 wastewater begins to accumulate or pond within the trench. The liquid level will rise above the growing biological mat and begin to flow through the trench sides where the soil is more permeable. The sides will then begin to clog. If the load of wastewater per square foot of infiltration surface is not too great, the trench system will reach a point where most of the effluent will soak through the biological mat on the bottom and partly up the sides. The occasional party may raise the CONVENTIONAL WASTE DISPOSAL SYSTEMS 67 level of wastewater such that the level rises above the biological mat on the sides of the trench and exits without filtration by the mat. As long as the trench is deep enough to accommodate these occasional excess loads without the wastewater coming to the surface, it mat- ters little how deep the trench is. In tighter soils, shallow trenches may allow more water to be evaporated and taken up by plant roots than do deep trenches. Deep trenches may allow for greater variation in the amount of wastewater that can be handled without creating a problem. Although soils are usually more permeable near the surface, deeper trenches can be used to get the trench bottom below a layer of clay, rock or other slowly permeable soil layer. After the debate over how deep the trench should be for best operation, we enter the debate over how wide it should be. Since a wide trench turns into a narrow bed as its width is increased, the issue is really between absorption trenches and absorption beds. Beds depend on their bottom area for absorption much more than do trenches, for their sidewall area is much less than that of trenches. Typically, a trench has 30% to 50% more absorption area than a bed with equal bottom area. The major advantage of beds is that they require less room for installation than does a series of trenches. A disadvantage, in addition to the loss of sidewall absorp- tion area, is that beds are more easily damaged by excavation equip- ment tires or metal tracks during installation. In summary