Anaerobic Digestion Fundamentals

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1 Anaerobic Digestion Fundamentals What are they designed for? Leonard E. Ripley, Ph.D., P.E., BCEE Senior Process Engineer / Freese and Nichols, Inc. 1

2 Introduction: Pancakes 2

3 Introduction: Eggs & Silos 3

4 Background Bug s-eye View SUBSTRATE Hydrolysis & Acidification ORGANIC ACIDS (ACETATE) Methanogenesis METHANE 4

5 Background A Closer View SUBSTRATE INERTS PROTEINS CARBOHYDRATES LIPIDS Hydrolysis AMINO ACIDS, SUGARS, LONG-CHAIN FATTY ACIDS, ALCOHOLS Fermentation VOLATILE ACIDS ACETATE PROPIONATE, BUTYRATE, VALERATE Homoacetogenesis HYDROGEN Anaerobic Oxidation CO 2 HCO 3 - Acetoclastic Methanogenesis METHANE 5 Hydrogenotrophic Methanogenesis

6 A Little More Background Mesophiles Preferred temperature: 95 F (35 C) Moderate activity rate More efficient at metabolizing carbohydrates Thermophiles Preferred temperature: 130 F (55 C) Higher activity rate More efficient at metabolizing proteins Higher potential for odor generation 6

7 Many Process Variations! Single-Stage Thermophilic Temperature Phased Anaerobic Digestion (TPAD) 7 F R W Batch-Fed Thermophilic: Feed React Withdraw

8 More Process Variations Thermal Hydrolysis (Cambi / Exelys ) 2-Stage Acid / Gas Digestion Sonication/Cavitation (Sonolyzer & Crown) Others: (BioCrack, O 3, Enzymes) Pretreatment & Single- Stage Mesophilic AD 8

9 Scope of This Workshop BIOGAS Single-Stage Mesophilic HEAT A simple process, with many design and operating complexities! 9

10 Why Digest? 10

11 Solids Destruction 70 Solids Before & After Digestion: 50 MGD Plant 60 Tons Solids per Day WAS VSS WAS FSS Primary VSS Δ = 33% VSS 10 0 Primary FSS Raw Sludge FSS Digested Sludge 11

12 Pathogen Destruction and VAR Required Time vs. Temp for Alternative 1 Required Contact Time (hours) Contact Temperature ( F) Vector Attraction Reduction Note: mesophilic digestion alone will not give Class A biosolids 12

13 Biogas Production / Purification Pipeline sale Vehicle fueling (See Aug WEF Highlights) Photos: City of Grand Junction, CO 13

14 Cogeneration a.k.a. Combined Heat and Power Reduce energy purchase Recover waste heat Sell power back to grid 14

15 Codigestion of trucked wastes Reduce organic loading to liquids treatment Tipping fees from diversion of MSW organics Generate more methane more electricity/steam Not limited to Fats, Oil, and Grease! 15

16 Improve (Usually) Dewaterability Less polymer consumption Better throughput Higher cake concentration (lower trucking/disposal $$) BUT higher potential for struvite formation w/ BNR! (Magnesium Ammonium Phosphate) 16

17 What Are Your Drivers? Solids Reduction Stabilization for Beneficial Reuse Sustainability Better Dewatering Energy Production Revenue (Tipping Fees) 17

18 AD Chemistry Food or Poison? Substrate (Sludge, Codigestion) Ammonia Sulfides Volatile Acids 18

19 Alkalinity and Buffering Water chemist: Titrate to ph 4.3, calculate [CO 3= ] and [HCO 3 - ] Anaerobic microbiologist: Multiple buffer systems Titration to 4.3 includes bicarbonate (good) and volatile acids (bad) Preferable to measure VFA s & HCO 3 separately Ammonia pk a = 9.3 Phosphoric Acid pk a = 7.2 Sulfide pk a = 6.9 Bicarbonate pk a = 6.4 Volatile Acid pk a s = Titration endpoint 19

20 From Bug s-eye to Big Picture Digester mixing Bring bacteria and food together Avoid grit deposition & loss of volume Prevent (reduce) foaming 20

21 Design Criteria Organic Loading Rate either VSS or COD Hydraulic Retention Time days OLR = Solids Concentration / HRT Feed Concentration (%) OLR in lb/1000 ft 3 -day: OLR vs. HRT & Feed Concentration Note: 1.0 kg/m 3 -d = 62.4 lb/1000 ft 3 -d HRT (days) HRT 21

22 Design Criteria Organic Loading Rate either VSS or COD Hydraulic Retention Time days OLR = Solids Concentration / HRT Feed Concentration (%) OLR vs. HRT & Feed Concentration OLR in g/l-day: Note: 1.0 kg/m 3 -d = 62.4 lb/1000 ft 3 -d HRT (days) 22

23 Design Criteria HRT and digester pass-through Serious implications for pathogens, re-growth, odors 100% Solids Residence Time in Completely Mixed Tank Fraction Retained 80% 60% 40% 20% Less than 40% of feed is retained for one full HRT. Short-circuiting and dead volumes worsen the problem. 0% (t / τ) - Number of Theoretical HRT's 23

24 Closing Thoughts Depending on design and operation, mesophilic digestion is a stable, robust process with many benefits & opportunities for codigestion & biogas utilization Digestion does have drawbacks: susceptible to toxins, foaming (Nocardia), and overloading. Mesophilic digestion alone will not yield Class A. Historical concerns about odors can be eliminated with fixed covers and management of off-gassing. 24

25 Lots of Options Optimum design depends on: Utility s objectives Capital availability Operating costs Energy costs Regulations Customer demands 25

26 Questions? Leonard E. Ripley, Ph.D., P.E., BCEE 26