Effectiveness of multi-soil-layering systems in the treatment of sodium chloride-contaminated wastewater

ABSTRACT

Multi-soil-layering (MSL) is a sustainable wastewater treatment method that uses natural materials. Taiwan has successfully employed MSL to treat rural domestic sewage for reuse and to improve the water quality. However, coastal and riverside areas in Taiwan face challenges with sodium chloride contamination due to seawater intrusion, making conventional purification methods less effective. This study examined the effectiveness of MSL in treating sodium chloride-contaminated wastewater using three experimental tanks. Results showed that when sodium chloride concentration exceeded 2%, the reduction rates for ammonia nitrogen (NH4+-N) and chemical oxygen demand (COD) dropped below 50% from 90%. However, phosphate (PO43−) and nitrate nitrogen (NO3−) reduction rates were less affected. Replacing gravel with zeolite in the MSL system's permeable layers increased sodium chloride tolerance from 2 to 3%, improving NH4+-N reduction to 50% but only slightly affecting COD reduction. Furthermore, low concentration (0.5%) sodium chloride solution to condition the MSL system, fostering the growth of salt-tolerant bacteria (halophiles). Subsequent, at a 50% pollution reduction rate, experiments showed that the tolerance of NH4+-N increased from 2 to 3%, and the sodium chloride tolerance for COD reduction improved from 2.5 to 3.8%. This significantly enhances the applicability of MSL in treating sodium chloride-containing wastewater.