Conference Agenda

Several WWTPs have been practicing co-digestion with FOG or food wastes to increase biogas production and subsequent energy. A main effect of co-digestion is the impact on biogas quality. Depending on the quality of the organic waste used, co-digestion may alter the concentrations and/or introduce additional impurities to the biogas. Such change in biogas quality can impact (i) compliance with regulatory requirements and (ii) treatment needs for various end uses of biogas such as co-generation, vehicle fuel, and pipeline injection. However, limited to no information is available on complete characterization of biogas produced from co-digestion of different feed stocks with wastewater sludge. The Water Research Foundation (WRF) project focused on investigating the relationship between a wide range of organic wastes and the resulting biogas quality from their co-digestion. This presentation will highlight: field and bench scale co-digestion of wide range of organic wastes and impact on biogas quality and quantity, complete biogas characterization including major components, siloxanes, VOCs, alkanes, ketones etc, guidance to estimate emissions more accurately from co-digestion and evaluation of biogas quality parameters to assist with permit compliance.

Clean Water Services (CWS) is pursuing an opportunity to use available digestion capacity of the Rock Creek Water Resources Recovery Facility (WRRF) by developing a Co-digestion Program. This program serves two purposes: (1) it allows CWS to better serve the district by creating and strengthening relationships with surrounding industries as well as with local contributors that can provide High Strength Wastes (HWS) and, (2) it increases the overall biogas generation to a quantity that allows CWS to consider Renewable Natural Gas (RNG). This is mutually beneficial, as this service can lower the discharge costs of the industries’ byproducts and their environmental impact. Furthermore, our biogas system infrastructure is aging, which makes shifting to RNG and partner with Northwest Natural an attractive prospect.

Multiple groups within CWS are collaborating in this program to systematically identify, characterize and select wastes that can contribute to gas production goals for RNG without compromising digestion capacity and stability. The evaluation process consists of:

1. Defining the capacity of the system and gas production goals. This has allowed us to stablish an initial requirement of 4.4 ft3of gas/gallon of HSW.
2. Identifying sources of HSW within our service district. We engage with contributors to determine the reliability of their practices and consistency of their product.
3. Assessing potential impacts to operations and maintenance of the digesters, and infrastructure requirements. This is achieved through: a) novel bench-scale testing approach, b) pilot testing and, c) evaluation of the physical characteristics of the HSW.
4. Selecting the HSW and negotiating with suppliers.

This presentation will focus on the challenges associated to implementing this program, which include: coordinating efforts from multiple groups within and outside CWS, pushing a recalibration of the organization’s culture to get staff buy-in, and to make data-driven decisions based on results from relevant testing. Additionally, we will talk about our operational experience using fats, oil and grease at the Durham WRRF that has helped define our HSW selection criteria.