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A vermifilter (also vermi-digester or lumbrifilter) is an aerobic treatment system, consisting of a Bioreactor containing media that filters Organic matter from wastewater. The media also provides a habitat for Aerobic organism bacteria and composting earthworms that purify the wastewater by removing pathogens and oxygen demand. The "trickling action" of the wastewater through the media Dissolved 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 being safely discharged into the environment. Treated effluent is disposed of to either surface or subsurface leach fields. 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 aerobic microorganisms biodegradation pathogens and other , resulting in treated wastewater.
Vermifiltration is a low-cost aerobic wastewater treatment option. Because energy is not required for aeration, vermifilters can be considered "passive treatment" systems (pumps may be required if gravity flow is not possible). Another advantage is the high treatment efficiency given the low space requirement.
When this kind of sanitation system is used to treat only the mixture of excreta and water from or pour-flush toilets (called blackwater) then the term "toilet" is added to the name of the process, such as vermifilter toilet.
Vermifilters are a type of wastewater treatment biofilter or trickling filter, but with the addition of earthworms to improve treatment efficiency. Vermifilters provide an aerobic environment and wet substrate that facilitates microorganism growth as a biofilm. Microorganisms perform biochemical degradation of 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. When this builds up, it can be removed and applied to soil as an amendment to improve soil fertility and structure.
Microorganisms present are and . 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. In a study where municipal wastewater was treated 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 meets World Health Organisation guidelines for safe re-use in crops.
Worms actively Digestion 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 buildup of solid waste as a pile. Although oxygen is excluded from the centre of this "wet" compost pile, worms work from the outside in and introduce air as necessary into the pile to meet their nutritional requirements. This food resource buffer ensures primary treatment vermifilters have a level of resilience and reliability, provided space is provided for a pile to build up. There is some evidence that the wet environment facilitates 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.
Primary treatment of wet mixed blackwater can also include Grey water containing food solids, Fat 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, with much of the solid organic material removed from the effluent. Because liquid effluent is discharged almost immediately on entering the digester, little dissolved oxygen is consumed by the wastewater through the filtration stage. However, oxygen demand is leached 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 digestion of solids by worms takes place rapidly in an aerobic environment.
The liquid effluent is either discharged directly to a drain field or undergoes secondary treatment before being used for surface or subsurface irrigation of agricultural soil.
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) can influence treatment efficiency.
Vermifilter reactors may comprise a single section packed only with organic media, or up to three filter sections comprising an Organic compound top layer that provides habitat for the earthworms, an inorganic upper layer of sand and lower layer of gravel. The filter sits on top of a sump or drainage layer of coarse gravel, rocks or pervious plastic drainage coil where the treated effluent is discharged and/or recirculated to the top of the reactor. Alternatively the filter media may be suspended above the sump in a basket. Synthetic fiber geotextile cloth is sometimes used to retain the filter media in place above the drainage layer. To remain aerobic, adequate ventilation must be provided, along with an outlet for the liquid effluent to drain away. Common filter packing materials include sawdust,Arora, S., Rajpal, A., Kumar, T., Bhargava, R., Kazmi, A.A., 2014. Pathogen removal during wastewater treatment by vermifiltration. Environ. Technol. 35, 2493–2499.Arora, S., Rajpal, A., Kazmi, A.A., 2016. Antimicrobial Activity of Bacterial Community for Removal of Pathogens during Vermifiltration. J. Environ. Eng. 142 (5). Woodchips, coir, bark, peat, and straw for the organic layer. Gravel, quartz sand, round stones, pumice, Clay balls, glass balls, Ceramsite sand and charcoal are commonly used for the inorganic layer. The surface area and porosity of these filter materials influence treatment performance.Dahab, M.F. 1982 Effect of media design on the performance of fixed-bed anaerobic filters. Water Science & Technology, 15, 369–383. Materials with low granulometry (small particles) and large surface area may improve the performance of the vermifilter but impede its drainage.
In principle, provided the environment is aerobic, the longer the wastewater remains inside the filter, the greater the BOD5 and COD removal efficiency will be, but at the expense of hydraulic loading. Wastewater requires sufficient contact time with the biofilm to allow for the adsorption, transformation, and reduction of contaminants.Hughes, R.J., Nair, J., Ho, G., 2008. The toxicity of ammonia/ammonium to the vermifiltration wastewater treatment process. Water Sci. Technol. 58, 1215–20. The hydraulic loading rate is an essential design parameter, consisting of the volume of wastewater that a vermifilter can reasonably treat in a given amount of time. For a given system, higher hydraulic loading rates will cause hydraulic retention time to decrease and therefore reduce level of treatment. Hydraulic loading rate may depend on parameters such as structure, effluent quality and bulk density of filter packing, along with method of effluent application.Siegrist, R.L., 1987. Soil Clogging During Subsurface Wastewater Infiltration as Affected by Effluent Composition and Loading Rate. J. Environ. Qual. 16(2): 181-187. Common hydraulic retention time values in vermifiltration systems range from 1 to 3 hours.Sinha, 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, 409–420. Hydraulic loading rates commonly vary between 0.2 m3 m−2 day−1,Li, Y.S., Xiao, Y.Q., Qiu, J.P., Dai, Y.Q., Robin, P., 2009. Continuous village sewage treatment by vermifiltration and activated sludge process. Water Sci. Technol. 60(11), 3001–10. 3.0 m3 m−2 day−1 Manyuchi, M.M., Kadzungura, L., Boka, S., 2013. Vermifiltration of Sewage Wastewater for Potential Use in Irrigation Purposes Using Eisenia fetida Earthworms, 538–542. or 10–20 g L−1.Lourenço, N., Nunes, L.M. (2017) Optimization of a vermifiltration process for treating urban wastewater. Volume 100, March 2017, Pages 138–146 Organic loading rate is defined as the amount of soluble and particulate organic matter (as BOD5) per unit area per unit time.Otis, R.J., 2001. Boundary Design: A Strategy for Subsurface Wastewater Infiltration System Design and Rehabilitation. On-Site Wastewater Treatment Procedures: Proceedings of the Ninth National Symposium on Individual and Small Community Sewage Systems. ASAE. St. Joseph MI. p 245-260.
Treatment efficiency is influenced by health, maturity and population abundance of the earthworms. Abundance is a fundamental parameter for efficient operation of a vermifiltration system.Li, Y.S., Robin, P., Cluzeau, D., Bouché, M., Qiu, J.P., Laplanche, A., Hassouna, M., Morand, P., Dappelo, C., Callarec, J., 2008. Vermifiltration as a stage in reuse of swine wastewater: Monitoring methodology on an experimental farm. Ecol. Eng. 32, 301–309. Different values are reported in the literature, usually in grams or number of individuals per volume of filter packing or surface area of filter packing. Common densities vary between 10 g L−1 and 40 g L−1 of filter packing material.Tomar, P., Suthar, S., 2011. Urban wastewater treatment using vermi-biofiltration system. Desalination 282, 95–103.Zhao, L., Wang, Y., Yang, J., Xing, M., Li, X., Yi, D., Deng, D., 2010. Earthworm-microorganism interactions: a strategy to stabilize domestic wastewater sludge. Water Res. 44(8), 2572–82.
An abundance of earthworms improves treatment efficiency, in particular BOD5, TSS and NH4+ removal. This is because earthworms release organic matter into the filter media and stimulate nitrogen mineralization. Earthworm castings may have substances which contribute to higher BOD5 removal.Lourenço, N., Nunes, L.M., Submitted. Optimization of a vermifiltration process for treating urban wastewater. Ecological Engineering.
Occasional topping up of organic materials may be required as these decompose and reduce in volume. The volume of Vermicompost increases only slowly and occasionally vermicompost needs to be removed from the vermifilter.
Solids accumulate on the surface of the organic filter media (or filter packing). The liquid fraction drains through the medium into the sump or equaliser and is either discharged from the reactor or recirculated to the top for further treatment. Wastewater is discharged to the surface of the filter material by direct application or by sprinklers, drippers or tricklers.
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