The Evaluation of Enhancing Biological Phosphorus Removal and Improving Settleability Using Mainstream Hydrocyclones for External Selection
Creators: name: Ford, Amanda Carrie
Publisher: name: Old Dominion University Libraries
Year: 2018

description: Hydrocyclones, which receive mixed liquor tangentially, separate lighter solids from more dense solids through their tapered shape. Increasing the velocity of liquid as it moves downward allows for the selection of a desired solids fraction. Limited research has been conducted utilizing 20 m3hr-1 hydrocyclones, with the intent of improving settleability and biological phosphorus removal (Bio-P) for mainstream processes. Improved settleability would allow for increased capacity in the secondary clarifiers which prevents the loss of biomass and subsequent treatment disruption, especially during wet weather scenarios. In addition, treatment intensification can be accomplished by maintaining a higher mixed liquor suspended solids concentration within the secondary process. The retainment of phosphate accumulating organism (PAOs) in the underflow can lead to stabilization in secondary treatment systems by maintaining the biomass population. By amassing more denitrifying PAOs (dPAOs), which utilize either nitrate or nitrite as their electron acceptor during phosphorus uptake, allows treatment to become more efficient by utilizing influent chemical oxygen demand (COD) for both nitrogen and phosphorus removal. Achieving reliable Bio-P allows for a decrease in metal salt addition and, if operating in a low alkalinity system, a further decrease in caustic addition.

The site of the research was the Hampton Roads Sanitation District’s James River Wastewater Treatment Plant located in Newport News, VA. This facility is rated for an annual average design flow of 20 mgd, utilizes a 4-stage Bardenpho configuration with an integrated fixed film system IFAS system, and has had historically poor settleability, SVI30 140 + 34 mL g-1, not associated with filaments, nutrient deficiencies, or elevated monovalent to divalent cation ratios. The influent soluble chemical oxygen demand (sCOD) of 250 to 350 mg L-1 allows for seasonal Bio-P without a formal anaerobic selector. In order to evaluate the potential to stabilize year-round Bio-P and improved settleability, eight hydrocyclones were installed and continuously operated from June 2015.

Hydrocyclone performance was evaluated with comparison of mixed liquor suspended solids (MLSS), mixed liquor volatile suspended solids (MLVSS), hydraulic and mass split for the overflow and underflow, and initial settling velocity (ISV) analysis. Granulation analysis was performed at discrete particle settling concentrations of ~150 mg L-1 MLSS to find the percentage of flocs, aggregates, and granules. Floc density was measured using the Percoll method with density beads. Activity measurements for ammonia oxidizing bacteria (AOB), nitrite oxidizing bacteria (NOB), ordinary heterotrophic organisms (OHO), PAO, dPAO, and glycogen accumulating organisms (GAO) were performed on the plant aeration tank mixed liquor, cyclone feed, underflow, and overflow. With continued operation, the underflow MLSS measured ~ 40 g/L with ISV measurements of greater than 20 m hr-1; however, the mass return was limited to less than 10 percent. Kinetic activity measurements indicated washout of AOBs, NOBs, and OHOs did not occur with hydrocylone operation. The aeration effluent MLSS density increased indicating the potential for improved settleability. Biological phosphorus removal stabilized as indicated by activity measurements with hydrocyclone operation and better management of sidestream TP load.