Grease Digestion for Renewable Energy, Sustainability, and Environmental Enhancement

Transcription

1 Grease Digestion for Renewable Energy, Sustainability, and Environmental Enhancement New England Interstate Water Pollution Control Commission i PRESENTATION October 7, 2008 Jonathan Keaney, PE Brown and Caldwell Co-authors Perry Schafer, PE Brown and Caldwell Don Trueblood, Brown and Caldwell Ken Fonda, PE, Brown and Caldwell Craig Lekven, PE, Brown and Caldwell

2 Grease Problems are Frequent in Wet Wells and Other Places

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8 Goals of FOG Digestion Provide long term sustainable means of disposal Maintain or improve current operations Generate beneficial resources

9 Anaerobic Digestion Background Biological Process Reduces total solids, generates biogas Can generate class A or B biosolid per 503 regs Common Biosolids Stabilization Process Worldwide Limited installations in New England

10 Benefits of Digestion of Fog Digestion occurs at elevated temperatures that assist in homoginizing FOG FOG would represent a small relative volume for most digestion operations Low energy process Increases production of Biogas

11 Anaerobic Digestion and Sustainability Sustainability of Digestion Net reduction of solids Low energy requirements Shared construction and operating costs Maximal beneficial reuse of waste materials Cogeneration of biogas Potential for beneficial reuse of biosolids

12 Drivers for Grease to Energy Minimize sewer problems blockages and odors Obtain renewable fuel and energy value of grease resources Reduce greenhouse gas emissions and air pollutants Achieve economic benefits Be Green and Sustainable

13 Yellow Grease

14 Brown Grease

15 Characteristics of Brown Grease % solids <1 to >15% BOD 10,000 to 130,000 mg/l ph Acidic (4 to 5) VS/TS 90 to 97 % VS reduction 80 to 90 % in digestion C/N ratio High

16 Digester Loading Rates for Brown Grease Consistent loading (no slugs) Research shows good stability up to ~30% Over 30 to 50 % grease load, more susceptible to upsets Limited experience at 100% grease loading Warmed grease frequently 1 to 30% of VS load to digester Wastewater sludges (manure/organics)

17 Proper Mixing is Crucial Prevent surface mat! Disperse incoming LCFAs Insure consistent temperature Surface withdrawal Significant surface turbulence required Various Mixing Systems Surface withdrawal Withdrawal Withdrawal Egg or Submerged Cover Conventional Fixed or Floating Cover

18 Theoretical Methane Content of Biogas Chemical Methane, Substrate Composition % of Total Gas Fat C 15 H 31 COOH 72 Protein C 4 H 6 ON 63 Carbohydrate C 6 H 12 O 6 50 From Jewell & McCarty work in 1960s and 1970s

19 Assumptions for Example Calculations Sludge Grease TS = 5% TS = 10% VS = 78% VS = 95% VSR = 55% VSR = 85% Biogas = 16 ft 3 Biogas = 20 ft 3 lb VSR lb VSR Methane = 64% Methane = 72%

20 Example with Standard Assumptions (add 25 % grease solids load) Sludge Sludge % only + grease increase Gallons fed 23,500 gpd 26,500 gpd 12.5 TS fed 10,000 lbs/d 12,500 lbs/d 25 VS fed 7,800 lbs/d 10,175 lbs/d 30 VS destroyed 4,290 lbs/d 6,310 lbs/d 47 Biogas 68,600 ft 3 /d 109,000 ft 3 /d 59 Methane 43,900 ft 3 /d 72,500 ft 3 /d 65

21 Example with Symbiotic Assumptions (same 25 % grease solids load) Sludge Sludge % only + grease increase Gallons fed 23,500 gpd 26,500 gpd 12.5 TS fed 10,000 lbs/d 12,500 lbs/d 25 VS fed 7,800 lbs/d 10,175 lbs/d 30 7,210 lbs/d 68 VS destroyed 4,290 lbs/d 6,310 lbs/d ,900 ft 3 /d 81 Biogas 68,600 ft 3 /d 109,000 ft 3 /d 59 82,400 ft 3 /d 88 Methane 43,900 ft 3 /d 72,500 ft 3 /d 65 Sludge VSR 55 65% Grease VSR 85 90%

22 Potential Reasons for this Symbiotic Response 1. Greater level of biological activity within the digesting mass 2. Trace metal deficiency (without grease) 3. Trace metal stimulation (with grease) 4. Improved C/N ratio with grease (ww sludge can have low C/N ratio).

23 Key considerations for implementation Determination of ideal loading rate and biomethane potential (BMP) Determination of toxicity Too high a dose can have detrimental impact Consideration of other digestion supplements Determination ti of nutrient t deficiency i

24 Summary 1. Limiting grease discharges to sewers well-recognized approach. 2. Disposing grease to landfills creates problems including GHG emissions. 3. Grease wastes are valuable feedstocks for renewable energy economically viable projects are being implemented. 4. Indications of symbiotic activities when co-digesting sludges and grease.