The treatment and hydraulic mechanisms in a septic tank-soil absorption system (SAS) are highly in?uenced 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- and 2-dimensional models were used to investigate the relative importance of sidewall and vertical ?ow rates and pathways in SAS. Results of 1-dimensional modelling show that several orders of magnitude variation in saturated hydraulic conductivity (Ks) reduce to a 1 order of magnitude variation in long-term ?ow rates. To increase the reliability of prediction of septic trench hydrology, HYDRUS-2D was used to model 2-dimensional ?ow. In the permeable soils, under high trench loading, ef?uent preferentially ?owed in the upper region of the trench where no resistant biomat was present (the ex?ltration zone). By comparison, ?ow was more evenly partitioned between the biomat zones and the ex?ltration zones of the low permeability soil. An increase in ef?uent in?ltration corresponded with a greater availability of ex?ltration zone, rather than a lower resistance of biomat. Results of modelling simulations demonstrated the important role that a permeable A horizon may play in limiting surface surcharge of ef?uent under high trench hydraulic loading.