Nitrogen and phosphorus removal by reactive filter as a tertiary treatment unit for isolated houses with high alkalinity groundwater source

Three slag reactive filters were fed with elevated alkalinity household secondary effluent containing bicarbonates, bicarbonates+soluble organic carbon, and bicarbonates+particulate matter, respectively, for 30 days. Water was sampled for nutrient and metal concentrations, alkalinity and pH. Slags were analyzed with scanning electron microscopy. The slags were exhausted by calcite precipitation throughout the entire filter. Despite calcite accumulation in the filter, no pressure build-up was observed. The saturation index and shape of calcite precipitates were influenced by the presence or absence of organic carbon. Major reactions leading to filter exhaustion were calcite formation (3.1–3.3 g), hydroxide release (1.7–2.4 g), and hydroxyapatite formation (0.82–0.94 g). The results showed effective removal of phosphate during the period tested since column outlet concentrations never exceeded 1 mg P-PO4/L. NH4+ removal of approximately 30 % was measured in the inlet zone of the filter fed with biodegradable organic carbon. Incomplete nitrification was observed in the inlet zone at pH between 8.75 and 9.5, while complete nitrification (formation of NO3−) was observed at pH between 8.5 and 8.75. Slag reactive filters are suitable for the tertiary treatment of high alkalinity domestic wastewater. The contribution of calcite to media exhaustion highlights the importance of alkalinity control processes coupled with reactive filters as a tertiary treatment.