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Co-Digestion – The Path to Success

Learn about the benefits of co-digestion, the addition of high-strength waste to anaerobic digesters, including increased revenue, enhanced digester performance, and overall sustainability. Explore the feasibility and potential challenges, as well as the importance of proper pretreatment and receiving stations. Discover the various organic feedstocks that can be used for co-digestion and how it can contribute to a zero-waste goal.

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Co-Digestion – The Path to Success

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  1. Co-Digestion – The Path to Success Blair Wisdom

  2. Agenda Co-Digestion Overview Feasibility Ensure Success of Receiving Stations and Pretreatment Safeguard for Stable Digester Performance

  3. Co-Digestion overview

  4. Black & Veatch | November 2015 What is Co-Digestion? It takes shape in many forms. Essentially, it is the addition of high strength waste (HSW) to anaerobic digesters Commonly practiced with the brown grease (BG) portion of fats, oils, and grease (FOG) Increasing focus on industrial and pre-consumer wastes • Dairy whey • Fruit/vegetable wastes • Institutional food wastes • Corn silage

  5. Black & Veatch | November 2015 Why would you Co-Digest? Excess capacity in digesters may afford opportunity for revenue from tip fees. Regional haulers (FOG and others) may need an outlet (encourage them to keep it out of collection system. • Control where HSW are injected in plant – turn from waste that consumes energy to an enhancement Increased load can make energy recovery more attractive Energy

  6. Black & Veatch | November 2015 DEDICATED DIGESTERS FLEET Organic Resources-to-EnergyCO-LOCATION/(CO)-DIGESTION AND GAS UTILIZATION GRID CO-GEN EXISTING DIGESTERS Sludge RNG GAS UPGRADING stabilization technologies COMMERCIAL ORGANICS WWTP PIPELINE SOURCE SEPARATEDORGANICS/ORGANIG FRACTION OF MSW FOG

  7. Black & Veatch | November 2015 Co-Digestion Can Enhance Overall Sustainability • Reliable systems provide valuable service to industries • Increased revenue • On-site power production provides greater reliability • Flexibility for multiple energy uses

  8. Black & Veatch | 2 September 2015 Pros/Cons of Co-digestion • ADVANTAGES • Synergistic effects (location, infrastructure) • Qualified & experienced personnel on site • Improves digester performance thanks to symbiotic behaviors • Increase in revenues through tipping fees • Self-sufficient generation of renewable energy • on-site achievable • Biosolids contain less contaminants, more nutrients, and are more suitable for agricultural use than conventional biosolids • Increased in gas production and quality • Improved economics, benefitting rate payers for publically owned treatment works + stabilization technologies Source: Braun, Wellinger (2003); Grasmug, Braun (2003); Schmelz (2007); STOWA, (2006) adapted

  9. Black & Veatch | 2 September 2015 Pros/Cons of Co-digestion • DISADVANTAGES • Capital costs; feedstock receiving station and conditioning/removal of contaminants before feeding digester andincreased mixing may be required • Increased chemical oxygen demand (COD)/nutrient load of digestate’s filtrate • Foam formation and scum layers in digester • Odors • Dewaterability: increased amount of biosolids • Hygienization may be required • Deposition of material (pipe & valve blockage; reduced digester volume) • Mixing Challenges - stabilization technologies Source: Braun, Wellinger (2003); Grasmug, Braun (2003); Schmelz (2007); STOWA, (2006) adapted

  10. Feasibility

  11. Process Capacity

  12. Black & Veatch | November 2015 Rock River Water Reclamation District Drivers: • Combat rising energy costs • Institute sustainable practices • Fully utilize equipment capacities Capacity evaluation: • Average 32 day SRT • 100 ppd of VS/kcf

  13. Black & Veatch | November 2015 Rock River Existing Stabilization and Gas Handling Excess Gas Treatment Capacity ExcessDigester Capacity ExcessEngine Generator Capacity Limited Gas Handling Capacity

  14. Availability of HSW

  15. Black & Veatch | November2015 Significant Biogas potential in high strength wastes

  16. Black & Veatch | November2015 Organic Feedstocks & Properties for AD • Increase in total solids (TS) content • Decrease in homogeneity • Increase in contamination(non-digestible) stabilization technologies

  17. Area Specific HSW Black & Veatch | November2015 • Survey industries in service area • Evaluate characteristics & value of available wastes • pH • Total and Volatile Solids (TS and VS) • Total and Soluble COD • Total Ammonia Nitrogen (TAN) • Total Kjeldahl Nitrogen (TKN) • Volatile Fatty Acids (VFA) • Long Chain Fatty Acids (LCFA) • Alkalinity • Gas Production and Composition • Capillary Suction Time (CST) and Sludge Dewatering • Biosolids Odors • Determine loading and any adverse effects

  18. Black & Veatch | November2015 Increased Competition for FOG – Long term outlook? Increased interest in yellow grease for biodiesel • Gasoline price fluctuations • Carbon footprint/offsets Increased use of yellow grease in • Animal feed • Cosmetics Increased consideration as substrate for industrial anaerobic Digestion Grant/offset benefits • Renewable energy generation Reduced restaurant generation from recession

  19. Black & Veatch | November2015 Commercial and Residential Source Separated Organics - Co-Digestion stabilization technologies OBJECTIVES & MEASURES: Increase of Waste Diversion towards Zero Waste Goal (meet local, state and federal landfill diversion and/or organics ban requirements) Closing Carbon and Nutrient Loop Cheap and plentiful feedstock for energy production Educate food establishments,food processors, retailers, andresidents of organics collection

  20. Black & Veatch | November 2015 Availability of HSW Contacted 50 companies in surrounding area Conclusions of HSW Survey • Abundant supply of Non-FOG high strength wastes • Thin stillage, dairy wastes, distillers syrups • 4,000 to 5,000 gallons per day of FOG waste generated within the district’s service area • Another 20,000 gallons per day of FOG waste generated Around the district’s service area • Adequate Supply for 20,000 GPD HSW Facility

  21. Assess the Benefits

  22. What are the Operational Costs and Benefits? Black & Veatch | November 2015 Operational Costs • Some added maintenance • Added loads to the plant • Potentially added biosolids disposal costs B&V - 22 Operational Benefits Increased biogas Increased waste heat from engine generators Tipping fees Service to customers

  23. Ensure Success of Receiving Stations and Pretreatment

  24. Black & Veatch | November 2015 Hauling Considerations • What type of wastes are being processed? • Processed source separated food wastes? • “Liquid” high strength wastes? • Efficient truck unloading • Hauling schedules? • Multiple trucks at a time? • Cleaning stations • Weigh scales • Hauled load management and billing

  25. Black & Veatch | November2015 Pre-treatment and materials handling issues • Pretreatment: Rock traps, screens, grinding, depackaging (industrial wastes) • Remove non-biodegradable materials • Protect downstream equipment • Reduce particle size to optimize digestion • Storage tanks • Mixing • Site specific heating • Heat tracing, HEX, mixing with warm sludge

  26. Black & Veatch | November2015 Debris in Hauled Wastes

  27. Black & Veatch | November 2015 FOG/BG CO-DIGESTION EXAMPLE: WATSONVILLE WWTP stabilization technologies Source: Kester, G. (2008) PLANT DESIGN CAPACITY: 12 MGD FOG CO-DIGESTION: 3,000 – 10,000 GAL/DAY

  28. Black & Veatch November 2015 Example – Des Moines WRA - 170,000 Gallon Receiving Tank

  29. Des Moines WRA - Receiving/Storage Tank Black & Veatch | November 2015 • Mixed via recirculation pumps and nozzles • Lined for corrosion control

  30. Black & Veatch | November 2015 Summary of receiving conceptsBasic or Sophisticated Systems – No two are the same! • Simple systems with rock-boxes - traps grit/debris before larger tank • Mechanical systems for removing debris • Some have storage while others go straight to digesters • Heating? Is it needed?

  31. Safeguard for Stable Digester Performance

  32. Black & Veatch | November 2015 Opequon WRF – Frederick Winchester Service Authority (FWSA) Current Solids Treatment: • Lime-Stabilized biosolids – landfill disposal • Frame and Plate Filter Presses Design Solids Process: • Two mesophilic primary digesters and one secondary digester

  33. Black & Veatch | November 2015 Solids Process

  34. Black & Veatch | November 2015 Solids Design Summary • Substrates for co-digestion: • Wastewater sludge • Kraft Foods waste • Dairy whey waste • GTW and DAF Float • Food waste

  35. Black & Veatch | November 2015 Pilot Testing - Digester Set up and Operation • 4 digesters with working volume of 9.75L, 15 day SRT • Mesophilic temperature (370C) • Seeded with 8 Liters of digester effluent from Christiansburg WWTP Schematic Diagram showing Digester setup

  36. Black & Veatch | November 2015 Parameters analyzed • pH • Total and Volatile Solids (TS and VS) • Total and Soluble COD • Total Ammonia Nitrogen (TAN) • Total Kjeldahl Nitrogen (TKN) • Volatile Fatty Acids (VFA) • Long Chain Fatty Acids (LCFA) • Alkalinity • Gas Production and Composition • Capillary Suction Time (CST) and Sludge Dewatering • BiosolidsOdors

  37. Black & Veatch | November 2015 Part I: Feed contained Juice Waste and Whey Waste only. Digester performance was assessed and the roles of alkalinity and VFA concentrations were evaluated Part II: Carried out in three phases with increasing concentrations of all the food wastes listed in the table above.

  38. Black & Veatch | November 2015 • Total Time of Operation: 310 Days • The above mentioned compositions were attained on Day 180 • Juice processing waste underwent a sudden, drastic change on Day 240 • New waste much “stronger” • Digester 4 which was receiving the highest HSW load by volume exhibited failure

  39. Failure Caused by Shock Load to Digester • Wastes being added were acidic in nature, especially the juice processing waste • The sudden change in the nature of the waste seems to have inflicted a “shock load” on the digester that exhibited failure

  40. Black & Veatch | November 2015 • Acidic wastes can reduce buffering capacity of anaerobic systems. • The nature of the wastes to be added as co-substrates to anaerobic digesters is therefore something that must be thoroughly studied • Similar Juice mix with supplemental ammonia did not show failure • Variability in the nature of the HSW must be monitored as sudden changes in the HSW feed might push digesters to failure

  41. Black & Veatch | November 2015 Biomethane Potential Analysis

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