Where Should my THP go? Drivers to Implementing Thermal Hydrolysis at Woodman Point, Perth, Australia

Introduction The Woodman Point (WP) Water Resource Recovery Facility (WRRF) is owned and operated by Water Corporation. The WP WRRF is the largest treatment plant in Western Australia and services industrial, commercial, and residential customers in the southern Perth metropolitan region. The WRRF liquid stream treatment process was upgraded in 2019 to provide 180 ML/d wastewater treatment capacity (approximately 900,000 equivalent population). The Water Corporation selected a 120 tDS/d design horizon for the next WP solids handling upgrade, which corresponds roughly to the 180 ML/d design horizon and includes dewatered solids imported from regional WRRFs. Growth in the WP WRRF catchment is projected to continue beyond 250 ML/d and 160 tDS/d (including imported solids) in 2060. The existing egg-shaped digesters (ESDs) at WP WRRF use mesophilic anaerobic digestion to stabilize combined thickened primary sludge (PS) and excess activated sludge (EAS) into digested sludge (EAS is equivalent to Waste Activated Sludge), which is then dewatered into biosolids cake, suitable for direct beneficial use in agriculture under the Western Australia (WA) Guidelines for Biosolids Management. Biosolids must be Pathogen Grade 3 (P3) (equivalent to Class B), Contaminant Grade 2 (C2) or of better quality to be suitable for direct beneficial use in agricultural land. Without an upgrade to the current solids handling process, achieving the P3 C2 requirement is at risk. The only alternative for management of off-specification biosolids is landfill disposal, which is financially and environmentally unsustainable. Water Corporation completed a planning study in 2019 that identified EAS only thermal hydrolysis process (THP) and EAS only carbonization process as preferred technologies for consideration. The principal driver for implementing THP is achieving a design minimum digester solids retention time of 15 days within the existing digesters to meet P3 requirements. Carbonization was identified as an technology to potentially augment digestion capacity and manage emerging contaminants of concern, notably Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) and microplastics. Methodology Six different process options utilizing one or both of Water Corporation's preferred technologies, THP and carbonization, were evaluated to select the preferred process configuration for the WP solids handling upgrade. These options included: -EAS only THP PS and hydrolysed EAS are digested in the existing ESDs. -EAS only carbonisation PS is digested in the existing ESDs and undigested EAS to diverted to a new carbonisation facility. -Full THP Hydrolysed PS and EAS are digested in the existing ESDs. -Full THP with digestate carbonisation Hydrolysed PS and EAS are digested in the existing ESDs, and digestate is diverted to a new carbonisation facility. -Intermediate THP (i-THP) PS is digested in a new ESD. Digested PS and EAS are hydrolysed and digested in the existing ESDs. -Post THP PS and EAS are digested in the ESDs, with additional ESD capacity provided. Digestate is hydrolysed and dewatered. Dewatering centrate is returned to the digesters for further digestion. Simplified block diagrams for each option are presented in Figure 1 through Figure 6. Mass and energy balances were completed, incorporating key inputs from vendor-specific equipment/processes. Key outputs from the mass and energy balances are presented in Table 1. Post digestion THP was not evaluated in detail as it required the greatest additional digestion capacity, despite potentially providing the highest dewatered cake solids concentration. Results The THP options were assessed to be the most favorable options based on a multi-criteria assessment. One of the key drawbacks considered for THP is that it cannot manage emerging contaminants (including, but not limited to, PFAS and microplastics). The current regulatory environment does not require management of these contaminants. As a result, carbonization was considered as a future augmentation if PFAS were to become strictly regulated. The key drawbacks of biosolids carbonization at the time of evaluation was the technology was considered to be embryonic and not sufficiently established at full scale to be implemented with confidence by 2024 without any onsite trials, and there is uncertainty in acceptable the biochar bi-product end-uses in Western Australia. EAS only THP was selected as the preferred option as it balances initial capital expenditure by intensifying the existing process capacity, enhances generation of biogas, and allows for future implementation of other process options (including energy neutral carbonization). EAS only THP also provides the greatest (net positive) energy balance, without requiring supplemental fuel for heat/steam during normal operations. The EAS only THP option provides the flexibility to increase future plant capacity with another THP train and/or construction of the fourth ESD and implementation of i-THP. The capacity can be augmented in stages to manage capital investments. Alternatively, digestate carbonization can be implemented at any stage, with the benefit of having fully or partially digested solids as the feed to the thermal dryer, reducing the drying capacity required. Carbonization can also be implemented to manage emerging pollutants or minimize transport of the end product off-site. Figure 7 show the concept design layout of the EAS only THP implementation, with areas for future expansion. Conclusion This paper will benefit the water utilities, and consultants as it demonstrated how both THP and carbonization technologies can be complementary. EAS only THP was determined to be the preferred option for the 120 tDS/d solids upgrade, and a staged implementation approach to carbonization was identified for consideration beyond the 120 tDS/d. EAS only THP maintains the current biosolids quality required for the projected growth in plant capacity and balances initial capital expenditure by intensifying the existing process capacity while allowing for future implementation of other process options such as carbonization if PFAS were to become strictly regulated.