Seeking Nitrification and Excess Biological Phosphorus Removal in Cold Weather Using Densified Sludge Control

Introduction The Gold Bar wastewater treatment plant, operated by EPCOR in Edmonton, is an advanced secondary treatment facility with a capacity to process an average of 310 MLD (82 MGD) and a peak capacity of 420 MLD (111 MGD). As one of the largest cold climate treatment plants, it faces operational challenges during temperature drops, particularly in maintaining nitrification processes, which has led to periodic issues with the sludge volume index (SVI) averaging 270 ml/g, prompting the testing of inDENSETM hydrocyclone technology to enhance treatment capacity and maintain low SVIs during colder weather. Continuous flow wastewater treatment processes using biological and physical selectors, including density selectors like hydrocyclones, optimize treatment by accumulating dense biomass while removing lighter forms, leading to improved process efficiency and the retention of crucial organisms for effective nutrient removal (Regmi, et al., 2022) (Gemza, Janiak, Zieba, Przyszlak, & Kusnierz, 2022) (Roche, Donnaz, Murthy, & Wett, 2021) (Daigger, et al., 2023) (Regmi, et al., 2022). This full-scale application will confirm the effectiveness of the plant's design that utilizes InDENSE technology. Additionally, the full-scale pilot seeks to investigate the impact of selective wasting on two process parameters: 1) Increasing the treatment capacity (flows and loads) of a continuous flow activated sludge system through improved settleability and improving the quality and kinetics of the biomass inventory. 2) Understanding the structure of the densified sludge and slow growers at a biological level through DNA sequencing and microbial ecology. Methodology The inDENSETM hydrocyclone technology has been added to Train #2 as depicted in Figure 1. The implementation of inDENSE technology in this full-scale pilot encompasses: 1) commissioning of the inDENSE system and gathering data, 2) Making operational changes to the return activated sludge (RAS) flow and adjusting the feast and famine ratio, and 3) Separating the anaerobic SRT by intermittently mixing for 30 minutes per day. This is aimed at enhancing the anaerobic mass fraction, sustaining nitrification, and improving treatment efficiency. One objective of the technology validation is to quantify the sludge settling speed and compactability improvement using tall settling column tests and developing the Vesilind relationship. At increased settling speeds and higher compaction, the secondary clarifier can operate safely at higher solids loading rates (SLR) without risk of thickening failure. At higher SLRs, the bioreactor MLSS concentration and biological treatment capacity can be increased. Results and Discussion The data from Train #2 (Figure 2) between June and December 2023 shows that using an external selector, such as a hydrocyclone, led to significant improvements in the quality and densification of the sludge. Initially, the sludge had bulking issues with SVI values ranging from 100 to 250 ml/g. However, in addition to inDENSE implementation, applying intermittent mixing and reducing RAS rates, the SVI improved dramatically, stabilizing in the range of 100-150 ml/g, even as the temperature dropped from 19ºC to 14º over the five-month period. Moreover, the hydrocyclone allowed for the separation of the SRT, resulting in the denser granular fraction having a higher SRT compared to the flocculent fraction. This separation, made possible by the hydrocyclone, improved the settling characteristics of the overall biomass as the flocculent particles were more effectively removed. The data clearly demonstrates that the use of an external selector, particularly a hydrocyclone, had a positive impact on the quality and densification of the sludge in Train #2, as seen in the improved SVI values and the enhanced settling behavior of the biomass. The data presented in Figures 3 and 4 demonstrates that the system effectively meets effluent nitrogen requirements despite variability, and the use of a hydrocyclone in enhanced biological phosphorus removal (EBPR) results in consistent effluent phosphorus concentrations below 1 mg P/L with a higher capacity and abundance of phosphorus accumulating organisms (PAOs). The hydrocyclone's operation allows for the retention of denser, more-desirable PAO fractions in the system, creating favorable selection pressure for enhanced biological phosphorus removal. To gain further insights into the EBPR stability and reliability and nitrification performance, this study will extend to perform various stages of tests which further intends to provide sufficient information to allow an assessment of the performance improvement. The information will allow a rational determination of the additional flows and loads that could be accommodated by a bioreactor/secondary clarifier system equipped with selective sludge wasting versus the baseline provided by a conventional treatment train. Conclusion The introduction of hydrocyclones led to an improvement and stabilization in settling, to reduce seasonal variations, allowing for increased process capacity, and supporting higher mixed liquor suspended solids (MLSS), while uncoupled SRT enhanced process resiliency and improved carbon and energy efficiency, although further research is required for validation.