DESIGN AND OPERATIONAL CONSIDERATIONS TO AVOID EXCESSIVE ANAEROBIC DIGESTER FOAMING

Transcription

1 DESIGN AND OPERATIONAL CONSIDERATIONS TO AVOID EXCESSIVE ANAEROBIC DIGESTER FOAMING ABSTRACT Neil Massart*, Black & Veatch Robert Bates, Louisville and Jefferson County Metropolitan Sewer District Blair Corning, South Adams County Water and Sanitation District Gary Neun, Black & Veatch *Black & Veatch 8400 Ward Parkway Kansas City, MO Excessive foaming in anaerobic digesters has been a problem for many years. All anaerobic digesters will foam to some extent, but excessive foam can be problematic. Excessive foaming is defined as foam that interferes with flow through the gas piping system and/or is not contained within the digester. Causes of foaming include presence of excessive filamentous bacteria, extreme amounts of oils and grease, and the feed sludge composition (primary sludge (PS) versus waste activated sludge), but the chief cause of excessive foaming is inconsistent feed to the digesters. Many utilities have evaluated technological changes to the digester facilities. While such changes may reduce/control foaming, they often involve expensive additions to the facility in terms of both capital and operation and maintenance (O&M) costs. Consequently, before investing in technology changes, utilities should consider operational changes that could mitigate foaming. In addition, facilities that are installing new digesters should consider design elements that can reduce the potential for foaming as well as the effects of foaming on ancillary equipment. Design considerations to avoid excessive digester foaming will be presented followed by three case studies of unstable operations at existing digester facilities, and operational considerations to avoid excessive foaming will be presented in relationship to the three case studies. KEYWORDS Anaerobic digester, foam, monitoring, primary solids, waste activated solids INTRODUCTION Foaming is a serious problem with many anaerobic digesters, and it seems to be becoming more prevalent across the country. No one knows the exact cause, as there is no single common thread; however, researchers believe that the increase in advanced biological treatment processes and the corresponding changes in the quantity and characteristics of the WAS are key factors. Foaming is reported to occur more frequently if the ratio of WAS to total sludge exceeds 40 percent. There are usually multiple factors that contribute to digester foaming: 2553

2 Slug feed to the digesters Intermittent feeding of digesters Separate feeding or inadequate blending of WAS and PS Insufficient or intermittent mixing of digesters Excessive grease and scum in digester feed, especially if fed in batches WAS content exceeds percent of the total amount of sludge fed Filaments, specifically Nocadia, in activated sludge fed to the digesters Presence of Organism B, an anaerobe filament believed to be generated in the digester, and thriving under conditions of high volatile solids loadings While some foaming always occurs, problematic foaming can result in loss of active digester volume, structural damage if it causes the digester cover to rise, spillage, and adverse impacts on gas handling equipment. Foaming is considered as long as it is contained under the digester cover and does not interfere with the working of the gas piping system. Foam is defined as excessive if it plugs piping and/or escapes the containment of the anaerobic digester. Excessive foaming is illustrated on Figure 1. Figure 1 - Excessive Digester Foaming Facilities that are planning to build new digesters should consider design elements that can reduce the potential for foaming as well as the effects of foaming on ancillary equipment. Many utilities have evaluated changes to their digester technology, such as adding ultrasound to pretreat the WAS ahead of digestion or conversion to two-stage digestion. While such changes may reduce or control the foaming, they often involve expensive additions in terms of both capital costs and O&M costs. Consequently, before investing in technology changes to existing facilities, utilities should consider operational changes that could mitigate the foaming. Anaerobic digestion systems are complex and do not run on autopilot; they require daily sampling, analysis, data review, and process control adjustments. Excessive foaming is 2554

3 controllable if the operator is provided the time and resources to monitor the system. The goal of the facility operator should be to minimize the factors that exacerbate foaming rather than to eliminate foaming altogether. A dedicated and knowledgeable staff is essential to the operation of the anaerobic digesters and related processes. It is imperative the staff be adequately trained to control digester performance. Regular staff meetings to discuss data trends and digester sampling protocol are essential for routine operation of the digesters. DESIGN CONSIDERATIONS Design considerations are essential to efficient operation of anaerobic digesters and control of excessive foaming, such as adequate mixing of primary sludge and WAS ahead of the digester, and in the digester; adequate digester volume; sludge feed and withdrawal schemes; and equipment such as foam separators, grit removal facilities, electrically operated drip traps, and sloped gas piping systems. Because the extent of foam generation is uncertain, its elimination and/or provision of gas utilization equipment that can handle foam must be a design consideration. The designer of an anaerobic digester system must think not only about the anaerobic digester system, but also about the other systems within the plant that will affect digester operation. For instance, if the plant receives hauled waste, the waste should be stored and metered into the digesters rather than being pumped at a high rate into the digester. Anaerobic digestion is not merely an add-on process for sludge treatment, but an integral part of the treatment system. The digester complex must be compatible with other components of the plant and the type of sludge being processed in the anaerobic digesters. The sludge must be properly blended before it enters the digester to assure consistent solids concentration in the digester feed, and thus, a more consistent feed of volatile solids. Upstream process such as grit removal is vital to effective digester performance. If the grit is not removed, it could pass into the primary clarifiers and be pumped to the digesters, where it could settle and effectively reduce the usable digester volume. To ensure that all solids that enter the digester will leave the digester, adequate mixing is necessary. Adequate mixing produces a uniform solids concentration throughout the digester, which is important for the reduction of volatile solids. The feed to the digester should be well mixed, especially if it consists of a combination of WAS and PS. The ratio of WAS to PS can affect the amount of foam formation, but the more critical parameter for controlling foaming is the volatile solids loading rate which can be partially controlled by proper blending of feed sludge. Withdrawal from the digester should be done by hydraulic overflow rather than by opening and closing a valve. The difference in hydraulics caused by opening a valve to withdraw sludge will result in the level and the volume of the digester contents to vary. By using a hydraulic overflow, the level of sludge in the digester and its volume will always be a constant. 2555

4 Other means of controlling foaming include ensuring that the gas piping is sloped properly and installing electrically operated drip traps at key points in the piping to remove condensed water from the piping system. Water in the gas piping can cause blockages that increase the pressure in the system. When the blockage is removed, foaming can occur in the digester. This is analogous to shaking a can of soda and then opening the top: The sudden decrease in pressure within the can will cause the contents to foam. Thus, it is imperative that the system is designed to allow the condensate to be quickly and effectively removed. Two other add-on features can be considered when designing a digester: ultrasonic pretreatment of WAS and foam separator. While both treat the symptoms rather than the cause, they can be effective in alleviating short-term process upsets. Pretreatment of WAS with ultrasonic energy causes cavitation within the microbe cell membrane and creates a soluble chemical oxygen demand (COD) that can be readily digested. Conditioning of thickened waste activated sludge (TWAS) using ultrasound before anaerobic digestion has been recently implemented in a few treatment facilities in both Europe and North America to achieve more complete digestion and to enhance volatile solids reduction. Increased volatile solids reduction translates to increased biogas generation (of relevance where biogas is reused), and decreased quantities of stabilized biosolids for disposal. In some cases, improved dewaterability of the sludge has also been reported. Many of the installations that use ultrasonic conditioning have also made anecdotal references to observing a decrease and in some cases complete elimination in foaming incidents. The drawback of ultrasonic conditioning is nutrients are released which can increase by 10 percent the amount of nitrogen recycled to the head of the plant. An ultrasound conditioning system is illustrated on Figures 2 and 3. Figure 2 Sonix Ultrasound System Figure 3 EMICO Sonolyzer 2556

5 The other add-on option, the foam separator, removes foam by drawing gas/foam from the top of the digester and beating it down using a series of water spray nozzles. The excess water flows through a P-trap to a drain leading to the headworks of the wastewater treatment plant. This option is attractive where there is inadequate blending of the feed sludge or where the feed rate to the digester is highly variable. A schematic and a picture of a foam separator are presented in Figures 4 and 5. Figure 4 Varec Foam Separator Figure 5 Foam Separator Schematic OBSERVATIONS Three case studies representing various process scenarios will be presented, demonstrating that the usual culprit for excessive anaerobic digester foaming is inconsistent feeding. Case Study No. 1 The Morris Forman WWTP in Louisville, Kentucky, has a treatment capacity of 105 mgd. This plant was upgraded in 2003 by converting the four sludge holding tanks to four 1.8 million gallon anaerobic digesters, followed by five high rate centrifuges and four triple pass drum dryer 2557

6 systems. The digesters have an associated heating system consisting of a recirculation pump, a spiral heat exchanger, and a heated water pump. Each digester is equipped with a gas mixing system consisting of a liquid ring gas compressor, gas distribution manifold, and 14 gas bubble cannons. Excessive foaming at the Morris Forman facility was a result of inconsistent organic loading (Figure 6). The four digesters were loaded with primary sludge plus an occasional load of hauled-in septic solids received as hauled waste. Foaming occurred whenever the mass of volatile solids to the digesters was significantly increased either by pumping too large a volume or by not maintaining the feed solids concentration at less than 5 percent, which caused the organic loading rates to exceed the targeted operating rate. Once the mass of volatile solids to the digesters was controlled, the excessive foaming events ceased. Figure 6 Excessive Foaming Event At the Morris Forman facility, the anaerobic digesters perform two functions: 1. Produce digester gas for use as fuel by the dryer system 2. Generate a uniform product for dewatering and drying Monitoring the digester is vital to proper operation, and has greatly enhanced the performance of the digester. Figure 7 shows the digester performance in

7 Figure 7 Digester Performance in Digester Performance 300, , VS Loading (lbs/day) 200, , , Volatile Acids (mg/l) , /1/2004 2/20/2004 4/10/2004 5/30/2004 7/19/2004 9/7/ /27/ /16/2004 Date VSS fed to digester Volatile Acids Foaming occurred when the feed to the digester was inconsistent and the percentage of change in the feed rate was greater than 20 percent. Once the feed rate was controlled, volatile acids formation remained consistent and no excessive foaming events were reported. Figure 8 represents a two-month period of inconsistent feed to the digester, and Figure 9 represents a twomonth period of controlled feed rate. 2559

8 Figure 8 Inconsistent Feed to the Digester, July 2004 to August 2004 Digester Performance July August , , VS Loading (lbs/day) 200, , , Volatile Acids (mg/l) , /1/2004 7/11/2004 7/21/2004 7/31/2004 8/10/2004 8/20/2004 8/30/2004 Date VSS fed to digester Volatile Acids 2560

9 Figure 9 Consistent Feed to the Digester, October 2004 to November 2004 Digester Performance October November , , VS Loading (lbs/day) 120, ,000 80,000 60,000 40, Volatile Acids (mg/l) 20, /1/ /11/ /21/ /31/ /10/ /20/ /30/2004 Date VSS fed to digester Volatile Acids Case Study No. 2 A WWTP in South Carolina was upgraded in 2000 to include three anaerobic digesters and ancillary equipment. The digesters measure 85 feet in diameter, with a sidewater depth of 30 feet and a volume of 1.4 million gallons each, for a total volume of 4.2 million gallons. Total design loading is 58,900 pounds per day under annual average conditions, with a detention time of 29 days for 5 percent total solids (TS), and 75 percent volatile solids (VS). The digesters are equipped with floating gasholder covers (arch type), which have a gas storage volume of 26,000 cubic feet, and are mixed by nine large-bubble gas cannons each. The digester gas is circulated by liquid-ring type compressors. Excessive digester foaming was a recurring problem and became the focus of an evaluation (Figures 10 and 11). The foaming caused sludge to overflow the walls of the digesters and to enter the biogas collection piping, blinding the flame traps and interfering with the operation of the gas mixing systems. 2561

10 Figure 10 Excessive Foaming Event Figure 11 Excessive Foaming Event Although excessive amounts of grease and insufficient blending were contributing to the foaming problems, they were determined not to be the primary cause of foaming. The primary cause was thickened sludge, both primary and WAS (which was routinely injected into the digester at concentrations greater than 5 percent) settling to the bottom of the digester, and the unstable operating conditions resulting from the inconsistent organic loading. The following recommendations for reducing the foaming included evaluation of the upstream processes, digesters, and digester support equipment. 1. Maintain solids concentrations in digester feed at 4.5 to 5 percent. Higher concentrations reduce the effectiveness of the digester mixing systems. 2562

11 2. Maintain digester loading rates from 100 to 150 pounds per 1,000 cubic feet of digester volume for mixed PS and TWAS. 3. Analyze the PS and TWAS characteristics and total solids, and volatile solids concentrations, at least daily to track variations in organic loading rates to the digesters and track the flow rates of PS, TWAS, and scum to the digesters. 4. Limit the daily variation in volatile solids load (organic load) to the digesters to ± 5-10 percent by trending volatile solids loading. 5. Trend the volatile solids reduction against the actual digester gas production, which typically averages cubic feet per pound of volatile solids destroyed. Foaming can reduce the effluent volatile solids concentrations below these values, as a result of entrapment of solids in the foam layer, causing abnormally high gas production rates. 6. Maintain lower sludge blankets in the primary clarifiers to improve removal efficiencies. Improved removal of biochemical oxygen demand (BOD) in the primaries reduces secondary sludge production for digestion. Since these recommendations were implemented, excessive foaming events have ceased. The impacts of large variations in volatile solids feed on the anaerobic digester are depicted on Figure

12 Figure 12 Effects on the Digester Variations in Volatile Solids Loading Digester Performance Inonsistent VSS Feed/ Excessive Foaming Consistent VSS Feed/ No Foaming Volatile Solids (lbs/day) Volatile Acids (mg/l) /24/2004 1/26/2004 1/28/2004 1/30/2004 2/1/2004 2/3/2004 2/5/2004 Time (Weeks) Volatile Solids Volatile Acids The plot on Figure 12, which represents the digester performance for a two-week period, illustrates the importance of controlling digester feed. Overfeeding the digesters caused excessive foaming. Once the volatile solids loading was controlled, the excessive foaming ceased. Trending the data became invaluable to the plant operating staff in controlling the feed, thus stopping the excessive foaming. Case Study No. 3 The Williams Monaco WWTP in Henderson, Colorado, is another plant where operational controls were implemented to eliminate excessive foaming and to restore an upset digester to proper operation. The plant consists of one primary and one secondary digester. PS and WAS are pumped to a gravity thickener to be co-thickened. The thickened sludge is pumped to the primary digester, and thickener overflow is pumped to the head of the plant. The primary digester overflows by gravity to the secondary digester. The gravity thickener is 45 feet in diameter, with a sidewall depth of 10 feet and a volume of approximately 136,000 gallons (Figure 13). The primary digester is 45 feet in diameter and 28 feet deep with a capacity of 344,000 gallons, and is equipped with three gas mixing cannons (Figure 14). The secondary digester has a capacity of 321,000 gallons. 2564

13 Figure 13 Gravity Thickener Figure 14 Primary Digester Overloading and excessive foaming of the primary digester would occur due to the large variations of thickened solids discharged from the gravity thickener (Figure 15). 2565

14 Figure 15 Dried Excess Foam The concentration of solids in the gravity thickener ranged from 4 to 8 percent, with the depth of solids varying accordingly, leading to organic overloading and excessive foaming. The gravity thickener not only thickened the solids, but also acted as a fermenter, producing volatile fatty acids (VFA), which would not normally be necessarily detrimental to the process, except that the volatile solids loading to the digester had been extremely erratic. To correct the problem, operators established and maintained a consistent solids concentration in the gravity thickener and prepared process control trend charts to correct any problems before the digester became upset. Figure 16 illustrates data trending and how to recognize inconsistent volatile solids feed to the digester. 2566

15 Figure 16 Volatile Solids Loading in the Digester Primary Digester Performance Inconsistent VSS Feed/ Excessive Foaming Upset Digester/ Excessive Foaming Corrective Action Taken Consistent VSS Feed/ No Foaming VSS Feed to Digester lbs/day Volatile Acids in Digester mg/l Time (Weeks) VSS fed to the digester Volatile Acids 0 From week 1 to week 19, inconsistent feed to the digesters caused excessive foaming and large variations in the volatile acids concentration, which led to decreased gas production and inability to restore the proper ph in the digester. As a corrective action, the feed to the digester was decreased to 20 percent of the design loading capacity to allow the methane formers to recover and to reduce the volatile acids concentration in the digester. When the excessive foaming ceased, the feed to the digesters was gradually increased to match the rate of feeding volatile solids into the gravity thickener, as shown for week 26. The plant has continued to maintain the volatile solids loading rate, operation of the digester has stabilized, and the excessive foaming has stopped. OPERATIONAL CONSIDERATIONS The causes of excessive digester foaming include an excessive amount of filamentous bacteria, excessive grease, and unstable operating conditions. In the three case studies presented, all excessive foaming events were caused by unstable operating conditions. Excessive amounts of filamentous bacteria in the WAS feed from secondary treatment process were not considered the primary cause of foaming in any facilities studied. While such bacteria may have been present and may have contributed to foaming, the intermittent nature of the foaming suggests other primary causes. Experience has shown that the effects of filamentous 2567

16 bacteria on foaming can be controlled as long as the basic principles of operating an activated sludge system are observed. Waste grease from grease traps and hauled waste are processed in the digesters in all three case studies. Greasy components of the scum in the digesters feed are hydrophobic and tend to float in the gravity settling basins. Septage wastes may contain large quantities of grit or inert materials that could ultimately settle in the digesters, and reduce their capacity. In all case studies, excess grease and oils at various concentrations were fed to the digesters with the PS and/or WAS. There was no apparent correlation between the amounts of foaming and the amount of oil and grease in the digester feed. If the sludge in the digester is not properly mixed, a scum layer may accumulate on its surface and adhere to the gas bubbles, forming a foam layer. The biogas generated will not be completely stripped off the digesting solids. Consequently, the gas bubbles can adhere to the solids, reducing their density and propelling them to the surface. Mixing can also have an adverse effect by increasing the entrapment of gas bubbles in the liquid, thereby generating foam. In all the case studies, the digesters were thoroughly mixed by observing the consistency of the digested solids from the digester to the dewatering processes over the time period. Organic overload on a digester generates more VFA than the methane-producing bacteria are able to consume. The production of VFA is accompanied by production of carbon dioxide gas. A biological overload caused by excess feeding will result in an increase in the carbon dioxideto-methane ratio in the digester gas. Depending on the composition of the feed, shock loading or overloading can lead to higher rates of digester gas production, causing foaming problems. Unstable operation resulting from inconsistent organic loading to the anaerobic digesters is the most common cause of excessive foaming. The factors contributing to unstable conditions encountered in the three case studies include the following: No blending tank or a blending tank of insufficient volume for mixing the various types of feed sludge, such as primary sludge, thickened WAS, and grease trap and septic tank sludges. Inadequate mixing of the digester contents. The gas cannon mixing systems, which are designed for use with 5 percent or lower total solids concentration in the digesters, are not always in operation because of foam binding of flame traps or leaky gas piping. The mixing rates are erratic, as balancing flows to the various guns is difficult to maintain. Failure to maintain digester contents at a stable temperature of 95 F. Failure to keep digester contents at a level to avoid foam overflow into the gas system and over the digester walls (at least 2 feet above the gas mixing system). If the level is too low, the effectiveness of the mixing systems or hydraulic detention time is reduced. Grit accumulation, which reduces both the effective volume of the digesters and the efficiency of the gas mixing system. Operational adjustments and recommendations for reducing or eliminating excessive digester foaming, including plant processes that affect digester operation in the three case studies, include the following: 2568

17 1. Maintain solids concentrations in digester feed at 4.5 to 5 percent. Control thickened WAS concentrations or bypass the gravity belt thickeners. WAS at concentrations higher than 5 percent reduces the effectiveness of the digester mixing systems. 2. Maintain volatile solids loading rates in the range of 100 to 150 pounds per 1,000 cubic feet of digester volume for mixtures of PS and TWAS (loading rates for PS only can be higher). 3. Analyze PS and TWAS for total and volatile solids concentrations at least once per day to track variations in organic loading rates, and track PS and TWAS flow rates. 4. Limit daily variations in volatile solids load (organic load) to the digesters to ±5 to 10 percent by analyzing the various types of sludges, and mixing as necessary. Trending of volatile solids loading to the digesters is desirable to minimize variations. 5. Track volatile solids reduction from plant data against digester gas production. Typically, the gas production averages 13 to 15 cubic feet per pound of volatile solids destroyed. Foaming can reduce the effluent volatile solids concentrations below the concentrations in the digester as a result of entrapment of solids in the foam layer, which will cause higherthan-average gas production. 6. Mix the various types of sludge in a blending tank before pumping them to the digesters to maintain a consistent feed. 7. Maintain the depth of sludge blankets in the primary and secondary clarifiers at less than 18 inches at peak flow to improve removal efficiencies. 8. Develop trend charts, including upper and lower limits for each parameter. Develop process control spreadsheets for weekly analysis and monthly summary reports. Some examples of process control spreadsheets developed at other facilities are presented on Figure 17. Combined sludge volatile solids loading and conversion to gallons. Conversion of gallons of WAS feed to a gravity belt thickener to the percentages of percent solids and liquid (gallons) discharge rate to the digesters. Moving average spreadsheets, 7, 15, and 30 days. Trend charts with upper and lower control points for all anaerobic digestion process control data points. 2569

18 Figure 17- Example of Volatile Solids Loading Rate Spreadsheet Volatile Solids Feed Calculations Volatile solids dose to be used INSTRUCTIONS ON HOW TO USE THIS SPREADSHEET Average 1. Blue cells are for data entry. VS loading 0.13 lb/ft 3 /day 2. Yellow cells are for calculated results. Percentage of Feed as TWAS 30 % 3. Enter volatile solids loading. 4. Enter VS concentration for primary and TWAS. Plant Flow 5. Enter amount of VS feed in lbs to be TWAS 6. Enter the digester volume. Primary VSS conc mg/l TWAS VSS conc mg/l 7. Spreadsheet calculates dosing rates based Digester Volume MG 6.6 on 24 hour pumping. 8. If required enter different pumping rate less than 24 hours. Dose to use in digester 9. Spreadsheet calculates pumping rates based Volatile Solids lbs/day on hours of operation to achieve VS loading rate. VSS Flows from Primary Only VSS Flows into Digester From TWAS Only Primary Flow into Digester 0.20 MGD TWAS Flow into Digester 0.09 MGD Pumping rates Pumping rates gpm gpm gpd 200,232 gpd 85,635 Pumping rates for less than a 24 hour day operation Pumping rates for less than a 24 hour day operation Hours per day in operation 24 Hours Hours per day in operation 24 Hours gpm gpm gpd 200,232 gpd 85,635 HRT = days 9. Perform system monitoring. The key to preventing instability of digestion is continuous monitoring to assess the health of the digester. The data collected should be used to set up trend charts in the form of a spreadsheet to monitor the solids loading, ph, and volatile acids and alkalinity in the digester; volatile acids to alkalinity ratio; and volatile solids destruction. The trends will show the condition of the digester and the effects of operational changes. Data collection is noted in Table 1. Flow rate Table 1. Data Collection Data Item Frequency Feed Digested solids Daily Daily Total solids Volatile solids Feed solids Digested solids Feed solids Digested solids Daily 3/week Daily 3/week Alkalinity Digested solids 3/week Volatile Acids Digested solids 3/week ph Digested solids Daily Temperature Digested solids Daily Ammonia Digested solids 1/week Gas production Digester gas Daily Gas Quality Digester gas 1/week 2570

19 The data collection listed in Table 1 is the minimum required for good digester operation. Additional monitoring would be necessary during digester upset or when sludge feed is unpredictable, such as hauled waste flows or intermittent thickening that interface with continuous feed to the digesters. Primary Clarifiers Septic conditions in the primary clarifier are conducive to excess VFA formation which will upset the digester operation. It is important to control sludge flow rate of the primary clarifier to maintain the depth of the sludge blanket during peak flow, as measured at the middle of the clarifier bridge at 12 inches or less. This will mitigate VFA formation that could upset the digester and cause excessive foaming. Digester Gas Collection System The operation and maintenance of the digester gas collection system directly affects digester operation. During normal operation, significant amounts of condensate or water carryover accumulate in the digester gas piping system. Manual drip traps are small and must be emptied frequently. The drip traps can be fitted with electric actuators that operate on timed cycles to automatically drain the liquid. The digester waste gas flare and gas relief valves must be set at the proper pressure. For instance, if the system is to operate at a pressure of 9 inches of water column, the waste gas burner should be set to fire at 9.2 inches of water column (Figure 18). The pressure relief valves should be set for a pressure approximately 5 percent lower than the design pressure. For instance, if the design pressure is 11 inches of water column, the relief valves should be set for 10.5 inches of water column. 2571

20 Figure 18 Waste Gas Flare A properly operated sediment trap provides sufficient protection system against carryover of water from the digester to the gas piping system (Figure 19). Operators should monitor the sediment trap sight glass (liquid level in tank) and observe the color of the liquid. A clear liquid indicates normal conditions; brown or black indicates foam carryover and the need to check the feed sludge VS loading. Figure 19 Sediment Trap 2572

21 The manometers located throughout the digester gas header system must be properly maintained, cleaned, and filled with the proper fluid. The operating staff should compare the manometer readings with the electronic pressure indicators to confirm proper operating gas pressures. Digester Mixing Systems Digester mixing is critical for a homogeneous mixture in the digester. The digesters evaluated in all three case studies were equipped with gas cannon mixing systems. These systems are effective for use with feed solids concentrations of 5 percent or less. At higher feed solids concentrations, their effectiveness is reduced. It is also important to minimize downtime of the mixing system, as there may be insufficient mixing energy to re-suspend large accumulations of settled solids. Mixing system recommendations are as follows: Operate the digester gas mixing compressors with the bypass valve completely closed and with all cannons producing approximately one bubble every 5 to 10 seconds. The compressors should always operate continuously, except during maintenance. The digester gas bypass valve should remain closed. Examples of a gas compressor and a manifold are presented on Figures 20 and 21. Figure 20 Digester Gas Compressor 2573

22 Figure 21 Gas Manifold The gas mixing system can be turned off for up to 8 hours without disrupting the biological process or the VS loading cycle. For periods longer than 8 hours, the VS loading should be stopped until the gas mixing system is re-started and operating for at least 12 hours. Before feeding is resumed, digester contents should be sampled to confirm the operating conditions, and the VS loading feed rates adjusted as needed. Spreadsheets similar to that presented on Figure 17 should be provided for calculating the VS loading to allow operators to determine the correct loading rates. CONCLUSIONS The root cause of most cases of excessive digester foaming is unstable operation. There are measures that can be utilized in the design of a digester process to mitigate foaming, but ultimately continuous monitoring and trend charting will ensure that the process operates at peak efficiency performance. While some foaming is to be expected, excessive foaming is a serious problem. Digester operation is not an automatic process. It requires time and resources to monitor the operation and evaluate the data generated. An adequate and well-trained staff is essential. It is not enough to simply collect data. It is important to understand the data and compare them with past records to improve digester performance and ensure good gas production and volatile solids reduction. Thomas Edison, America s most prolific inventor, best describes this philosophy: Machines are no better than the skill, care, ingenuity, and spirit of the men who operate them. All three plants evaluated in the case studies are fortunate to have well-qualified operators who, once the problems of the unstable operation were recognized, were able to meet or exceed Mr. Edison s expectations, by operating and maintaining their digester systems through monitoring and establishing operational protocols. 2574

23 REFERENCES Massart, Neil, Robert Bates, Blair Corning, Gary Neun (2006). The Root Cause of Excessive Anaerobic Digester Foaming. Proceedings of the Residual and Biosolids Management Conference, Water Environment Federation, Cincinnati, Ohio, March Sandino, Julian, Hari Santha, Steve Rogowski, Wendy Anderson, Shihwu Sung, Ferit Isik (2005). Applicability of Ultrasound Pre-Conditioning of WAS to Reduce Foaming Potential in Mesophilic Digesters, Proceedings of the Residuals and Biosolids Management Conference, Water Environment Federation, Nashville, Tennessee, March