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Subject: search.filters.subject.Septic tanks

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    Nitrogen removal in a foam media biofilter for on-site wastewater treatment systems : a thesis presented in partial fulfilment of the requirements for the degree of Master of Engineering in Environmental Engineering at Massey University
    (Massey University, 2005) Miller, David Rei
    Discharges of nitrogen can contaminate groundwater, and cause algal blooms or eutrophication in surface waters. On-site wastewater treatment systems (OWTS) have been identified as significant sources of nitrogen. Homeowners and manufacturers are under increasing pressure to install OWTS capable of effective nitrogen removal. Biological nitrogen removal in OWTS usually takes place in a fixed growth biofilter, following primary treatment in a septic tank arrangement. Three configurations of OWTS using foam media biofilters were assessed in the field. Foam media has advantages over sand, as high porosity and large air gaps allow the simultaneous flow of wastewater and air, thus reducing clogging and allowing higher loading rates. Septic tank effluent had lower concentrations of TSS, COD and TN in configurations with larger tank volume. Biofilters provided additional removal of TSS and COD to give effluent concentrations as low as 9 mg/L and 36 mg/L respectively. TN concentration in the effluent varied from 41-53 mg/L depending on configuration. The least nitrogen removal occurred in the configuration with the highest loading rate (in terms of L/m2/d). A bench-scale biofilter constructed using a single foam block (200 x 160 x 60 mm) achieved TN removal up to 10.7 mg/L (0.024 g-N/d at a dosing rate of 2.2 L/d). It was observed that nitrification and denitrification can both occur in a single foam block. Assimilation was also a significant nitrogen removal mechanism, accounting for up to 49 % of total removal. DO concentrations at microenvironments within the bench-scale biofilter were determined using a miniature membrane electrode. A syringe needle and custom-made plunger with the electrode fitted inside allowed DO concentration to be determined in sample volumes as small as 1 mL. The empirical equation derived to calculate DO concentration was accurate to within ± 2.9 %. The extent of nitrification was greatest after an overnight rest period. At microenvironments within the bench-scale biofilter, nitrification increased at longer hydraulic residence time. Nitrification increased at high feed concentrations of carbon, which was not expected, and did not decrease at DO concentrations as low as 0.88 mg/L. Denitrification was greatest when feed was high in carbon and low in DO, but was not affected by DO concentrations as high as 2.70 mg/L. The effects of loading rate, biofilter depth, recirculation ratio and flooding need to be investigated further to optimise the design of biofilters in the field.
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    Improving the performance of septic tank soil absorption systems : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology at Massey University
    (Massey University, 1996) Forsyth, Kirsten Anne
    This thesis is primarily a literature review of research and information about septic tank soil absorption systems and their combined performance as on-site sewage treatment systems. The purpose of the review is to analyse the functions of the tank and the soil absorption area and assess how aspects of their design affect their overall performance. Design variations that optimise septic tank soil absorption system performance have been determined so that they can be applied in New Zealand. An assessment of New Zealand septic tank soil absorption systems with regard to their treatment capabilities shows that they are designed to fail. That is, the tanks are designed to produce a poor quality effluent, and the absorption areas are designed to produce saturated conditions so that contaminants in the effluent are transported to groundwater. Inadequate systems are installed and used because there has been little New Zealand research into the environmental and public health effects of their use and how these effects could be reduced, and there has been limited application of knowledge gained from overseas research. Septic tank design features that optimise wastewater treatment are dual or multi-chambered tanks, outlet tees, outlet baffles, anaerobic filters, and large sludge storage volumes. These features improve the physical processes of sedimentation and separation, the biological process of anaerobic decomposition, and provide storage for accumulated sludge and scum. Incorporating these features can achieve consistent, significant biochemical oxygen demand and suspended solids reductions and assist in retaining pathogenic organisms and nitrogen in the sludge at the bottom of the tank. Dual or multi-chambered tanks, outlet tees, and outlet baffles are effective because wastewater short-circuiting is avoided or decreased and sludge is retained in the tank. Anaerobic filters provide effective treatment of low strength wastewater and are insensitive to variable loading rates. This makes them suitable for the types of wastewater and loading use generated by individual households. Improved septic tank effluent quality in terms of BOD and suspended solids concentrations extends the life of the soil absorption area. Soil absorption area design features that optimise wastewater treatment are pressurised effluent distribution in all soils, effluent application at hydraulic loadings appropriate to the soil type, the presence of a biological mat, particularly in sandy soils, and passive de-nitrification components. These features ensure that effluent is applied to the entire infiltrative surface at the design hydraulic loading, assist in removing pathogenic organisms in the effluent, and avoid or decrease adverse effects on groundwater quality from nitrates. Critical aspects of the soil absorption area design are the hydraulic loading, the method of distribution, and components that promote de-nitrification. Potential adverse effects on groundwater quality and public health are reduced by maintaining unsaturated conditions under the soil absorption areas and removing nitrates. Uniform effluent application at a rate compatible with the infiltration capacity of the soil is essential to avoid saturated conditions. Uniform application is achieved by pressure distribution.