STARTUP OF ANAEROBIC MESOPHILIC DIGESTERS

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1 STARTUP OF ANAEROBIC MESOPHILIC DIGESTERS Marialena Hatzigeorgiou P.E./CH2M HILL Brian Owsenek P.E./Upper Occoquan Sewage Authority Tom Alkema/CH2M HILL Ronald Sieger P.E. BCEE/CH2M HILL and Karen Pallansch P.E./Alexandria Sanitation Authority *Ron Sieger passed away on May 3 26 ABSTRACT Many operators have started up mesophilic digesters but everyone has differing stories and experiences. This paper is a combination of the startup of several mesophilic anaerobic digesters with very low seeding to fully seeded conditions. Upper Occoquan Sewage Authority (UOSA) operates the Millard H. Robbins Water Reclamation Plant. UOSA started three anaerobic digesters between August 2 and March 21 as part of a major expansion which brought the permitted capacity of the facility from 32 to 54 million gallons per day (mgd). Alexandria Sanitation Authority (ASA) operates the ASA Advanced Wastewater Treatment Plant with an average plant influent flow of 54 mgd. ASA started four anaerobic digesters between May and October 25 as part of a solids upgrade that added pre-pasteurization and two new anaerobic digesters. This paper will provide a useful roadmap for digester startups for all operators. KEYWORDS Mesophilic anaerobic digestion biosolids solids startup. INTRODUCTION Case Studies. The plant expansion for UOSA involved conversion of one unheated unmixed secondary digester (8/1) to a primary digester by installation of mixing cannons heating equipment digester gas system and foam separation equipment. In addition the floating cover was fixed in place and new gas purifier systems and other miscellaneous instrumentation and automation equipment were installed. Digester 8/1 along with the retrofitted digesters 7/1 and 7/2 were startup as primary digesters between August 2 and March 21. The plant upgrade for ASA involved addition of a pre-pasteurization system two new anaerobic digesters and the retrofit of the two existing anaerobic digesters. For all four anaerobic digesters the work included installation of mixing cannons heating equipment digester gas collection and utilization system foam separation equipment and instrumentation and automation equipment. In addition the floating cover of the one existing digester was fixed and the two new mesophilic anaerobic digesters were built with structural thermophilic capability. The startup discussions for the pasteurization system and the anaerobic digesters were initiated in early fall of 24. The pasteurization system was performance tested on water and then operated on water during winter of By early spring of 25 a detailed startup plan 415

2 was provided for the startup of the anaerobic digesters with pasteurized solids (PSLDG) in order to achieve Class A biosolids pathogen density levels. The four digesters were started up as primary digesters between May and October 25. Tables 1 and 2 below provides the digesters information for UOSA and ASA respectively. Table 1 UOSA Anaerobic Digesters Parameter Dig. # 8/1 Dig. # 7/1 & 7/2 Maximum Capacity (gal) Diameter (ft) 8 8 Heat Exchangers (MBtu/hr) Spiral 2. Spiral 2. Sludge Recirculation (gpm) Type of Cover Fixed Metal Fixed Concrete Mixing System IDI Gas Cannons IDI Gas Cannons Mixing Capacity (gpm) Target Temperature (C) Table 2 ASA Anaerobic Digesters Parameter Dig. # Maximum Capacity (gal) 15 Diameter (ft) 95 Heat Exchangers MBtu/hr Sludge Recirculation (gpm) Type of Cover Mixing System Mixing Capacity Concentric tube counter flow gpm Fixed Concrete IDI Gas Cannons 564 gpm Target Temperature (C) 35 Paper Objectives. Mesophilic anaerobic digestion is a biological process which during startup requires time and careful management. Good planning continuous monitoring and slow changes are the three key factors for a successful startup. This paper provides a comparison in terms of 416

3 % TS increase stability and performance of anaerobic digesters started up from very low to fully seeded. It can be used as a guide for starting digesters at any plant. METHODS AND MATERIALS General. Anaerobic digestion seed includes raw sludge (primary sludge or scum) digested solids and pasteurized solids. Digested solids are preferred but raw sludge can be used when there is no other operating digester on site or close by from a nearby wastewater treatment plant. Pasteurized solids have not been used either in the United States or in Europe. Theoretically pasteurized solids will work for digesters seed but the startup time is expected to be far more lengthy and operational intensive. Upper Occoquan Sewage Authority (UOSA) VA. Solids processing at UOSA consists of feeding waste activated (WAS) and primary sludge (PSD) to the digesters. The stabilized solids are then get dewatered and dried. Alternatively WAS can be mechanically thickened dewatered dried and disposed after lime addition. The project was phased such that the secondary digester (8/1) was taken out of service first retrofitted and returned to service as a primary digester. Once the biological processes had stabilized an existing primary digester (7/1) was taken out of service retrofitted and returned to service. Finally the remaining primary digester (7/2) was retrofitted and returned to service. Startup conditions for the two operations differed substantially. In startup of digester 8/1 the digester was filled with plant water heated to 36C seeded from another digester and then fed with primary sludge. For the second startup that of digester 7/1 the digester was filled with primary sludge and seed. Unfortunately there are no data available on the exact amount of seed used for this startup. All that is available is the monitoring data during the startup that indicate that the amount of seed used was not adequate for the feed rates to that digester. Alexandria Sanitation Authority (ASA) VA. The normal operation solids processing at ASA consists of: Primary (PSD) and tertiary (TSD) sludge to gravity thickeners Waste activated sludge (WAS) to mechanical thickeners Gravity and mechanically thickened sludges blended and pasteurized Pasteurized solids (PSLDG) to anaerobic digesters Digested solids (DS) to dewatering At the time of the start up of the four high rate mesophilic digesters of 1.5 mgd capacity each ASA had no operating digesters to seed the new digesters and the plant was in operation at all times so there could be minimum interruption to the daily solids process. In addition the volume of seed hauled in from a nearby plant had to be minimum in order not to contaminate the volume of the newly built digesters with other than Class A pathogen levels. The digesters were started one at a time. The very first digester required the most attention and took the longest. It was filled to the top of the canon mixers with plant effluent that was then 417

4 heated to mesophilic digestion temperature. At the same time the headspace was purged with nitrogen gas as was the associated digester gas piping. Once purged and heated the digester was seeded with digested biosolids from the Upper Occoquan Sewage Authority (UOSA) plant. The volume of the seed solids was equal to 4.5% of the total volume of one digester. Once the digester was seeded PSLDG was added at a very slow rate. The digester had to be carefully managed in order for the slow-growing methane formers to populate the digester and to balance the acid formers. The digester parameters (temperature total solids volatile solids volatile acids alkalinity ph VSR) along with the feed parameters (total and volatile solids) were monitored and evaluated based on the ranges provided below: Temperature between 34.5 and 35.5 C with 35 C target value. Ratio of volatile acids/alkalinity less than.35 ph between 6.5 and 8. with target value of 7.1 VSR higher than 4% Ratio of TS feed/ts in digester between.5 to.1 Ratio of VS feed/vs in digester between.3 to.1 Digester gas methane concentration higher than 55% by volume concentration Every time the digester feed rate was modified a new period of the startup began. Each period was based upon an average week. There were specific goals for feed loading and digester results. Decisions on whether to proceed to the next period or not were made based on the monitored parameters. It took a couple of periods longer to start the first digester than anticipated because the desirable solids concentration of the feed PSLDG was not achieved until later in the startup. Table 3 is the planned table for the first digester to start up (Digester 4) provided with the startup plan in March of 25. The digested solids in (lb/day) calculated in the digester at the end of the period includes the volatile solids reduction plus the solids lost from the digester through the transfer. Table 3 - Feed Rates and Time from Startup to Achieve Solids Concentration in Digester Period Vol in start (gal) Feed Vol/day (gal/day) Feed Rate (gal/hr) Vol in Dig after 7 days DS in start (lbs) DS added per day (lbs) DS in Dig after 7 days (lbs) DS in Dig after 7 days with VSR + Transfer (lbs) % TS in DS after 7 days Adjustable Values Seed Volume fed Digester volum % Solids in seed Digester deten % Volatile solids % Volatile solids full % Solids in PSLD Once the first digester reached the overflow level digested solids were then transferred to the next dormant digester for startup. Solids were transferred every day from the first digester to the 418

5 dormant digester equal to the daily feed to the first digester. The dormant digester headspace was isolated from the rest of the digester gas piping and did not require purging. Once the dormant digester level reached the cannon mixers temperature was confirmed mixing started and raw solids were fed to the digester at much higher feed rates. Once the second digester reached the overflow level the same concept as above was applied to the third dormant digester by transferring from the first two digesters that were now in service. Finally the dormant digester concept was applied to the last digester by transferring digested solids from the other three digesters in service. Table 4 is one of the tables created during startup to assess the current period based on the parameters monitored and calculate the next period s feed rate (this is the table created during periods 17 and 18 for the first digester). The actual test analysis results for the % TS and % VS are used for both the digesters feed and the digesters contents in order to plan the next period s feed rate and calculate the expected % TS by the end of that period. Similar tables were created for the other three digesters as well. Table 4 - Actual Feed Rates and Time from Startup to Achieve Desired Solids Concentration in Digester Period Days in start of Period Vol in start (gal) Feed Vol/day (gal/day) DS in Vol in start of end of period period (lbs) TS in seed/feed per period (%) VS in seed/feed per period (%) TS added per day (lbs) Actual DS Actual % TS in Adjustable in end end of Values of period period (lbs) Seed % 6.2% % % 75.7% % % 74.1% % Dig vol. gal % 73.9% % % 73.9% % % 72.9% % 6 full % 71.5% % 7 ovflow % 71.3% % 8 ovflow % 73.9% % 9 trsfr % 75.5% % % 75.4% % % 74.9% % % 74.6% % /16/ % 75.4% % /25/ % 75.9% % /2/ % 75.7% % /6/ % 76.8% % /13/ % 78.6% % Planning Feedrates and Time from Startup to Achieve Desired % Solids in Digester (With VSR) Period Days in Period Vol in start (gal) Feed Vol/day (gal/day) Actual DS in Vol in start end of period of period (lbs) DS added per day (lbs) DS in of period (lbs) Calculated DS in end of period with VSR + Transfer (lbs) Calculated % TS in end of period Adjustable Values 14 trsfer % 15. Dig SRT day 15 trsfer 4 9/2/ % 78.6% VS in feed 16 trsfer 7 9/6/ % 61.6% VS in digeste 17 trsfer 1 9/13/ % 56% VSR 17 trsfer 1 9/14/ % 18 prg/dwtr 7 9/24/ % 4.48% TS in feed 19 dwtr 7 1/1/ % 2 dwtr 7 1/8/ % There was a series of items or obstacles that had to be considered or overcome respectively. In reference to the digester gas (DG) system the purging of the digester headspace with nitrogen had to be scheduled and coordinated so that the required amount of nitrogen was pressurized in the digester in a timely manner. A leaking gasket of the liquid relief port was not detected during the tank air test and was addressed during startup. As a result the liquid relief port for the first digester that was started up had to be clamped down and was no longer an effective means 419

6 of digester s roof structural safety device. This liquid relief port gasket was changed to the appropriate one at a later date when the digester was taken out of service for maintenance. The enclosed flares operating parameters had to be modified in order to burn the initial low flow DG produced. Safety was the first priority for all actions and especially for when handling digester gas (DG) equipment. In reference to the solids operation all activities had to be coordinated based on continuous plant operation. The operation of the pasteurization facility was coordinated such that to efficiently and effectively ensure the production of the required daily digester feed. The ASA digesters temperature is controlled with the daily PSLDG feed temperature. The digester heat exchangers are not in service under normal operation. Upstream solids processes were operated such that PSLDG feed to the digesters was high in % TS (between 4.5 and 5.5 %TS). Purging and cleaning of equipment downstream of the digestion process were conducted in order to achieve Class A biosolids pathogen levels throughout. The moderate foaming that was developed after high filament activity in the biological reactors and high digester feed rate (see period 13 under Table 4) made it necessary to decrease the digester feed rate relative to the initial planned feed rate. RESULTS Alexandria Sanitation Authority (ASA) VA. For all four digesters volatile acids and alkalinity have remained well within appropriate ranges demonstrating that the digesters had no signs of difficulty such as foaming or low ph. The first digester was put in service (started feeding PSLDG) on June 3rd 25 and reached 2.3% total solids (TS) by the time we started transferring digested solids (DS) to dewatering which was the last week of September from all four digesters. The second digester was put in service on August 3th 25 and reached 2.2% total solids (TS) by the time we started transferring DS to dewatering. The third digester was put in service on September 14th 25 and reached 1.9% total solids (TS) by the time we started transferring DS to dewatering. The fourth digester was put in service on September 24th 25 and reached 1.8% total solids (TS) by the time we started transferring DS to dewatering. The success of the ASA digesters startup was a result of detailed preparation thorough equipment testing and operation of equipment with plant effluent water before start up training and working as a team with the operators often monitoring and evaluating the digesters in service and patience. DISCUSSION The charts below provide a comparison and information on anaerobic digesters start up based on the case studies seed and feed total solids concentration summarized in Table 5. 42

7 Table 5 UOSA/ASA Digesters Startup Seed Summary Parameter by Digester ASA UOSA Dig. 4 Dig. 3 1 Dig. 8/1 Dig. 7/1 Seed DS (% digester volume) n/a Feed (type) PSLDG PSLDG PSD PSD Feed (gal/day) Feed % TS % TS in digester /3/5 6/18/5 7/3/5 7/18/5 8/2/5 8/17/5 9/1/5 9/16/5. Feed % TS Feed: 7 day avg % TS: 7 day avg Figure 1 ASA digester 4 startup feed rates and % TS % TS Feed (gal/day) Feed % TS in digester /24/5 1/9/5 1/24/5 11/8/5 11/23/5 12/8/5 12/23/5 Feed % TS Feed: 7 day avg % TS: 7 day avg Figure 2 ASA digester 1 startup feed rates and % TS 421

8 Feed (gal/day) % TS Feed % TS in digester /1/ 8/16/ 8/31/ 9/15/ 9/3/ 1/15/ 1/3/ 11/14/ 11/29/ Feed % TS Feed: 7 day avg % TS: 7 day avg Figure 3 UOSA digester 8/1 startup feed rates and % TS The feed rate increase through the start up of ASA digester 4 is considerable less than the ones indicated for ASA digester 1 and UOSA digester 8/1 because of the small amount of seed solids that this digester received (only 4.5% of total volume). In addition the feed rate in UOSA digester 8/1 is higher than the feed rate of ASA digester 1. UOSA feed solids had an over 1% less concentration than the ASA feed solids therefore higher feed rates were acceptable. UOSA generally tends to be more aggressive in reference to the digesters startup feed rates Alkalinity (mg/l) ph Alaklinity ph 6 3 VAs 6/3/5 6/18/5 7/3/5 7/18/5 8/2/5 8/17/5 9/1/5 9/16/ Figure 4 ASA digester 4 startup stability Alkalinity VAs ph ph: 7 day avg Alkalinity: 7 day avg VAs: 7 day avg. 422

9 Alaklinity Alkalinity (mg/l) ph ph VAs /24/5 1/9/5 1/24/5 11/8/5 11/23/5 12/8/5 12/23/5 Alkalinity VAs ph Alkalinity: 7 day avg VAs: 7 day avg. ph: 7 day avg Figure 5 ASA digester 1 startup stability Alkalinity Alkalinity (mgl/l) ph ph VAs /1/ 8/16/ 8/31/ 9/15/ 9/3/ 1/15/ 1/3/ 11/14/ 11/29/ Alkalinity TOAs ph ph: 7 day avg Alkalinity: 7 day avg TOAs: 7 day avg Figure 6 UOSA digester 8/1 startup stability The slow increase in alkalinity for ASA digester 4 along with the ph stabilizing is indicated on figure 4. ASA digester 1 and UOSA digester 8/1 indicate a more stable operation from the beginning of startup. ASA tests their samples for volatile acids versus the total organic acids that UOSA perform analysis for. Both perform the analysis based on the recent standard methods. The total organic acids include the volatile acids along with crotonic adipic pyruvic phthalic fumaric lactic succinic malonic gallic aconitic and oxalic acids. For both plants the indicated acids do not raise any concerns. For UOSA digester 8/1 ph actually reached a minimum of 6.56 S.U. while TOA was decreasing from its peak. A ph at this level is normally 423

10 considered undesirable and potentially inhibitory of methanogens but no obvious ill effects were observed DG Produced x 1 ft3/day %VSR DG % VSR /3/5 6/18/5 7/3/5 7/18/5 8/2/5 8/17/5 9/1/5 9/16/5 4 DG Produced %VSR % VSR: 7 day avg DG produced: 7 day avg Figure 7 ASA digester 4 startup performance DG Produced x 1 ft3/day %VSR DG % VSR /24/5 1/9/5 1/24/5 11/8/5 11/23/5 12/8/5 12/23/5 DG Produced %VSR % VSR: 7 day avg DG produced: 7 day avg Figure 8 ASA digester 1 startup performance 424

11 % VSR 45 4 %VSR /1/ 8/16/ 8/31/ 9/15/ 9/3/ 1/15/ 1/3/ % VSR % VSR: 7 day avg Figure 9 UOSA digester 8/1 startup performance The performance for the digesters start up was evaluated based on the percent volatile solids reductions (% VSR) and the DG production. Once the digester reached its loading capacity the % VSR stabilized between a range of high 4s and low 6s. For UOSA digester 8/1 there is an indication of some low % VSR values at the end of September 2 that are not supported by the available data. The DG production indicated for ASA digester 4 actually commenced before August 25. DG was produced as early as the third day of startup but actual data were not available before August 25. The amount of DG produced represent the total amount produced daily by all digesters in service. Data in reference to DG produced from the UOSA digesters during startup are not available. 425

12 % TS Feed (gal/day) Feed % TS in digester /6/1 3/21/1 4/5/1 4/2/1 5/5/1 5/2/1 6/4/1 6/19/1. Feed % TS Feed: 7 day avg % TS: 7 day avg Figure 1 UOSA digester 7/1 startup feed rate and % TS ph Alkalinity (mgl/l) Alkalinity VAs /6/1 3/21/1 4/5/1 4/2/1 5/5/1 5/2/1 6/4/1 6/19/1. Alkalinity TOAs ph ph: 7 day avg Alkalinity: 7 day avg TOAs: 7 day avg Figure 11 UOSA digester 7/1 startup stability 426

13 %VSR 5 % VSR /5/ 3/2/ 4/4/ 4/19/ 5/4/ 5/19/ 6/3/ % VSR % VSR: 7 day avg Figure 12 UOSA digester 7/1 startup performance Figures 1 through 12 provide information on the startup of UOSA digester 7/1. The exact quantity of seed provided for the startup of digester 7/1 back in Spring of 21 is not available. Based on the steep slope of the % TS along with the high amounts of total organic acids as shown in Figure 11 and the low %VSR it is seen that digester 7/1 was overloaded at the beginning of April 21. Digester 7/1 was started under emergence conditions; there was not sufficient time for planning and monitoring the results were the overloaded digester. A total of 33 lbs of sodium bicarbonate were added between April 4 th and April 6 th 21. The sodium bicarbonate provided the required buffering for the digester to establish stable conditions (total organic acids decreased and VSR % increased). CONCLUSIONS Whether a digester is being put in service for first time or back in service after maintenance planning monitoring and patience are the key factors for its startup. A digester can be started up with minimum seed (less than 5% of total volume) or a lot of seed (over 5% of total volume) the less the seed the longer it takes and the more careful management it requires. If multiple digesters are to be started up the best practice is to start up one at a time and to use the dormant digester concept. Per the dormant digester concept when the starting up digester reaches the normal operating level then digesting solids equal to the daily feed of the seed digester are transferred to the next digester to start up. During filling the dormant digester is not being mixed neither fed. Anaerobic digesters react slowly to any adverse changes but also require time in order to correct any missteps. Chemicals like sodium bicarbonate are available in order to add buffering 427

14 capacity but they should be applied only when really needed and not as means to speed up the start up. ACKNOWLEDGMENTS Credits. The authors thank the operators and lab personnel of the two wastewater treatment plants who performed the start up and collected the data that formed the basis of this document. Authors. A big thank you to Ron Sieger for his assistance with the ASA startup. He is greatly missed. REFERENCES U.S. Environmental Protection Agency (23) Environmental Regulations and Technology Control of Pathogens and Vector Attraction in Sewage Sludge; EPA-625/R Water Environment Federation (1998) Design of Municipal Wastewater Treatment Plants 4th ed.; Manual of Practice No. 8; Alexandria Virginia. Standard Methods Committee (1996) 556 Organic and Volatile Acids 428