o crown the whole area. After you know the dimensions of your trench, the next consider- ation is layout. The absorption system should be located SO as not to pose a threat to building foundations or to wells or streams. State or local codes may specify the distances to various landmarks. If there are no specific requirements for setback distances in your area, the Table 4.3 provides reasonable distances to observe in locating your system. These distances can be modified according to site specific conditions such as depth of ground water, direction of ground water flow, and soil texture. The trench bottom should be as level as you can get it SO that some areas of the trench will not be overloaded while other areas never see a drop of effluent. The maximum slope allowable for both the trench bottom and the distribution lines is about 3 in. per 100 ft. By following the contours of the land, the bottom can be kept level while maintaining the same trench depth throughout. There are three different ways to design the trench system (Figure 5.5). Each is for a specific purpose. Parallel trenches are commonly used on land that is fairly level. Effluent from the septic tank is fed to the distribution lines through a distribution box (Figure 5.6). The CONVENTIONAL WASTE DISPOSAL SYSTEMS Figure 5.4 Installing filter fabric. Parallel Serial Figure 5.5 Distribution pipe configurations. ONSITE WASTEWATER DISPOSAL Figure 5.6 Distribution box. outlets of the distribution box are placed at equal heights to ensure that each trench in the distribution system gets the same amount of effluent. However, the distribution box sometimes settles after installation, which can result in uneven flows to the laterals or even broken pipe connections. A bed of sand and a concrete slab below the box may help to prevent this, as well as make the initial leveling easier. But because of the problems associated with distribution boxes, their use is not recommended. If your plumbing inspector mentions that it's required by the Uniform Plumbing Code f