Comparative Study of Two Standard Septic Tank Drain Fields Using Different Sand with Recirculation for Nutrient Removal

On-site Wastewater Treatment System (OWTS) using a septic tank followed by a recirculating sand filter (RSF) with effluent discharged to an unlined standard drain field is a typical performance-based treatment facility for nutrient removal. The type of effluent distribution in the standard drain field systems include gravity systems, low pressure dosed systems, drip irrigation systems, etc. The standard drain field that is constructed by a series of parallel, underground, perforated pipes allows the septic tank effluent to percolate into the surrounding soil in the vadose (unsaturated) zone where most of the residual nutrients may be assimilated. The inclusion of RSF may improve the nitrification to some extent so as to promote denitrification in the drain field. With such design, most residual nutrients in wastewater are expected to be consumed as the wastewater passes through the soil. This author's aim of this paper is to present the effectivenss of RSF and a comparative study with these two different drain field sands for nutrient removal in vadose zone. They include astatula sand (i.e., citrus grove sand) and washed building sand, popular in Central Florida for drain field use. The use of a gene identification method is novel in supporting the biological removal. Research findings show that the performance of washed building sand and the astatula sand in terms of ultimate nutrient removal is about the same. Because astatula sand is less costly it can possibly be considered for replacement of the more expensive washed building sand.