Conference Agenda

Anaerobic digesters are typically operated with limited process information and rely on industry standard values and long detention times to minimize the impact of any perturbations. Understanding the risk of failure is particularly important when these are operated under variable loading conditions or close to their design capacity. Our ability to identify the causes of upset events and remedy them, is affected by the limited number of online parameters that can be used to characterize digestion performance, the restricted ability to make visual inspections, and the use of laboratory measurements that can sometimes be unreliable and take time to be completed.

Over the previous years, Clean Water Services has developed and implemented a bioassay to monitor the digesters of the Durham and Rock Creek Water Resource Recovery Facilities to better understand digestion performance and health as it relates to capacity. The bioassay consists of measuring the ability of the microbial communities to use a key intermediate over time, such as acetate, and can help identify conditions of stress caused by organic or hydraulic overloading. The indicators generated can provide a unique insight about operational strategies that help maintain stability. This bioassay has been proven to be reproducible, relatively easy to implement, and it can generate indicators of digestion health within 12 hours.

The experimental development of the bioassay and initial results were presented at the Pacific Northwest Clean Water Association Conference in 2022. This presentation will include an evaluation of the bioassay indicators against conventional full-scale operation metrics and will focus on addressing the following questions:

- What are some of the challenges CWS has had using traditional metrics to identify unstable conditions?

- What are some of the insights that have been generated from using the bioassay over the past year of operation?

- How can these indicators enhance the information provided by conventional metrics?

During water resource recovery, primary and secondary solids are commonly stabilized in anaerobic digesters utilizing bacteria to convert organic materials to biosolids and biogas, both having beneficial uses. The digestion process runs optimally with near uniform conditions, requiring complete mixing of digester contents with minimal short-circuiting and maintaining contact between the tank active biomass and incoming feed. Mixing efficiency is assessed post construction through tracer testing, and measurement of temperature and solids profiles. Industry has reported similar levels of mixing for a wide range of power inputs and types of mixing systems. Oversized mixing systems lead to high construction and operating costs, while under-sized systems lead to subpar performance. Understanding mixing in a more fundamental way can reduce costs while delivering desired performance aiding in design of right-size mixing systems that reduce energy consumption while maximizing biogas production and organics destruction. In addition, newer types of mixers have entered the market that may have high efficiencies. Understanding actual mixing provided by systems allows designers and utilities to compare technologies, determine the limits of mixing innovation, and ultimately select the best solution that balances cost, performance, and risk. A recent new digester installation in South San Francisco used two different mixing technologies in two otherwise similar tanks and included detailed startup testing, providing ideal data for developing computational fluid dynamics (CFD) modeling approaches to study internal mixing details.

CFD is a promising tool to optimize mixing system selection and design, as it allows us to “see” inside the tank, visualize the mixing, identify dead spots or areas of over-mixing, and provide an easy platform to customize and compare different technologies or designs. Development of CFD models for different types of mixing systems and comparison of model outputs to actual field-produced data could allow for refinement of this tool and for expanded use in digester system design and optimization. This talk will focus on the development of CFD modeling approaches for evaluating digester mixing. Key physics will be reviewed. Model results will be presented comparing model simulated mixing with field measurements.

Anaerobic digesters are the most common solids stabilization technology in municipal wastewater treatment facilities. Although they are relatively low maintenance, routine cleaning every 5-10 years helps maximize performance and prolongs the useful life of the tank. However, restarting a digester is a complex process requiring careful oversight and detailed knowledge of the plant processes. This presentation will cover best engineering practices for taking a digester out of service and restarting the process safely and efficiently. Stopping and restarting a digester require careful coordination of digester gas, mechanical processes, solids management protocol, and digester biological health. Specific procedures vary depending on the presence of other operating digesters, fixed vs floating covers, the type and quality of solids produced at the facility, and other factors. Step-by-step instructions for managing digester maintenance will be provided, along with a case study of a recent digester startup.

The Grant’s Pass Wastewater Treatment Plant in Oregon recently restarted their digester using seed sludge from a nearby wastewater treatment facility. The startup presented several challenges, including concurrent upgrades to the digester heating system, cold weather, and a floating cover digester. To maintain safety, a water seal was formed using primary effluent and seed sludge was added to the digester in batches over a two-week period. A temporary recuperative thickening process was installed to expedite the startup process. Key digester health parameters such as digester gas production, digester gas characteristics, volatile acids, alkalinity, etc. were tracked to guide the startup process.