Clarification of water movement properties in a multi-soil-layering system

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In multi-soil-layering (MSL) systems, the effect of the materials and structural differences on the wastewater treatment performance and clogging mechanisms are not fully understood because of insufficient quantitative evaluations of water movement properties inside the system. In the present study, water movement properties were examined using laboratory-scale MSL systems consisting of one to six soil mixture layers (SML). The aims were to determine (1) changes in water movement characteristics inside the systems by varying the hydraulic loading rate (HLR), (2) the relationship between the number of inflow points and water movement characteristics, and (3) water movement characteristics and clogging mechanisms under wastewater treatment conditions.

As the HLR increased, the flow rates in the SML increased linearly, followed by a slight decline in the rate of increase, whereas flow rates in the permeable layers (PL) between the SML (PLb) increased exponentially except in the system with one layer. Water movement inside the systems with one inflow point inclined toward the central part of the system compared to that with three inflow points. Under wastewater treatment conditions, flow rates in the first SML decreased to 437 L m−2 day−1 from 1433 L m−2 day−1 in three days after applying wastewater. However, as the number of layers increased, high flow rates in the SML were maintained for longer periods. In the first to third SML, a decreasing trend of oxidation–reduction potential (Eh) values was correlated with a decrease in flow rates. Partial clogging in the first PLb was associated with Eh values in the PL of the upper part beginning to decrease. The study indicated that establishment of an optimum structure, materials, and aeration was required to maintain a proper level of permeability and anaerobic conditions inside the SML, which would facilitate simultaneous removal of organic matter, nitrogen and phosphorus.