Onsite Septic Systems ASTM STP 901, M.S. Bedinger, A.I. Johnson, and J.S. Fleming, Eds., American

Society for Testing Materials, Philadelphia, 1997. Page 12 >SEE FULL PAPER PDF<

Compartmentation

Over the years there has been continuing controversy over single-compartment versus two-compartment tanks.

Evidence of significant benefits to effluent quality that would support compartmentation of tanks, as they are

presently constructed, is inconclusive. The Public Health Service concluded its study by stating, “It cannot be

stated conclusively that there was any significant difference in the operation of the one- and the

two compartment tanks.”

Winneberger (1984) explains the effect that velocities and turbulence have on the migration path of particles

traveling through septic tanks and concludes, like Seabloom (1982), that slow velocities through long tanks

yield the highest effluent quality. Winneberger makes two generalizations. First, “the geometric shape of a tank,

as such, seems not to be critical. It is the management of flow-through that is of concern” and, second, “the size

of that second chamber matters little.” However, the duration of these studies is insufficient for long-term

predictions. Also, the studies have not adequately addressed how effluent quality is affected as sludge and

scum accumulate in the primary compartment. An observation common to all the reports is that, as the

hydraulic retention time increases, performance improves (i.e., larger compartments or tanks yield better

quality effluents).

Regardless of the number, size or shape of supplemental compartments the primary or first compartment’s

capacity should be designed based on hydraulic loading, velocity through the tank, reserve capacity, solids

storage capacity and hydraulic retention time. Too little primary capacity can lead to excessive pump-out

frequencies—a costly disaster for the community or individual that has to deal with the mess and pay for the

corrective measures. The difference in cost between a 5768 L (1500 gal) single compartment tank and a

smaller 3785 L (1000 gal) two compartment tank is negligible. A larger such tank reduces pumping

occurrences by a factor of four or more when servicing a family of three. Ultimately there will be less organic

matter to dispose due to more complete digestion. Excessive hydraulic loads on holiday weekends or wash

days will have less effect on the surge capacity of the larger tank. The money saved on unnecessary or less

frequent pumping could wisely be spent on servicing and monitoring.



Terry Bounds did further research and kindly provided >this information< which is a 6 Mb PDF file. It is well worth browsing as the important parts have been highlighted by Terry; once again showing advantages of single compartment tanks.