Characterization of treatment processes and mechanisms of COD, phosphorus and nitrogen removal in a multi-soil-layering system

Characteristics of the treatment processes inside a MSL system were investigated by using a laboratory-scale MSL system, which was set up in a D10 x W50 X H73 cm acrylic box enclosing "soil mixture blocks" alternating with I permeable zeolite layers. For the study of the treatment processes inside the system, wastewater, with mean concentrations (mg L-1) of COD: 70, T-N: 12, T-P: 0.9, was introduced into the system at a loading rate of 1,000 L m(-2) d(-1). Treatment processes in the MSL system were, different for the COD, P and N pollutants. Eighty percent of COD was removed in the 1st soil layer among the 6 layers, and the removal rate increased as water moved down and finally reached 90% in the last layer of the system. Phosphorus concentration was lower under the soil mixture layers than under the permeable layers, presumably because P was adsorbed mainly by soil and mixed iron particles. The P concentration in water gradually decreased in the lower layers of the system. The concentration of PO43--P was generally lower in the aerated MSL system than in the non-aerated one. NH4+-N was adsorbed and nitrified in the upper part of the system. The NO3--N concentration was lower in water under the soil mixture layers than under the permeable layers, indicating that denitrification mainly occurred in the soil mixture layers.