Conference Agenda

As wastewater treatment facilities are being rebranded as water resource recovery facilities, carbon is being viewed in a new light. Traditionally, the removal of carbon, expressed as biochemical oxygen demand (BOD) or chemical oxygen demand (COD), is the primary objective in wastewater treatment. However, with recent nutrient removal requirements and energy recovery incentives, carbon is now viewed not as a waste, but as food for the biological nutrient removal process and a source of renewable energy by capturing and re-using the digester gas. The increased attention on carbon benefits is now putting a spotlight on how it is managed across the entire treatment facility process. Understanding the nuances of carbon management is a critical issue for many municipalities, as more treatment facilities in the Puget Sound area are required to provide nitrogen removal, while others have both effluent nitrogen and phosphorus limits. To gain insight on how carbon can be used, it is critical to know the factors that affect carbon management. This presentation will provide a discussion of these factors including presence and types of primary treatment, biological nutrient removal requirements, energy recovery potential and methods, fermentation use, and external carbon sources. In the first case study, a treatment plant currently without primary treatment and digestion is being retrofitted to include primary treatment and digestion. While carbon re-use was not originally the main driver for the plant expansion, carbon management has become an important topic during the planning and design process. The implications of adding primary treatment and potential future fermentation are presented. In the second study, the carbon values of different digester gas uses are compared to the offsets of external carbon addition with sludge fermentation. This study allows a comparison of both costs and greenhouse gas emissions based on the different carbon management schemes. The findings from these examples can be applied to many municipalities in the Northwest as they are faced with the need to meet more stringent nutrient limits and to become more energy-neutral.

This presentation provides an overview of Pima County RWRD Biosolids and Biogas Management Program’s two sustainability projects: beneficial biogas utilization and controlled struvite formation. The Pima County RWRD serves the City of Tucson Arizona and the surrounding communities. The PCRWRD serves 900,000+ wastewater customers within a service area of approximately 700 square miles. It owns and operates 3,400 miles of sewer pipes, 66,000 manholes, 29 active lift stations, and two major regional Water Reclamation Facilities and several small sub-regional WRFs.

PCRWRD’s has been proactively planning for biosolids and biogas beneficial utilization. Land application continues to be the preferred option for its Class B biosolids due to lower cost, simpler to operate, satisfies current regulations, and is consistent with current market conditions. For biogas utilization, different options have been considered, such as CHP for onsite plant use, CNG for fleet use, recovery of carbon dioxide, district heating system, use of excess thermal energy to generate ice for local skating rink, among others.

In the past, biogas was captured and used for onsite cogeneration. An Energy Study for the WRF concluded that a higher value of the one million cubic feet per day of biogas was to purify the biogas to natural gas quality and sell the product in the renewable gas market. Sale of renewable natural gas is scheduled for early 2021.

Dilution of dewatered sludge centrate, and/or chemical addition to the digesters had long been practiced to suppress struvite formation. Planning studies evaluated the pathways of struvite formation and recommended struvite sequestration to control unintended struvite formation. A struvite sequestration process has been constructed after digestion and before dewatering of the biosolids. This facility was brought into service in the fall of 2020.