March 17, 2017
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Oakley, S. and von Sperling, M. 2017. Media Filters: Trickling Filters and Anaerobic Filters. In: J.B. Rose and B. Jiménez-Cisneros, (eds) Global Water Pathogen Project. http://www.waterpathogens.org (J.R. Mihelcic and M.E. Verbyla) (eds) Part 4 Management Of Risk from Excreta and Wastewater) http://www.waterpathogens.org/book/media-filters Michigan State University, E. Lansing, MI, UNESCO.
Acknowledgements: K.R.L. Young, Project Design editor; Website Design: Agroknow (http://www.agroknow.com)
|Last published: March 17, 2017|
Media filters are a sanitation technology that use microorganisms that are attached to a high surface area medium to primarily remove soluble organic matter (measured as BOD or COD) as wastewater passes through the medium. Trickling filters use aerobic processes for treatment while anaerobic filters operate under strict anaerobic conditions. Media filters are typically used for the treatment of wastewater in centralized sewerage systems serving urban areas. They can also be used in on-site wastewater treatment systems serving individual dwellings, industries, apartment complexes, and housing clusters. All media filters require primary treatment of suspended solids to avoid clogging of the filter media. Because media filters are designed to remove soluble organic matter, they should not be expected to have high pathogen removal rates. The few available data for properly designed and operated trickling filters integrated with primary and secondary sedimentation suggest removal, at best, of 1.0 log10 removal of bacterial pathogens (Salmonella), 0.5 log10 removal of viruses, 0.8 log10 removal of protozoa cysts/oocysts, and 1.4 log10 removal of E. coli and thermotolerant coliforms. Tricking filters integrated with primary and secondary sedimentation and chlorine disinfection are reported to remove up to 2.8 log10 viruses and 1.5 log10 protozoan cysts (Giardia). Anaerobic filters preceded by primary sedimentation (septic tanks) are reported to remove up to 1.9 log10 of fecal coliforms.
Media filters use microorganisms that are attached to a high surface area medium to primarily remove soluble organic matter (measured as biochemical oxygen demand (BOD) or chemical oxygen demand (COD)) from wastewater as it passes through the medium. Trickling filters use aerobic processes for treatment while anaerobic filters operate under strict anaerobic conditions (Metcalf & Eddy/AECOM, 2014; Chernicharo and Goncalves, 2007; Chernicharo, 2007). Trickling filters and anaerobic filters are also referred to as biofilm reactors and attached-growth processes, which also include rotating biological contactors (biodiscs), submerged aerated biofilters, and various emerging and proprietary technologies (Chernicharo and Goncalves, 2007; Metcalf & Eddy/AECOM, 2014). Constructed wetlands, which are a planted media variant of media filters are covered in detail in Section 60I. Media filters are used for the treatment of domestic and industrial wastewaters in centralized sewerage systems serving urban areas; they can also be used in onsite wastewater treatment systems serving individual dwellings, industries, apartment complexes, and housing clusters. (Figure 1 shows where media filters are used within the sanitation service chain.) Due to a lack of data in the literature on pathogen removal in the various media filters, this chapter will focus on the two most common and studied technologies: trickling filters and anaerobic filters. Historical information on pathogen fate in media filters is available in Feachem et al. (1981, 1983).
Figure 1. Locations where media filters are used within the sanitation service chain
All media filters require primary sedimentation (suspended solids removal) of the influent to avoid clogging of the filter with solids. In a trickling filter, the influent wastewater is distributed on the top surface and passes vertically downwards (trickles) through a permeable medium (e.g., rocks or plastic). Figure 2 shows a schematic of a trickling filter and Figure 3 shows the components. As the water flows downwards, soluble organic matter is removed by aerobic heterotrophic microorganisms that are contained in a biofilm attached to the medium. The biofilm gradually grows as it comes into contact with the passing wastewater. Aeration occurs through natural convection of air through ventilation ports connected to the underdrain system at the filter base. The filter medium is unsaturated, that is, after the liquid has trickled down, the porous spaces are occupied by air, thus guaranteeing aerobic conditions. As the biofilm grows parts of the biofilm periodically fall off and leave the filter with the effluent through the underdrain system, a process called sloughing. As a result, trickling filters require secondary sedimentation to remove the sloughed biofilm which are measured as suspended solids. Figures 4 and 5 provide examples of trickling filters in operation.
Figure 2. A schematic diagram of a trickling filter
Figure 3. Drawing of a trickling filter that shows a rotating distribution system, filter packing medium, and underdrain collection for effluent (Reprinted with permission of Eawag: Swiss Federal Institute of Aquatic Science and Technology Department Water and Sanitation in Developing Countries (Sandec); from Tilley et al. (2014).
Figure 4. a) A plastic media trickling filter with a rotary hydraulic distribution arm designed for a flowrate of 40,000 m3/day (Cuzco, Peru). Note the ventilation ports around the base in the top photo. (b) close up of distribution system (photos by Stewart Oakley)
Figure 5. A volcanic rock trickling filter with a fixed hydraulic distribution system. This system has been in operation for 35 years with an average daily flow rate of 500 m3/day. Filter located at the University of San Carlos, Guatemala City, Guatemala (photo by Stewart Oakley)
In an anaerobic filter the influent wastewater passes vertically through a submerged medium that maintains anaerobic conditions (Figure 6). The anaerobic filter can be run in a downward (Figure 6) or upward hydraulic flow pattern (Figure 7). Soluble organic matter is removed as it comes in contact with the anaerobic biofilm; low concentrations of suspended solids can also be removed by being retained within the interstices of the medium and subsequently biodegraded (Chernicharo, 2007). For the treatment of domestic wastewater anaerobic filters have been most commonly used in Brazil as a secondary treatment process for septic tank effluents and UASB reactor effluents (Chernicharo, 2007). Biofilm sloughing occurs in anaerobic filters but to a lesser extent than in trickling filters; as a result, anaerobic filters do not require secondary sedimentation but do require periodic removal of solids within the filter (Chernicharo, 2007). In anaerobic filters the influent is usually distributed in the bottom part, follows an upward flow and leaves from the top, with the medium remaining saturated (void spaces occupied by liquid). There are also down flow versions of anaerobic filters. In both designs, since there is no entrance of oxygen, anaerobic conditions prevail in the liquid and biofilm.
Figure 6. A schematic of a downflow anaerobic filter with submerged media. Anaerobic filters can be designed in downflow or upflow configurations
Figure 7. Schematic of an upflow anaerobic filter (Reprinted with permission of Eawag: Swiss Federal Institute of Aquatic Science and Technology Department Water and Sanitation in Developing Countries (Sandec); from Tilley et al. (2014).
Detailed information on the design and operation of trickling filter processes can be found in Metcalf & Eddy/AECOM (2014) and von Sperling (2007). Chernicharo (2007) presents detailed information on the design and operation of anaerobic filters.
Trickling filters and anaerobic filters are used to treat the following liquid waste streams: domestic wastewaters, a large variety of high strength industrial wastewaters (e.g., pulp and paper wastes, brewery wastes, textile wastewaters),and combined domestic/industrial wastewaters. Figure 8 shows the principal inputs and outputs for media filters. Trickling filters receive primary-treated wastewater effluent or upflow anaerobic sludge blanket (UASB) reactor effluent, while anaerobic filters commonly receive septic tank effluent or UASB effluent. The outputs from media filter processes include secondary effluent and secondary sludge, both of which require further treatment for stabilization and pathogen removal. Secondary effluent can be disinfected before discharge or reuse. Secondary sludge from trickling filters is most commonly stabilized by anaerobic digestion with primary sludge and then dewatered. Secondary sludge from anaerobic filters is directly dewatered when removed. Sludges will likely contain high concentrations of pathogens and must be treated if they are to be used in agriculture.
Figure 8. Typical inputs and outputs for media filter processes
Trickling filters and anaerobic filters are designed specifically for organic matter removal; therefore, any removal of viral, bacterial, protozoan or helminth pathogens in treated effluents is coincidental to the design objectives. For trickling filters that are combined with secondary sedimentation, the reduction of pathogens has been reported to range from 0 to 2 log10 units for viruses, 1 to 2 for bacteria, 0 to 1 for protozoa, and 1 to 2 for helminths (WHO, 2006). There are few data on pathogen removal in anaerobic filters. However, Oliveira and von Sperling (2008) reported a geometric mean of 0.9 log10 removal of thermotolerant coliforms (25 percentile = 0.55; 75 percentile: 1.02) for fourteen systems in Brazil that consisted of a septic tank followed by an anaerobic filter.
The principal removal mechanisms for pathogens in trickling and anaerobic filters are: 1) retention in the biofilm by adsorption and 2) sedimentation in the sloughing biofilm (Figure 9).
Figure 9. Major factors affecting pathogens in (a) trickling filters and (b) anaerobic filters. Note that the media in the anaerobic filter are submerged to maintain anaerobic conditions
Media filters remove soluble organic matter as the wastewater passes through the medium and comes in contact with the attached biofilm. Pathogenic microorganisms in the wastewater can also be adsorbed to the biofilm during this process. When the biofilm sloughs (discussed previously), the adsorbed pathogens will either leave with the suspended solids, where they may be removed by sedimentation or be released to the water that makes up the effluent. Also, in trickling filters some pathogens may be removed on the biofilm by predation by other organisms.
The physical factors that reduce the performance of media filters in treating organic matter removal can also be expected to influence pathogen removal by adsorption to the biofilm. These include (USEPA, 2000; Chernicharo, 2007):
Table 1 presents a summary of the main factors and mechanisms associated with pathogen removal in media filters.
Media filters are designed specifically to remove soluble organic matter and there are no design guidelines for pathogen removal. The design engineer should therefore ensure that wastewater treatment systems using media filters also have downstream treatment processes to remove pathogens from the final effluent to the extent necessary for its safe reuse or discharge; these downstream processes include secondary sedimentation of sloughing sludge and disinfection of the final effluent. Media filter sludge will likely contain elevated concentrations of pathogens and must also be treated appropriately before reuse or final disposal.
Table 2 presents a summary of key factors that potentially could influence the partial removal of the four major groups of pathogenic organisms in a trickling filter and anaerobic filter.
Figure 10 and Table 3 summarize the literature data on pathogen removal in trickling filter and anaerobic filter processes. There are a scarcity of data on pathogen removal from individual media filter unit processes at full-scale wastewater treatment plants; as a result the removal data for bacteria, fecal coliforms, viruses and protozoa include primary and secondary sedimentation for trickling filters and primary sedimentation (septic tanks) for anaerobic filters.
Figure 10. Reported log10 removal of pathogens and fecal coliforms (including E. coli) in trickling filter and anaerobic filter systems. Data for trickling filter systems include primary and secondary sedimentation; data for anaerobic filters include a septic tank followed by anaerobic filter. (Sources of data: Kitajima, et al., 2014a; Lewis, et al., 1986; Marin, et al., 2015; Oliveira, 2006; Oliveira and von Sperling, 2008; Robertson, et al., 2000; Yaziz and Lloyd, 1979)
Media filters are designed for the removal of soluble organic matter and cannot be expected to have high pathogen removal rates. The few available data from the literature for trickling filters with primary and secondary sedimentation suggest that, at best, a 1.0 log10 removal of bacterial pathogens (Salmonella), a 0.5 log10 removal of viruses, a 0.8 log10 removal of protozoa cysts/oocysts, and a 1.4 log10 removal of E. coli and thermotolerant coliforms can be obtained. Tricking filters with primary and secondary sedimentation and chlorine disinfection have been found to remove up to 2.85 log10 viruses and 1.5 log10 protozoan cysts (Giardia). Anaerobic filters preceded by primary sedimentation (septic tanks) have removed up to 1.9 log10 of thermotolerant coliforms. All media filter effluents require further treatment such as disinfection for adequate pathogen removal to meet regulatory or reuse requirements.
Sludges from media filters can be assumed to contain elevated concentrations of pathogens and must be managed accordingly to protect public health before reuse or disposal.