The Biosolids Odor Reduction Roadmap (BORR): A Decision Tool to Reduce Cake Odors

Transcription

1 The Biosolids Odor Reduction Roadmap (BORR): A Decision Tool to Reduce Cake Odors Presented at Dewatering 201: Technology, Evolutions, and Trends A webinar sponsored by the Water Environment Association of Texas by Bob Forbes, P.E. Regional Technology Leader CH2M HILL Charlotte, NC Bob.forbes@ch2m.com 1

2 WERF Biosolids Odor Projects History/Overview Phase 1 ( ): An extensive literature survey of odors in the municipal wastewater environment. Product: Literature Search & Review (00-HHE-5A) Phase 2 ( ): Explored the causes of odors from biosolids based on field studies. Product: Impacts of In-Plant Parameters on Biosolids Odor Quality (00-HHE-5T) Health Effects Addendum ( ): Reviewed evidence surrounding potential health effects of biosolids odors. Product: Health Effects of Biosolids Odors: Literature Review and Analyses (00-HHE-5C) Phase 3 ( ): Increased our understanding by developing more quantitative odor reduction measures. Product: Biosolids Processing Modifications for Cake Odor Reduction (03-CTS-9T) Phases 4a, 4b, 4c ( ): More laboratory-scale studies and development of Biosolids Odor Reduction Roadmap (BORR) ROSI (Regrowth, Odors, and Sudden Increase, ): Connected biosolids odors with pathogen-indicator regrowth and sudden increase phenomena. Led by Bucknell University, with DC Water, Virginia Tech, CH2M HILL & others.

3 Phase 2 (Biosolids Odors) was a Field Study of 11 WWTPs across North America E A WAS Thickener & Storage B D Anaerobic Digester F Dewatering Process H G C1 Primary Thickener & Storage C2 Conveyance System I

4 Mean Peak Sulfur Concentration (mg S/m3) Cake Odors are the Worst in the Biosolids Processing Chain Primary & Secondary Clarification/Thickening Digester Feed Dewatering Liquid, Digested Solids A B C1 C2 D F1 F2 G I1 I2 Sampling Locations Cake Conveyance, Storage H2S Methane thiol Total sulfide species

5 Some of the Findings from WERF Phase 3 Research on Biosolids Odor Reduction: Odors from anaerobically digested and dewatered biosolids correlate strongly with total, volatile organic sulfur compounds (TVOSC) emissions Bioavailable protein is the primary precursor of TVOSC emissions from digested, dewatered biosolids Odors and TVOSC emissions tend to rise and fall over time (usually 2-3 weeks) during cake biosolids storage Longer sludge-residence times (SRT) in mesophilic anaerobic digestion result in lower TVOSC emissions from biosolids Higher shear during dewatering results more bioavailable protein and higher TVOSC emissions from dewatered biosolids Addition of metal salts in appropriate amounts pre- or post-dewatering can help bind bioavailable protein and reduce TVOSC emissions

6 Case Study: Electro-dewatering Trials EIMCO s Cinetik Electrodewatering unit was piloted on digested, dewatered cake from two Virginia WWTPs in April 2010 The electro-dewatering unit increased cake solids from 18-19% TS range to 25-35%%TS range in 7 minute to 10 minute batch times 6

7 Case Study: Electro-dewatering Trials Pathogen Indicator Results: Electro-dewatered cake showed 3-4 log reduction of fecal coliform and 6-7 log reduction E. coli Salmonella also < 2 MPN/ 4 gm TS meets Class A pathogen reduction criteria Electro-dewatered cake was susceptible to FC and E. coli regrowth if contaminated Tab# Sample Log E. coli MPN/g DS 1 Plant 1 Electro-dewatered Cake < Plant 1 Centrifuge Cake Plant 2 Electro-dewatered Cake < Plant 2 Centrifuge Cake Plant 2 Cinetik Cake + Plant 2 Centrifuge Cake

8 Case Study: Electro-dewatering Trials Odor Results In general, the electro-dewatered cake had noticeably higher and more persistent odors than centrifuge cake Odor levels ranged 50% to 100% higher from electro-dewatered cake after one day of storage, and remained higher for 80+ days Hedonic tone of dewatered cake and electro-dewatered cake were roughly the same (-6 to -7 range) 8

9 The BORR Compiles Information from a Decade of WERF Research on Biosolids Odor Reduction: Primary goal is to produce dewatered biosolids cakes that have less odors, through reducing bioavailable protein by: Better Digestion Chemical addition Reduced shear during dewatering and handling These approaches are complementary, and several may be needed together for significant odor reduction in biosolids Next slide translates these objectives into a matrix or roadmap of options to consider

10 Decision Matrix Potential Reduction Measures for Biosolids Cake Odors Process Equipment Anaerobic Digestion Digester pretreatment Improve mixing Increase VS reduction Operate in series Multiple phases Increase temperature Other? Dewatering & Cake Conveyance/ Storage Decrease centrifuge bowl speed Decrease centrifuge torque Increase cake moisture content Consider BFPs Belt Conveyors Cake storage/aging Other? Operational Decisions Chemical Addition Pre-Digestion Aluminum Iron Other cations ph control CEBAs Other? Pre- or Post- Dewatering Aluminum Iron Other cations ph control Polymer dose and type CEBAs Other? Design Decisions Anaerobic Digestion Digester pretreatment Better mixing Longer SRTs Egg-shaped Acid-Gas (multi-phase) Thermophilic Temperature-phased Other? Dewatering & Cake Storage Belt Filter Press Rotary Press Variable-speed centrifuges Adjust cake solids Minimize conveyance Belt conveyors Enclose cake storage areas Other? Each decision must consider any other direct or indirect impacts of design and operational changes on wastewater treatment and solids handling processes

11 Decision Tree for Reducing Biosolids Odors Raw Wastewater Pre-Secondary Treatment System YES YES NO NO Nutrient Removal? NO VSR >50%? NO High Shear? YES On-site Storage? Secondary Treatment Anaerobic Digestion Dewatering Final Handling Al/Fe Addition? NO Al/Fe Addition? NO Al/Fe Addition? NO Al/Fe Addition? NO YES YES YES YES YES Digestion pre-treatment Pre- or post-dewatering Fe/Al addition Polymer dose and type adjustment Optimize cake solids content Digestion pre-treatment Improve digester mixing Reduce digester loading Decrease centrifuge speed/torque Consider BFPs or rotary press Pre- or post-dewatering Fe/Al Optimize cake solids content Consider extending storage time Digestion pre-treatment Improve digester operation Multi-phase digester operation Minimize dewatering shear Pre- or post-dewatering Fe/Al addition Optimize cake solids content Consider extending storage time Optimize chemical addition Improve digester operation Multi-phase digester operation Provide ph control Minimize dewatering shear Optimize cake solids content Consider extending storage time

12 Decision Tool The decision tool prompts the user to the following subject areas for guidance: Biosolids odor sampling and testing Treatment and handling options for reducing odors prior to digestion Effects of digestion process variables on odors Effects of dewatering process options on odors Chemical addition approaches Other operational suggestions Design suggestions

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14 Case Study Sydney Australia Problem Digested, dewatered biosolids causing odor complaints when hauled through a resort area Causes Identified Digestion SRT <20 days High %VS in digested solids High-shear centrifuges Shear in screw conveyors Cake storage of 3-5 days 14

15 Case Study Sydney Australia Recommended Odor Mitigation Measures Short Term Improve sludge digester feed - Give it more consistent quality (mostly %TS and %VS) and more consistent feed rate (improved results) Decrease the duration of dewatered cake storage: Odors increase in 3-5 day period, so load and transport biosolids after < 1 day storage time when possible (improved results) Reduce the speed of centrifuge and optimize polymer dose: So far these adjustments have shown mixed results 15

16 Case Study Sydney Australia Recommended Odor Mitigation Measures Mid-term Improve digester-feed thickening process: An optimization project is proposed to improve the reliability and performance of the Rotary Drum Thickeners of the digester feed Recuperative thickening digestion: This process aimed to increase the digester solids retention time by increasing the solids content in the digesters (in the planning phase) Increase capacity of screw conveyors to reduce conveyor speed and shear: A belt conveyor system was desired but is not possible due to space constraints. However, the existing conveyer system is being replaced with a higher capacity system so it can run at slower speed 16

17 Case Study Sydney Australia Recommended Odor Mitigation Measures Long Term These measures target improvements to long term efficiency, reliability, and redundancy of anaerobic digestion. They would likely provide the most substantial reduction in frequency and potential of odorous biosolids. They include: Build an additional mesophilic digester Convert to temperature-phased or two-phased (acid-gas) anaerobic digestion Both options are costly and not being considered at this stage. 17

18 The Biosolids Odor Reduction Roadmap (BORR): A Decision Tool to Reduce Cake Odors Presented at Dewatering 201: Technology, Evolutions, and Trends A webinar sponsored by the Water Environment Association of Texas Questions? Bob.forbes@ch2m.com 18