WRF Research Highlights: New Discoveries and Tools to Support Your Co-Digestion Program

Functioning at the nexus of the food, energy, and wastewater systems, co-digestion is one of the linchpin processes used to produce renewable energy from biogas and treat and transform food and municipal wastes into marketable products (WRF, 2019). Co-digestion with organic feedstocks can improve digester performance, increase methane production for energy generation, and decrease operating costs. US EPA has identified food waste as one of the least recovered materials in the municipal solid waste industry, making up 20% of national municipal solid waste. Because of this and other independent drivers, states such as California and Massachusetts have adopted initiatives to reduce the amount of organic waste accepted at municipal landfills. A shifting regulatory landscape is leading WRRFs to evaluate co-digestion with residential and commercial food waste from post-consumer sources. For over a decade and with over seventeen publications to date, The Water Research Foundation (WRF) has advanced the science and shared the knowledge through collaborations and funding co-digestion research covering US and international case studies, operational side-effects and other impacts, business case analyses, biogas production, and even co-digestion of manure. WRF is the leading research organization advancing the science of all water to meet the evolving needs of its subscribers and the water sector. WRF is a nonprofit, educational organization that funds, manages, and publishes research on the technology, operation, and management of drinking water, wastewater, reuse, and stormwater systems-all in pursuit of ensuring water quality and improving water services to the public. WRF serves approximately 1,200 subscribers, hosts an online research library of more than 2,300 projects, and manages an innovation platform with a database of more than 140 innovative technologies. In 2016, WRF began focusing research on source separated organic (SSO) feedstocks. Co-digestion with organic feedstocks can improve digester performance, increase methane production for energy generation, and decrease operating costs. SSO feedstock is defined as originating from commercial generators such as restaurants (excluding grease), commercial kitchens and cafeterias, grocery stores, and residential generators separated from other wastes at the source. The purpose of this research is to provide utilities and practitioners with tools to access and communicate the applicability, costs, contractual scenarios, and benefits associated with source separated organic pre-treatment for co-digestion and various source separated organic supply management strategies. This presentation will share highlights, along with guidance and tools, from past research and focus on three recently completed WRF projects touching on different but interconnected aspects and stages of co-digestion: pre-treatment, contamination and characterization testing, and biogas quality. A summary of each project and key findings are presented below. Project 5037 (PI: Mohammad Abu-Orf, Hazen and Sawyer): Evaluation of Existing Source Separated Organic (SSO) Feedstock Pre-Treatment and Management Practices. Existing research on source separated organic co-digestion focuses on anaerobic digestion operational strategies and processes. There is minimal information on feedstock pre-treatment, characterization and management of contaminants, overall pre-treatment implementation and monitoring, and assessment of pre-treatment impacts on anaerobic digestion performance. This project provides the industry with a technical foundation, associated guidance materials, and utilitarian tools to more comprehensively understand the various methods currently in use or potentially suitable for pre-treatment of source separated organics. Key Outcomes: An SSO pretreatment database was constructed in PowerBI including: -Literature surveys and case studies to summarize the implementation and effectiveness of numerous pretreatment technologies. -Factsheets to expand the accessibility of technology information. -Detailed case studies with anecdotal recommendations which were formulated into figures to guide SSO pretreatment selection. -Experience of currently operating SSO pretreatment facilities including successes and challenges. Project 4915 (PI: Tanja Rauch-Williams, Carollo Engineers): Characterization and Contamination Testing of Source Separated Organic Feedstocks and Slurries for Co-Digestion at Resource Recovery Facilities. A key challenge with SSO feedstock co-substrate is that its composition, quality, and characteristics differ between geographical locations and can change over time. This causes challenges and uncertainties for pre-treaters, substrate brokers, facilities accepting this material, operators, engineers, and regulators. This project addresses these challenges by establishing guidelines for characterizing SSO feedstocks; standardizing sampling protocols for rapid and comprehensive monitoring of feedstocks; and developing recommendations for minimum feedstock quality requirements. Key Outcomes: There are four separate documents/reports included in this project: 1) Characterization of Source Separated Food Waste for Co-digestion in Water Resource Recovery Facilities aka Literature Review. This document provides a summary of background information on pertinent regulations, policies, SSO feedstock sources, and typical SSO food waste quality characteristics reported in literature. 2) Analytical Method Guidance for the Characterization of Source Separated Food Waste for Co-digestion in Water Resource Recovery Facilities. This document provides operators with a summary of recommendations on the sampling, preservation, and analysis of these parameters in SSO food waste feedstock to help overcome common issues with this challenging waste stream. 3) Towards a Uniform Basis for Substrate Characterization and Modeling of Anaerobic Co-Digestion and Anaerobic High-Strength Waste Treatment Workshop Summary. This document provides co-digestion practitioners with a focused discussion on the current status, achievements, and remaining challenges of using process modeling to simulate and predict co-digestion of HSW in anaerobic digesters at WRRFs. 4) The Relevance of Food Waste Quality for Co-Digestion Facility Planning, Design, and Operation. This document provides practitioners with tools and guidance to assess the quality characteristics of different types of SSO feedstock and their impact on process performance, stability, regulatory compliance, and the quality of end products, including biosolids, biogas and air emissions, and final effluent. Project 4892 (PI: Ganesh Rajagopalan, Kennedy Jenks Consultants): Quality of Biogas Derived from Wastewater Solids and Co-Digested Organic Wastes: A Characterization Study. Co-digestion allows utilities to beneficially reuse organic matter and produce renewable energy. However, adding organic matter can result in unintended consequences to facility operation, such as digester foaming, sludge dewaterability, and odor. This project developed the scientific information needed to more accurately estimate emissions from anaerobic digesters that practice co-digestion, understand conditioning/pretreatment of raw biogas, and support the air quality permitting process for co-digestion. The research team created a predictive model to relate organic waste, including high-strength waste, and feed sludge characteristics to biogas quality. Key Findings: -Co-digestion of organic wastes with wastewater solids in anaerobic digesters in water resource recovery facilities (WRRFs) can enhance biogas yield (cu.ft./lb volatile solids reduced) as well as total biogas production. -Addition of organic wastes can alter biogas quality that may potentially impact regulatory compliance and/or treatment requirements. -Some organic wastes increased hydrogen sulfide levels in the biogas while others lowered the concentration. -Ammonia levels in biogas during co-digestion increased or decreased depending on the percent nitrogen content in the organic waste. -In general, several organic wastes appeared to increase the levels of volatile organic compounds (VOCs) in the biogas. In particular, the levels of acetone, butanone, ethanol, and butanol increased due to addition of the organic wastes tested in this study. -In most cases, addition of organic wastes appeared to lower the siloxane levels in the biogas.