Code of practice covering the design, construction, and installation of secondary treatment systems for septic tank effluent, detailing the use of dispersion trenches, soak pits, biological filters, and upflow anaerobic filters.
Deals with methods and requirements for providing secondary treatment to effluent from septic tanks.
Quick Reference Values
Minimum depth of water table below trench/pit bottom1.5 m
Minimum distance from drinking water well18 m
Maximum length of a single dispersion trench30 m
Width of dispersion trench0.3 m to 1.0 m
Depth of dispersion trench0.5 m to 1.0 m
Minimum clearance between adjacent trenches2.0 m
Key Formulas
Q = 130 / √t — Maximum rate of effluent application (l/m²/day), where t is the standard percolation rate (minutes)
Practical Notes
Always perform standard soil percolation tests before selecting and designing soil absorption systems like dispersion trenches or soak pits.
Ensure the groundwater table is sufficiently deep (at least 1.5m below the absorption system) to prevent groundwater contamination.
Upflow anaerobic filters are an excellent alternative where soil absorption is very poor, the water table is high, or space is highly restricted.