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How San Francisco Septic Systems Work

A septic tank is a large, underground, watertight container, typically about 9 feet long, 4-5 feet wide and 5 feet tall that is connected to the home’s sewer line. While typically designed with a 1,500- gallon liquid capacity, the number of bedrooms in the home legally determines the size of the tank. Septic tanks may be rectangular or cylindrical and may be made of concrete, fiberglass or polyethylene. Older tanks may also be redwood or metal.

A-1 Septic Tank Service Truck

The Process

Raw wastewater from the bathroom, kitchen and laundry room flows into the tank where the solids separate from the liquid. Light solids, such as soapsuds and fat, float to the top and form a scum layer. This layer remains on top and gradually thickens until you have the tank cleaned. The liquid waste goes into the drain field, while the heavier solids settle to the bottom of the tank where they are gradually decomposed by bacteria. But some non-decomposed solids remain, forming a sludge layer that must be pumped out.

Septic tanks may have one or two compartments. Two-compartment tanks do a better job of settling solids and are required in some areas for new installations. Tees or baffles at the tank’s inlet pipe slow the incoming wastes and reduce disturbance of the settled sludge. A tee or baffle at the outlet keeps the solids or scum in the tank. All tanks should have accessible covers for checking the condition of the baffles and for pumping both compartments.

Storm Drain

The Treatment

Further treatment of wastewater occurs in the soil beneath the drain field. The drain field consists of long underground-perforated pipes or tiles connected to the septic tank. The network of pipes is laid in gravel-filled trenches (2-3 feet wide), or beds (over 3 feet wide) in the soil. Liquid waste flows out of the tank and is evenly distributed into the soil through the piping system. After the effluent has passed into the soil, most of it percolates downward and outward. A small percentage is taken up by plants through their roots, or evaporates from the soil. The size and type of drain field depends on the estimated daily wastewater flow and soil conditions.

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