 either the septic tank or drain field in a surface depression. Where there is the potential for surface runoff to enter the septic tank or drain field, provide diversion channels or berms. This prevents hydraulic overloading of the field by rainwater. 4. Locate the drain field SO as to take full advantage of the capacity of the sun and wind to evaporate water. This reduces the amount of water which must be absorbed by the soil surrounding the drain 40 ONSITE WASTEWATER DISPOSAL field area. Where there is a slope, the chance of sewage coming to the surface downhill from the drain field is reduced. 5. Know and observe all setback requirements, such as separation distance of the drain field from wells, bodies of water, watercourses, buildings, property lines, etc. This eliminates the potential need for expensive lawyers and system relocation. PART II System Design, Construction, and Maintenance Waste Process SEPTIC TANKS Application The septic tank provides three services to the drain field: solids settling, solids storage, and solids digestion. Without these services, the drain field would become clogged with particulates from the wastewater in a very short time and the wastewater would either back up into the house (normally into the shower or bathtub, the lowest drain in the house) or rise to the surface of the ground. The septic tank, then, conditions the raw sewage before it is released to the drain field, SO that the life of the drain field is greatly prolonged. Process Wastewater consists of three types of solids: dissolved (such as salts), suspended (such as gelatin and soap), and settleable (such as hair and grease- the floating solids along with those that sink are classified as settleable). As the wastewater enters the tank, its veloc- ity is slowed SO that, during the time it takes to flow through the tank to the outlet, the larger solids have a chance to settle to the tank bottom, and the light solids, such as grease, float to the surface. The organisms which live in the septic tank digest a significan