Long-term phosphorus sorption and leaching in sand filters for onsite treatment systems

• Published in: The Science of the total environment 2022-08, Vol.833, p.155254-155254, Article 155254
• Main Authors: Hamisi, Rajabu, Renman, Agnieszka, Renman, Gunno, Wörman, Anders, Thunvik, Roger
• Format: Article
• Language: English
• Subjects: Adsorption; Constructed wetlands; Leaching; Phosphorus; Sand column
• ISSN: 0048-9697; 1879-1026; 1879-1026
• DOI: 10.1016/j.scitotenv.2022.155254

The sorption capacities of sand filters used for onsite wastewater treatment and their associated risks of phosphorus (P) leaching on contact with rainwater were investigated in column experiments and with modelling tool for over 300 days. Columns packed with sand were exposed to real domestic wastewater of different characteristics and hydraulic loading modes. The wastewater fed into the columns was effluent collected from three different treatment units in the field: a septic tank (ST), biofiltration tank (BF) and Polonite® filter bag (PO). The risk of P leaching to groundwater and surface water was also assessed, by exposing the same sand columns to natural rainwater. Overall results indicated that sand soils can exhibit different adsorption and desorption capacities for electrical conductivity (EC), Total-P, phosphate-P and total suspended solids, depending on the characteristics of influent wastewater, loading rate and total operation time. The removal efficiencies of the sand columns increased in the order ST (98.16%) > PO (93.36%) > BF (81.57%) for PO4-P and slightly decreased ST (97.11%) > PO (92.06%) > BF (76.76%) for Total-P columns. All sand columns loaded with actual wastewater solutions from septic tanks and biofiltration tank have demonstrated high risks of phosphorus leaching (>99.99%) to the groundwater. The modelling was successful captured behavior of EC tracer and adsorption of PO4-P with acceptable prediction uncertainty in the PO < 8% columns. The modelling results indicated that the decrease of loading rate from 83.3 mL d-1 to 20.83 mL d-1 led to an average increase of removal efficiency and prolong operational lifetime and mass of adsorbed Total-P in the sand soil. This study concludes that sand is a valuable filter medium at low loading rate for phosphorus removal in full-scale operations of onsite treatment systems, however very vulnerable for leaching P when in contact with rainwater. [Display omitted] •Sorption capacities of sand filter were determined in the column experiments using the real domestic wastewater of various quality parameters.•Phosphorus adsorption increased with the bioavailability of biofilm and microbial growth.•Well-sorted sand achieved higher efficiency of P adsorption than unsorted sand.•Binding affinity of sand is not strong enough to withstand the influence of rainfall and snow melting on the P leaching.•Leaching did not control the pH of the effluent leachate.