The hydrology of septic tank – soil absorption systems: investigation and prediction of hydraulic failure

Beal, Cara Diane (2007). The hydrology of septic tank – soil absorption systems: investigation and prediction of hydraulic failure PhD Thesis, School of Land, Crop and Food Sciences, University of Queensland.

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Author Beal, Cara Diane
Thesis Title The hydrology of septic tank – soil absorption systems: investigation and prediction of hydraulic failure
School, Centre or Institute School of Land, Crop and Food Sciences
Institution University of Queensland
Publication date 2007
Thesis type PhD Thesis
Supervisor Assoc Pr Neal Menzies
Abstract/Summary The mechanisms governing hydraulic failure of septic tank – soil absorption systems (SAS) are not well understood. The low permeable biomat zone that develops on the infiltrative surface is a key component of the hydraulic and treatment performance of a SAS. In this study, the hydrology of SAS, including biomat development, the role of soil texture and key effluent flow pathways, was investigated. A survey of 19 councils in south-east Queensland was undertaken to obtain data on aspects of SAS management, and to identify the number and type of SAS failure rates. A survey of this nature had not been carried out in Queensland before, and it illustrated the importance of field inspections of every non-sewered allotment to achieve meaningful compilation of data. The number of on-site systems (in 2003) was estimated to be 127,000 with septic systems accounting for approximately 80%. Over 75% of SAS were split systems (separate greywater/blackwater treatment). A common SAS failure was surcharging trenches, which was observed predominantly in soils of low permeability. Whilst the reported frequency of SAS failures was quite low (usually < 1% per year) there was a widespread opinion by councils that this was a substantial under reporting of the real situation. The effects of biomat development on long-term infiltration rates in different soil textures was investigated. Septic tank effluent was applied to repacked sand, Ferrosol and Vertosol soil columns over 16 months, at equivalent loading rates of 50, 35 and 8 L/m2/d, respectively. Biomats 1 to 2 cm thick developed in all soils with corresponding hydraulic resistances of 27 to 39 days. These biomats reduced a four order of magnitude variation in saturated hydraulic conductivity (Ks) between the soils to a one order of magnitude variation in long-term acceptance rate (LTAR). A relationship between biomat resistance and organic loading rate was observed in all soils. Results show that whilst initial soil Ks is likely to be important in the establishment of the biomat zone in a trench, LTAR is predominantly influenced by the biomat resistance, and to a lesser extent the unsaturated soil hydraulic conductivity, and not the Ks of a soil. One dimensional and two dimensional hydraulic models were used to investigate the relative importance of sidewall and vertical flow rates and pathways in SAS. In the permeable soils, under high trench loading, effluent preferentially flowed in the upper region of the trench where no resistant biomat was present (the exfiltration zone). In comparison, flow was more evenly partitioned between the biomat zone and the exfiltration zone of trenches in the low permeable soils. An increase in effluent infiltration corresponded with a greater availability of exfiltration zone, rather than a lower resistance of biomat. Field studies were conducted on two permeable soils (clay loam and loamy sand) to characterise the biomat physical properties and to calibrate 2D modelling. Field results indicated that sidewall flow above the biomat during high trench loading was a major flow pathway in the soils. Hydraulic transport and Ks parameters were calibrated using inverse modelling procedures in HYDRUS-2D. There was good agreement between measured and predicted matric potentials, and the optimized Ks biomat parameters of 0.003 and 0.004 m/day fell well within the range reported in previous field-scale studies. Predictive modelling indicated that during extreme loading conditions, the exfiltration zone is the major flow pathway for trenches in permeable soil. The influence of soil texture on SAS hydraulic failure was shown to be more important in situations of extreme hydraulic loading into a trench, rather than on the normal functioning of a trench. Soil column studies indicated that the biomat resistance is the key driver of long-term vertical flow rates, regardless of initial saturated hydraulic conductivities. Modelling and field studies demonstrated that during extreme trench loadings (such as extended rainfall events or episodic shock loads), the permeability and extent of the biomat-free upper sidewalls is the ultimate determinant of trench hydraulic failure.

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Created: Fri, 21 Nov 2008, 15:16:58 EST