Bench-Scale Digestion Evaluation of Potential Co-Digestion Waste Streams

Authors:

  • Richard Porter and Holly Elmendorf- Gwinnett County Department of Water Resources
  • Madan Tandukar and Spyros G. Pavlostathis - Georgia Institute of Technology
  • Scott A. Hardy - Hazen and Sawyer

When utilities undertake a co-digestion program with fats, oils, and grease (FOG) or other high strength waste (HSW) to boost digester gas production rates, there are many unknowns:

• How much FOG or HSW can be fed to the digesters?
• Will there be impacts on the digestion of the municipal sludge?
• What will be the actual digester gas production from a particular FOG or HSW?
• Will nutrient loading increase in the plant recycle streams?
• Could hydrogen sulfide and siloxanes levels increase in the digester gas?
• How much will it cost and what is the benefit of co-digesting with a particular HSW and FOG substrate?

Many of these unknowns can drive the business case evaluations for selecting a waste feedstock for co-digestion program. Bench-scale testing using samples of potential waste streams can give answers to all of these unknowns and help guide the decision process.

The answers to these unknowns are critical for Gwinnett County Department of Water Resources (GCDWR) in developing their FOG and HSW Receiving Program. GCDWR recently installed a 2.1. Megawatt biogas engine generator at the F. Wayne Hill Water Resource Center (FWHWRC), for which the current digester gas production is about 70% of the full load digester gas demand for the engine.

GCDWR desired to increase digester gas production via co-digestion to make up the digester gas difference and increase economic benefit created from the engine-generator. However, the FWHWRC has limited excess digester capacity, and digester gas production above that required for the biogas engine, has adverse air permit implications. GCDWR needed to know which waste produces the most gas while taking up the least digester capacity, while not exceeding gas production needed for the engine. GCDWR also evaluated unique high strength industrial waste streams, and needed to know if there were any adverse impacts to using a particular HSW on liquid and solids treatment processes.

To answer these questions, Georgia Institute of Technology and Hazen and Sawyer were contracted to perform bench-scale studies on potential sources of FOG and HSW. The testing included:

1. Waste stream characterization – measuring various parameters, such as pH, COD (total and soluble) ammonia, total solids, volatile solids, and volatile fatty acids. Analysis included qualitative compatibility tests of waste mixtures.

2. Ultimate Digestibility – included evaluating the ultimate digestibility of different combinations of primary sludge (PS), thickened waste activated sludge (TWAS), FOG, and HSW during a 90-day incubation period. During the test, total gas volume, gas composition, VFA, and pH were measured. At the conclusion of each test, pH, TS, VS, COD, VFA, ammonia and total and dissolved sulfides were measured. The kinetics and production rate of digester gas were evaluated.

3. Bench-Scale Reactor Study – included multiple runs of combinations of FOG, HSW along with primary and waste activated sludges at a 15-day solids retention time (SRT). During the test, gas production and composition, pH, soluble COD, VFAs, TKN and ammonia were measured every 3 days. At the conclusion of the test, pH, TS, VS, total and soluble COD, VFAs, TKN and ammonia were measured. The supernatant and digested sludge were tested for total and soluble phosphate, and the dewaterability of the digested sludge was evaluated by the time-to-filter test. The digester gas was characterized, including hydrogen sulfide and siloxanes.

4. BioWin Process Modeling – Utilizing the laboratory results and the existing FWHWRC calibrated BioWin model, the plant-wide impacts from co-digesting the proposed waste streams was evaluated.
Results and Findings

The detailed results from each testing phase will be presented. The results of the ultimate digestibility provided gas production estimates for various waste streams relative to their COD concentrations. Some wastes showed inhibition during digestion, while others showed immediate degradation. The initial results suggest an increased destruction of volatile solids in the primary and waste activated sludge components with the addition of the FOG/HSW waste streams. This beneficial effect is being further explored in a follow up study that is currently under way. Results from the follow up study will be presented in the paper.

The bench-scale results provided accurate digester gas production levels with digester gas testing showing no additional hydrogen sulfide or siloxane concentrations. The increase in digester gas production was confirmed and quantified for each of the waste streams.

The information obtained, such as gas production per gallon of FOG or HSW, is currently being used by GCDWR to evaluate various waste streams for co-digestion and to determine tipping fees for FOG/HSW to meet digester gas production requirements for the biogas engine generator.

In summary, this paper will present the methodology and results utilized to screen a wide range of streams for possible co-digestion at the F. Wayne Hill WRC using bench scale screening tools. These methods can be utilized to evaluate ultimate digestibility, gas yield (CFT per gallon treated), and identify potential adverse or synergistic effects associated with co-digestion of FOG and/or HSW substrates with a blended municipal primary and waste activated sludge stream.

To request a copy of the full presentation, please contact the author at shardy@hazenandsawyer.com.

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