Cooking Without Gas: Reducing sludge management costs using THP (Thermal Hydrolysis Process) in plants without anaerobic digestion

1. Introduction Since 2019, 4.1M wet tons of sludge cake were produced annually at WRRFs in South Korea. Following the elimination of ocean dumping as a disposal strategy in 2006, sludge management costs have climbed significantly. As of 2018, sludge disposal reached an average of $120 USD/wet ton (MOE & KECO, 2021). In 2019, only 67 out of 4,216 Korean WRRFs larger than 500 m3/d (132,000 gpd) included anaerobic digesters (MOE, 2020). Since digestor projects cannot be deployed quickly or cheaply enough to keep up with rising disposal costs, alternative sludge reduction processes are being sought by public agencies. The thermal hydrolysis process (THP), traditionally applied as a pre-treatment for anaerobic digestion, can also be applied as a sludge cake minimization strategy in the absence of digestors. This paper describes recent pilot and full-scale demonstrations of this concept, including identification of new optimal operating setpoints specific to this application, as well as downstream dewatering performance and economic impacts. 2. Materials and Methods 2.1 Thermal Hydrolysis Process This paper describes a new thermal hydrolysis process (THP) developed and commercialized by BKT/Tomorrow Water. The process, trade-named Draco, is a three-stage, batch process comprised of a pre-heating tank followed by reactor and decompression tanks. Steam is used to build heat and pressure in the reactor vessel, causing cell lysis and improving the dewaterability of cellular sludges. The Draco process is differentiated by the inclusion of a paddle mixer in the pre-heating and reactor tanks, novel steam sparger configuration, and the use of novel temperature setpoints (as described below). 2.2 Pilot testing A containerized Draco THP pilot was deployed to a municipal WRRF in Dangjin (Korea) to process 2 wet tons/d of undigested biological sludge cake from August to October, 2021 (Figure 1). The pilot was fed mixed primary and waste activated sludge (WAS) which had been dewatered to 20.0±1.2% dry solids (DS) and fed without dilution into the Draco process. To optimize the temperature setpoint for hydrolysis of undigested sludges, influent dewatered cake was processed at either 170, 180, 190 or 200oC for 30 min. Capillary Suction Time (CST) was measured following THP as a measure of dewaterability in thermally processed sludges. To assist in optimization and selection of dewatering equipment, the 190oC-processed sludge product from the Draco THP pilot was fed into either a plate-and-frame filter press (operated at 15 bar) or a belt filter press (operated at 15, 30 and 50 bar.) The plate-and-frame filter press was operated without any chemical addition, while the belt filter press was operated with addition of cationic polyacrylamide polymer at 0.64%/kg dry solids. 2.2 Full scale operation A full-scale Draco THP system combined with a plate-and-frame filter press was installed at a slaughterhouse wastewater treatment plant in Gimhae, S. Korea. This integrated sludge minimization system, operating since June of 2019, processes roughly 50 wet tons/d of dewatered WAS containing 14.8±0.5% DS. Upstream, the slaughterhouse's wastewater is treated by a sequencing batch reactor (SBR), and sludge wasted from this SBR has similar characteristics to undigested municipal WAS. Sludge was fed undiluted into the Draco THP and processed over more than 2 years. 3. Results 3.1 Pilot Results Over three months of pilot testing, normalized capillary suction time (NCST) decreased as the processing temperature increased, while specific capillary suction time (SCST) increased with temperature (Figure 2). This suggests that higher THP temperatures produce better downstream dewatering performance. At the same time, the marginal improvements in dewatering indicators (NCST, SCST) between 190oC and 200oC were relatively minor, while the added steam energy required to meet the higher setpoint was significant. As such, 190oC was selected as the optimal temperature setpoint for this application. In subsequent dewatering testing with hydrolyzed pilot sludge, dewatered cake from the plate-and-frame filter press averaged 52.8% DS, while cakes from the belt filter press ranged from 37-41% DS, dependent on belt pressure (Table 1). Based on the higher dewatering performance and lack of chemical inputs required for the plate-and-frame press, this equipment was selected for an upcoming full-scale installation over the belt filter press. Table 2 lists the average filtrate quality of the plate-and-frame press in the pilot, which needs to be sent back to the head of the wastewater plant for treatment in full-scale installations. 3.2 Full-scale results Over 27 months of operation, the full-scale Draco process increased the dry solids content of dewatered WAS from 14.8±0.5% to 53%±1.1%. The intermediate-stage solids content of the thermally hydrolyzed sludge was 11.8±0.7% at the output of the decompression tank, and was subsequently dewatered in the press to over 50% solids. Dewatered cake following the Draco process was compact and easily handled (Figure 3). By installing a THP plus filter press, this plant reduced its overall sludge cake output to disposal by 80% (by volume). This translated into a 67.6% reduction in sludge disposal costs, even after energy costs of the THP process were factored in (Table 3). 4. Discussion and Conclusions This paper describes the optimization and application of a combined THP and dewatering process intended for use at WRRFs which do not anaerobically digest their sludge. Pilot testing demonstrated that dewaterability of municipal THP sludge increases with increasing THP temperatures from 170oC up to 190oC. At higher temperatures of 190-200oC, the marginal benefit in dewaterability was outweighed by the marginal cost in energy requirements. The combination of Draco THP at 190oC, coupled with a plate-and-frame dewatering press, was able to consistently achieve sludge cake dryness over 50% in both pilot and full-scale installations. Sludge cakes above 50% DS are much easier to process in thermal sludge dryers, due to the avoidance of a plastic 'sticky phase' occurring between DS 35-50% (Peeters, B., 2010). As this study demonstrates, plants which utilize sludge dryers can benefit significantly from the addition of a THP, since THP allows dryers to be fed a much lower-moisture cake (up to 30% less moisture), reducing the amount of energy required in thermal drying and dramatically reducing the size of dryer required. Compared to the energy required to dry a typical dewatered cake from 20% to 90% DS, drying post-THP cake (at >50% DS) to the same moisture content saves about 40% in total energy inputs, even after factoring in the energy required for hydrolysis and dewatering. These results suggest that Draco THP, paired with the right dewatering press, can serve as a cost-effective strategy for reducing sludge volumes and disposal challenges, even for WRRFs without anaerobic digestors. At the Gimhae plant, application of this process reduced sludge volumes by 80% and cut disposal costs by 68%, achieving a simple payback of 4-6 years.