INTRODUCTION


        Septic tank/soil absorption systems are economic alternatives to full-scale wastewater

treatment systems for homes in sparsely populated rural areas as well as in numerous suburban

counties. Geraghty and Miller (1978) indicated that in the USA there were four counties with more

than 100,000 and an additional 23 counties with more than 50,000 septic systems. Septic systems

have been indicted as the most frequently reported source of groundwater contamination (USEPA,

1977). Treatment of domestic wastewater therefore is receiving more attention, especially as the land

suitable for conventional septic systems continuously decreases nowadays (Reneau et al., 1989).

       The majority of septic systems in the USA are conventional gravity-flow systems. This

system includes a septic tank, sometimes a distribution box, and a subsurface soil absorption field

which may be in the configuration of a bed or an array of parallel trenches. In addition to the

conventional design, other alternative systems have been created to selectively place the septic tank

effluent (STE) in the soil profile or in fill materials above the soil profile in an attempt to overcome

certain soil limitations. One common alternative for this purpose distributes the STE through a low-

pressure distribution (LPD) system, i.e. a dosing system. The advantages of a dosing system are that

the effluent can be applied uniformly to the soil at a given rate and at specific time intervals so that

a better wastewater renovation can be achieved (Degan and Reneau, 1991). Another alternative

utilized in situations where the local soils have limitations, e.g. short depth to a high or seasonally

fluctuating water table, low soil permeability, or shallow soil underlain with a creviced bedrock, is

the mound system. A mound system consists of an artificial mound filled with materials (normally
sand) adequate for wastewater purification as an additional treatment before the natural soil is

encountered. Harkin et al. (1979) discovered that the mound type system was superior to the