Field Testing Texas' Largest Fine Grit Removal System: Implementational Insights

As the industry shifts its focus to removal of fine grit particles at wastewater treatment facilities, Village Creek Water Reclamation Facility (VCWRF), one of the largest wastewater treatment plants in the Dallas/Fort Worth area, has completed field performance testing on their 369 million gallons per day (MGD) fine grit removal facility. VCWRF is Fort Worth's only wastewater treatment plant which serves Fort Worth residents and some surrounding small cities. It is a conventional plant with fine screening, primary clarifiers, activated sludge secondary treatment, tertiary filters, and chlorine disinfection. VCWRF does not currently possess an influent grit removal process. Screened influent is sent directly to the primary clarifiers, where grit is settled and removed with the primary sludge and de-gritted before thickening and anaerobic digestion. The primary sludge de-gritting equipment is capacity-limited, outdated, and inefficient, resulting in a substantial amount of grit accumulation in the anaerobic digesters. VCWRF conducted multiple grit characterization studies which are summarized in Table 1 to determine the size and amount of grit entering the plant. This data shows that VCWRF has a significant amount of fine grit entering the plant where roughly 46% is finer than 297-micron and 20% is finer than 150-micron. These findings are consistent with other Texas wastewater treatment facilities. A grit particle's specific gravity and, consequently, its settling velocity is affected by attached surface active agents (SAA) that include fats, oils, greases, and other organic materials. The shape and composition of grit and inert solids also considerably affect settling velocities. Material with similar effective specific gravities may have very different settling velocities due to the shape of the particle. The sand equivalent size (SES) is the sand particle size having the same settling velocity as the actual grit particle and should be considered when designing a grit removal system. Table 2 summarizes the average grit particle distribution and the estimated removal efficiency of a system designed to remove the grit of a specific SES. On average, roughly 80% of the grit in raw wastewater influent has a physical particle size of 150-micron or greater. However, a grit removal system designed to remove 150-micron and larger particles will only have an efficiency of 78% because some grit particles that are 150-micron and greater do not settle as expected. As observed from the data, finer grit particles settle as expected and therefore their physical particle size and SES are very similar. This is why we see higher removal efficiencies for a grit removal system designed to remove 105-micron and larger. Average grit concentrations in the raw wastewater influent varied throughout the grit characterization studies. The difference in concentrations between the sampling days is likely a result of rain events that preceded some of the sampling dates. Average grit concentrations in the raw wastewater influent samples are summarized in Table 3. At the permitted annual average daily flow (AADF) of 166 mgd and average grit concentration of 186 lb/mg the plant is expected to receive 30,900 lbs of grit a day in the influent raw wastewater. Grit handling mass balance flow charts (Figure 1) were developed to illustrate how much grit will be handled daily at the permitted AADF for a system designed for 105-micron. The mass balance shows solids loading, retention, and disposal requirements if 95% efficiency is maintained across the entire grit removal system. If efficiency is compromised, more grit will pass through the grit removal system and be retained in the downstream processes. Given the grit characteristics and quantities, we evaluated various removal technologies and established design recommendations for the grit removal system. Specifically, to address fine grit accumulation throughout the plant, a grit removal system must be capable of meeting the following performance criteria: 95% removal of all grit 105-microns (150 mesh) and greater at the AADF of 166 mgd. 95% removal of all grit 150-micron (100 mesh) and greater at SPTC of 255 mgd. 95% removal of all grit 212-micron (70 mesh) and greater at 2HPF of 384 mgd. Various grit removal system components (separation, pumping, processing) and technologies were evaluated separately to allow a selection of the most appropriate type of equipment for every stage of the grit removal system. Free vortex grit separation units (Hydro International HeadCell(R)) with dry pit recessed impeller grit pumps and cone washer/screw conveyor grit processing system (Huber Technology Coanda) were selected to provide the desired grit removal performance of 105 microns with cleaner, dryer grit for disposal. Table 4 summarizes the performance of eight HeadCell units at the various plant design flows. We established performance testing criteria and required the contractor to complete field performance testing upon start-up of the HeadCell and Coanda Units. Field performance testing was completed in September 2023. Table 5 summarizes the results. This paper will go into detail about the design and performance criteria selection, field-testing details, how the performance results compare to other facilities', and how this effort sets the stage for the future of fine grit removal systems and performance testing.