Septic System Pumping

Traditionally, the most common domestic wastewater treatment system used in rural areas is the septic tank-soil absorption system. The septic tank removes settleable and floatable solids from the wastewater. The soil absorption field then filters and treats the clarified septic tank effluent and distributes it through the soil. Removing the solids from the wastewater protects the soil absorption system from clogging and failure. In addition to removing solids, the septic tank also promotes biological digestion of a portion of the solids and stores the remaining undigested portion.

The first stage of the treatment system, the septic tank, removes solids by holding wastewater in the tank. This allows the heavier solids to settle as sludge and the lighter particles to form scum at the top. To accomplish this, wastewater should be held in the tank for at least 24 hours. Up to 50 percent of the solids retained in the tank decompose; the remainder accumulates in the tank. Biological and chemical additives are not needed to aid or accelerate decomposition.

As the system is used, sludge continues to accumulate in the bottom of the septic tank. Properly designed tanks should have enough space for up to three years safe accumulation of sludge. When the sludge level increases beyond this point, sewage has less time to settle before leaving the tank and more solids escape into the absorption area. If too much sludge accumulates, no settling occurs before the sewage flows to the soil absorption field. Infiltration of sludge into the soil absorption field can cause system failure. To prevent this, the tank must be pumped periodically. The material pumped is known as septage.

The frequency of pumping depends on several factors:
1.capacity of the septic tank
2.volume of wastewater (related to size of household)
3.amount of solids in wastewater (e.g. garbage disposals produce more solids)
Table I lists estimated pumping frequency according to septic tank capacity and household size. The frequencies were calculated to provide a minimum of 24 hours of wastewater retention assuming 50 percent digestion of the retained solids.

TABLE 1 Estimated septic tank pumping frequencies in years
(for year-round residences)

Tank Household size (number of people)
Tank Size 1 2 3 4 5 6 7 8 9
gals. (Years between pumping listed below)
500 6.0 2.5 1.5 1.0 0.5 0.5 0.3 0.2 0.1
750 9.0 4.0 2.5 2.0 1.5 1.0 0.5 0.5 0.5
900 11.0 5.0 3.5 2.5 1.5 1.5 1.0 1.0 0.5
1000 12.5 6.0 3.5 2.5 2.0 1.5 1.0 1.0 1.0
1250 15.5 7.5 5.0 3.5 2.5 2.0 1.5 1.5 1.0
1500 19.0 9.0 6.0 4.0 3.5 2.5 2.0 2.0 1.5
1750 22.0 10.5 7.0 5.0 4.0 3.0 2.5 2.0 2.0
2000 25.5 12.5 8.0 6.0 4.5 3.5 3.0 2.5 2.0
2250 28.5 14.0 9.0 6.5 5.0 4.0 3.5 3.0 2.5
2500 31.9 15.5 10.0 7.5 6.0 5.0 4.0 3.5 3.0



* Below the minimum size allowed by rule.

Note: More frequent pumping is needed if a garbage disposal is used. Generally the indicated frequency will vary with the volume of solids (+20% for high volume, -20% for low volume). Also, more frequent pumping will be needed if your tanks are not water tight, and allow the infiltration of storm or ground water.



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