Columbus Southerly WWTP Dual Phase Digestion Success 5 Cities Conference St. Louis MO Presented by: Darin Wise, Plant Manager September 4, 2019 Process Schematic Diagram Distribution of Primary Solids Delivered into Columbus Southerly WWTP. Values are in GPD at 5% Total Solids. Slide 2 Columbus Southerly WWTP Dual Phase Digestion Provide energyObjectives from
methane gas for building and process boilers Reduce mass of solids for dewatering and disposal. Reuse existing tanks and buildings Columbus Southerly WWTP Slide 3 Columbus Southerly WWTP The Southerly WWTP is the larger of two WWTPs operated by the City of Columbus Rated capacity of 114 MGD Sewer system is mostly
separate but with some combined areas Columbus Southerly WWTP Slide 4 Columbus Southerly WWTP Dual Phase Digestion Overview Slide 5 Columbus Southerly WWTP Dual Phase Anaerobic Digestion System Looking North. The 3 Acid Phase Digesters and 6 Methane Phase Digesters are visible Background Methane gas is a source of energy with a typical heating value of approximately 650 BTU/CF, which can be compared to 1,000 BTU/CF for natural gas
Digester gas can be used for process and building heating and for possible conversion to electrical power. Focus on renewable resource is a trend to offset operational costs and become more green. Slide 6 Project Features Two digester feedwells mix Thickened Waste Activated Sludge (TWAS) with Thickened Primary Sludge (TPS). 31 ft. diameter concrete tanks with fixed insulated steel covers
Gas piped to Waste Gas Burners This is the front of the digestion process Columbus Southerly WWTP Digester Feed Wells Slide 7 Project Features Three Acid Phase Digesters (APDs), first phase of dual phase
digestion provide detention time of approximately 2 days. New 33 foot diameter concrete tanks with fixed insulated covers and insulated side walls (Acid) Gas piped to Waste Gas Burners Columbus Southerly WWTP Acid Phase Digesters Slide 8 Project Features
Six conventional Methane Phase Digesters (MPDs) Retrofitted existing 85 feet diameter concrete tanks with insulated steel floating covers, which hold gas MPDs are fed using most open valve control scheme
Gas piped to boilers under normal operational conditions. This is the good (methane rich) gas. Slide 9 Columbus Southerly WWTP Methane Phase Digesters Project Features Pumped mixing system for feed wells, APDs, and MPDs. A Computational Fluid Dynamics (CFD) study was conducted during
design to determine mixing nozzle placement Mixing pumps are located next to the tanks in the tunnel area. No moving parts are within the tanks themselves. Columbus Southerly WWTP Mixing System Piping Slide 10 Project Features Sludge heated using direct steam injectors by Hydrothermal.
Steam supplied from boilers, which are fueled by digester gas Main (Raw) steam injectors located upstream of APDs Additional steam injectors located on MPD mixing system to maintain constant temperature in MPDs Slide 11
Columbus Southerly WWTP Steam Injector Project Features Digester gas handling system includes waste gas burner, sediment traps, foam separators, and digester gas compressors. Digester gas is used to fuel steam boilers and previously plant incinerators
At this point there is no other gas cleaning to remove siloxanes for electrical generation Slide 12 Columbus Southerly WWTP Waste Gas Burner The Digestion Process Three classes of reactions occur: Hydrolysis: breaking apart complex organics and solids into simple, liquid phase organics. Kinetically, this reaction occurs in hours. Acid Forming: Conversion of simple
organics and acetate. Some gas is formed. Kinetically, this reaction is also fast, and occurs in about 1-2 days. Methane Forming: Conversion of acetate, carbon dioxide, and hydrogen into methane gas. Kinetically, this reaction is slow and occurs over > 10 days. Slide 13 Process Schematic of Key Reactions Good Design Features
SRT > 25 days Even loading Steady Temperature at about 95 deg. F Good Mixing No toxicants in the system Sufficient Alkalinity for pH control Slide 14 History Project initiated in 1998 to rehabilitate 6 existing tanks constructed in the 1960s Value Engineering was performed on preliminary design in 2001 Design commenced in late 2004 with design revisions including dual phase Construction began in March 2006 and was substantially
complete in May 2009 Digester foaming at the MPDs occurred soon after start up. Slide 15 History Optimization Study initiated in 2010 and finalized in 2013. Study focused initially on SRTs in APDs and volatile acid generation that promote cell lysis. Initial belief that SRT should be maintained in a range of 1.54.0 days. No consensus on exact time but range. Study attempted to identify primary and secondary causes of foaming Ultimately S-90 was commissioned to add in a new digestion tank and convert APDs to in line flow storage. Slide 16 Digester Foaming Begins
The Methane Phase Digesters have foamed periodically since the system was started in 2009 Foam builds up in the annular space between the digester covers and the tank wall During severe foaming episodes, foam spills over the
tank walls Slide 17 Columbus Southerly WWTP Residue from Foaming Episode Foam Mechanics Foam is a byproduct of the surface chemistry gas, liquid, and solids phases Historically not fully understood but many theories given
All agree that the potential for foaming robs a system of capacity, which in turn hinders the production of methane gas Simple Foam Slide 18 Identified Causes of Foaming POSSIBLE ISSUE Excessive Mixing COMMENTS Could contribute to frothing
Sludge Temperature Variations >1 C per day can upset the process. Intermittent or slug Feeding Interruptions to the feed can lead to foaming The most open valve fed system feeds all tanks evenly. Filaments: Nocardia and Microthirx pavacella Hydrophobic composition of filaments may alter surface chemistry at foam interface
Possible minor or contributing factor Volatile Solids Loading variation Recommended daily variation in volatile solids load (organic load) stay within 5-10 percent of the previous days loadings. Daily Variations in volatile solids loading are noted to exceed 30% on a daily basis. This is far outside of the desired range of 5-10%. A strong correlation was discovered between high volatile solids variation and foaming events. Short-Circuiting
Short-circuiting of sludge within the digester by inadequate mixing or poor pipe placement There are no obvious flaws in the piping arrangement to support this as a likely cause. Slide 19 DETERMINATION One mixing pump impeller was changed out to reduce the applied mixing energy with no difference noted. While problems were noted during start up, temperature fluctuations do not appear. Primary Sludge Daily Flows
Study Findings High variation of primary sludge delivered to the plant due to wet weather flows scouring interceptor sewers Little process storage ahead of digestion or within process for buffering. Operational recommendation was to attempt to vary VS loading by < 5% - 10% per day and TWAS < 40% of total mass loading.
Led to recommendation to provide load leveling system and more capacity in S-90 Slide 21 A New Approach In 2018 I was hired as the new Plant Manager with experience in the successful control of Microthrix parvicella foaming by feeding polymer to the RAS. I wanted an opportunity to optimize operations of the APDs to control foaming based on lessons learned from managing a previous plant Requested construction of S-90 be delayed 6 months to make a final attempt at optimizing the dual phase system, and permission was granted Slide 22 Premise
The premise was based that the primary cause of foaming was Microthrix parvicella. Literature stated that by controlling SRTs in the APDs to 1.8 days, the SRT would weaken thick hydrophobic cell walls associated with Microthrix parvicella but not lyse (burst) them in the APDs Hostile environment in APDs is to create an environment where cells are almost lysed. The weakened but not lysed cells pass into the MPDs and become digested without foaming. Slide 23 Premise The premise is that the primary cause of foaming was Microthrix parvicella and the generation of Mycolic Acid upon lysis of the cells in the APD. Literature states that by limiting the SRT in the APDs to 1.8 days it will not foam later in the methane phase digesters. The tight control on SRT weakens thick hydrophobic cell
walls associated with Microthrix parvicella to prevent the release of mycolic acid into the APDs Slide 24 Findings The New Optimization Process began on October 31, 2018. Prior to the study only 10% TWAS sent to digestion. Now 100% is sent to digestion. Over the next few months, operations staff increased WAS by 1% each week for the first four months and then twice each week after that. TWAS is fed as a percentage of the whole with 3550% is 100% of the TWAS. System operations are stable, and are able to handle variations of feed sludge. Slide 25 Benefits
No foaming has occurred A HUGE SUCCESS! Enhanced volatile solids destruction System is stable and can withstand loading variations Digester gas is much cleaner to the point where no ferric chloride is needed to reduce sulfide emissions. No TWAS is hauled to our composting facility or offsite digester Less odor from sludge being hauled Digestion gas production has increased from 700,000 CF/day to over 1,000,000 CF/day Slide 26 Further Areas of Investigation A better understanding of the detailed mechanism of why this is successful. Ammonia in the MPDs increased from 900 mg/l to 1,700 mg/l
Alkalinity in the system has increased (ammonia). Siloxanes in the sweet gas increased from 2000 ppm to 20,000 ppm when Acid Phase Digesters went on line, preventing use in our boilers until we scrub it. Slide 27 Other Factor Contributing to Success In Mid February 2019, the aeration basins were configured for Phosphorus removal by creating a large anaerobic zone at the beginning of each basin The Step-Feed was also shut off at this time Soon afterward, the heavy viscous brown foam disappeared from the
surface of the aeration basins This may have contributed to lack of foaming in the digesters, as this was my indicator at my previous plant Before After Quantification of Benefits Should be Further Developed Better quantification of cost savings and operational benefits attributable to dual phase digestion: Slide 29
Solids reduction cost savings for handling and disposal Class A possibilities should be explored further Side stream benefits (recovery of brushite) Bio P removal in the activated sludge process Questions? Presented by: Darin E. Wise (614) 645-3248 [email protected]
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