ain field, used where site conditions are marginal, to high technology systems which could be used where nothing but bedrock and ice are present. Table 3.4 presents commonly prescribed alternative systems and alternative disposal fields which are suitable to be used to overcome a variety of site limitations. Full descriptions of each of these sys- tems are given in Part II. The following general descriptions provide a starting place for deciding which system might be best for any particular application. The complete system description in Part II should be read before a final decision is made. Alternating fields are useful where water takes a long time to filter into the soil. As wastewater begins to accumulate in one field, another is put into use to allow more time for infiltration and the drying out of the first field. Alternating any type of drain fields should result in extending the lifetimes of those fields. Beds are standard drain field trenches squeezed together SO closely SELECTION OF SITE AND SYSTEM Table 3.4 Site Conditions and Alternative Systems Site Condition Shallow Shallow Steep Small System Percolation Percolation Bedrock Ground Water Slope Space Conventional trenches Deep trenches Seepage pits Sand mounds Lined evapotranspiration Unlined evapotranspiration Exposed sand filters Buried sand filters Underdrained sand filters Land application Grey water system Enclosed system Water conservation that there is no space left between them. While this obviously saves space, beds have the disadvantage of having very little sidewall area available for absorption as compared to trenches. The Uniform Plumbing Code requires that bed bottom areas be 150% of the area required for trenches. Deep trenches save space by substituting sidewall absorption area for bottom area. The same amount of absorption can be accom- plished with shorter trenches. Deep trenches deliver the sewage effluent deeper into the soil than do standard drain fields, SO that on steeply sloping land, the sewage is less likely to reach the sur