Page - 86 - in Clean Water Using Solar and Wind - Outside the Power Grid

86 Clean Water Using Solar and Wind: Outside the Power Grid simple in design and operation and possibly affordable for low-income people. Primarily, we consider the energy aspect. 7.2.1 Septic tanks A septic tank consists of a concrete or plastic tank. The design of the tank usually includes two chambers separated by a dividing wall with openings located about midway between the floor and roof of the tank. Used water enters the first chamber where solids will settle. These are anaerobically digested (see below), reducing the volume of solids. The liquid flows through the dividing wall into the second chamber, where further settlement takes place. The excess liquid, now in a relatively clear condition, then drains from the outlet into a septic drain field. The remaining impurities are trapped and eliminated in the soil, with the excess water eliminated through percolation into the soil, through evaporation and by uptake through the root systems of plants and eventual transpiration, or entering the groundwater or surface water. The required size of the drain field must increase with the volume of used water. On the other hand, if the soil has a higher porosity, the drain field can be smaller compared to a field with lower porosity. The entire septic system can operate by gravity alone. A lift pump will be required for certain places. Obviously, a septic tank has two major drawbacks: the inherent energy in the organic matter is not used and the effluent water is not reused. The organic content of the waste is converted to biogas (mainly methane) during anaerobic digestion. The gas will escape into the air, unnecessarily contributing to the greenhouse effect in consideration of the fact that the Global Warming Potential (GWP) of methane is 30–90 times higher than that of CO2 depending on the considered timescale. The other drawback is that the effluent water is not reused. Naturally, a septic tank can be considered the first step in a water reuse scheme, but the loss of organic energy is still there. Furthermore, the septic tank must be emptied from the settled sludge, typically every three to five years. 7.2.2 Activated sludge systems The traditional process for removing organic matter is biological oxidation, which involves microorganisms feeding on the carbon and Downloaded from https://iwaponline.com/ebooks/book-pdf/520710/wio9781780409443.pdf by IWA Publishing user