Quantitative evaluation of treatment processes and mechanisms of organic matter, phosphorus, and nitrogen removal in a multi-soil-layering system

The water treatment performance of multi-soil-layering (MSL) systems has been mainly evaluated by measuring differences in the quality of wastewater and treated water. Changes in water quality inside the system have not been assessed together with the water movement. In this study, therefore, quantitative evaluation of the treatment processes inside the MSL system was conducted using laboratory-scale MSL systems, which were set up in 10 cm (depth) × 50 cm (width) × 23–73 cm (height) acrylic boxes enclosing “soil mixture blocks” alternating with permeable zeolite layers. Six MSL systems consisting of one to six soil mixture layers (SML) were constructed, and treated water from the SML and the permeable layers between the SML (PLb) was collected separately at the bottom of each system. In order to determine the treatment processes inside the system, wastewater, with mean concentrations of biological oxygen demand (BOD) 28.1 mg L−1, chemical oxygen demand (COD) 65.7 mg L−1, total nitrogen 9.8 mg L−1, total phosphorus 1.0 mg L−1, was introduced into the system at a hydraulic loading rate of 1000 L m−2 day−1.
Concentrations of both BOD and COD in the SML were lower than those in the PLb. As the flow rate in the SML decreased and the rate in the PLb increased, concentrations increased in the PLb in each system. Although the removal rate of BOD always exceeded 80% in the systems with more than three layers, six layers were required for COD to be reduced to the same extent as BOD. Phosphorus concentrations were also lower in the SML than in the PLb due to the adsorption of phosphorus by the soil and mixed iron particles. Therefore, phosphorus removal efficiency was strongly influenced by flow rate in the SML. In this study, the ammonium adsorption and nitrification were almost completed down to the third layer of the MSL system. However, the removal of nitrogen did not proceed much below the fourth layer due to low denitrification efficiency.