ems in the distribution or disposal system develop. The space between the alarm level and the "pump on" level should be small 1-about - 50 gal of volume. The volume of the permanent reser- voir at the bottom is determined by the height of the suction line of the pump, which should always be submerged. If the pump itself is always supposed to be submerged, then the pump height itself determines this volume. Often the pump is set on blocks for adjust- ment of the hydraulic head and/or positioning the pump's intake port. Having the pump intake elevated somewhat above the bottom of the chamber reduces the amount of solids which pass through the 56 ONSITE WASTEWATER DISPOSAL pump, and therefore extends the life of the pump. The material collecting on the chamber bottom must be removed periodically. Since dosing siphons are used only where the drain field is at a lower elevation than the chamber, an overflow can be installed SO that, in case of a malfunction, the siphon is automatically bypassed. Where a pump dosing unit is installed at an elevation considerably higher than the disposal field, as well as where a gravity dosing system is used, the need for the space above the high water alarm is reduced. Be sure to check the chamber periodically to verify proper operation. Dosing volume is dependent upon the type of disposal system used. The methods of calculating desired dosing volumes for sand mounds, sand filters, land application (pressure distribution), and evapotranspiration systems are described in their respective sec- tions of this book. Dosing volumes where the effluent is delivered through standard drain field tiles or pipes (usually 4-in. diameter) to absorption trenches or beds are easily determined. In general, one wants more frequent dosing (four to six times per day) in soils with rapid percolation and less frequent dosing in tight soils. The size of the drain field and, therefore, the length of the distribution pipe is proportional to the soil percolation rate (see the next chapter). A rule of thumb is 