Sustainable, Low-maintenance Disinfection for Onsite Systems

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Courtesy of Premier Tech Aqua

Most of the regulations applicable for onsite wastewater treatment system are based on the use of native soil for effluent dispersal. However, there are more and more situations where dispersal of effluent in native soil is impossible: repairs of existing systems on undersized lots for implementation of required dispersal fields: sites located in soils having low permeability: or where the bedrock or groundwater level is near the surface. For most of theses cases, the wastewater treatment solution must include a disinfection unit to assure health and safety protection. This is particularly important for sites where the treatment system is installed 011 imported soil over fractured bedrock allowing the possible direct discharge of treated effluent into the groundwater. Chlorination and UV light are currently the most common methods used for disinfection of wastewater. While these processes are generally well-adapted to community or municipal installations, their operation and maintenance needs present some limitations in order to ensure sustainable performance for individual home applications (ex: chlorine addition, cleaning of the quartz sleeve in UV systems, etc.). Management of such individual disinfection systems can be costly and. if neglected, may present risks to both human health and receiving water quality. In view of these limitations, a need remains for a low maintenance disinfection system suitable for individual home applications.

I11 2003. Premier Tech Environment (PTE) started a research and development project to develop a passive disinfection approach meeting this need. We evaluated the concept of 'sand mantle' prescribed in Ontario regulation and the horizontal sand filter approach used many years ago in France. Based on this evaluation, we designed a new concept of horizontal sand filter corresponding to an optimized version of the 'sand mantle' but with a lower footprint. The passive disinfection system developed is essentially a horizontal sand filter (HSF). which is fed vertically by the advanced secondary effluent produced by the peat based Ecoflo Biofilter. Hie biofilter effluent is evenly applied 011 the first section of the horizontal sand filter by a low pressure distribution system or by using an open bottom Ecoflo* Biofilter module. The HSF consists of a 30 cm layer of filtration sand (D10 = 0.15 to 0.5. Cu = 3 to 4) spanning over an impermeable liner with a length of 10 111 and a slope of 3 to 5 %. The filter width is sized 011 a linear loading rate (LLR) of 225 L/m.d. The horizontal filter operates in aerobic conditions and air renewal is provided by interconnection of the gravel layer over sand with the Ecoflo* Biofilter aeration system. At the downstream extremity of the horizontal filter, a collection and sampling device is provided. The treated effluent is collected by a perforated pipe located in a gravel trench surrounded by geotextile and followed by a sampling port and a diffusion pipe for final dispersion.

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