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

Authors: TABATABAEEFAR, AMIN - Polytechnique Montreal; Penn, Chad; CLAUVEAU-MALLET, DOMINIQUE - Polytechnique Montreal

Submitted to: Journal of Water Process Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/24/2024
Publication Date: 4/30/2024
Citation: Tabatabaeefar, A., Penn, C.J., Clauveau-Mallet, D. 2024. Nitrogen and phosphorus removal by reactive filter as a tertiary treatment unit for isolated houses with high alkalinity groundwater source. Journal of Water Process Engineering. https://doi.org/10.1016/j.jwpe.2024.105319.
DOI: https://doi.org/10.1016/j.jwpe.2024.105319

Interpretive Summary: Excess phosphorus (P) in surface waters leads to degradation of ecological systems and increases expense for drinking water treatment. One of the major sources of P to surface waters is on-site domestic wastewater systems. Traditional systems use soil to filter secondary household effluent (i.e. leach bed), but increasingly strict regulations demand improvements in such systems. Steel slag has been shown to be effective at filtering P from effluent, but problems can occur due to clogging. The objective of this study was to investigate potential clogging and P and nitrogen (N) removal in slag effluent filters. Effluents were created to simulate three main aspects of effluent: chemical, biological, and physical, as it is possible for clogging to occur from any of the three avenues. Filter columns were constantly fed simulated effluent for 30 days and monitored for changes in flow rate (indicating potential clogging) and removal of N and P. No clogging occurred and P concentrations were maintained below the regulatory threshold of 1 mg/L. Effluent P removal occurred over a front as the pH buffering capacity of the slag was slowly neutralized. 30% of N was removed, likely by biological processes. The results suggest that steel slag may be effectively used in tertiary filters for wastewater, thereby reducing nutrient pollution.

Technical Abstract: 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 was 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.