 contact chamber inlet. The liquid disin- fectant pumping system can pump a chlorine mixture directly into the dosing chamber. The system can be set to pump the total efflu- ent dose volume just when the pump finishes its cycle and there is only a small amount of sewage in the bottom of the chamber. Then, as the dosing chamber fills up, the proper concentration is reached. One problem with having the liquid disinfectant pumped directly into the dosing chamber, or the dry feed outlet flow directly into the dosing chamber, is that while most of the effluent in the dosing chamber has plenty of contact time with the chlorine, the last few gallons that activate the dosing chamber pump may have only sec- onds of contact prior to being pumped away. This should not be a significant problem, but if you run into trouble because of a required high frequency of dosing, you should add a contact chamber. ONSITE WASTEWATER DISPOSAL Baffle Figure 7.16 Contact chamber. The contact chamber should provide about six hours of contact time. Calculate the chamber volume using: Volume (gal) = 0.25 days X flow (gal per day). It should have baffles to prevent flow of effluent directly from the inlet to the outlet (Figure 7.16). Materials For a chemical pump system, check with a swimming pool equip- ment and supply store. The system will require a source of electric- ity, a pump that will withstand the concentrated disinfectant, plastic tubing, a bucket for mixing and holding the disinfectant solution, and a delay switch. You may also want a shelter for the switch and bucket, and an electrician to wire the time delay. For a dry fed system you can go to the swimming pool supplier, a supplier of aerobic home sewage treatment units, or you can make one. If you decide to make one, be sure to use materials that will withstand the corrosive action of the sewage plus the disinfectant. This means glass, plastic, plexiglass, or fiberglass. Clear material is best SO that you can see whether the tablets are feeding properly. You will need: 1. six ft2