| Summary: | Contamination of water sources by inappropriately disposed poorly treated wastewater from countryside establishments is a worldwide challenge. This study tested the effectiveness of retrofitting sand (Sa)- and gas−concrete (GC)-packed reactors with biochar (C) in removing turbidity, dissolved organic carbon (DOC), phosphate (PO<sub>4</sub><sup>3−</sup>), and total phosphorus (P<sub>tot</sub>) from wastewater. The down-flow reactors were each intermittently loaded with 0.063 L/d for 399 days. In general, all reactors achieved <3 NTU (Nephelometric Turbidity Units) effluent turbidity (99% efficiency). GC reactors dominated in inlet PO<sub>4</sub><sup>3−</sup> (6.1 mg/L) and DOC (25.3 mg/L) reduction, trapping >95% and >60%, respectively. Compared to Sa (PO<sub>4</sub><sup>3−</sup>: 35%, DOC: 52%), the fortified sand (SaC) filter attenuated more PO<sub>4</sub><sup>3−</sup> (>42%) and DOC (>58%). Student t-tests revealed that C significantly improved the Sa PO<sub>4</sub><sup>3−</sup> (<i>p</i> = 0.022) and DOC (<i>p</i> = 0.034) removal efficacy. From regression analysis, 53%, 81%, and 85% PO<sub>4</sub><sup>3−</sup> sorption variation in Sa, C, and SaC, respectively, were explained by variation in their effluent pH measures. Similarly, a strong linear correlation occurred between PO<sub>4</sub><sup>3−</sup> sorption efficiency and pH of fortified (r > 0.7) and reference (r = 0.6) GC filters thus suggesting chemisorption mechanisms. Therefore, whereby only sand may be available for treating septic tank effluents, fortifying it with biochar may be a possible measure to improve its efficacy.
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