Vol. 24 No. 11 November 1998 American Water Works Association Dedicated to Safe Drinkinq Water Benefiting From Biological Growth in Filters by E. Franklyn Smith and Monica B. Emelko Filtration is the final barrier between suspended particles and the consumer in many water treatment plants. As a result of the use of ozonation for meeting increasingly stringent finished water quality requirements and continuing concern about microbiological growth in distribution systems, many utilities are considering the use of biological filtration. Biological filtration, or biofiltration, utilizes innocuous microorganisms for the breakdown and removal of the biodegradable component of water biodegradable organic matter (BOM). Typically, water reaching a biofilter does not contain a disinfectant residual, although a biofilm can be established in the presence of very low levels of disinfectant. Biologically active filters have been used in Western Europe for more than two decades. Typical biofiltration scenarios in Europe consist of dual-stage rapid filtration with biological filters located downstream of the main filtration processes designed for particle removal. Although these operations produce water that is low in turbidity and biologically stable, separate and sequential filtration optimized for particle and BOM removal may not be feasible at many North American utilities because of the associated cost and space requirements. Biological filtration can be applied at the single-stage level for effective removal of both particulates and BOM. Biofiltration is similar to conventional filtration in that biological filters can provide excellent removal of particulates and turbidity. As with conventional filters, well-operated biological filters can achieve effluent turbidities below 0.1 ntu, even from source waters for which treatment is relatively challenging, such as municipally and agriculturally impacted waters. Process Need Not Affect Design Biological filtration processes do not necessarily have to be included in water treatment plant design. Many plants can benefit from biological filtration by simply disinfecting downstream of filtration. Several investigations have indicated little or no difference between biological and nonbiological filters in terms of filter effluent turbidity. Although biological filtration has produced higher particle counts in at least one study, the composition and effect of those higher A truck unloads filter media into a particle counts hopper for the trip via eductor hose to have not been the filter beds. identified. In a biofilter, the innocuous microorganisms form an indigenous biofilm that consists of various living species (including heterotrophic bacteria and higher life forms) and the materials they excrete. These biofilm-forming microorganisms can use natural sources of organic substances as their sole source of carbon and energy. Naturally occurring sources of carbon include algal by products and humic substances, which include a variety of large organic structures that are often color-forming. In most natural waters the majority of organic carbon consists of humic substances. Humic substances are soluble in aqueous systems and can be further categorized as fulvic and humic acids. Humic acids represent about 10 percent of continued on page 4 Benefiting From Biological Growth in Filters continued from page dissolved organic carbon (DOC) and are not soluble in acidic ozonation prior to biofiltration enhances the rate and extent of biodegradation by breaking down large-sized humic conditions, whereas fulvic acids represent about 40 percent of DOC and are soluble in both acidic and basic conditions. compounds to smaller-sized, more readily biodegradable The remainder of DOC includes compounds that are not organic matter or BOM. Biofilms can also act to remove inert particles. Backwash biodegradable (non-BOM). conditions may affect the amount of biofilm present in a BOM describes a group of biodegradable compounds, including but not limited to ozonation by-products. Organic biological filter; however, the effects of specific backwashing strategies on biofilm presence and BOM removal are not compounds that have been identified as ozonation by fully known. Figure 1 is a schematic that depicts one products include aldehydes, oxo-acids, ketones, and carboxylic acids. Several procedures have been developed specifically for potential treatment scenario for postozonation biological measuring ozonation by-products and other components of filtration. BOM. Assimilable organic carbon (AOC) and biodegradable Key to Success dissolved organic carbon (BDOC) are the most common parameters; however, both suffer from some limitations. This The key to successful biofiltration processes lies in has prompted efforts that focus on quantifying the major accumulating and maintaining a sufficiently large mass of components of BOM (humic substances, amino acids, microorganisms that will consume BOM. Figure 2 depicts the carbohydrates, and carboxylic and oxo-acids). accumulation of biomass at the upper portions of a dualMany of the ozonation by product components of BOM are Figure 1. Schematic of a Water Treatment Plant With Post-Sedimentation readily biodegradable and can be Ozonation and Biological Filtration targeted for removal in the biofiltration process. Some of Rapid Raw Water Storage Tank Mix these BOM compounds can Ozonation Sedimentation Flocculation contribute to the formation of potentially harmful disinfection Tank No. 1 by-products following •V disinfection with chlorinated compounds. These compounds 777777777 can act as a food source for both the innocuous microorganisms in the filter biofilm and the To Sludge undesirable regrowth biofilm in 03 Treatment some distribution systems. Raw Distribution system biofilms can Water also act to shelter coliforms and other pathogens. "Y Biofilm Problems Biological instability or the Alum Reservoir No. 2 non-negligible concentration of materials that can support microbiological growth is a common problem faced by drinking water treatment utilities. Microbiological growth can affect treatment operations by decreasing the hydraulic capacity of the treatment system or accelerating pipe corrosion. By-products of microbial metabolism can also form taste and odor compounds. In the past, water treatment processes addressed these issues by using disinfectants whenever possible. Biological filtration contains microorganisms within a given process so that they can deplete the available BOM, and it will less likely support microbiological activity between the plant and the consumer. Biological filtration has also been shown to be effective for the removal of chlorination by-product precursors and the reduction of chlorine demand. Process Effectiveness Biofiltration processes can typically remove 5-30 percent of the humic substances present in water. The practice of tf: Tank No. 2 To Sludge Treatment media filter (the concentration of biomass substantially decreases with media depth; however, it is typically present throughout the depth of biofilters). The microorganisms preferentially attach to the filter media and form a biofilm through accumulation and reproduction. Biofilm processes favor the accumulation of one or more species of bacteria that can degrade a wide range of compounds that are present at relatively low concentrations. In general, the microorganisms released from biofilms are not pathogenic and of no known health concern. The health effects of the by-products of microbial activity (e.g., lipopolysaccharide complexes derived from the outer layer of cell walls of certain bacteria and soluble microbial products) are mostly unknown but have not been demonstrated to be of concern at the levels seen in most water treatment systems. Opflow Monitoring and Evaluation Development and implementation of a successful monitoring program for evaluating biological filtration processes should include testing for dissolved organic carbon (DOC). This measurement gives a snapshot of a filter©s capacity for further adsorption of organics. As indicated above, BOM is also undesirable in treated water because of its potential to result in distribution system regrowth. The removal of BOM from water as it passes through a filter bed is an excellent barometer of the biological efficiency of a filter. This type of evaluation can be extended to evaluate the BOM removal efficiency of an entire treatment facility. Figure 3 depicts the production (by ozonation) and removal (by biological filtration) of BOM through a typical water treatment plant with postsedimentation biological filtration. The figure shows that biofiltration successfully removes nearly all of the BOM produced during ozonation, thereby resulting in a net removal of carbon from the system. The removal of carbon can be seen as a decrease in DOC. In general, the concentrations of BOM compounds will decrease as water percolates through a biofilter. The established biological population "feeds" on the material, thereby effectively removing BOM. Ideally, samples of representative BOM compounds such as aldehydes, carboxylic acids, and oxo-acids should be collected from just above the media layer and at graduated points down through the filter bed at monthly or other specific intervals. At minimum, these types of analyses should be performed for filter influent and effluent samples so that the overall BOM removal efficiency of the filters can be tracked. Biological filters also require monitoring typical of nonbiological filters, including measurements of accumulated head loss, filter effluent turbidity, and filter flow rate. Filter beds that act as biofilters also require maintenance such as backwash optimization, just as do filters intended to act only as a physical barrier to particles. Particle counting is another useful method of ensuring optimal operation of both biofilters and conventional filters. Occurrence in Various Media Biofiltration will occur in different types of filter media such as sand, garnet, anthracite, granular activated carbon, or combinations of these materials. GAC is a popular choice of media since that type of media maximizes the surface area for bacterial attachment. Heterotrophic bacteria and November 1998 nitrifying bacteria grow in the large macroporous structure of the carbon, thereby removing biodegradable organic matter, ammonia, and chlorine demand. Depending on water quality and filter design (e.g., filtration rate, media type, and empty-bed contact time), filter biomass may need to be either "enhanced" or "suppressed." For example, more vigorous backwashing (backwashing with air scour or higher rise rates) may be necessary in warm waters rich in BOM than in colder water, as biological activity is generally greater in warmer water than in colder waters. Also, use of chlorinated backwash water will result in impairment or preclusion of biomass development in the filter. In the past, the idea of a biological population existing in a filter bed may have concerned plant operators. By practical experience, a biological mode of filter operation is no cause for alarm. Because disinfection downstream of filtration prevents further passage of viable microorganisms, the maintenance of such a biological population within filters will enhance plant performance. Normal filter backwashing, along with a straightforward monitoring program, ensure that biological populations in filters function as intended that is, the utilization of innocuous microorganisms to feed off or metabolize BOM. Process Based on Historical Use Biofiltration is not a brand new process. It has existed in various forms for years in sand filters, rapid conventional filters, and granular activated carbon filter adsorbers. Processes such as ozonation act to further enhance the positive effects of biofiltration. Whether intentional or not, Figure 3. Production and Removal of BOM Through a Water Treatment Plant •5.00 •BOM I -DOC 1-4.00 0.00 -f—^™——I——^™——I——™™-^——™™——I——^™—f- 0.00 Plant Influent Post Sedimentation Post Ozone Post Filtration Post Clearwell Treatment Step biofiltration will continue to be an integral component of many filtration processes. Franklyn Smith is the process-SCADA supervisor at the Region of Waterloo's Mannheim Water Treatment Plant in Kitchener, Ontario, Canada. He can be contacted by telephone at (519) 571-6206 or by fax at (519) 743-5654. Monica Emelko is a graduate student working with the Natural Sciences and Engineering Research Council chair in water treatment, Peter M. Huck. She is pursuing her PhD in the department of civil engineering at the University of Waterloo in Ontario, Canada.