Comprehensive Assessment of the Benefits and Challenges of Converting an Ozone Facility to BAF
- David Briley, Erik Rosenfeldt, Thomas Worley-Morse - Hazen and Sawyer
- Julius Patrick, Jesse Chadwick - Greenville Utilities Commission
Biological Filtration has shown promise as a technology capable of providing water quality benefits, including removing total and dissolved organic carbon (TOC/DOC), reduce disinfection byproduct (DBP) precursors, and contributing to distribution system stability by consuming assimilable organic carbon (AOC). However, biofiltration also has shown some operational issues, including decreased unit filter run volumes (UFRV), increased backwash waste water, and biology impacts. Additionally, implementation of biofiltration can be as simple as a media replacement and thus, it is difficult to determine if biofiltration is a suitable filtration alternative without an evaluation that includes pilot testing and a comprehensive analysis of the whole facility impacts.
To assist Utility A in evaluating ozone-biofiltration, a multiple-step biofiltration analysis was performed. This analysis included a review of the drivers and risks associated with BAF, a review of full-scale BAF demonstration results with two different filter media (sand/anthracite and sand/GAC), use of the Water Research Foundation’s (WRF) Biofiltration Evaluation Tool (WRF 4496), an analysis of the water quality in the distribution system, and an eight-column ozone-BAF pilot with chemical addition.
Key considerations for Utility A include improving water quality parameters and water stability; reducing BDP precursors, such as THMs, HAA, and NDMA; minimizing manganese breakthrough; the impact of ozone-biofiltration on operational requirements, such as maintaining the higher UFRVs that are achieved with abiotic filtration and the ability to switch to free chlorine; and the capital expenses associated with upgrading to biofiltration. To evaluate these considerations, the ozone biofiltration pilot had eight columns with two distinct control columns representing the exhausted GAC media used in the pilot and sand/GAC from the full-scale biofilter at Utility A that was recently replaced (with a remaining adsorptive capacity). The remaining six columns tested the effects of pH adjustment (caustic), phosphorus supplementation, peroxide addition, and combinations of these strategies for improving water quality performance (particularly TOC and Manganese), and improvements in hydraulic performance. The pilot results suggested that caustic and peroxide individually improved hydraulic performance, while a dual-chemical addition strategy did not provide additional benefits either single chemical addition. Overall, the benefits of chemical addition were minimal compared to the non-chemical control.
A concurrent comprehensive distribution system water quality evaluation was also conducted to quantify the distribution system water quality concerns that would be drivers for biological filtration. A detailed evaluation of the available hydraulic head for biofiltration and the impacts to net water production were assessed to define alternatives to ensure sustained capacity with biofiltration such as intermediate pumping, additional filters, or a hybrid approach with a portion of the WTP converting to biofiltration. The results suggest that ozone-biofiltration is a suitable alternative for Utility A that would improve some water quality parameters.
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