatant from 1 Centrifuge Recycled to the Head of the Plant 20 hours/day, 7 days/week 9/5/2001 9/6/2001 TIME 9/7/2001 9/8/2001 9/9/ Total N (mg/l) /2/2001 Dewatering Begins on Monday 9/3/2001 9/4/ day AVG Supernatant from 2 Centrifuges Recycled to the Head of the Plant 20 hours/day, 5 days/week 9/5/2001 9/6/2001 TIME Dewatering Ends on Friday 9/7/2001 9/8/2001 9/9/2001 Figure 2: The Effect of Different Dewatering Schedules on Liquid Treatment Performance Dedicated Treatment Train for Sidestreams Some plants chose to designate a single treatment train to receive all sidestreams. This isolated train must have an adequate aeration system, sufficient alkalinity, and enough rbcod for denitrification to handle the additional ammonia load. A benefit of having a dedicated treatment train is that if effluent quality does suffer as a result of sidestream loads, the dedicated treatment 5288

8 system will have removed much of the nitrogen mass. The effluent of the dedicated train is diluted with the rest of the plant flow, which decreases the chance of a permit violation. Fermenters for Biological Phosphorus Removal Bio-P plants concentrate phosphorus in the waste activated sludge (WAS) and much of this phosphorus is released into the digester. Some of the phosphorus is precipitated as struvite and is removed during dewatering as a solid. Approximately 40 percent of the phosphorus is precipitated but 60 percent is returned in the sidestream to be re-treated in the liquid treatment plant. The 60/40 split of phosphorus may change as the water supply hardness varies. Usually, the Bio-P system is balanced with the volatile fatty acids (VFA) in the incoming wastewater to efficiently remove phosphorus. In some instances, a fermenter is needed to release additional VFA from the volatile solids in the incoming wastewater to make the Bio-P mechanism work. Primary solids are pumped to fermenters at dilute concentrations (less than 1 percent total solids), where the solids are pre-digested to release VFA to support the Bio-P process. The fermenters also serve as gravity thickeners. If additional phosphorus is imported in the biosolids from other regional plants, it will be released from the solids during digestion and will further increase the phosphorus load to the Bio-P plant. The net result is that more VFA is needed to maintain a low effluent phosphorus concentration in the plant effluent. Without sidestream treatment, the phosphorus concentration at a Bio-P plant can increase significantly (often more than doubling), which can also double the required concentration of VFA or rbcod. As a rule of thumb, it has been found that VFA concentrations 4 to 5 times the influent PO 4 -P concentrations are required for successful biological phosphorus removal. Barnard and coauthors report that the minimum rbcod to P ratio should be between 11 and 15, depending upon the plant (Barnard et al., 2005). Chemical Phosphorus Removal Chemical phosphorus removal facilities are also affected by phosphorus-rich sidestreams. In essence, more chemical is needed to remove the additional phosphorus load. The addition of more chemical raises the plant s chemical costs, increases sludge production, and consumes more alkalinity. If the plant also nitrifies, the alkalinity balance must be checked to determine if alkalinity supplementation is necessary. The phosphorus content of the mixed liquor suspended solids will also rise as more phosphorus is precipitated and effluent filtration may have to be considered if it is not already practiced. Online Instrumentation Several of the treatment strategies discussed above can be optimized by using online instrumentation, particularly if flow equalization is not used. Two types of online monitoring and control that can be used to control the sidestream return flow rate are respirometry and nutrient analysis. An online respirometer can be used to measure the oxygen demand in the activated sludge basin and the sidestream flow can be adjusted to balance the sidestream loads with the main influent loads. Commercially available online respirometers include ESCOR A 2 C, Capital Controls Respirometer, STIP Biox 1010 and Rho Environmental SAS. Similarly, online 5289

9 nutrient analyzers can be used to measure and balance nitrogen and phosphorus loads. The analyzers are available from many suppliers, including ChemScan, Hach, Danfoss, and Myratek. Separate WAS Processing During anaerobic digestion of WAS from a Bio-P plant, stored polyphosphate is released in the digester, as volatile suspended solids (or biomass) are destroyed. The problem with this released phosphorus is that it remains in solutions after the dewatering process unless chemicals are added. Therefore, almost all of the phosphorus that the liquid treatment facilities has worked so hard to remove from the solution, is released and recycled back in the dewatering sidestream. A strategy to avoid this is to process the WAS separately, since primary solids contain only about 2 percent phosphorus, rather than the 6 to 8 percent phosphorus in WAS from Bio-P plants. Most nutrient removal plants operate with a long SRT so the WAS is partially or nearly completely digested already (however it will not meet 503 sludge stabilization criteria, defined by EPA - 40 CFR Part 503). The WAS could be stabilized in an aerobic digester which would not release as much phosphorus. Alternatively, composting or alkaline treatment could be used which would minimize nutrient recycle. If biosolids processing regionalization is considered, composting or heat drying would be more appropriate stabilization processes. One thing to remember is that two separate stabilization processes will require more staff, more equipment, and will probably have higher capital and O&M costs. However, separate treatment of WAS separately breaks the nutrient cycle by removing the accumulated phosphorus from the system. Lime Stabilization of WAS One approach to breaking the nutrient cycle is to stabilize WAS with lime, which will prevent the release of phosphorus and nitrogen into the liquid to be recycled. The most common method of lime stabilization is adding dry lime to the dewatered cake. The lime will fix the phosphorus into the cake; however, some ammonia from the cake will be volatilized and returned in the scrubber blowdown. Most of the nitrogen will remain in the cake, and there will be an ammonia sidestream only if ammonia scrubbers are installed. Many operators elect to vent the ammonia without treatment. To meet Class B stabilization requirements, a dose of lime equal to 20 percent of the dry weight of sludge is typically sufficient. To meet Class A requirements, a dose of lime equal to 100 percent of the dry weight of sludge is needed. There are several proprietary processes that use other additives that may reduce the lime dose to meet the time and temperature criteria of higher than 70ºC for longer than 30 minutes. In either case, a lime storage silo and a lime/sludge mixer are needed. While lime stabilization is a commonly-used approach, the reality of separate stabilization of WAS is that the plant will have two biosolids processing systems, which means duplication of effort, extra record-keeping and additional equipment to be maintained. An additional concern is that in areas where the soils are alkaline, land application of lime sludge is inappropriate. It is important to compare the costs of separate lime stabilization with the costs of other alternatives to verify that the savings are real and worth the extra effort. 5290

10 OPTION 3: TREAT THE SIDESTREAM For certain projects, it may be more cost-effective to implement sidestream treatment facilities, than to accommodate the nutrient load at the liquid treatment facility. These facilities can be small conventional activated sludge systems with or without chemical addition, or more advanced proprietary systems. Several basic facts must be faced when considering sidestream treatment. The sidestream resulting from anaerobic digestion will contain higher concentrations of ammonia, BOD, alkalinity and phosphorus than typical wastewater. However, just because the concentrations are high, they are not necessarily properly balanced. For example, a typical sidestream from anaerobic digestion may have a BOD concentration of 1,000 mg/l, an ammonia concentration of 1,000 mg/l, an alkalinity concentration of 3,500 mg/l and a phosphorus concentration of 500 mg/l. If the sidestream is fully nitrified by treatment, roughly 1,000 mg/l of nitrate will be formed and 7,100 mg/l of alkalinity will be consumed. Roughly half this alkalinity will be present in the sidestream, and about 3,500 mg/l of alkalinity will have to be added to maintain a stable ph. Since the sidestream has a high BOD concentration it would be advantageous to denitrify to recover alkalinity. The 1,000 mg/l BOD would denitrify about 350 mg/l NO 3 -N and recover about 1,250 mg/l of alkalinity, which is not enough to avoid the need for alkalinity supplementation. If enough supplemental carbon is added, the 3,500 mg/l of alkalinity would be recovered at the expense of a higher aeration demand to remove the BOD added to recover alkalinity. Ultimately, more solids are formed for disposal from the sidestream treatment facility. Biological phosphorus removal in the sidestream treatment facility is not recommended. While the anaerobic digester will produce VFA, most are destroyed and converted into methane. The mg/l VFA in the effluent from an anaerobic digester are not nearly enough to support Bio-P in a sidestream treatment system. In addition, there is very little if any rbcod left in the digester effluent, so there is virtually no potential for additional VFA formation in a sidestream treatment facility. One advantage of sidestream treatment is the elevated liquid temperature. At 30ºC, the rate of nitrification is maximized. One feature of the sidestream treatment processes is temperature control: cooling in the summer and heating in the winter. Operating at temperatures above 35ºC should be avoided, since higher temperatures will begin to inhibit the rate of nitrification so good temperature control is important. Conventional Activated Sludge for Nitrogen Removal A sidestream treatment system can consist of conventional small-scale activated sludge facilities for nitrification and denitrification, with ferric chloride addition if phosphorus removal is needed. An advantage of conventional activated sludge is that the process should be familiar to plant staff and its operation and maintenance should not involve tremendous amounts of additional labor. A disadvantage is that if the sidestream treatment facility is located at a regional biosolids processing facility, the processes are quite different from the biosolids processes and additional training could be required. 5291

11 SHARON Process for Nitrogen Removal Several proprietary treatment processes are available as alternatives to conventional sidestream treatment. The SHARON process was developed for treatment of ammonia-rich wastewater (Hellinga et al. 1998). SHARON stands for Single-reactor High Activity Ammonia Removal Over Nitrite. A schematic of the SHARON process is shown on Figure 3. Influent passes through heat exchangers to the aerated portion of the basins where ammonia is converted to nitrite. The SHARON process is designed to promote Nitrosomonas sp. selection, and not Nitrobacter sp., which completes the nitrification reaction to nitrate. The process is controlled with online ammonia, nitrite, and nitrate analyzers, in addition to monitoring dissolved oxygen and temperature. By limiting the conversion of ammonia to nitrite only (rather than going all the way to nitrate), less oxygen is required which results in savings in the cost of electricity. The nitrite produced is then denitrified with methanol in the anoxic zone, and the alkalinity that is recovered is recycled to the oxic zone. Figure 3: SHARON Sidestream Treatment System Another advantage of the SHARON process is its smaller footprint compared to conventional sidestream treatment, since it is a flow-though process only that does not include secondary clarifiers or return activated sludge pumping. It requires only low head mixed liquor recycle pumping at 6-9 times the influent flow. There are several operating SHARON installations in Europe, and one under design in New York, NY which is expected to be operational in The three facilities that have been in operation the longest (Rotterdam, Utrech and Zwolle; all in the Netherlands) were contacted to obtain operator feedback on process performance, ease of operation, and information on process stability/reliability. In general, the comments about the SHARON process were very positive, and operators claimed that it is very easy to operate. Operators of all three facilities said that they would choose the SHARON process again, knowing what they know now and having several years of experience with it. 5292

12 ANAMMOX Process for Nitrogen Removal Another very new process is ANAMMOX, which is an acronym for ANaerobic AMMonia OXidation. The ANAMMOX process uses autotrophic bacteria that convert ammonium directly into nitrogen gas under anaerobic conditions, using nitrite in a partial nitritation step (van Loosdrecht and Salem, 2005; Strous et al., 1998). A benefit of the ANAMMOX process is that because the bacteria are autotrophic, there is no need for an external carbon source for denitrification. The main disadvantage is that the anammox bacteria have a slow growth rate of 0.069/day (van de Graaf et al., 1996), so methanol may be needed during startup until the anammox bacteria are well-established. Because nitrite is needed, the SHARON process partners well with the ANAMMOX process for sidestream treatment. Sidestreams from anaerobic digestion do not contain much nitrite, but the ammonia that they do contain can be converted to nitrite with the SHARON process. The ANAMMOX bacteria can then oxidize the remaining ammonia directly to nitrogen gas in the presence of nitrite. This SHARON /ANAMMOX combination has been recently implemented in Rotterdam, the Netherlands at the Dokhaven WWTP (van Loosdrecht and Salem, 2005). The ANAMMOX process has also been piloted in a fixed film moving bed bioreactor (MBBR) at the Himmerfjärden WWTP in Stockholm, Sweden (Gut et al., 2005). While the process is still somewhat in its development stages, the research that has been conducted in the past few years shows that the process has a lot of promise for certain applications. Figure 4: SHARON /ANAMMOX Sidestream Treatment System In-Nitri Process for Ammonia Removal The In-Nitri, or Inexpensive Nitrification process is also relatively new and it creates a synergistic relationship between the liquid processing and biosolids processing facilities. The In- Nitri reactor removes nitrogen from the dewatering sidestream, while seeding the activated 5293

13 sludge system with nitrifying bacteria generated within the reactor. The process was developed for plants in northern climates that needed to upgrade to year-round nitrification or total nitrogen removal (Kos et al., 2000). The challenge for these plants is that the liquid treatment process did not have adequate aeration tank volume to maintain the long SRTs needed at low temperatures to keep an adequate population of nitrifiers. At the operating SRTs that they could maintain without overloading their clarifiers, nitrifiers would wash out faster than they could re-grow. The InNitri process solves this problem by growing abundant nitrifying populations at higher temperatures (30-35ºC) to process ammonia-rich sidestreams from biosolids processing, and recycling them to the aeration basin. The InNitri process, which is essentially a small-footprint activated sludge facility with an aeration basin and a clarifier, is shown on Figure 5. Primary Effluent (or Raw Influent) Secondary Clarifier Effluent Aeration Basin Nitrified Clarifier Overflow Waste Solids with High Nitrifier Concentrations Alkalinity, Primary Effluent Return Activated Sludge Waste Activated Sludge Nitrification Ammonia-Rich Sidestream Anaerobic Digester, Dewatering Primary Solids InNitri System 30-35ºC mg/l NH 3 -N Figure 5: InNitri Sidestream Treatment System Cake Alkalinity is added to the InNitri reactor to support nitrification, and primary effluent (or raw influent) is added to maintain an ideal temperature and to provide some alkalinity. By seeding the main activated sludge facility with nitrifiers, InNitri provides a 50 percent reduction in SRT and tankage compared to the conventional activated sludge process. Other reported advantages include lower capital and O&M costs, and simplicity of operation. The InNitri process was recently piloted at the Ina Road WPCF in Pima County, Arizona (Riska et al., 2004). BABE Process for Nitrogen Removal The Biological Augmentation Batch Enhanced, or BABE process is another new method of nitrogen removal in wastewater treatment. The process has been tested full-scale at the Garmerwolde WWTP in the north of the Netherlands. The concept of the BABE process is 5294

14 similar to that of the In-Nitri process in that it treats the sidestream, while also augmenting the liquid treatment facility with nitrifying organisms. The processes differ in the way that the sidestream is treated. With the BABE process, the ammonia-rich sidestream from dewatering is processed with a limited amount of return activated sludge in a batch process that includes denitrification. Using a batch reactor, long sludge retention times are achieved, allowing the growth of specialized nitrifying and denitrifying bacteria in the BABE reactor. In the reactor, ammonia is nitrified to nitrate and nitrite, generating acid as a byproduct (alkalinity destruction). To rapidly convert ammonia, the acid is neutralized by adding caustic soda. Denitrification will recover some alkalinity, as previously discussed. The effluent MLSS is recirculated to the mainstream BNR process where it augments nitrifiers in the BNR process. A schematic of the The BABE process is shown on Figure 6. Primary Effluent (or Raw Influent) Secondary Clarifier Effluent Aeration Basin Return Activated Sludge BABE Effluent BABE Reactor 30ºC > 750 mg/l NH 3 -N RAS fraction to BABE Reactor Ammonia-Rich Sidestream Waste Activated Sludge Anaerobic Digester, Dewatering Cake Figure 6: BABE Sidestream Treatment Process Primary Solids Steam Stripping for Ammonia Removal Another option for removing ammonia in certain applications is steam stripping, which involves removing the ammonia from solution in gaseous form at high temperatures. Because of the high temperature requirements, steam stripping is typically used only in specific industrial applications. Steam stripping also has high energy requirements, and the elevated temperatures will cause calcium scaling which will cause a continuous problem for operating staff. Since ammonia gas is also a hazardous air pollutant (HAP), the facility could be regulated by the EPA, depending on the quantity of ammonia discharged into the atmosphere. While it is technically feasible for ammonia removal, there are too many problems for this process to be used in most biosolids processing facilities. 5295

15 Chemical Precipitation of Phosphorus The easiest way to remove phosphorus from sidestreams is to add iron (as ferric chloride, FeCl 3 ) or aluminum (as alum, or Al 2 (SO 4 ) 3 12H 2 O), to form metal salts which can be removed in a small clarifier or other solids separation device. A benefit of using ferric chloride rather than alum is that iron has agronomic value; when it is applied to land, plants can take up iron, while aluminum has no agronomic value. Another benefit of using ferric chloride is that ph adjustment is not needed. Lime can also be used to precipitate phosphorus. For efficient precipitation, the sidestream ph must be raised to 8.5 or 9. Because of the high alkalinity in anaerobic digester sidestreams, a large amount of lime must be added to raise the ph. Chemical phosphorus removal using either iron or aluminum is a well-known and proven technology and can be easily applied to sidestream treatment (U.S. Environmental Protection Agency, 1987). Controlled Struvite Precipitation for Phosphorus and Ammonia Removal Struvite (magnesium ammonium phosphate, MgNH 4 PO 4. 6H 2 O) is a crystalline material consisting of equimolar concentrations of magnesium, ammonium, and phosphorus. Struvite precipitation is usually limited by the magnesium concentration, since ammonia and phosphorus are present in excess. Struvite is typically viewed as a problem, since it can develop downstream from anaerobic digestion processes and form deposits in pipes and on dewatering equipment, plugging pipes and causing damage to equipment. However, if precipitated in a controlled manner, struvite formation is beneficial for BNR plants since it removes both ammonia and phosphate from solution. As ph and temperature rise, the potential for struvite precipitation increases. Conditions in anaerobic digesters are conducive to struvite formation high temperature, high ph, available ammonium and phosphate; so struvite precipitation can be increased by adding magnesium. Fullscale processes for controlled struvite precipitation have been operational in Japan since the mid- 1980s, but are currently not available in the U.S. These processes involve magnesium addition and ph adjustment, and the recovered struvite is typically dried and sold as a source of phosphorus for industrial or agricultural applications. Controlled precipitation through CO 2 removal has also been investigated. Turbulence created by pumping or dewatering tends to strip CO 2 from the biosolids, resulting in a rise in ph and a greater potential for struvite precipitation. Treatment processes that remove CO 2 through the application of a vacuum or through air stripping would be expected to enhance struvite formation. While predicting the amount of struvite formation can be difficult, and using controlled struvite formation in BNR applications is relatively new, the concept has a lot of potential. Crystalactor Process for Phosphorus Removal The Crystalactor technology, which was developed in the Netherlands, is a crystallization process. It consists of treating the sidestream with milk of lime and passing it through an upflow fluidized bed of Ca 3 (PO 4 ) 2 particles where calcium phosphate is precipitated and plates out on the calcium phosphate crystals already in the reactor, producing an effluent with a low phosphorus concentration. The end product can be washed and bagged and either sold or given away, which eliminates disposal costs. The system uses very high upflow velocities of 5296

16 approximately 60,000 gpd/ft 2 /day, and there is no need for mixing and flocculation since the precipitated crystals are deposited on seed crystals. Very fine sand is added continuously to serve as seed for the growth of crystals. A schematic of Crystalacter is shown on Figure 7. Summary Figure 7: Crystalactor Reactor Considerable research is being done on sidestream treatment processes; in fact, most of the processes discussed above have been developed in the past decade. Several processes that are currently being developed are not discussed here. If sidestream treatment is needed, several things in addition to cost should be considered when selecting a process. If the operators prefer a process they are familiar with, then a conventional system may be best. However, if the community wants to be the first to implement and document a new process, then the proprietary technologies have much to offer. One of the case studies summarized below found that the proprietary processes can be cost-competitive with conventional sidestream treatment, so the choice is really a matter of comfort level and site-specific decision factors. CASE STUDIES This section highlights the challenges presented by sidestreams at regional biosolids processing facilities. Both of the case studies described below involve regionalization of solids processing with anaerobic digestion for biosolids stabilization, and both plants practice biological phosphorus removal. While both plants have imported nutrients that ultimately tripled their influent phosphorus load, each found a different solution to its sidestream challenges based on detailed mass balances and economic evaluations. 5297

17 Case Study 1: Sidestream Treatment Two neighboring communities decided to form a regional biosolids processing facility, since evaluations showed that regionalization results in significant cost savings over investing in separate solids processing facilities. One community has an 80 mgd WWTP while the other a 30 mgd WWTP. However, the 30 mgd WWTP has a much higher influent waste strength than the 80 mgd WWTP, which results in a biosolids contribution of approximately 40 percent of the total biosolids load to the regional facility, rather than the expected 30 percent (30/110). So the first lesson learned is to carefully examine the biosolids production from each WWTP to determine the total mass load of solids for design. The 80 mgd WWTP would be the host site for the new regional biosolids processing facilities and would treat all sidestreams. Both WWTPs were considering improvements to convert from chemical phosphorus removal to Bio-P. A comprehensive GPS-X model was developed that included the two plants, the regional biosolids processing facility, and the sidestream treatment facility. GPS-X is a widely-used, commercially available software package from Hydromantis. A sampling study determined that the influent VFA concentration at the 80 mgd WWTP was too low to support Bio-P without VFA supplementation. A fermenter was added to the model to determine the VFA needs and to identify the impacts of a higher BOD load to the activated sludge facility. A layout of the model is shown on Figure 8. A second model was built without sidestream treatment to determine the effects that the sidestreams had on the 80 mgd WWTP. 80 mgd WWTP Fermenter Flow EQ Anaerobic Digestion Dewatering FeCl 3 Nitrif./Denitr. Regional Biosolids Processing Facility Sidestream Treatment 30 mgd WWTP Figure 8: GPS-X Model for Regional Biosolids Processing with Sidestream Treatment Several operating and design conditions were evaluated with the models, and the impacts of sidestream return on the 80 mgd Bio-P plant are shown on Figure 9. Figure 9 shows a phosphorus concentration profile through the liquid treatment processes, with sidestream treatment (in blue) and without sidestream treatment (in red). In the anaerobic zone of the Bio-P 5298

18 process, phosphorus is released and then re-assimilated in the oxic zone of the activated sludge process. Both model simulations show that soluble phosphorus is efficiently removed by the activated sludge process, since a fermenter and VFA supplementation were included in the design. The major difference in the two simulations can be seen in the first column of Figure 9 which represents the aeration basin influent. Without sidestream treatment (the red bar), the influent phosphorus concentration is three times the concentration in the simulation with sidestream treatment (the blue bar). To process this additional phosphorus load, significantly more VFA is required. 35 TP or PO 4 -P Concentration (mg/l) Without sidestream treatment, the TP load more than triples. Additional P-release required for Bio-P to work without sidestream treatment. (Approximately twice as much VFA needed, which the fermenter cannot produce alone; additional carbon supplementation needed.) BUT, even with additional VFA to remove most of the PO 4 -P, the effluent TP > 1.0 mg/l. Tertiary filtration will be required without sidestream treatment. 5 0 Aer.Basin Inf TP PreAX Zone PO4-P AN Zone PO4-P Oxic1 Zone PO4-P Oxic2 Zone PO4-P Oxic3 Zone PO4-P Plant Effluent TP Without Sidestream Treatment, With VFA Addition With Sidestream Treatment, With VFA Addition Figure 9: Phosphorus Profile in a Bio-P Plant with and without Dewatering Sidestreams The model simulation without sidestream treatment indicated that VFA supplementation was needed (in addition to what the fermenter could produce) to ensure adequate phosphorus removal. Approximately twice as much VFA was needed for the alternative without sidestream treatment, compared to the alternative with sidestream treatment. This can be seen in the anaerobic zone on Figure 9 (the third pair of bars) where over twice as much phosphorus must be released for bio-p to work without sidestream treatment (red), compared to with sidestream treatment (blue). However, even with the addition VFA to compensate for the lack of sidestream treatment, the plant effluent TP concentration is higher than the permit limit of 1.0 mg/l (the last pair of bars on Figure 9). With the addition of VFA, the increased BOD load causes a carryover of 10 to 15 mg/l TSS into the plant effluent, and the phosphorus in these solids produces an effluent total phosphorus concentration of greater than 1 mg/l. This is caused by the fact that the activated sludge MLSS must assimilate approximately twice the mass of phosphorus that the 5299

19 MLSS with sidestream treatment must assimilate. In the Bio-P process phosphorus is not destroyed; it is merely incorporated into biomass (solids) and the success or failure of the secondary treatment process is shifted to the solids capture efficiency of the clarifiers. This modeling has shown that the phosphorus content of the MLSS without sidestream treatment is in the 7-8 percent range, so the loss of 10 mg/l of MLSS would equal mg/l of total phosphorus in the effluent. This is an extreme case. It is common for Bio-P solids to carry at least 4 to 5 percent phosphorus. From these simulations, it was concluded that without tertiary filtration or chemical polishing in the activated sludge, sidestream treatment would be needed. The modeling also showed that more air would be needed in the activated sludge basins to handle the sidestream ammonia load. The total oxygen required for the cases with and without sidestream treatment was roughly the same (since the same mass of ammonia was being oxidized in both cases); however the blower capacity had to be evaluated in either case. The major difference between the two cases was the addition of sidestream treatment facilities or effluent filtration. In this evaluation, sidestream treatment was less costly than the addition of an 80 mgd effluent filtration facility. Sidestream treatment alternatives, which included conventional nitrification/denitrification and the SHARON process were compared in a detailed present worth analysis that included capital costs and operational and maintenance costs. Both alternatives included chemical phosphorus removal with ferric chloride. The analysis indicated that the two processes had essentially equal present worth. Case Study 2: Sidestream Management In this example a plant is being upgraded to meet new permit limits for total nitrogen and phosphorus. The previous permit required only seasonal nitrification for nutrient control. The upgrading will require full biological nutrient removal (BNR) capabilities. Currently, biosolids are stabilized by anaerobic digestion. The anaerobic digestion complex is being expanded to accommodate stabilization of biosolids from a sister BNR plant. The imported thickened biosolids, which will be hauled to the site and fed directly into the anaerobic digesters, will increase the influent total phosphorus load significantly. A mass balance of the plant that was created using BioWin software (Envirosim) to evaluate Bio-P. A schematic of the model is shown on Figure

20 Figure 10: BioWin Model of a Plant using Phosphorus Removal and Regional Biosolids Processing The mass balance for phosphorus and nitrogen is shown on Figure 11. The results are similar to those from the previous case study in that the influent phosphorus load to the activated sludge process is approximately doubled. Once again the Bio-P process required supplemental VFA, and the phosphorus content of the MLSS was again high (in the 7-8 percent range). In this case, the final solution was to add iron at the dewatering system and ahead of the primary clarifiers to remove the phosphorus mass contributed by the imported biosolids. The plant was already designed to handle the sidestream ammonia and BOD loads from its existing anaerobic digesters and had the capability to nitrify year-round. Since the activated sludge basins were being modified for denitrification and Bio-P, the blower systems were sized to handle the additional oxygen demand from the ammonia released from the imported sludge. In this case, the target effluent quality was 1.5 mg/l TP so effluent filtration was not necessary. 5301

21 PC Inf, ppd AB Inf, ppd Influent, ppd 620 TP 4,346 TN Total Recycle, ppd 1,216 TP 1,840 TN 1,836 TP 6,186 TN PC GT Overflow, ppd 161 TP 404 TN Primary Solids, ppd 704 TP 1,589 TN GT 1,132 TP 4,597 TN Aeration Basin Thickened Solids, ppd FC Cent Effluent, ppd 149 TP 1,427 TN WAS 983 TP 942 TN Centrate, ppd 393 TP 549 TN 543 TP 1,185 TN Thickened Biosolids, ppd 1,133 TP 1,734 TN Filtrate, ppd 662 TP 887 TN Hauled WAS BFP AN DIG Cake, ppd Digested Biosolids, ppd Hauled WAS, ppd 705 TP 1,100 TN 1,367 TP 1,986 TN 234 TP 305 TN Figure 11: Nutrient Mass Balance in a BNR Plant with Regional Biosolids Processing Case Study Conclusions Both case studies indicated that importing biosolids from other treatment facilities has significant impacts on Bio-P processes, and that importing 30 to 40 percent more phosphorus into the system can stress Bio-P processes to the point that new structures or processes are needed. For both case studies, detailed mass balances and cost evaluations were conducted to identify the impacts of nutrient-rich sidestreams. These mass balance tools help identify and quantify the hidden costs associated with sidestreams that are often overlooked. SUMMARY Sidestream nutrient loads do affect the design and operation of the liquid treatment processes and can affect the ability of a plant to maintain compliance with its permit. It is crucial to develop a thorough understanding of the hidden economic factors associated with sidestream loads. Plantwide mass balances that include both liquid treatment and biosolids processing facilities can help identify some of these hidden costs. An issue that often arises with Bio-P plants, as it did with the examples in this paper, is effluent filtration. If a WWTP has a stringent TP limit and is also processing phosphorus-rich solids from other Bio-P plants, a cost evaluation will likely be needed to compare the cost of tertiary 5302

22 filters with the cost of sidestream treatment. Similarly, if the WWTP has a stringent TN limit, the cost of sidestream treatment must also be compared to the hidden costs of handling the additional ammonia load at the liquid treatment facilities. The cost evaluations must include all capital and O&M costs, including labor, chemicals, electric power, and solids disposal. Identifying these costs and comparing them on a present worth basis will help communities make important decisions about their long-term WWTP planning. As more WWTPs receive permits that require nutrient control, sidestream management will become an increasingly prominent issue. Sidestreams become a more important issue when regional biosolids processing is involved, since imported nutrient loads are often not taken into account in the design of the liquid treatment processes. Each project is unique and plant capacity (both aeration basin capacity and blower capacity) can dictate whether sidestreams can be managed with the existing facilities, or whether separate sidestream treatment is necessary. Permit criteria differ from state to state and even within the same watershed. It is evident after examining these issues that the sidestream load impacts must be examined for every project. Some solutions will be simple, and others are more complex. However, even the simple solutions will increase the capital and operating costs of a project. Therefore, it is recommended that every project begin with a solids and nutrient mass balance to assess the impact of the sidestream return loads. Improvements to accommodate the sidestream nutrient loads must also be taken into account in the project budget. REFERENCES Barnard, J.; Shaw, A.; Lindeke, D. (2005) Using Alternative Parameters to Predict Success for Phosphorus Removal in WWTP s. Proceedings of the Water Environment Federation 78 th Annual Technical Exhibition and Conference. Gut, L.; Plaza, E.; Trela, J.; Hultman, B.; Bosander, J. (2005) Combined Partial Nitritation/Anammox System for Treatment of Digester Supernatant. Proceedings of the IWA Specialty Conference: Nutrient Management in Wastewater Treatment Processes and Recycle Streams. Hellinga, C.; Schellen, A.A.J.C.; Mulder, J.W.; M.C.M. van Loosdrecht; Heijnen, J.J. (1998) The SHARON Process: an Innovative Method for Nitrogen Removal from Ammonium- Rich Waste Water. Water Sci. Technol. (G.B.), 37 (9), 135. Kos, P.; Head, M.A.; Oleszkiewicz, J.; Warakomski, A. (2000) Demonstration of Low Temperature Nitrification with a Short SRT. Proceedings of the Water Environment Federation 74 th Annual Technical Exhibition and Conference. Riska, R.; Husband, J.A.; Kos, P.; Johansen, R. (2004) Pilot Scale Tests of a Unique Approach for BNR Upgrade of a Short SRT High Purity Oxygen System at Pima County, AZ. Proceedings of the Water Environment Federation 77 th Annual Technical Exhibition and Conference. Strous, M.; Heinen, J.J.; Kuenen, J.G.; Jetten, M.S.M. (1998) The Sequencing Batch Reactor as a Powerful Tool for the Study of Slow Growing Anaerobic Ammonium-Oxidizing Microorganisms. Appl. Microbiol. Biotechnol. 50, U.S. Environmental Protection Agency (1987) Design Manual: Phosphorus Removal. EPA/625/1-87/001; Cincinnati, OH. 5303