A vermifilter (also vermi-digester or lumbrifilter) is an aerobic treatment system, consisting of a biological reactor containing media that filters organic material from wastewater. The media also provides a habitat for aerobic bacteria and composting earthworms that purify the wastewater by removing pathogens and oxygen demand. The "trickling action" of the wastewater through the media dissolves oxygen into the wastewater, ensuring the treatment environment is aerobic for rapid decomposition of organic substances. Vermifilters are most commonly used for sewage treatment and for agro-industrial wastewater treatment. Vermifilters can be used for primary, secondary and tertiary treatment of sewage, including blackwater and greywater in on-site systems and municipal wastewater in large centralised systems. Vermifilters are used where wastewater requires treatment before it is discharged into the environment. Solid material (such as fecal matter and toilet paper) is retained, de-watered and digested by bacteria and earthworms into humus that is integrated into the filtration media. The liquid passes through the filtration media where the attached organisms biodegrade pathogens and other organic compounds, treating the wastewater. Vermifiltration is a low-cost aerobic wastewater treatment option. Because energy is not required for aeration, vermifilters qualify as passive treatment systems (pumps may be required if gravity flow is not possible). Another advantage is the treatment efficiency in limited space.
Terminology Alternative terms used to describe the vermifiltration process include aerobic biodigester, biological filter with earthworms, or wet vermicomposting. The treatment system may be described using terms such as vermi-digester and vermi-trickling filter. When this kind of sanitation system is used to treat only the mixture of excreta and water from flush toilets or pour-flush toilets (called blackwater) then the term "toilet" is added to the process name, such as vermifilter toilet.
History Vermifiltration was first proposed by researchers at the University of Chile in 1992. A 2017 study treated municipal wastewater in a vermifilter, removal ratios for biochemical oxygen demand (BOD5) were 90%, chemical oxygen demand (COD) 85%, total suspended solids (TSS) 98%, ammonia nitrogen 75% and fecal coliforms eliminated to a level that met World Health Organisation guidelines for safe use in crops. Vermifilters offer treatment performance similar to conventional decentralised wastewater treatment systems, but with potentially higher hydraulic processing capacities. Vermifilters are a type of biofilter or trickling filter that adds earthworms to improve treatment efficiency. Vermifilters provide an aerobic environment and wet substrate that facilitates microorganism growth as a biofilm. Microorganisms biochemically degrade organic matter present in wastewater. Earthworms regulate microbial biomass and activity by directly or/and indirectly grazing on microorganisms. Biofilm and organic matter consumed by composting earthworms is then digested into biologically inert castings (humus). The vermicast is incorporated into the media substrate, slowly increasing its volume. It can be removed and used as a soil amendment to improve fertility and structure. Microorganisms present are heterotrophic and autotrophic. Heterotrophic microorganisms are important in oxidising carbon (decomposition), whereas autotrophic microorganisms are important in nitrification. As a result of oxidation reactions, biodegradation, and microbial stimulation by enzymatic action, organic matter decomposition and pathogen destruction occurs in the vermifilter.
Process types Vermifilters can be used for primary, secondary and tertiary treatment of blackwater and greywater.
Primary blackwater treatment
Vermifilters can be used for aerobic primary treatment. Untreated blackwater enters a ventilated enclosure above a bed of filter medium. Solids accumulate on the surface of the filter bed while liquid drains through the filter medium and exits the reactor. Solids (feces and toilet paper) are aerobically digested by aerobic bacteria and composting earthworms into castings (humus), thereby significantly reducing the volume of organic material.
Primary treatment vermifilter reactors are designed to digest solid material, such as contained in raw sewage. Twin-chamber parallel reactors offer the advantage of being able to allow one to rest, while the other is active, in order to facilitate hygienic humus removal. Worms digest the solid organic material. Over time, an equilibrium is reached in which the volume digested by a stable population of worms is equal to the input volume of solid waste. Seasonal and environmental factors (such as temperature) and variable influent volumes can cause solid waste buildup as a pile. Although ambient oxygen cannot the centre of this (wet) compost pile, worms work from the outside in and introduce air as necessary to meet their nutritional requirements. This food buffer provides a level of resilience and reliability, provided space is provided for a pile to develop. The wet environment may facilitate digestion of solid waste by worms. The volume of vermicast humus increases only slowly and occasionally needs to be removed from the primary treatment reactor. Treatable blackwater can include greywater containing food solids, grease and other biodegradable waste. Solid material is reduced to stable humus (worm castings), with volume reductions of up to tenfold. The process produces primary treated blackwater, removing much of the solid organic material. Because liquid effluent is discharged almost immediately on entering the digester, little dissolved oxygen is consumed through the filtration stage. However, oxygen demand leaches into the wastewater flow through the filter as worms digest the retained solids. This oxygen demand can be removed with secondary treatment vermifilter reactors. Primary treatment vermifilters provide a similar level of liquid effluent treatment to a septic tank, but in less time because solid digestion by worms takes place more rapidly in an aerobic environment. The liquid effluent is either discharged directly to a drain field or undergoes secondary treatment.
Secondary treatment
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