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.Meiyan Xing, Xiaowei Li and Jian Yang. Treatment performance of small-scale vermifilter for domestic wastewater and its relationship to earthworm growth, reproduction and enzymatic activity, African Journal of Biotechnology, November 2010
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.Jiang, L., Liu, Y., Hu, X., Zeng, G., Wang, H., Zhou, L., Tan, X., Huang, B., Liu, S., Liu, S., 2016. The use of microbial-earthworm ecofilters for wastewater treatment with special attention to influencing factors in performance: A review. Bioresour. Technol. 200, 999–1007 Biofilm and organic matter consumed by composting earthworms is then digested into biologically inert castings (humus).Liu, J., Lu, Z., Yang, J., Xing, M., Yu, F., Guo, M., 2012. Effect of earthworms on the performance and microbial communities of excess sludge treatment process in vermifilter. Bioresour. Technol. 117, 214–21 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.Rajiv K. Sinha, Gokul Bharambe, Uday Chaudhari. Sewage treatment by vermifiltration with synchronous treatment of sludge by earthworms: a low-cost sustainable technology over conventional systems with potential for decentralization, Environmentalist, 2008 28:409-420 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.C. Furlong, W. T. Gibson, M. R. Templeton, M. Taillade, F. Kassam, G. Crabb, R. Goodsell, J. McQuilkin, A. Oak, G. Thakar, M. Kodgire, R. Patankar. The development of an onsite sanitation system based on vermifiltration: the "Tiger Toilet", Journal of Water, Sanitation and Hygiene for Development, January 2015 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.C. Furlong, M.R. Templeton, W.T. Gibson. Processing of human faeces by wet vermifiltration for improved on-site sanitation, Journal of Water, Sanitation and Hygiene for Development 4(2):231, June 2014
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.Taylor, M. Clarke, W. P., Greenfield, P. F. The treatment of domestic wastewater using small-scale vermicompost filter beds, Journal of Ecological Engineering, December 2003 21: 197–203 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,A.S. Molla, P. O. Antwi, R.A. Buamah, H.M. Essandoh, E. Awuah The Potential of Subsurface Infiltration for the Treatment of Biofil Toilet Technology Effluent, Management Studies, December 2015 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
Secondary and tertiary treatment vermifilters can be stacked underneath the primary vermifilter in a single tower, but are typically separate. Multiple reactors can be chained in series. Drainage within the reactor is provided by filter media packed according to the hydraulic conductivity and permeability of each filtration media. The filter retains the solid particles, increases the hydraulic retention time and provides a suitable habitat for sustaining composting earthworms. This population requires adequate moisture levels within the filter media, along with adequate drainage and oxygen.
Sprinklers or drippers can be used in secondary and tertiary treatment vermifilter reactors (see image).
Hydraulic factors (hydraulic retention time, hydraulic loading rate and organic loading rate) and biological factors (earthworm numbers, levels of biofilm) influence treatment efficiency.
Design Vermifilter reactors are made from durable materials that exclude vermin such as plastic or concrete. Ventilation must be sufficient to ensure an aerobic environment, while also excluding flies. Temperature within the reactor needs to be maintained within a range suitable for the worms and microorganisms.
Design parameters include stocking density of earthwormsSinha, R.K., Bharambe, G., Chaudhari, U., 2008. Sewage treatment by vermifiltration with synchronous treatment of sludge by earthworms: a low-cost sustainable technology over conventional systems with potential for decentralization. Environmentalist, 28(4), 409–420. (over time earthworm population tends to self-moderate), filter media composition,Cardoso-Vigueros, L., Ramírez-Camperos, E., Garzón-Zúñiga, M., 2013. Evaluation of a pilot vermifilter for the treatment of wastewater. Ingeniería Agrícola y Biosistemas, 5(2), 33–44. hydraulic loading rate,Kumar, T., Bhargava, R., Prasad, K.S.H., Pruthi, V., 2015. Evaluation of vermifiltration process using natural ingredients for effective wastewater treatment. Ecol. Eng. 75, 370–377. hydraulic retention timeArora, S., Rajpal, A., Kumar, T., Bhargava, R., Kazmi, A.A., 2014. A comparative study for pathogen removal using different filter media during vermifiltration. Water Sci. Technol. 70, 996–1003. and organic loading rate. Hydraulic retention time and hydraulic loading rate both affect effluent quality. Hydraulic retention time is the actual time the wastewater is in contact with the filter media and is related to the depth of the vermifilter (which may increase over time due to the accumulation of earthworm vermicastings), reactor volume and type of material used (porosity). The hydraulic retention time determines wastewater inflow rate (hydraulic loading as influent volume per hour).
Influent Influent enters from above the filter media. Side entry is possible for full-flush toilets, while micro-flush toilets do not provide sufficient water to convey solids through sewer pipes and generally are installed directly above the reactor. Primary treatment reactors must provide sufficient vertical space for growth of the pile. This is dependent on the volume of influent solids and the presence of slower decomposing materials such as toilet paper. Secondary and tertiary treatment reactors can use sprinklers or tricklers to distribute the influent wastewater evenly over the filter media to improve treatment efficiency of the filter media.
Filter substrate Drainage within the vermifilter reactor is provided by the filter media. The filter media has the dual purpose of retaining the solid organic material, while also providing a habitat suitable for sustaining composting worms. This population requires adequate moisture levels within the media, along with good drainage and aerobic conditions.