Source Separated Organic Food Waste Characteristics and Implication for Co-digestion Performance, Operation, and Design

Relevance Organic food waste is an underutilized energy and nutrient source in North America that is mostly disposed of in landfills. This common practice contributes to global warming and wastes precious carbon and nutrient resources. Some private and public entities have started to separate, collect, pretreat, distribute, and recover so-called 'source separate organics' (SSOs) at water resource recovery facilities (WRRFs). Many more WRRFs are interested in evaluating similar practices for their future operation. The substrate quality of SSOs added to anaerobic digesters requires focused evaluation: It varies between locations and might contain contaminants of potential concern to digester stability, operation, equipment, or final quality of biogas and biosolids. To date, we have a few but no standardized industry specifications to monitor and control the quality of SSO feedstock or pre-treated slurries accepted by WRRFs. To address these challenges, this project aims to develop accurate and reproducible analytical methods for food waste characterization. These methods are applied in this project to various engineered bioslurries produced from SSO food waste. This presentation will present the characterization results and implications for co-digestion performance, operation, and design. Results point to practical guidance for the wastewater industry to create cost-effective and meaningful sampling strategies as well as acceptable minimum quality specifications for accepting SSO substrates for co-digestion. Introduction The Water Research Foundation (WRF) Project #4915 called 'Characterization and Contamination Testing of Source Separated Organic Feedstocks and Slurries for Co-Digestion at Resource Recovery Facilities' aims to develop guidance for the characterization and assessment of SSO food waste quality for WRRFs who are planning or practicing co-digestion. The quality of accepted SSO foodwaste may impact the stability of digester operation, biogas production, quality of the final end products (such as biogas and biosolids) and even longevity of infrastructure used for receiving, pre-treating, storing, conveying, and digesting this material. SSO food waste may comprise of heterogeneous material from private households, restaurants, grocery stores, food producers and industry, and displays variability in quality characteristics depending geography, season, collection practices and pretreatment processes. Current laboratory methods applied by commercial producers of engineered bioslurries and by WRRFs have limitations and are not tailored to this type of material. For these reasons this project has the following objectives: 1. Identify, evaluate, and develop techniques for characterizing SSO feedstocks and slurries. Develop an Industry Guidance Document for a systematic, tiered, and comprehensive approach to characterizing feedstocks and slurries. Assess current and emerging concerns for potential SSO feedstock contamination. Define analytical methods for relevant feedstock characteristics needed for process modeling. 2. Standardize sampling protocols for rapid and comprehensive monitoring of feedstocks. Develop simple and robust methods for representative sample collection, preparation, and analysis. Recommend a timely and cost-effective quick-test strategy for WRRFs to assess relevant SSO feedstock characteristics. 3. Develop guidelines for minimum feedstock quality standards for various product goals. Provide guidance on potential contamination of SSO wastes and slurries, depending on their sources and pre-treatment process. Develop guidance on typical quality parameter ranges. Recommend limits for aesthetic, operational, and health-related parameters of concern, as well as final products and objectives. 4. Link characteristics to final product quality. Summarize cause-effect relationships for practitioners on how SSO feedback quality affects final effluent quality, process performance and stability, digester loading capacity, biogas/energy production, biosolids and biogas quality, and downstream treatment processes. The results for goal 1 have been previously presented. This presentation will focus on the results related to goals 2, 3, and 4. Methods Parameter Selection A broad literature review was conducted in the first part of the study from which our team identified relevant SSO food waste parameters for the operation, design and stability of co-digestion food waste systems. These parameters were then grouped into five tiers representing logical analytical parameter groups that can help utilities answer specific operational questions. 1. Tier 1: Immediate inspection. Parameters based on operators' fast inspection upon arrival of SSO to WRRFs. SSO delivery might be rejected or accepted based on this inspection. 2. Tier 2: Frequent routine monitoring screens. Parameters that are relevant for daily anaerobic digester operation and SSO feed control to the digesters. 3. Tier 3: Monitoring for final product quality. Parameters that are relevant for biogas, effluents, emissions or biosolids for land application. These parameters might be tested quarterly or when accepting material from new sources as they are more expensive and time consuming. 4. Tier 4: Comprehensive assessments. Parameters that are of interest for process modelling or research. These might need to be analyzed by commercial labs since they require specialized equipment. Table 1 lists the suggested parameters under each of the four tiers. Analytical Methods Analytical methods for SSO feed stock analysis were compiled from established standard methods currently employed by practitioners for the analysis for SSO feedstock bioslurries. A utility survey was conducted that solicited feedback on which protocols were challenging to conduct or resulted in poor analytical reproducibility or accuracy. The analytical protocols were established at the University of Michigan lab and were tested and refined on an exemplary sample received from a commercial producer of Engineered Bioslurry. Methods that resulted in poor reproducibility were refined until reproducible results were achieved. The written protocols were compiled and documented in a draft deliverable for publication by WRF upon completion of this project. Bioslurry Sample Sites and Monitoring Plan Four U.S. sampling locations were selected for sample collection and SSO feedstock characterization applying the analytical methods selected earlier in this project. The sample locations are in general terms listed in Table 2 and provide a diverse representation of bioslurries in the U.S. that reflect A geographical variety (within the U.S. and Canada); Pre-consumer and post-consumer SSO feedstock; A variety of pre-processing slurry technologies o onsite pretreatment process o commercial small scale pre-treatment o Commercial large-scale pre-treatment using various different technology approaches Good coverage of established slurry manufacturers Names and locations of the sample locations are kept anonymous by request while results can be published and shared. Samples are collected by the participant facilities on a monthly basis until early 2021. The samples are shipped to University of Michigan for analysis. and split samples are shipped to Eurofins for certain specialized tests. QA/QC procedures for sample preservation are followed prior to analysis. Split samples are shipped to Eurofins for certain specialized tests. Guidelines for Minimum Feedstock Quality Standards Monitoring data from the SSO bioslurry characterization is evaluated along with available full-scale data from the co-digestion facilities to assess cause effect relationships between the SSO food waste quality and co-digestion stability, operation, and quality of the final end products biogas, biosolids, and dewatering recycle streams. Table 3 summarizes relevant effects for WRRFs that could result from different SSO food waste characteristics. Conclusions Co-digestion of SSO food waste at WRRFs is quickly becoming industry practice in densely populated areas in the US where landfill space is limited and the sensitivity surrounding negative impacts of GHG emissions through climate change is rising. Co-digestion is an attractive option for WRRFs to increases energy recovery through enhanced biogas production and can generate economic benefits from tipping fees and reduced energy costs. This project fills an important need of the wastewater industry by developing robust analytical methods for the characterization of SSO food waste and helping WRRFs to understand cause-effect relationships between food waste quality and digester stability, operation, and design recommendations.