the accumulation of floating scum. Figure 4.1 shows the basic design for septic tanks. If preferred, the efficiency of a double compartment tank can be realized by connect- ing two single compartment tanks. Further, if pumping or dosing the effluent to a disposal system is necessary or desired, a third compartment can be attached to the outlet end of the tank to serve as a dosing chamber. Premanufactured tanks may be horizontal or vertical cylinders or rectangular parallelepipeds (boxes-try asking your grocer for a cardboard parallelepiped). They may be made of concrete, steel, fiberglass, or plastic. If you buy a premanufactured tank, check to see if the manufacturer is licensed, the necessary design features mentioned above are present, the tank is strong enough to with- stand the weight of the rear wheel of a septic tank pumper truck, and the exposed surfaces are not subject to excessive corrosion. If you choose a steel tank, be sure to provide corrosion protection. ONSITE WASTEWATER DISPOSAL If you want to make your own septic tank, the information below will help you produce a first-rate unit. Septic Tank Materials 1. 4 in. diameter inlet and outlet piping 2. 2-4 in. diameter sanitary tees 3. 1-4 in. diameter ell 4. 4 U bolts for manhole covers 5. 5-20 ft long #4 steel reinforcing rods 6. 60 ft of 10-gauge 6" X 6" wire reinforcing mesh 7. 4 cubic yd of concrete 8. mortar for sealing around inlet and outlet pipes 9. 2.5 gal of bituminous waterproofing These materials are sufficient to construct a 1000-gal, two- compartment septic tank, coated on the inside only. If concrete blocks are used for the vertical walls and partition, about 155 8 by 16 in. blocks and approximately 1.5 cubic yd of concrete are required. If you have no experience in making forms, pouring concrete, or laying block, it would be wise to visit a public library or an experienced friend. Since premanufactured septic tanks are universally available and relatively inexpensive, few attempt to make their own. For this reason, the construction de