ectant is almost always chlorine in the form of sticks or pellets that slowly dissolve or erode in the liquid. See the next section for specification information. Materials There is a broad range of materials and material costs, depending on the particular conformation of system components used. See the sections on Septic Tanks, Dosing Chambers, Sand Filters, Land Application, and Disinfection for materials details. LOW-IMPACT SYSTEMS Pea Gravel Medium Gravel Coarse Gravel Wood Chips Pea Gravel Coarse Sand Fine Sand Pea Gravel Coarse Gravel Sprinkler Coarse Sand Pea Gravel Coarse Gravel Figure 7.13 Simple sand filters. Cartridge Filter Septic Tank Contact Chamber Dosing Chamber Figure 7.14 Cartridge filtration. ONSITE WASTEWATER DISPOSAL Construction Again, see the above-mentioned sections. Operation and Maintenance In general, grey water is of higher quality than combined waste- water, and this translates into less maintenance for the grey water system. Depending on the size of the septic tank used as a grey water tank, and depending on whether the kitchen sink (and gar- bage disposal) are plugged into the system, you should check the sludge level and have it pumped out every five years, or as needed. The accumulation of a floating scum layer may mean that pumping will be necessary as frequently as with a combined wastewater sep- tic tank. Operation and maintenance of the dosing chamber is described under that section. Sand filter maintenance is the same as described in the Sand Filter section, except that the required frequency may be cut approximately in half. Operation and maintenance of swimming pool filters is described in literature received with these units. Hair, lint, and grease are wastes found in swimming pools, and these filters are efficient in trapping them. But they are designed for rea- sonably clean, disinfected water, not reasonably dirty, germ-laden grey water. Therefore, maintenance on these units will probably be several times more frequent than indicated, not SO much from the particulate ma