carbon in the septic tank effluent was probably the limiting

factor to greater denitrification (Lamb et. al., 1990).

     Swanson and Dix modified the traditional recirculating sand

filter.  Instead of using a recirculation tank, they put gravel

in the bottom of the sand filter which served as the

recirculation tank.   They also used bottom ash, a waste product

of coal-fired power plants, instead of sand as the filter media.

The filter plus the gravel storage is referred to as a batch

recirculating bottom ash filter (BRBAF).

    The system includes a septic tank, a BRBAF, and an

ultraviolet disinfection unit. The 2.4 m x 4.5 m x 1.4 m BRBAF

is filled with 80 cm of screened bottom ash overlying 15 cm of

peagravel atop 40 cm of washed gravel. These layers are enclosed

within treated plywood walls and posts and sealed with three

layers of 6-mil plastic.

     They concluded that the bottom ash recirculating sand filter

system produced a good quality effluent consistent with effluent

from other RSF's in terms of pH, BOD5, SS, and TKN. However,

nitrates and nitrites were not monitored and thus they were

unable to determine the nitrogen removal of the system.

      Sandy (1987) modified the BRBAF system and monitored its

potential for nitrogen reduction. The main modification was that

a provision was made to recycle the highly nitrified filter
effluent back to the septic tank.    Since the amount of organic

carbon is often a limiting factor in achieving nitrogen removal,

it was felt that the "sink of carbon" in the septic tank could be