The efficiency of a multi-soil-layering system on domestic wastewater treatment during the ninth and tenth years of operation

Abstract

A multi-soil-layering (MSL) wastewater treatment system has been operating at Matsue City, Japan, since July 1990. This study investigates the efficiency of the MSL treatment system during the ninth and tenth years of operation. The MSL system consists of soil mixed with forest floor litter, pelletized jute and iron particles in the ratio of 60:10:15:15 by weight, respectively. The soil mixture is arranged within a 1.75×2.5×1.2 m 3 durable plastic box as blocks thus forming an alternating brick layer-like pattern with zeolite inter-layers. Household wastewater was pre-treated using the traditional Japanese septic tank system (Tandoku Johkasou) with integral aeration facility, before being gravitationally discharged continuously into the MSL system. The flow rate of pre-treated wastewater into the MSL system varied from 1.2 to 113.4 l h −1 as a function of household water consumption. The results indicate that during the period of investigation, the MSL system was still effective at removing biochemical oxygen demand (BOD 5) and total phosphorus (TP). The removal rates of BOD 5 and TP were up to 91.0±4.5 and 89.0±4.4%, respectively, in the ninth year and 95.2±3.8 and 82.9±11.9%, respectively, in the tenth year of operation. Total nitrogen (TN) removal was significantly dependent on aeration of the MSL system, quality of pre-treated wastewater and temperature. Although the continuous aeration of the MSL system for 24 h d −1, which was equivalent to 2.8×10 4 l m −3 d −1, was effective in facilitating the removal of BOD 5 and TP, it was inappropriate for TN removal. However, after aeration was stopped, TN removal improved considerably, increasing to 89%.