North Texas Municipal Water District (NTMWD) developed a Biosolids Master Plan to evaluate potential sustainable biosolids management improvements. This paper describes that evaluation, the planning process utilized, and results. The methodology for evaluation includes
multi-attribute utility analysis including both financial and non-financial criteria.
This paper summarizes results from the Biosolids Master Plan, which evaluated solids handling approaches for all of the NTMWD wastewater treatment plants. The District’s service area is located northeast of Dallas, TX, and has a current drinking water service population of approximately 1.6 million, of which the majority also receives wastewater service. NTMWD currently treats an average of 378.5 megaliters per day (ML/d) (100 million gallons per day (mgd)) of wastewater, with anticipated future average flows of 700 ML/d (185 mgd) in 2040. The District currently operates fourteen (14) wastewater treatment plants that each serve populations ranging from less than 1,000 to 400,000. Wastewater residuals from the facilities are
currently thickened, dewatered, and landfilled, with most of the residuals placed in a landfill also owned by the District.
Multiple residuals management technologies were evaluated, with considerations for capital cost, operation and maintenance cost, and non-monetary factors such as odors, noise, permitting, and beneficial reuse. In addition, the District-owned landfill enabled consideration of options that utilize landfill biogas to offset the cost to purchase natural gas for energy intensive processes.
An evaluation of each WWTP facility and a projection of anticipated loads revealed that approximately eighty percent of all residuals are generated by three of the fourteen WWTPs, so most capital improvement options were focused on the three largest WWTPs. Interplant material transfers were considered as well to centralize residuals processing taking into account the district geography, existing sewer lines connecting some plants, and the proximity of adjacent WWTPs. For each facility a mass balance approach was taken to determine design parameters, and costs associated with each alternative were developed.
The combination of non-financial scores resulted in a cumulative “Benefit” score for each biosolids option. The relative contribution of each criterion will be presented. Without consideration of the project cost, Thermal Drying received the highest total benefit core, followed by Thermal Hydrolyis and Composting, with Landfilling receiving the lowest nonmonetary score.
These non-monetary scores were combined with the estimated net present value for each alternative to develop a relative benefit-cost score for each option. The rankings revealed that digestion-based approaches would be the most favorable long term biosolids management
option for the District. The paper will provide valuable insight into trends in the residuals industry including the drivers for change to more sustainable systems, and technological advances enabling facilities to adapt more efficiently.
Keywords: Residuals, Biosolids, Management, Planning, Benefit-Cost, Alternatives Analysis
