Intensification at the Ajman STP using inDENSE...TM Gravimetric Selection Technology-Long term full scale demonstration

Introduction Intensification is a new term describing upgrades at WWTP's which increases treatment capacity in existing tankage. Waste Activated Sludge (WAS) using hydrocyclones increase process throughput, provide more stable performance via the selection of dense sludge aggregates with improved settling rates and promotes enhanced biological phosphorus removal (EBPR). Increased density leads to improved settling characteristics, preventing loss of biomass and subsequent treatment disruption, especially during wet weather scenarios. There are over 50 installations worldwide using the WAS hydrocyclone technology (inDENSETM), however none have been installed at STP's with an operating SRT between 2 and 2.5 days and warm operating conditions (25oC - 35oC). The purpose of this project was to verify if WAS hydrocyclone technology could improve sludge settleability operating under warm operating conditions, thereby demonstrating a higher flow could be treated with the existing infrastructure. Project Background Ajman Sewerage Private Company Limited (ASPCL) was founded in 2002 and was granted by the Government of Ajman Emirate (United Arab Emirates) a Concession Contract to Finance, Build, Own, Operate, Maintain and Transfer its main wastewater infrastructure. ASPCL shareholders are the Government of Ajman, Besix and Veolia. ASPCL appointed Moalajah as its Operator. The infrastructure constructed by ASPCL since its inception includes: 950 km of network, 220,000 properties connected (a connection rate of 92%), 35 pumping stations, and a centralised wastewater treatment plant (Ajman Sewerage Biorefinery) that treated the wastewater and produced more than 15 GWh of electricity in 2023. Ajman Sewerage Biorefinery WWTP was developed in two main phases (Phase 1 and Phase 2), which can treat a combined maximum volume of 137 MLD. Phase 2 of Ajman WWTP was commissioned in January 2017. Phase 2 consists of two aeration tanks and four rectangular clarifiers. Due to the increase in flows to Ajman WWTP, ASPCL investigated an option to increase the capacity of Phase 2 from 40 MLD to 60 MLD. Due to poor sludge settleability the maximum capacity of Phase 2 was 45 MLD was unable to be reached. As an interim solution, ASPCL investigated if WAS hydrocyclone technology could improve the poor sludge settleability and increase the plant capacity. As part of the full-scale implementation, four (4) inDENSE skids were installed. Each hydrocyclone treats 10 m3/hr, resulting in a total capacity of 200 m3/hr. A new higher capacity of pump (i.e. 200 m3/hr, 2.1 ~2.4 bar operating pressure) was installed. Figure 1 shows an inDENSE skid, the influent manifold piping, overflow piping collecting in the effluent waste and underflow collection pipe which returns the heavier biomass back to the system. Figure 2 shows a process flow diagram of Ajman WWTP (Phase 2) with inDENSE System implementation. The hydrocyclone skids were installed and commissioned in October 2022 with minimal impact to the operating plant. Figure 3 shows the hydrocyclone skids installed at Phase 2, Ajman WWTP. Full Scale Demonstration Results The hydrocyclones at Phase 2, Ajman WWTP started up in November 2022. Figure 4 shows changes in the mixed liquor SVI since the hydrocyclones were commissioned. Prior to hydrocyclones, the mixed liquor SVI varied significantly between 50 to 250 mL/g. Since the hydrocyclones started up, SVI improved significantly averaging around 80 mL/g and remained below 100 mL/g. Figure 5 presents the changes in solids loading rates (SLR) after the installation of the hydrocyclones. Solids loading rate (SLR) is a key parameter used to assess the capacity of secondary clarifiers. Higher SLR indicates higher flows can be conveyed without impacting the effluent quality. Since the hydrocyclones have been in operation, the SLRs have increased from 3.8 kg/m2/hr to 5.8 kg/m2/hr while maintaining the effluent quality target. Figure 6 presents the MLSS concentration trends since the hydrocyclones were commissioned. It should be noted that increasing the MLSS was not intentional. The early increase of the MLSS from Feb to May was mostly due to insufficient wasting due to frequent clogging of the cyclones (e.g. issue with the inlet screens). One key project objective was to see how much more flow could be treated in Phase 2 while maintaining effluent compliance. Since the hydrocyclones have been in operation, the influent flow rates to Phase 2 have gradually increased from 40 MLD and reached 58 MLD within 6 months operations of hydrocyclones. It is to be noted that the limitation to 58 MLD does not come from the clarifiers (settleability) but from the downstream disc filters capacity. It was believed that the biological treatment capacity could be increased above 58 MLD if more disc filters were added. Table 1 summarizes the impact of hydrocyclone operation at Phase 2, Ajman WWTP. Since the hydrocyclones are in operation, the capacity of Phase 2 has been increased by approximately 45% as the SVI improved significantly. This resulted in significant CAPEX and OPEX savings for Ajman WWTP. Conclusions The long term hydrocyclone operation at Phase 2 demonstrated that the improved SVI resulted in treating more flow in the existing secondary treatment infrastructure without major capital expenditure. The outcomes of the lessons learned from the long term hydrocyclone operation from Phase 2 will be incorporated into a Phase 3 upgrade.