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.