Removal of Escherichia coli in sand columns supplemented with hydrochar derived from sewage sludge

The need to provide safe drinking water to people living in less developed communities without access to treated piped water supply has promoted the use of sand filtration units in households in such areas. Hydrothermal carbonization (HTC) technology provides a simple, low cost, energy efficient and environmental friendly means to convert various types of waste biomass such as sewage sludge into a carbon rich by-product referred to as hydrochar, which can serve as low-cost adsorbent. The major objectives of this research were to: (1) assess the removal efficiency of Escherichia coli using sewage sludge hydrochar in sand columns (2) assess the effect of chemical activation with potassium hydroxide (KOH) and also, the effect of hydrochar surface charge modification with Polydiallyldimethylammonium chloride (p-DADMAC) on E. coli removal efficiency and (3) assess the potential for field application of activated sewage sludge hydrochar using large columns and the effect of idle time on E. coliremoval efficiency. This study evaluated Escherichia coli (E. coli) removal efficiency of two sewage sludge hydrochar type with 10 cm sand columns supplemented with 1.5% (w/w) hydrochar, followed by a 30 days E. coli removal assessment with larger (50 cm) columns. Chloride tracer breakthrough tests indicate that the hydraulic flow in the hydrochar sand columns was similar to plug flow with some effect of dispersion. The highest E. coli removal efficiency recorded with small (10 cm) columns was 90.3% (1.01 log10) for the activated hydrochar column compared to 99.7% (2.35 log10) achieved with the large columns. The mean total E. coli log removal for the 30 days E. coli flushing for the sand, raw, and KOH activated large columns were 0.20 log10, 0.10 log10 and 1.26 log10 respectively. While KOH activation increased the adsorption capacity of the raw hydrochar, coating with p-DADMAC did not increase E. coli attachment to the activated hydrochar as it was not substantially retained in the columns after demineralized (DI) flushing. The effect of idle time on E. coli removal efficiency was significant in the sand column (P 50% of the total removal efficiency, but not in the raw (P = 0.750) and KOH (P = 0.865) hydrochar supplemented columns. Biofilm formation/filter maturation in hydrochar sand columns, the direct synthesis of meso-macroporous hydrochar particles without chemical activation, and simultaneous removal of microbial pathogens and heavy metals such as arsenic and chromium are areas of further research