ree-plus gallon per flush toilets, SO if you are just building and want low-flush toilets, consider using 3-in. diameter drainpipe. It is obvious that the toilet contributes a large fraction of the pollu- tants generated in a home. Elimination of toilet waste from the septic tank/drain field system can significantly reduce the impact of onsite waste disposal, and can be very important where a high density of such systems presents a significant pollution potential. LOW-IMPACT SYSTEMS Process Holding tanks provide some minimum amount of treatment of sewage in that bacteria dine on the organic solids, some separation of solids and liquids occurs, and some pathogens are eliminated. But the holding tank does not produce a quality of effluent suitable for discharge into a drain field. One should not automatically presume that the septic tank pumper has a safe and legal place to deposit his aqueous acquisi- tion. Ask. Design Materials and Construction A holding tank should hold at least 1000 gal of sewage effluent. A 2000-gal tank will take twice as long to fill up as a 1000-gal tank, but the septic tank pumper truck which has a 1200-gal capacity will have to make two trips instead of one. Holding tanks are designed just like septic tanks, only without the partition, the second access port, and the exit pipe. One- or two- compartment commercial septic tanks with the outlet sealed off will serve just as well, if a holding tank is not available. The materials are basically the same, as are the construction techniques. Because holding tanks require frequent service, it is particularly important to locate them SO that the pumper truck has easy access, the access port is easy to find, and the lid is easily removed. A high water alarm, as described in the chapter on dosing cham- bers, can be installed SO that a timely appointment with the pumper can be scheduled. Sewage backing up into the shower stall is a cheap and effective alarm, but is not recommended for obvious health reasons. Operation and Maintenance The frequent mai