Pima's Plural Purposes for PONDUS

LEARNING OBJECTIVE: Pima County's Tres Rios Water Reclamation Facility (TRWRF) recently evaluated process benefits and cost effectiveness of including PONDUS into their process, ultimately deciding to proceed with the design and construction based on several advantages. Participants of this presentation will learn the findings of the evaluation including present-value and life-cycle costs of implementing the new process, positive impacts to digestion capacity, biogas production and sales, and solids loading rates at dewatering and the future Class A biosolids solar dryer system. INTRODUCTION | BACKGROUND The TRWRF, owned by The Pima County Regional Wastewater Reclamation Department (County), serves as the centralized solids handling and treatment system for the entire county, offering a treatment capacity of 50 MGD. The TRWRF currently produces approximately 180 wet tons per day (WTPD) of Class B biosolids at approximately 18-20% total solids content. With the goals to reduce TRWRF energy requirements, lower solids disposal costs, and address future biosolids disposal regulations the County is planning for a solar dryer to produce Class A biosolids. An opportunity to maximize these goals is to introduce a hydrolysis process that can further reduce biosolids volumes and reduce the size of the solar dryer. In addition, a hydrolysis process would create additional needed digester capacity, increase biogas production, and create a dryer cake going into the solar dryer amongst other potential TRWRF benefits. The thermochemical hydrolysis process PONDUS was specifically targeted based on limited additional operational requirements and small footprint. METHODOLOGY The PONDUS system is a thermal/alkaline hydrolysis process designed for breaking down bacteria cell walls from thickened waste activated sludge (TWAS). The cell membrane breakdown releases organic acids into the downstream anaerobic digester that ultimately feed methanogens to produce greater methane production. Note that this process is not designed to treat primary sludge (PS) that also goes into the anaerobic digester, as that material does not contain a high fraction of bacteria for lysing. The process combines the TWAS at around 6% TS with caustic soda (NaOH) to a pH of approximately 10. The TWAS is then heated to between 140o F and 160o F for 30 minutes for the reaction to take place. The system is offered from a single manufacturer Centrisys-CNP as a package which includes the chemical storage and delivery system, TWAS recirculation pumping and heat exchange, and instrumentation and controls. A preliminary mass balance and process flow diagram of the system is represented in Figure 1 below. Figure 1 PONDUS Mass Balance and Process Flow Diagram Potential Benefits The PONDUS system creates operational savings with increased biogas production and reduced total solids needing to be processed, which is the primary driver for using the system. Other benefits of PONDUS include the following: - Lower Viscosity: The hydrolyzed sludge reduces the digester feed sludge viscosity by over 50%. The biosolids can be pumped, mixed, and heated with lower energy requirements and allows for a higher TS loading rate into the digesters. A higher loading rate allows for reduced total digester volume required to meet minimum solids retention times (15 days), greater storage capacity, and allows greater flexibility for a digester to be taken offline. Additionally, the higher total solids feed rate to the digesters reduces the water heating requirement, and therefore reduces natural gas heating water needs on the digester boilers at an anticipated savings of approximately $80,000/year. The lower digester viscosity could also reduce energy demands on pumping and mixing systems. - Achieving Class A Biosolids: The PONDUS thermal/alkaline hydrolysis process will result in higher volatile destruction, resulting in more biogas and less sludge to manage. A secondary benefit to the volatile solids destruction is a further reduction in fecal coliform, salmonella, enteric viruses, and viable helminth ova in the biosolids exiting digestion. This further reduction will increase the ability for the Solar Dryer to achieve Class A biosolids under the 503 Regulations at a lower total solids concentration. The lower total solids exiting the future solar dryer will reduce dust production and create a better working environment. - Total Solids Cake Dryness: With a breakdown and reduction in volatile solids, a dryer dewatered cake total solids concentration can be assumed. Manufacturer data and an independent study of 17 different samples shows that the total solids of cake will increase between 3% and 6%. The independent study data from KBKopp Sludge Analytics is provided in Table 1. - Polymer Demand: Manufacturer data shows that a 20% decrease in polymer can be expected when using PONDUS. Table 1 implies that primary sludge subjected to PONDUS has a marginal polymer demand increase. Given the high WAS:PS fraction at TRWRF, and the nature of PONDUS breaking down volatile solids it can be assumed polymer demand will be reduced. - Dewatered/Dried Biosolids Odors: The PONDUS biosolids cake is reported to have a lower odor concentration than traditional anaerobically digested sludge. This is due to the greater volatile solids reduction. The benefits include reduced odor concentrations to treat, increased biofilter bed-life, and reduced risk of odor complaints. Operations and Maintenance Considerations The largest direct operational costs of the PONDUS system is caustic soda consumption. Daily caustic soda use is expected to be 155 gallons resulting as high as $280,000 yearly cost. Heating the TWAS in the PONDUS Lysing Reactor adds minimal cost from an overall perspective, as the heat energy required to heat the TWAS from 140o F to 160o F is eventually used in the downstream anaerobic digester process. Table 1 PONDUS Independent Study: Sludge Effect on PONDUS (Dr Julia Kopp, 2022) DISCUSSION Revised Mass Balance Estimate With a PONDUS system, the increase in biogas is expected to be 22% higher than the existing TRWRF anaerobic digester system. The daily biogas production would increase by 91 million BTU (MMBTU). The sell back value of those 91 MMBTU, with the EPA value of 11.7 Renewable Identification Numbers (RINs) per MMBTU and at $2.30 per RIN results in an additional $2,463 in biogas sales per day, or $899,000 in biogas sale per year. The expected increase in biogas production also reduces the total solids needing to be dewatered, dried through the solar dryer, and hauled away. Coupled with an expected dryer cake, the current 180 WTPD of solids through the existing plant processes ending at dewatering would be reduced to 124 WTPD of solids after dewatering. This is a 53 WTPD total solids reduction, or about two hauling trucks per day of reduction. At current costs per wet ton disposal of $27.20, this results in a potential savings of $526,000 per year. Implementation Risk The PONDUS process includes risks that must be recognized and managed to the extent possible. Key implementation risks include the following: - Full-Scale Implementation: The primary risk of the process is that PONDUS has limited installations to use as proofs and lessons learned. There are over 20 installations in Germany, with one fully operational facility in the US. While the thermal/alkaline process is well understood and proven, the unknowns of full-scale may reduce the estimated savings. To mitigate against this uncertainty, the County and its design team conduct a site visits and spend time at the existing US facility to help understand full-scale operations, maintenance, and data comprehension of the process. - The lysing process creates an opportunity for a lower centrate/pressate capture rate of solids downstream of dewatering due to the high volatile solids destruction rate. This can be addressed by using total solids metering on centrate/pressate to identify and fix under performance. - Increased nutrient ratios within the centrate/pressate from PONDUS is expected. Table 1 shows a 28% chemical oxygen demand (COD) increase, an 18% ammonia increase, and a 26% orthophosphate increase. Updated BIOWIN modeling may be necessary to to confirm impacts on centrate/pressate deammonifiaction and secondary treatment processes. RESULTS & STATUS Installation of PONDUS is anticipated to create an operational savings of $1,060,000 per year and a system payback in under 10-years. The County determined these savings opportunities along with added digester capacity and future solar dryer benefits justifies continuing with the project. The new PONDUS facility is now in design at TRWRF with a construction start date of 2025.