or flat ground, ask for special permission from the "administrative author- ity" to use an alternative. Your request is supported by a Federal Housing Administration study showing that distribution boxes may be harmful to system performance. Looped distribution systems overcome some of the limitations of the parallel system, but proper operation is dependent on having all the laterals at the same elevation. If properly installed, the looped system is more likely than the parallel system to provide equal dis- tribution between the laterals. Also, the expense of buying and installing a distribution box might be saved. But use of the looped system is also limited to relatively flat terrain. There should be no more than a 6-in. difference in surface elevation over the area where the field will be installed. Serial distribution systems are for use where the land slopes. Trenches are laid out parallel to the land contours in a downhill CONVENTIONAL WASTE DISPOSAL SYSTEMS Figure 5.7 Drop boxes. series. The trenches are connected by solid pipe SO that as the first trench accumulates sewage effluent to a certain depth, the effluent begins to overflow through the connecting pipe to the next downhill trench. Drop boxes (Figure 5.7) are used to set the overflow level. That level should be just a few inches below the level of the distribu- tion box inflow pipe SO that maximum use of the upper trench sidewall will be gained before the lower trenches are used. In this way, all the absorption potential of the system will be used and failure will be less likely. If codes do not specify otherwise, an informal rule of thumb is to allow three times the width of the trenches as a separation distance between them. The Uniform Plumbing Code requires 4 ft plus 2 ft for each foot of trench depth below the distribution pipe in excess of 1 ft. Greater separation allows easier construction and provides a buffer zone between trenches, SO that effluent from two parallel trenches will not accumulate between them. ONSITE WASTEWATER DISPOSAL Som