Effect of Aeration and Material Composition in Soil Mixture Block (SMB) on the Removal of Colored Substances and COD in Livestock Wastewater by Multi-soil-layering (MSL) Systems

To investigate the efficiency of multi-soil-layering (MSL) systems on the removal of colored substances and chemical oxygen demand (COD) from livestock wastewater, four MSL systems with different soil mixture block (SMB) compositions were constructed in four 50 cm × 10 cm × 68 cm acrylic boxes. Livestock wastewater (diluted 10-fold) with an absorbance of 0.9215 at a wavelength of 406 nm and a COD concentration of approximately 3,000 mg L−1 was applied to the systems at a hydraulic loading rate (HLR) of 250 L m−2 day−1. Aeration pipes were set in the water permeable layers (PL) in MSL 1–3 and in the SMB layers in MSL 4. The results showed that MSL systems could keep mean decolorization rates of 60.7–67.1% and COD removal rates of 48.8–58.0% for 6 weeks of operation. The different aeration pipe positions did not have any significant influence on the removal efficiency of the systems. However, an increase in aeration intensity from 1,000 to 2,000 L min−1 per system (27.4–54.8 L min−1 L−1) increased decolorization rates by 3.0–12.1%. For COD removal, both an increase in aeration intensity and temperature enhanced the removal rates by 23.0–43.3%. The addition of sawdust and iron into the SMB of MSL 1 improved the system's decolorization rate by 9.1% and COD removal rate by 12.0% compared with MSL 2 during the fifth and sixth months of operation. Interruption of MSL systems for 1 month could recover the decolorization and COD removal rates to over 50% and 80%, respectively.