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Knowledge Base

Design Phase

Studies and full-scale operation over the last few decades have shown that biofiltration can be successfully practiced using a wide range of design configurations, most of which are held in common with conventional filtration. This makes biofiltration amenable to conventional treatment retrofit as well as to new construction application. However, there are a few key considerations that should be factored when designing a biofiltration process including underdrain type, empty bed contact time, use of oxidants, media type, media depth, backwash practices, need for nutrient addition, headloss control measures, algae control measures, and process monitoring. Some of these design criteria are developed during the evaluation phase while others are recommended based on full-scale operational experience. Frequently asked questions are provided below.

1. Are biofiltration facilities predominantly retrofits or new construction?

Based on the current Knowledge Base, over half of full-scale facilities incorporated biofiltration as a retrofit of an existing treatment facility.

2. What are the typical requirements/standards for biofilter designs?

Standards are not uniform from state to state. Please consult your primacy agency for guidance.

3. What type of media is most widely used in biofilters?

Over half of respondents report using sand mono-media or anthracite/sand dual media filters. Slightly less than half reported using GAC mono-media or GAC/sand dual media filters. Recommendations will differ depending on the desire to leverage adsorptive capacity of GAC and variability of water quality. The performance of GAC media configurations appear to be less sensitive to changes in water quality.

4. What empty bed contact time should I use for my biofilters?

EBCT is a key factor to determine during the evaluation phase of implementation, however common values range from 5 to 10 minutes.

5. What is the range of hydraulic loading rates associated with biofilters?

Knowledge Base respondents have reported effective treatment under a wide range of loading rates. Values have been reported as low as 2.5 gpm/ft2 and as high as 10 gpm/ft2. Initial loading conditions will most likely be dictated by the primacy agency, while target loading conditions will need to be established during the evaluation phase.

6. What type of water should I use for filter backwashing and at what rate?

Some studies have suggested that biomass in GAC-based biofilters is relatively insensitive to a chlorine residual in the backwash supply, while biomass in non-GAC biofilters is sensitive to chlorine. That said - it is recommended that a backwash supply without a disinfectant residual be available for regular backwashing. Knowledge Base respondents have reported effective backwash rates between 11 and 22 gpm/ft2.

7. How and how often should I backwash my filters?

Utilities have reported using conventional backwash protocols triggered by typical filter performance indicators (e.g., headloss, effluent turbidity). Consideration should be given to limiting or eliminating disinfectant residual in the backwash water Air scour is recommended. Reported biofilter run time ranges from 24 to greater than 72 hours

8. What type of media extraction should I use to replace old media?

Frequent media change-out is not generally required. Some utilities have reported media useful life spanning decades. Design considerations could include yard hydrants for vendor provided eductors, truck access adjacent to the filter boxes, and hose access to individual filter boxes.

9. What monitoring tools do I need and where should I locate them?

Developing an effective monitoring approach is critical to successful biofilter operation. WRF 4231 provides a guide to developing a monitoring strategy based on treatment scheme and treatment objectives..

10. What are the key design provisions for biofilter underdrains?

Underdrains should be designed to address the potential for headloss development due to excessive extracellular polymeric substance (EPS) production. EPS buildup in the underdrain, specifically in media retention caps, has led to underdrain failures during backwash. Utilities have various means to address this including use of steel underdrains anchored to the filter box floor, pressure relief piping that discharges to the filter box, use of oxidants (e.g., chlorine or H2O2) in the backwash water, and underdrain pressure sensors that can provide an early indication of underdrain fouling.

11. Can you design a filter for both conventional and biofiltration operation?

Yes, provisions for biofiltration can be integrated into a conventional filter design.

12. What are the major cost factors for a biofilter design?

The cost of designing and constructing a biofilter is essentially the same as that for a conventional filter, though testing may indicate the need for a deeper bed depth or GAC media. These factors can increase cost. Other cost factors may include additional piping for pressure relief on backwash lines or nutrient addition systems, if recommended during the evaluation phase.

Water Research Foundation Reports

Biological Drinking Water Treatment Perceptions and Actual Experiences in North America
Engineered Biofiltration for Enhanced Hydraulic and Water Treatment Performance
Ozone-Enhanced Biofiltration for Geosmin and MIB Removal
Occurrence, Impacts, and Removal of Manganese in Biofiltration Processes
Cost-Effective Regulatory Compliance With GAC Biofilters
Cometabolism of Trihalomethanes in Nitrifying Biofilters
Removal of Natural Organic Matter in Biofilters
Microbial Impact of Biological Filtration
Design of Biological Processes for Organics Control
Colonization of Biologically Active Filter Media With Pathogens
Ozone-Enhanced Biofiltration for Geosmin and MIB Removal
Optimizing Filtration in Biological Filters
Use of Chlorine Dioxide and Ozone for Control of Disinfection By-Products
Removal of DBP Precursors by GAC Adsorption
Microbial Activity on Filter-Adsorbers
Advances in Taste-and-Odor Treatment and Control
Treating Algal Toxins Using Oxidation, Adsorption, and Membrane Technologies
Colonization of Biologically Active Filter Media With Pathogens
Biological Treatment and Downstream Processing of Perchlorate-Contaminated Water