t amount of the solid material and release it in the forms of dissolved solids and gases, thereby slowing the accumulation rate of solids in the tank. Gas bubbles rising from the bottom of the tank where the bacteria are working can lift solid material to the liquid surface where it could be carried over into the drain field. This potential problem will be discussed later. Only the dissolved and suspended solids should be delivered to the drain field. The septic tank design ONSITE WASTEWATER DISPOSAL Table 4.1 Recommended Septic Tank Capacities No. of Bedrooms Septic Tank Capacity (gal) 1 or 2 1,000 1,200 5 or 6 1,500 and the degree of maintenance it receives determine how well these functions are performed. Design Sizing of the septic tank requires determination of a minimum residence time of wastewater in the septic tank. Residence time is the time it takes a molecule of water to move from the inlet to the outlet. This should be at least a day and a half. The volume of the tank compared to the rate of wastewater coming into it determines the residence time. Some additional tank volume should be pro- vided to compensate for the reduction in residence time as the solids accumulate in the tank and its effective volume decreases. A tank volume of two to three times the design daily flow is common and a good investment. For example, a three-bedroom house can be assumed to produce around 450 gallons of wastewater per day (gpd). 3 bedrooms X 2 persons/bedroom X 75 gpd/person = 450 gpd A retention time of one and one-half days would necessitate a septic tank volume of 675 gal. 450 gpd X 1.5 days = 675 gal Allowing for sludge accumulation, a three-bedroom house should be served by a 1000-gal septic tank. In fact, this is the national standard as established by the Uniform Plumbing Code. The Code recommends the septic tank capacities shown in Table 4.1 for single family dwellings. These recommended sizes accommodate the use of garbage dis- posals, which tend to increase the rate of solids accumulation. The two-compartmen