Combination of Metabolic, Kinetic and Physical Selection to Achieve Successful Full-Scale Continuous Flow Densification of Activated Sludge at Robert W. Hite Treatment Facility

Abstract
– In this work, we document successful densification of activated sludge at the Robert. W. Hite Treatment Facility utilizing continuous flow bioreactors and secondary clarifiers. Further, we discuss key factors that influence densification and nutrient removal performance.
#Introduction - The Metro Water Recovery (Metro) Robert W. Hite Treatment Facility (RWHTF) is a 220 million gallon per day (MGD) facility that currently achieves effluent total inorganic nitrogen (TIN) < 10 mg/L and total phosphorus (TP) < 1 mg/L. The RWHTF is faced with progressively tightening of effluent nutrient limits over the next 20 years. As part of facility planning, Metro proactively identified continuous flow densified activated sludge (DAS) as a technology that would help to address treatment requirements for the tightening nutrient standards. Metro commissioned a full-scale continuous flow DAS demonstration train in 2018 to: i) assess the feasibility of achieving densification, ii) inform treatment capacity rating and expected performance of full-scale DAS, and iii) gain insights into long term DAS operations. After several years of piloting with various operational configurations combined with physical selection the test train has demonstrated the ability to develop DAS. Once densified, SVI30 was maintained <115 mL/g (Figure 1), maximum solid loading rates capability increased from 30 to 60 (ppd/sf), and the RAS flow were reduced from 8.2 to 4 MGD providing additional secondary clarifier flexibility and eliminated the need for facility expansion. This work documents findings from the full-scale demonstration facility. Specifically, we will discuss: - Factors that have led to the development of DAS, - Insights from long term operation of DAS with physical selection. Although the pilot successfully densified, the impacts of densified solids across the entire facility must be considered before the technology is widely adapted. This study evaluates the impacts of densified sludge on various processes, including, thickening/rheology, E.coli removal, solid retention time, microbial community of AS, nutrient removal efficiency, and effluent suspended solids concentrations. A second paper will discuss how findings from the full-scale demonstration are being leveraged to maximize nutrient removal treatment capacity of the existing aeration basins (Abs) and secondary clarifiers (SCs) at RWHTF.
Methods
– The full-scale DAS 'test' facility consisted of an isolated paired aeration basin (2.05 million gallons (MG)) and secondary clarifier (13,270 ft2) that was equipped with separate primary effluent pumping station designed to mimic full-scale diurnal flow patterns. The test facility was equipped with a hydrocyclone skid comprised of eight 10 m3/h hydrocyclones that was used for physical selective wasting. The test facility has been in operation since May 2018 and been used to test eight (8) distinct metabolic and kinetic selector configurations. Metro has also operated 'control' trains that do not have similar metabolic, kinetic and physical selection capabilities but represents current full-scale operation. Sampling and data collection of the test and control train occurred during densification start-up and continued into steady state operations.
Piloting, Sampling and Data Collection- Over the course of the demonstration operation, field sampling was performed at the facility to characterize performance as follows:
- Routine influent and effluent concentrations (COD, sCOD, NH4, PO4, TSS) within the test and control facilities,
- Profile sampling was performed across the test and control trains to characterize nutrients and organic carbon removal,
- Column settling tests and clarifier stress testing were performed to characterize performance under peak flow/load conditions
- DAS size distribution analyses was performed via particle size analyzer to determine how particle size changes over time in the MLSS, hydrocyclone overflow, hydrocyclone underflow, and secondary clarifier effluent.
- Microscopy analyses were performed to qualitatively characterize filaments and distribution of phosphorus and glycogen accumulating organisms
- Biomass activity testing was performed to identify changes to biomass function (e.g., nitrifier, phosphorus uptake)
- Weekly measurement of E. coli were taken. Grab samples were collected at both test and control secondary clarifier effluents. E.coli was quantified via the IDEXX method - Piloting of the centrifuge thickening system on densified stream. The system was operated with a variety of polymer doses and throughputs to evaluate effect on solids percentage and percent capture.
Results and Discussion
“ Key results from the demonstration are summarized below:
- The test train had significantly improved settling characteristics versus the control train (Figure 1). Observations suggested a 40% reduction in SVI30, and SVI5 for the test versus the control train. These results confirmed that the combination of metabolic, kinetic and physical selection employed in the test train allows for continuous flow densification.
- Size distribution analyses indicated that the test train MLSS frequently comprised larger particles that the control train (Figure 2). This can be partially attributed to the benefits of the hydrocyclones. Depending on the specific configuration tested and the degree of kinetic selection provided, the size distribution in the test train was also variable. In general, periods with more defined and stronger metabolic and kinetic selection resulted in the generation and retention of larger particles in the test train. Full discussion of these findings will be provided.
- Rheology testing confirmed that both DAS and CAS behaved as non-Newtonian fluids with pseudoplastic properties (Figure 3). Results suggested that there was limited different between DAS and CAS at MLSS concentrations less than 1%. DAS appeared to have a lower viscosity than CAS at solids concentrations between 1 and 2%. At 5% solids , DAS was observed to have a higher viscosity than CAS.
- Qualitative microscopic analyses indicated that both test and control trains had strong clusters of phosphorus accumulating organisms (PAOs; Figure 4). However, the test train was observed to have lower levels of filaments relative to the control train.
- Results also suggested that the test train had lower secondary effluent E. coli concentrations relative to the control train (Figure 5). Cumulatively, these results indicate combining metabolic, kinetic and physical selection could allow for stable densification in continuous flow applications at the RWHTF. The full scale plant evaluation indicates that densification continues to be a viable option to increase secondary clarifier capacity while delivering enhanced E. coli removal and minimal negative secondary impacts such as increased secondary clarifier effluent TSS. The test train operation is currently ongoing to verify performance through seasonal changes. An additional mixing evaluation is also currently being conducted to determine minimal mixing requirements to ensure even suspension of the densified AS in anaerobic and anoxic zones. The presentation and paper will contain updated data from all sampling performed.