Effluent flux prediction in variably saturated soil zones within a septic tank—soil absorption trench
Australian Journal of Soil Research
The treatment and hydraulic mechanisms in a septic tank -soil absorption system (SAS) are highly influenced by the clogging layer or biomat zone which develops on bottom and lower sidewall surfaces within the trench. Flow rates through the biomat and sub-biomat zones are governed largely by the biomat hydraulic properties (resistance and hydraulic conductivity) and the unsaturated hydraulic conductivity of the underlying soil. One dimensional and two dimensional models were used to investigate
... sed to investigate the relative importance of sidewall and vertical flow rates and pathways in SAS. Results of one-dimensional modelling show that several orders of magnitude variation in saturated hydraulic conductivity (Ks) reduce to a one order of magnitude variation in long-term flow rates. To increase the reliability of prediction of septic trench hydrology, HYDRUS-2D was used to model two dimensional flow. 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 zones and the exfiltration zones of the low permeable soil. An increase in effluent infiltration corresponded with a greater availability of exfiltration zone, rather than a lower resistance of Modelling effluent flow in septic systems biomat. Results of modelling simulations demonstrated the important role that a permeable A horizon may play in limiting surface surcharge of effluent under high trench hydraulic loading.