A Beautiful Biosolids Product in the Making: Dewatering Optimization and Aerobic Curing of Thermally Hydrolyzed Solids at HRSD's Atlantic Treatment Plant

Background In mid-2020 Hampton Roads Sanitation District (HRSD) transitioned to Class A biosolids production via thermal hydrolysis process (THP) using a Cambi Mark II B6-4 unit at the Atlantic Treatment Plant (ATP). THP increased solids capacity by allowing for higher digester loading, increased volatile solids reduction (VSR), and significantly improved final dewatering performance. This additional capacity will allow ATP to receive solids from other HRSD facilities, which will provide the opportunity to shut down multiple hearth incinerators at other plants. In addition, THP is near energy neutral and can be energy positive, especially with the addition of fats, oil, and grease (FOG) receiving at the ATP installation. The B6-4 system relies on steam injection within batch reactors to thermally hydrolyze solids prior to anaerobic digestion. Not only does this process kill pathogens within the solids, but it also reduces the viscosity of feed solids and allows for higher digester loading. Initial startup and seeding of the digesters with DC Water digestate occurred in May 2020; it took approximately six months to startup three anaerobic digesters after disinfection and seeding. Using the same dewatering centrifuges, final dewatering cake solids content increased from the baseline prior to THP (14-16% total solids (TS)) to a range of 20-25% TS with minimal dewatering optimization. Additional optimization at final dewatering resulted in reliable 30% TS final cake solids production without any change in polymer selection. Additional aerobic windrow turning of solids was shown to produce a low-odor product that was 55-60% TS. Purpose This presentation will focus on final dewatering centrifuge optimization that was conducted to reliably produce a 30% cake product. In addition, a series of curing pilots were conducted using the 30% cake and a windrow turner to determine the most efficient method for producing an aerobically cured product with low odor. Data from final dewatering optimization and the curing pilots were used to evaluate cost savings associated with each improvement. Final Dewatering Optimization Initial dewatering optimization focused on evaluating solids loading rates after transitioning to a thicker feed coming out of the THP digesters. After achieving optimal loading rates, staff focused efforts on polymer dosing locations and sludge-polymer mixing upstream of the final dewatering centrifuges. Adding 100% of the polymer upstream of the centrifuge at the sludge-polymer inline mechanical mixers allowed plant staff to minimize the mass of polymer required to get optimal capture. In addition, after testing various sludge-polymer mixing intensities, it was found that the optimal mixer speed was 900 rpm. This mixing intensity achieved the same or better recoveries with an even lower polymer usage rate (Figure 1). After capture optimization was completed, staff focused on cake dryness. To do this, the torque setpoint on the centrifuge was increased progressively and additional polymer was added to maintain equivalent capture. These data were used to compare the cost benefit of polymer usage vs. reduced hauling rates with increased cake dryness (Figure 2). Additional data will be presented to show the dewatering benefits of ferric addition upstream of the primary clarifiers. While plant staff initially added ferric to help minimize reduced sulfur compound odors in the final cake product, it was found that half of the ferric costs could be recouped with polymer savings in final dewatering. Curing Pilots Various iterations of curing pilots were executed to answer two main questions: (1) What are the characteristics of the final cured end-product? and (2) What is the most efficient way to achieve that end-product? Additionally, data collected during pilot studies were used to evaluate the business case for curing THP solids. End-Product Characteristics Figure 3 shows %TS and percent volatile solids (%VS) from several curing pilot experiments in which solids were aerated with a windrow turner at various frequencies, with and without the addition of cured cake to piles to 'seed' the process. When evaluated holistically, it appears that the final cured product tended to be 55-60% TS and 55-60% VS. Once the piles achieved those endpoints, plant staff found that the product was too dusty to continue turning. Seasonal VSR was evaluated using the Van Kleek method (Figure 4). During the winter, addtional VSR due to curing hovered around 10-15%, whereas it was seen as high as 40% in the spring and 45% in the summer. Production Efficiency Two main variables, seed addition and frequency of turning, were evaluated to determine if they impacted the speed at which cured cake could be produced. Seed addition consisted of adding variable amounts of already cured product to fresh cake by volume. Although this study did not determine the mechanism by which back mixing cured product with fresh product, two main explanations are likely: (1) Seed addition creates additional pore space for oxygen penetration into piles, similar to wood chip addition in the composting process, and (2) Seed addition kick starts the curing process by adding a large quantity of microorganisms adapted to aerobic curing. A variety of turning scenarios were employed within the five-day work week (10 times, 5 times, 3 times, and 2 times per week). Observations and data collected during these phases indicated that turning 5 times a week (once a day Monday to Friday) was sufficient to achieve the end-product. Seeding appeared to accelerate the curing process (Figure 5). While it took fresh cake-only piles approximately four weeks to reach the end point, the addition of 25% cured cake by volume reduced the curing time by at least one week. It should be acknowledged that using 25% of pad capacity for a 25% reduction in curing time is a breakeven point, however, qualitative observations during the process indicated that odors emanating from the curing piles were less offensive when 45% TS was achieved in the piles. Seed addition may therefore offer an additional benefit if that threshold is achieved faster in the process. Conclusions The combination of final dewatering centrifuge optimization and aerobic curing have been utilized at HRSD's Atlantic Treatment Plant to produce a low odor, high value soil amendment. Ultimately, dryer cake with lower odors will result in lower hauling costs and less truck traffic through neighborhoods surrounding the plant site.