design includes a belowground sand filter with collection lines beneath it. With this design, the effluent from the septic tank or aerobic unit could drain by gravity onto the sand filter surface, and then be collected from beneath the sand filter and sent by gravity through a disinfection unit into a deep dosing cham- ber. Although a second dosing chamber is not necessary with this design, any problem with the sand filter surface clogging or the collection system plugging up would be more difficult to correct than if an aboveground sand filter was used. Design of the septic tank, dosing chamber, and disinfection units are described in their respective sections. Design of the flood irriga- tion plots is as follows. First, the system design flow must be known. This is normally calculated assuming 75 gal/day/person, and two persons per bed- room. But, because the plants to be irrigated cannot use theoretical water, the actual flow must be approximated. Moderate usage is about 40 gal/day/person, and if there are multiple bathrooms, a laundry room, a dishwasher, and garbage disposal, assume 50 to 55 gal/day/person. Next, find out the length of the growing season and the water in excess of net rainfall needed by the plants in your plot. Also, ask about whether overwatering could be a problem. The county exten- sion service, the state agricultural university, the state department of agriculture, or the U.S. Soil Conservation Service should have ONSITE WASTEWATER DISPOSAL Table 6.7 Irrigation Efficiencies for Various Soils Soil Texture Efficiency data on various crop requirements in your area. The local airport should have rainfall data. With this information, plus an appraisal of the soil texture, you can calculate the daily water requirement. Divide the seasonal water consumption in inches of water by the number of days in the growing season. Then convert from inches to gallons per day per square foot by multiplying by 0.63. Finally, divide by the irrigation efficiency for the type of soil being irrigated to correct f