Wide-Reaching Improvements Through Direct TWAS Dewatering

Introduction: The Truckee Meadows Water Reclamation Facility (TMWRF), serving the cities of Reno and Sparks, Nevada, has had historic difficulty of their dewatering operation with their biosolids averaging just over 16% total solids (TS). This poor performance is largely the result of TMWRF running an enhanced biological phosphorus removal (EBPR) process, which has limited their maximum mechanical dewaterability due to the large amount of dissolved phosphate and volatile solids in their centrifuge feed. Centrifuge replacement in 2015 further exacerbated the dewatering performance with the newer machines requiring high polymer dosages and delivering consistently poor centrate quality. Since June of 2020, TMWRF has carried out an alternative strategy to their dewatering operation by sending the thickened waste activated sludge (TWAS) produced in their EBPR process directly into their centrifuges, bypassing all digestion. The impacts of this change have been major and wide-reaching with effects on biosolids character and volume, chemical usage, plant wide nutrient removal, biogas production and quality, and centrate treatment. This presentation will discuss the background of the facility including their historically low-quality performance as well as the many facets that have been positively impacted by carrying out this change to direct TWAS dewatering, This presentation will also examine the role of this operating strategy in the long-range planning of the facility. Dewatering Background: TMWRF, a 30 million gallon per day (MGD) facility, had operated the same three centrifuges since 1983, producing just over 15% TS biosolids with a centrate quality of slightly over 400 mg/L total suspended solids (TSS). To improve both process parameters and energy demand, a centrifuge replacement took place in 2015. While the centrifuges were specified to produce a 22% TS biosolid from a polymer input of 23 active pounds per dry ton of solids, these machines have averaged 16% TS biosolids with a typical dosage of 44 active pounds per dry ton of polymer. While some marginal improvement occurred in the biosolids, the centrate produced from these machines worsened, averaging approximately 1,100 mg/L TSS which has burdened downstream centrate handling processes. With continued struggles to produce a higher solids content product from the centrifuges, in August of 2019 TMWRF submitted their sludge for further laboratory analysis to determine the maximum mechanical dewaterability. The results indicated that the sludge grab sample had a maximum achievable TS content of 15.8%. Also noted was the high phosphate concentration (292 mg/L) and volatile solids content (73.8%) present in the sludge liquor. Streaming current detector measurements were carried out to quantify polymer demand and indicated an optimal polymer dosage of 40 active pounds per dry ton. Direct TWAS Dewatering Study and Results: Given that many of the factors negatively affecting dewaterability stemmed from the TWAS sludge and were independent of the centrifuge, a study commenced to determine to what extent this sludge could be dewatered directly. With no beneficial reuse customers, the only standard to meet for TWAS biosolids was to pass a Paint Filter Liquids Test in order to be landfill compliant. After several discrete trials demonstrating this was possible, full-scale direct TWAS dewatering was implemented on June 9, 2020. After the passing of several detention times within the digesters, TWMRF experienced marked improvement in sludge dewaterability of their digested sludge with it being solely comprised of primary sludge. This digested primary sludge demonstrated the ability to consistently produce over 20% total solid biosolids with the highest achieved solids content of 28%. The waste activated sludge has shown to have nearly the same level of dewaterability as the historic sludge, averaging near 15% total solids without presenting any disposal concerns. In the time since beginning the experiment, there have been profound impacts throughout the treatment plant. The centrate, which had historically provided nearly 30% of the ammonia loading to the facility, saw a rapid and sustained decrease of 47%. This has significantly improved the nitrification and denitrification operations present at TMWRF as well as enabled more capacity for their nitrifying trickling filters. This reduction in ammonia translates to a lower loading into the denitrification process, resulting in an annual methanol savings over $200,000. Through the direct dewatering of the TWAS, a sharp decline in concentration of orthophosphate within the centrate has also been observed. This concentration has reduced from approximately 250 mg/L to less than 100 mg/L and as a result has removed the need for centrate sidestream phosphorus removal. The decommissioning of this additional treatment and associated chemical consumption has created a benefit of nearly $250,000 per year. The other large impact to chemical usage that TMWRF has experienced is a reduction in polymer usage. Waste activated sludge is thickened in a dissolved air flotation thickener and had previously utilized polymer to maximize TWAS total solids. This dosing has since been discontinued since the centrifuge design parameters require a lower feed solids content. Overall, this has led to a 20% reduction in polymer use, creating an annual savings of approximately $170,000. The continued duration of this project has identified other numerous benefits such as a 26% reduction in the hydrogen sulfide of the digester biogas, consistent phosphorus loading reducing aluminum sulfate demand, ability to achieve higher centrifuge hydraulic loads, and realized capacity of the digestion process. The demonstrated benefits of this full-scale experiment have resulted in its recommended implementation in TMWRF's latest facility plan, prompting an addition to TMWRFs Capital Improvement Program to begin analysis for making it a permanent process. This paper will outline the common challenges that EBPR sludge has on dewatering and the historical poor performance solids processing of TMWRF. Insight will be provided on a variety of operating configurations such as pure TWAS dewatering versus TWAS/digested primary sludge blended centrifuge feeds and the impacts of these on centrifuge outputs and total biosolids volumes produced. Detailed discussion will occur on the utilization of direct TWAS dewatering as an effective nutrient removal process not only for ammonia and phosphorus, but also for organic nitrogen.