Recirculating sand filter design and operating criteria for removal of nitrogen from domestic septic tank effluent

A physical model of a recirculating sand filter (RSF) system was constructed to determine the ability of the system to remove nitrogen from domestic septic tank effluent and to learn the effects on the removal ability of the specific operating parameters depth of sand, recirculation ratio, loading rate.

The RSF system model consisted of a sand filter reactor, denitrification chamber reactor, and a polishing sand filter reactor. A research investigation was conducted with the help of twelve model units in which domestic septic tank effluent was treated on a continuous basis. Then, optimum and appropriate combinations of operating parameters were found and the systems were evaluated on the ability to remove mineral nitrogen, TOC, coliform and streptococcus bacteria.

To measure RSF system performance, water quality samples were collected weekly from the effluent of the three reactors and analyzed for concentration of several quality parameters. The samples collected were analyzed for concentrations of NH₃-N, NO_x^--N, TOC, and bacteria in the form of total coliform, fecal coliform, and streptococcus populations.

The research model units were successful in achieving biological treatment of domestic septic tank effluent. Prediction equations developed from model system data indicated that maximum mineral nitrogen removal for circulation factors (R) of 4 and 6, should be 73.69% at a sand bed depth of 16.50 cm and a wastewater loading rate of 40. 74 cm/day. Results revealed that an increase in circulation factor (R) from 4 to 6 produced no significant impact on the overall system mineral nitrogen removal efficiency.

In the recirculating sand filter reactor system, TOC removal responded significantly to variations in R value. For an R factor of 6, a maximum TOC removal efficiency of 80.70% at a sand depth of 17.00 cm and loading rate of 22.92 cm/day can be expected. The reduction of bacterial count in the RSF system model was in the order of 99%.

The investigation showed that redox potential and NO₃^--N concentration have a definite relationship. An estimate of the obtainable maximum NO₃^--N concentration can be predicted by measuring the redox potential in the DNC water. Measurements of redox potential below -150 mV gave indication of excellent denitrification potential with very high removal efficiency.