Taking Intensification One Step Further: Can We integrate Advanced Secondary Treatment Processes after Advanced Primary Treatment ?

Introduction:
The main purpose of this study is to reduce the energy demand of water resource recovery facilities (WRRFs) through intensification of both primary and secondary treatment processes. Three Advanced Secondary Treatment (AST) Technologies (Aerobic Granular Sludge (AGS), Membrane Aerated Bioreactor (MABR) and MicroVi ) were integrated after an Advanced Primary Treatment (APT). A full scale Cloth Disk Primary Filter was used for APT technology. The performance of three advanced secondary treatment technologies were also evaluated with primary clarifier effluent (conventional primary treatment) to be able to understand the impact of advanced primary treatment on advanced secondary treatment process. The project is funded by the California Energy Commission (CEC) Project EPC-20-044.

The specific objectives of this project are to:
- Establish the design criteria, ability to meet effluent objectives, and determine supplemental carbon addition requirements when treating advanced primary treatment (APT) effluent.
- Quantify the differences in design capacity, energy and supplemental carbon demand when the process is treating primary clarifier effluent.

Methodology:
This demonstration project was conducted at the Linda WWTP located in Olivehurst, CA. AGS and MicroVi units were evaluated at demonstration scale (Figures 1&2), whereas MABR was installed at full scale (Figure 3). The technology demonstration period was at least (8) months for AGS, MABR and around 2 years for MicroVi process. During the demonstration period, the performance of these technologies were monitored with the effluent from an AquaPrime APT and baseline performance following primary clarifier. To monitor the performance of the demonstration units, a comprehensive sampling and analyses plan was prepared. Analyses performed at the laboratory were assessed with process and real time sensor data on a weekly basis.

The basis for evaluations were dictated by the effluent requirements. Hence, COD and cBOD removal, Nutrient removal (Nitrogen and Phosphorus) and TSS removal rates were monitored throughout the project. Samples were taken from influent, effluent of each process.

Results: The results from the AGS system tested downstream of the APT system (Reactor A) were compared with operating AGS with Conventional Primary Treatment (CPT) effluent (Reactor B). The main observation was that the AGS system can create aggregated microbial granules in both Reactor A and B. The faster settling rates of the sludge granules and higher biomass concentrations in reactors were observed. Figure 4 clearly shows the SVI values below 60 mL/g all the times for Reactor A, and most of the times for Reactor B. When both reactors were operated at steady state, SVI's were averaging around 40 mL/g. Improved settling characteristics leads to significant secondary treatment footprint and energy savings. The nutrient removal performance was assessed in detail in both Reactors. The preliminary results clearly showed full nitrification in both reactors. Effluent ammonia numbers consistently stayed below 1 mg/L in both Reactors. (Figure 5). Effluent nitrate numbers were kept below 10 mg/L and nitrite was lower than 1 mg/L. Effluent total nitrogen have been below 15 mg/L without any carbon addition. The study also showed promising results in Phosphorus removal. The full paper will elaborate on the carbon demand for Reactor A and B.

AGS performance was aimed to be improved further with APT effluent consisting of soluble BOD and smaller particulates rather than insoluble BOD associated with larger particulate material of primary clarifier effluent. Colloidal and smaller particulates are readily hydrolysable by the AGS bacteria that are responsible for denitrification of oxidized nitrogen forms. As a result, the larger organic particles that represent less efficient carbon sources for the denitrifying bacteria are sent to the digester (via primary filtration) to produce energy rather than consume energy within the AGS reactor. The reduced quantity of larger organic particulates is anticipated to improve the AGS efficiency and eliminate the additional oxygen demand required to oxidize the solids. Hence, the airflow into the reactors were also compared to determine the energy demand of the reactors.

Similarly, the nutrient removal performance of MicroVi and MABR technologies were evaluated. Combination of Advanced Primary Plus Microvi Biocatalyst provided a very compact Total Nitrogen Removal System with reactor Nitrogen Loading Rates 3 — 4 times higher than conventional Activated Sludge process. However, particulate removal may require additional mechanisms to meet target effluent quality. MABR technology also provided benefits with increase nitrification rate.

The project was initiated in 2021 and will be completed in March 2025. Our full paper will provide detailed performance analyses for AGS, MABR and MicroVi. The final findings and conclusions will be available in the full paper.