DAS in cold wastewater BNR to increase capacity at Gold Bar WWTP — myth or reality?

ABSTRACT: This paper presents two years of operational data at a large full-scale facility in cold climate for implementation and optimization of DAS.

BACKGROUND: The Gold Bar WWTP in Edmonton Canada, operated by EPCOR, treats average and peak flows of 78 and 264 MGD, respectively. Secondary treatment consists of 11 BNR bioreactors each with their own clarifier. Seasonal settleability constrains system capacity with poor settling (SVIs over 250 mL/g - Figure 1) occurring during the cold wastewater period (Figure 2) when spring run-off and wet weather events occur. With the WWTP nearing capacity and facility expansion constrained, DAS was identified as the preferred intensification approach. However, DAS is an emerging technology with site-specific results and limited full-scale applications for cold wastewater BNR facilities the size of Gold Bar WWTP. To address the risk, EPCOR proceeded with a full-scale DAS pilot on a single train. Installation and commissioning of the inDENSE™ hydrocyclone technology, as depicted in Figure 3, was completed in June 2023 and the technology validation program commenced. Methodology and findings from Year 1 of the full-scale trial has been presented (Hunter, et al. 2024). As per the Year 1 results, the test train average SVI during cold wastewater was 150 mL/g, consistently 50 to 90 mL/g lower than the control. Sludge settling speed analysis indicate significant improvement and potential capacity increase of 12.5 to 22.5%. However, stress testing was limited to a 10% increase before unforeseen circumstances were encountered. To confirm the capacity increase, EPCOR extended the full-scale pilot to address the unforeseen challenges and stress test the system in cold wastewater. The intent of this paper is to present the objectives, methodologies, and findings of Year 2 of the testing program.

METHODOLOGY: Year 2 of the testing program includes four phases: 1) Optimization, 2) Benchmarking, 3) Stress Testing, and 4) Wet Weather. Stress Testing includes a phased approach to increase the flow and load to the test and control train. Wet Weather operates at the newly defined treatment capacity during the extended spring run off period. Monitoring of parallel operation is completed with routine analysis and intensive testing as described in the Year 1 testing work. In addition, bioreactor sCOD profiles were added for Year 2 testing to indicate carbon utilization through the bioreactor. Monthly biomass microscopic analysis (imaging, morphology, filamentous) and particle size distributions (PSD) were completed on the test and control ML and the inDENSE™ underflow and overflow.

RESULTS: The first two phases of the Year 2 testing program have been completed. The Year 1 detrimental challenges were addressed including:
- Challenging SRT control. With high RAS solids concentrations and long SRTs, the wastage rate reduced below pump system limits. Intermittent feeding was implemented but SRT control was inconsistent. Dilution water was incorporated into the inDENSE™ feed sludge line.
- Unconditioned sludge transfer. When a parallel secondary treatment train was removed from service, a significant portion biomass was transferred to the test train and negatively impacted settleability and clarifier blanket stability. New dewatering pumping systems were employed.

Process optimization changes included:
- Reduced RAS rates were employed in Year 1 of testing. With notable DAS performance improvements including increased RAS compaction, reduction in nitrate load to the anaerobic zone, and improved plug flow conditions, the Year 2 testing program continued with reduced RAS rates.
- Intermittent mixing of the first two unaerated zones was implemented in Year 1 of testing to improve biological phosphorus removal by decoupling the anaerobic SRT and providing inline fermentation. Without notable improvements to phosphate release, the zones were returned to continuous mixing for Year 2 to target a higher F/M ratio.
- Partial swing zone aeration was implemented in Year 2 of testing. The second anoxic zone has mixers and two aeration grids but is typically mix only. By aerating the second grid, the aerobic SRT increases for more stable nitrification and enhanced feast to famine. In addition, partial nitrification and free ammonia have been correlated to the growth of M. Parvicella which is the dominant cold wastewater filament at the facility.
- High DO concentration in the first aerobic zone was implemented in Year 2 of testing to mitigate M. Parvicella growth. High DO concentrations in the swing zone target immediate oxidization of residual carbon following the biological phosphorus and denitrification processes.
- Reduced NMLR rates were employed in Year 2 of testing to improve plug flow conditions and limit continuous anoxic/aerobic sequencing. NMLR rates were reduced by 50% to match average primary effluent rates. The settleability results for Year 2 (Figure 4) with a notable SVI reduction of 30 to 80 mL/g (15 to 45%). Microscopic imaging completed in Dec 2024 (Figure 5), clearly reveals the presence of granules in the test train and an abundance in the inDENSE™ underflow. PSD analysis confirms the microscopic imaging results.

SUMMARY: Year 2 results clearly demonstrate that the use of a hydrocyclone and biological selection result in denser sludge with increased settling speeds. With promising results, the Year 2 Stress Testing is targeting 20% capacity at 12 C.