Characteristics of Wastewater Treatment Using a Multi-Soil-Layering System in Relation to Wastewater Contamination Levels and Hydraulic Loading Rates.

Characterization of wastewater treatment using a multi-soil-layering (MSL) system at two levels of wastewater contamination (a low level [LWW] and a high level [HWW]) with a mean biochemical oxygen demand (BOD) of 30 and 70 mg L−1, respectively, and at hydraulic loading rates (HLRs) of 500–2,000 L m−2 day−1 was conducted. Despite fluctuations in the wastewater contamination levels, the BOD and total phosphorus (T-P) concentration in the treated waters remained constantly lower than those in the wastewaters. However, the total nitrogen (T-N) concentration in the treated water was influenced by the level in the wastewater and fluctuated. In general, the two different levels of wastewater contamination did not influence the mean concentrations of the contaminants in the treated waters and the mean removal percentages in the MSL system. Mean removal percentages ranged from 90 to 96% for suspended solids (SS), from 88 to 98% for BOD, from 83 to 94% for CODcr, from 44 to 57% for T-N, and from 63 to 89% for T-P. Removal of SS, BOD, chemical oxygen demand (CODcr) and T-N was not appreciably influenced by changes in the HLR. For the removal of nitrogen, a negative correlation was observed with the oxidation–reduction potential (ORP) in the treated water, suggesting that the development of reduced conditions in the system was the key factor for the removal of T-N through the enhancement of the denitrification process. The mean removal percentage of phosphorus tended to be higher at lower HLRs. An increase in the HLR probably reduced the contact time of phosphorus and materials in the MSL system, which resulted in the decrease in the removal percentage. Removal rates (g m−2 d−1) of BOD, CODcr, nitrogen and phosphorus were highest at the highest HLR in the HWW treatments. However, higher HLRs, especially in the HWW treatments, caused clogging of the system during the early periods of operation. Clogging was alleviated after an 8-week rest of the system. An operation cycle in which periodical resting time of the system is used to prevent or alleviate clogging could be recommended for the use of the MSL system.