Comparing the performance of prescribed septic tanks to long, narrow, flooded designs

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Courtesy of Water Environment Federation (WEF)

About 30% of all sewage generated in North America passes through septic tanks, but there is a lack of study to improve these important treatment vessels, and there is no performance-based standard. A survey of existing literature shows that septic tank effluent quality is improved in tanks designed for quiet, laminar fluid-flow and minimum ‘dead space’ such as in longer, narrower tanks. A correlation is suggested between formation of unwanted heavy scum and the presence of tank airspace where vegetative molds live and trap rising sludge particles. The presence of partitions with small orifices worsens effluent quality by setting up turbulent flow and short-circuiting between orifice and outlet, as seen in dye and surrogate solids testing.

Side-by-side testing at the Massachusetts Buzzards Bay Test Facility of a prescribed 1500 gallon single-compartment tank and a long, shallow, flooded tank with no airspace confirms that airspace and shorter flow length are not advantages in septic tank design. During normal testing conditions in the 14 month Study 1 dosed at 750 gpd, the flooded tank performed better in organics removal (~23% better cBOD removal in summer; ~6% in winter) and in solids removal (~30% better TSS in summer; ~18% in winter). Stress tests with heavy laundry detergent added lessened that differential. Over the first 7 winter months of ongoing Study 2 dosed at 660 gpd the flooded tank had both cBOD & TSS removal rates of 30-35% better than the prescribed tank. Solids accumulation was higher in the conventional tank (average 54%) compared to the flooded tank (25%) in both studies.

Septic tank design for thorough contact time and laminar flow can improve effluent quality, and perhaps lessen pumping out requirements. Prescriptive regulations and standards should be reviewed with treatment and maintenance considerations in mind.

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