Enhanced Treatment of Decentralized Domestic Sewage Using Gravity-Flow Multi-Soil-Layering Systems Coupled with Iron-Carbon Microelectrolysis

Abstract

Soil-based decentralized treatment technology has become increasingly popular as an ideal solution for water pollution control in rural areas. It is very necessary to optimize the removal mechanisms and performance of such technologies on rural domestic sewage treatment. This was the first study of a gravity-flow multi-soil-layering (MSL) system coupled with iron-carbon microelectrolysis (ICM). Influent COD/TN (C/N) ratio and bottommost soil mixture block (SMB) submersion were selected as the operating factors relevant to the ICM in MSL systems. Such two key factors were investigated in the factorial experiment. The removal efficiencies of COD, TP, NH3-N, NO3−-N, and TN could be reached up to 96.3, 100, 95.4, 93.8, and 79.6%, respectively. Different levels of factors could comprehensively drive the performance variation. The factorial analysis indicated that the bottommost SMB submersion had the most significant and dominant negative effects on aerobic processes. The ideal TP removal attributed to the presence of the bottommost SMB submersion. It played the dominant role for the bottommost SMB submersion in facilitating an electrochemical reaction through the ICM. Zero-valent iron or ferrous ions could be transformed to final ferric ions more efficiently during the period of the ICM reactions. The ICM could promote the capability of a SMB for removing nutrients in sewage, especially provide electron donors to denitrifying bacteria in MSL systems. However, there were non-significant effects of the influent C/N ratio on the removal performance of MSL systems. This study can help enrich the pollutant removal mechanisms in MSL systems.