Effect of structural difference on wastewater treatment efficiency in multi-soil-layering systems: Relationship between soil mixture block size and removal efficiency of selected contaminants

Multi-soil-layering systems composed of soil mixture blocks (SMB) arranged in a brick-like pattern and surrounded by permeable zeolite layers (PL) were designed for domestic wastewater and polluted river water treatment. To investigate the efficiency of MSL systems in relation to structural differences, five MSL systems with different sizes and layers of SMB and PL were constructed in five 50 cm x 10 cm x 60 cm acrylic boxes. The SMB comprised sandy soil, charcoal, sawdust and iron on a dry weight ratio of 7:1:1:1. Diluted domestic wastewater with an average suspended solids (SS), biological oxygen demand (BOD5), total phosphorus (T-P) and total nitrogen (T-N) concentration of 73, 34, 1.3 and 15 mg L-1, respectively, was applied at a hydraulic loading rate (HLR) ranging from 1,000 to 3,000 L m(-2) day(-1). At HLR of 1,000 and 2,000 L m(-2) day(-1), systems with larger SMB surface area had higher removal rates for SS, BOD5, chemical oxygen demand (COD) and T-P because of the enhanced contact efficiency between wastewater and SMB. The structural difference was greatest at a HLR of 2,000 L m(-2) day(-1), especially for COD and T-P removal. An increase in the top surface area of SMB was found to affect the efficiency of MSL systems more than an increase in side surface area. When the HLR was increased to 3,000 L m(-2) day(-1), the effect of structural differences on MSL systems was not as great, probably because of preferential water flow into PL as well as the oxidation-reduction potential ORP change caused by the accumulation of SS at high HLR. For T-N removal, no structural difference was found at a low HLR of 1,000 L m(-2) day(-1) and when HLR increased to more than 2,000 L m(-2) day(-1), a secondary effect of SS accumulation and ORP change appeared to influence the systems' performance more than the expansion of SMB surface area.