What Goes in Must Come Out: An Approach to Developing a Mass Balance Tool Encompassing Three WRRFs and a Biosolids Processing Facility

Issue: While managing solids is integral to all Water Resource Recovery Facilities (WRRFs), evaluating whole facilities and systems from a mass balance perspective is often challenging. All WRRFs must manage their biosolids whether it be for treatment or processing/handling at the end of the treatment process. To understand the quantity of biosolids that must be managed, utilities need a proper biosolids inventory. To further enhance the understanding of a biosolids inventory, a biosolids mass balance tool can be developed and applied to monitor performance. However, quality biosolids data is often not available at utilities through all process points, rendering a mass balance tool difficult to develop and calibrate. Objectives: This presentation will provide an approach to developing a biosolids mass balance tool to identify potential process limitations, support solids processing optimization and capital planning, and to evaluate data improvements. This approach will be demonstrated with a case study of a biosolids mass balance tool that was developed for the Philadelphia Water Department's (PWD's) three WRRFs and centralized biosolids processing facility. Utilization of PWD and its facilities within this case study provides a unique opportunity to address not only the benefits, but the challenges that present when constructing a biosolids mass balance tool for a large, public utility. Lessons learned throughout the process will be included for application to any utility seeking to develop a similar tool. Approach for Developing a Biosolids Mass Balance Tool: The development of an informative solids mass balance requires data gathering, quality assurance/quality control (QA/QC) of data, and annotation of assumptions. First a request for information is generated based on the unit processes utilized at the facilities of interest. Primary and secondary treatment data is gathered and reviewed by plant staff and engineers. This request is prioritized by grouping parameters into categories (e.g., 'must have', 'nice to have', etc.). QA/QC of the data is done by evaluating historic data trends to identify outliers or possible analytical errors. Following the initial data screening, a mass balance is built for each process based on the data received. When the mass balances around a process do not close (i.e., solids in does not equal solids out), further investigation is required to identify potential reason(s) for the error. Reasons may include metering errors, analytical errors, an indication of process inefficiency, etc. Assumptions must be made using engineering judgment and industry standards to calibrate the model based on observed data, fill in knowledge gaps, or resolve issues discovered during data analysis. Assumptions must be well documented to inform the mass balance user for current and future applications as more data/information becomes available. Once the mass balance is developed, key process metrics of the unit processes of interest can be compared to industry standards to determine process capacity/limitations. The mass balance can also be used to evaluate optimization/upgrade alternatives for planning purposes in a timely manner. Case Study: PWD, established in 1801, is a public water utility, providing integrated drinking water, wastewater, and stormwater services to the City of Philadelphia and several neighboring communities. PWD owns and operates three WRRFs with a combined permitted maximum daily average flow capacity of 783 MGD. Biosolids from PWD's WRRFs are anaerobically digested and sent to a central biosolids recycling center (BRC). The BRC is located on PWD property but operated by an outside entity. The BRC produces Class A pellets that are land applied throughout the east coast. To better characterize PWD's biosolids handling, understand system limitations, and support planning efforts, PWD collaborated with consulting teams HDR and AECOM on the development of a biosolids mass balance tool. The tool was developed to assess the individual WRRFs as well as the overall solids strategy among the three WRRFs and the BRC. Figure 1 presents a process flow diagram of the three WRRFs and the solids flows among facilities. As shown, this system is complex with respect to solids processing in that one of the three plants sends their sludges to another plant for processing, and the third plant conveys solids via barge to the processing facility. Like other utilities of its age and size, PWD faces the challenges of aging infrastructure and a changing regulatory environment that drives optimization of existing processes and adoption of new processes. These forces, coupled with the ever-present requirement of meeting NPDES permits, can make the prioritization of biosolids data collection and system optimization difficult. PWD and HDR employed creative collaboration strategies to secure the support and cooperation of PWD stakeholders as well as the data required to develop the biosolids mass balance tool. Within Figure 2 is the preliminary output from the mass balance tool calibration that tracks solids production, transport, and destruction across PWD WRRFs. During calibration, several processes were identified for further evaluation due to the mass balance not closing. Further investigation of these processes may lead to important improvements in process performance or process monitoring. Observed key process parameters were also compared to industry standards to identify system limitations and identify optimization opportunities. In doing this, potential inefficiencies were identified such as anaerobic digesters being organically underloaded but hydraulically limited, indicating potential improvement in performance and energy efficiency through further digester feed thickening. This presentation will discuss a process for developing and utilizing a biosolids mass balance to evaluate WRRF biosolids management, as well as present lessons learned from PWD's biosolids mass balance development and evaluation.