e amount of disinfec- tant you should use to start is lb of chemical per day per 0.00000834 X gal day X mg/L % availability 216 ONSITE WASTEWATER DISPOSAL For example, if you have a three-person family, estimate 50 gal per person per day for a total average flow of 150 gal per day. You may be using hypochlorite granules with 65% available chlorine, and trying for a concentration of 5 mg/L in the effluent. Then you would 0.00000834 X 150 gal per day X 5 mg/L 0.65 availability or about 0.01 lb, or about 0.15 OZ of dry chemical per day. One 3-in. tablet weighs about 4.5 oz, SO theoretically, one tablet should last 30 days. In practice, a three-person family may use about 0.6 to 0.9 tablets per 1000 gal of flow from the sand filters, giving a single tablet a lifetime of 7 to 11 days. So the typical result is overchlorina- tion, not insufficient disinfection. To determine if the disinfection unit is performing correctly, wait until the whole sewage disposal system has been in operation for a week or so, then collect a sample of sewage as it exits the dosing chamber or contact chamber. If your laboratory will give you a chemical (sodium thiosulfate) to get rid of the residual chlorine, pick some up or ask them to send you some. They will know how much you need. The reason for adding this chemical is that you don't want the disinfectant working on the bacteria any longer than the normal contact time. Put the proper amount of chemical into one of three well-washed and rinsed screw top bottles or jars (of about 1 qt capacity each). Try not to leave any bubbles in the jars when you put the lids on, and be sure to label them so that the laboratory knows which is which. One will be dechlorinated, one will be chlorinated, and one will be for your own chlorine test. Be careful when you handle the effluent, and wash your hands when you are through. Take three samples of your disinfected effluent. Measure the com- bined residual chlorine in one jar with your test kit (do not worry about free available chlorine) and make a note o