Septic tank systems are used at 50 percent of roadside rest areas in the United States for onsite disposal of wastewater generated from restrooms and from recreational vehicle waste holding tank dump stations. survey results are presented from 28 California roadside rest areas of the use of rest areas, and of the volume and strength of wastewater generated at restrooms and dump stations. Traffic densities in peak months averaged 24 percent higher than the annual mean, while peak holiday weekend densities averaged 86 percent higher for facilities serving one direction of traffic. A mean of 12 percent of mainline traffic used the rest areas, and of the traffic using rest areas that provided dump stations, 2 percent were recreational vehicles that actually dumped. Restrooms generated 5. 5 gal of waste per vehicle, and dump stations generated 12 gal of wastewater plus 9 gal of washdown water per dump. Restroom wastewater is comparable in strength to domestic wastewater, but dump station wastewater(diluted by washdown water) produces about 20 times the quantity of sludge as the same volume of domestic wastewater. Depending on the proportion of dump station waste and the frequency of pumping the septic tank, rest area septic tanks should be sized to provide 1.5 to 30 days detention of diluted dump station wastewater, compared to 1.5 days for a domestic septic tank. Septic tank-leach field system design procedures consider the risk of overload for a particular design, or permit design to a selected acceptably low risk of overload.

Restroom toilets so predominate among roadside rest area amenities that a rest area may have to be closed if its waste disposal system fails. Being distant from city sewers, most rest areas must dispose of the wastewater they generate onsite. One-half of the roadside rest areas surveyed in the United States used septic tank systems for wastewater disposal (l), The design of onsite wastewater disposal systems for roadside rest areas are addressed in this paper, with emphasis on septic tank systems.