Conference Agenda

Many WRRFs do not have consistent or sufficient VFA content available in the influent wastewater because of a low organic content or seasonal variability of the influent characteristics. The lack of influent VFA content has driven facilities to modify plant operations to generate their own VFA, utilizing embedded carbon, ensure consistent EBPR and maintaining low effluent phosphorus concentrations.

This presentation reviews two WRRFs that recently upgraded their traditional anaerobic selectors to inline fermentation reactors to produce VFA and improve EBPR.

In November 2021, South Granville Water and Sewer Authority in NC upgraded their anaerobic selector to an intensified fermentation tank by alternating a short mixing cycle with a long non-mixed deep anaerobic cycle. The generation of additional VFA lead to the proliferation of PAOs, which stabilized the EBPR process, resulting in lower and more consistent effluent total phosphorus. In addition, the plant realized a 90% reduction in mixing energy demand for the anaerobic reactors. In November 2022, having gained confidence in EBPR process, the plant initiated a plan to lower the alum feed rate 10% each month, and continues to monitor performance and lower the alum to further optimize savings.

The Warren, MI Water Recovery Facility performed a similar upgrade in 2021. The system maintained exceptionally low ORP, generated excess VFA, and utilized a unique mixing regime to transport VFA throughout the reactor without disturbing the fermentation process. In addition to achieving consistently low phosphorus effluent, well below the 1 mg/L requirement, the facility also reduced ferric consumption by 73%.

While generating VFA in the fermentation blanket is important, it is equally important to transport VFA to the PAOs throughout the reactor. The unique approach used by both plants included an update to the control logic to switch from continuous mixing to intermittent mixing. This new mixing approach operates cyclically, with complete mixing events occurring every 8-12 hours, and intermittent low-energy pulses occurring hourly during the unmixed phases. The periodic gentle pulses of low energy mixing from the fermentation blanket are used to transport excess carbon to the bulk liquid providing availability to PAOs throughout the anaerobic reactor.

Enhanced biological phosphorus removal (EBPR) is a critical component of modern wastewater treatment strategies to protect aquatic ecosystems and capture valuable nutrients. However, EBPR process instability can create challenges for meeting stringent effluent phosphorus limits. Previous work has shown that monitoring phosphorus uptake kinetics can provide an early warning of instability events. While these methods provide a useful indicator of the functional health of the EBPR system, they do not always elucidate possible mechanisms responsible for instability events. Accordingly, we compared microbial population dynamics and their gene expression during periods of stable and unstable EBPR operation.

MLSS samples were collected from the Rock Creek Advanced Water Resource Recovery Facility during both a process upset and a stable period in 2021. Relevant process performance data at the time of sampling was also recorded. Nucleic acids were extracted and sent for shotgun metagenomic (MG) and metatranscriptomic (MT) sequencing. Metagenome assembled genomes (MAGs) were assembled from the MG data to allow for more complete identification of the microbial community. Good quality draft MAGs served as the references to determine relative abundance and gene expression rates from the MG and MT data, respectively.

Bioinformatic analysis highlighted two novel microbial populations (strain 1 and strain 2) that were very abundant across both MT and MG datasets. Both were distinct members of the Accumulibacter genus based on analysis of their respective MAGs, including genes for polyphosphate accumulation as well as volatile fatty acid (VFA) metabolism and PHA synthesis. Further, strain 2 appears to be capable of denitrification. Gene expression for VFA uptake and carbon metabolism appeared depressed in the unstable samples relative to the stable samples, while stored phosphorus utilization activity remained somewhat stable. This could be due to decreased total VFA load during the upset event. Chain elongation activity also dropped substantially, further indicating insufficient carbon availability. Overall, gene expression profiles during the upset are supported by process data.

This presentation will offer an introduction to the use of genomics for process stability, followed by a case study describing key EBPR-related populations at Rock Creek and the response of these organisms to a process upset.