Optimizing the Performance of Anaerobic Digestion

Transcription

1 Optimizing the Performance of Anaerobic Digestion Marialena Hatzigeorgiou, P.E. Senior Technologist _WWGST_RRR_PPT_Template_01.pptx

2 Agenda Basics of Anaerobic Digestion Biological reactions & phases Temperatures ph range Detention time Loading rates (volatile solids, nutrients) Anaerobic Digestion Issues Foaming and gas holdup Acidification (going sour) Toxicity Mixing (under-mixing and over-mixing) Maximizing VS reduction and gas production Variants to Mesophilic Anaerobic Digestion Pretreatment processes (i.e., hydrolysis, pre-pasteurization) Two-phase (acid-gas) digestion Thermophilic and temperature-phased digestion Post-aerobic digestion _WWGST_RRR_PPT_Template_01.pptx

3 3 Residuals Resource Recovery Anaerobic Digestion - 4 Phases 1 st (Hydrolysis) - Extracellular enzymes (outside cell) break down solid complex organics 2 nd (Acidogenesis) - Acid forming (acid production and acid regression) microorganisms convert products of first phase to organic acids, carbon dioxide & hydrogen 3 rd (Acetogenesis) Organic acids further broken down into mostly acetic acid, ammonia, carbon dioxide, and hydrogen 4 th (Methanogenesis) - Methane forming bacteria converts acetic acid, hydrogen and carbon dioxide into methane _WWGST_RRR_PPT_Template_01.pptx

4 Hydrolysis Acidogenesis Acetogenesis Methanogenesis 4 Residuals Resource Recovery Conventional, Mesophilic Anaerobic Digestion (MAD) Typically digests primary sludge (PSD) and waste activated sludge (WAS) at C ( F) for 15 to 30 days Phase 1 Phases 2 & 3 Phase 4 Proteins Amino Acids, Sugars Acetic Acid Carbohydrates Volatile Acids Methane Fats Fatty Acids Hydrogen _WWGST_RRR_PPT_Template_01.pptx

5 ADM1 - Anaerobic Digestion Process Model Complex Particulate and Dead Biomass Disintegration First-order Hydrolysis Acidogenesis Carbohydrates Proteins Lipids Monsaccharides Amino acids LCFA Inert - Particulate - Soluble Uptake, growth, decay HPr, Hbu, HVa Acetogenesis HAc H 2 Methanogenesis CH 4, CO _WWGST_RRR_PPT_Template_01.pptx

6 6 Residuals Resource Recovery Anaerobic Digestion Methanogenisis First 3 phases generally take 1-2 days, while 4 th phase methanogenesis can take 8-20 days Methanogenesis controls the process overall and determines digestion effectiveness Sensitive to environmental factors (ph, temperature, toxins, competition) Methanogens reproduce very slowly, are easy to kill and difficult to grow _WWGST_RRR_PPT_Template_01.pptx

7 7 Residuals Resource Recovery Physics of Anaerobic Digestion Grit density greater than water Biomass density ~ slightly greater than water Substrate (proteins & carbohydrates) density ~ slightly greater than water Substrate (lipids/acids) density less dense than water Biochemical transformations Grit stripped of grease >> increases in density Precipitates >> converts soluble matter to suspended inert matter Carbs, proteins, lipids >> int polymer >> VA >> acetic acid >> biogas Biogas, VA, AA LCFA, Lipids Anaerobic Biomass, WAS, Proteins, carbohydrates Grit Precipitates _WWGST_RRR_PPT_Template_01.pptx

8 8 Residuals Resource Recovery Anaerobic Digestion Uniformity and consistency in operations are key Sudden changes to solids feed (rates or characteristics) can inhibit performance Criteria that govern capacity: Volatile solids (VS) loading rate Energy loading rate (COD) Hydraulic detention time (HRT) Solids residence time (SRT) Higher SRT increases VS reduction, to a point _WWGST_RRR_PPT_Template_01.pptx

9 Volatile Solids Reduction Residuals Resource Recovery Effect of SRT on VS reduction (based on 4 labscale pilot tests on the same substrate) Part 503 reg. requirement = 38% minimum November October September December Digestion SRT _WWGST_RRR_PPT_Template_01.pptx

10 Full-scale Performance can be Erratic _WWGST_RRR_PPT_Template_01.pptx

11 Performance parameters to monitor during operation: Solids concentration feed to the digester of 4 6 % solids. Feeding dilute sludge will reduce the retention time and decrease the digester sludge operating temperature. This will result in reduced VS destruction and reduced biogas production. Solids concentration of digested sludge of % solids. The digested sludge concentration should be within the operating range described in the table above. A low solids concentration can be a indicator of dilute sludge feed to the digester, and a high solids concentration can indicate reduced digestion efficiency. Operating temperature of degrees F. Fluctuations of more than 3 to 5 degrees can shock the microbial population, leading to a lower overall growth rate and reduced digestion efficiency Temperature consistency is key to maintaining methane formers _WWGST_RRR_PPT_Template_01.pptx

12 12 Residuals Resource Recovery Anaerobic Digestion - Temperature Ranges Psychrophilic 50 to 68 degrees F 50 to 180 days HRT/SRT Occurs most often in nature Generally not effective in engineered systems Mesophilic 85 to 100 degrees F 15 to 30 days HRT/SRT Most stable and popular for engineered systems Thermophilic 125 to 135 degrees F Faster rates = shorter HRT/SRT, days Not as stable as MAD and may be more odorous Can produce Class A biosolids if certain time-temperature and/or pathogen density criteria are met _WWGST_RRR_PPT_Template_01.pptx

13 Items to monitor during MAD operation (continued): ph range of 6.8 to 7.2. The methane forming microbes are sensitive to ph. Lower ph will cause destruction of methane forming microbes. This will result in an increase in VFA concentration, which in turn will further reduce the ph and cause a digester souring or upset. Bicarbonate Alkalinity range of 2,500 3,500 mg CaCO 3 /L. Alkalinity provides buffer capacity to prevent drop in ph as VFAs are formed. A drop in alkalinity is an indicator of high VFA concentration, and an early warning sign of upset. VFA to Alkalinity Ratio < 0.3. A higher value is an indicator of high VFA concentration which can lead to ph drop and digester upset. Ammonia concentration of 800 1,200 mg NH3-N/ L. Ammonia is produced as a byproduct of methane fermentation and supports ph control (ammonium alkalinity). Ammonia exceeding the recommended range can prove toxic to microbes due to NH4+. Toxicity causes Soluble heavy metals (sulfate precipitation is a digester s friend) Metal salts Organic compounds _WWGST_RRR_PPT_Template_01.pptx

14 Toxicity Salts and Heavy Metals Source: WEF Manual of Practice _WWGST_RRR_PPT_Template_01.pptx

15 Toxicity (Inhibition) Organic Compounds Source: WEF MOP _WWGST_RRR_PPT_Template_01.pptx

16 16 Residuals Resource Recovery Digester - Foaming Foam is fine bubbles of gas trapped in a semi-liquid matrix Foam commonly forms during startup and when the digester is under stress (such as when carbon dioxide to methane ratio not being stable) Foam is sometimes carried over from activated sludge process Can occur with aerobic or anaerobic digesters Possible Causes of Foaming: Filamentous organisms from the biological treatment system Continuity (or discontinuity) of digester feeding Addition of co-digestates that stimulate biogas formation Over-mixing Variety of causes = variety of solutions _WWGST_RRR_PPT_Template_01.pptx

17 Digester Foaming and Mixing Too little mixing can allow pockets of gas to accumulate, creating density gradients Too much mixing can entrain gas on a wider scale, creating density gradients Temperature gradients and variability in feeding, increasing density gradients _WWGST_RRR_PPT_Template_01.pptx

18 Digester Foaming and Mixing (continued) There will be some foaming in a digester under the best of circumstances Co-digestates like FOG and food wastes add to the potential for nuisance foaming Digesters should be designed to accommodate some nuisance foaming Increasing mixing energy input has been a tempting panacea for digester designers, but too much mixing energy is detrimental _WWGST_RRR_PPT_Template_01.pptx

19 19 Residuals Resource Recovery Mixing System Objectives Improve contact between microorganisms and solids in the digester Homogenize tank contents Dilute incoming toxicants Minimize foam formation Minimize scum accumulation _WWGST_RRR_PPT_Template_01.pptx

20 Some Mixing System Options Mechanical Draft Tube Pumped Draft Tube Pumped Recirculation _WWGST_RRR_PPT_Template_01.pptx

21 Conclusions Digester Foaming and Mixing Digester over-mixing is a wide-spread concern and contributes to foam production. Detailed information and CFD modeling required to select appropriate digester mixing systems. Better performance results from feeding digesters as continuous as possible, especially with FOG and high-strength organic wastes added to the feed. Can expect to see More pumped hydraulic jet mixing systems with VFDs (variable frequency drives) Linear motion mixers (LMMs) as a path forward (especially when grit removal implemented upstream) _WWGST_RRR_PPT_Template_01.pptx

22 22 Residuals Resource Recovery The shape of the digester affects these critical anaerobic digestion system design aspects Digester cover Mixing system Biogas storage Foam management Grit management Spherical Egg Pancake Silo with steep cone bottom Tall Narrow Silo _WWGST_RRR_PPT_Template_01.pptx

23 While pancake shaped digesters typically cost less, there are operational challenges Large liquid/gas surface area Shallow slope Grit accumulation Less effective volume Frequent cleaning Scum accumulation Requires more mixing energy Large footprint _WWGST_RRR_PPT_Template_01.pptx

24 24 Residuals Resource Recovery Eggs and silos are similar, but silos are less expensive Silo Smaller footprint, more compact; can conflict with height restrictions Smaller liquid gas surface area makes allows scum wasting Aspect (height:diameter) minimizes mixing intensity Steep sloped bottom allows for grit concentration and removal Less redundancy required Effective volume is maximized Egg _WWGST_RRR_PPT_Template_01.pptx

25 Anaerobic digester designs are evolving. Riverside Park WRF, Spokane, WA F. Wayne Hill Water Resources Center Gwinnett County, GA South Cross Bayou WRF Pinellas County, FL Oceanside WPCP, San Francisco Rock Creek AWT, Hillsboro, OR _WWGST_RRR_PPT_Template_01.pptx

26 High Performance Anaerobic Digestion Enhance operating characteristics Increase stability Reduce and manage foaming and volume change Manage struvite and vivianite Improve biosolids quality Reduce pathogens & odors Increase biomass conversion More biogas, less biosolids Improve financial performance Increase capacity Produce and conserve energy _WWGST_RRR_PPT_Template_01.pptx

27 Advanced Digestion Technologies Two-phase (Acid/Gas) Temperature Phased (TPAD) Extended or staged thermophilic Batch thermophilic Pre-pasteurization Enhanced hydrolysis Biological Chemical Physical (pressure, sound, electrolysis) Thermal Ability to Meet Class A Pathogen Density Levels Low or unknown Meets Class A _WWGST_RRR_PPT_Template_01.pptx

28 Anaerobic Digestion after Thermal Hydrolysis Process (THP) versus Conventional Digestion Conventional Digestion: VS loading lbs/ft 3 Feed at 4-6% DS HRT 20 days ph Temperature F Product at 3-4% DS Typical cake solids ~20% Digestion after THP: VS loading 0.33 lbs/ft 3 Feed at 10-12% DS HRT 15 days ph Temperature 102 F Product at 6-7% DS Typical cake solids ~30% _WWGST_RRR_PPT_Template_01.pptx

29 Anaerobic/Aerobic Digestion, aka Post- Aerobic Digestion (PAD) Pro2D Model Results Kumar (2006) Parravicini (2008) Anaerobic SRT, days Anaerobic VSR 55% 50% Unknown Aerobic SRT, days Aerobic VSR 15% 23% 16% Total VSR 62% 60% Unknown _WWGST_RRR_PPT_Template_01.pptx

30 Acknowledgments Peter Burrowes, P. Eng., Fellow Technologist, CH2M Bob Forbes, PE, Senior Technical Consultant, CH2M Dave Parry, PhD, Principal Technologist, CH2M Tim Shea, PhD, retired Fellow Technologist, CH2M Dru Whitlock, PE, Principal Technologist, CH2M The CH2M Residuals Resource Recovery Team _WWGST_RRR_PPT_Template_01.pptx

31 Optimizing the Performance of Anaerobic Digestion Questions? Marialena Hatzigeorgiou, P.E. Senior Technologist _WWGST_RRR_PPT_Template_01.pptx