Phosphorus and Nitrogen Removalin Modified Biochar Filters

Abstract: Onsite wastewater treatment systems in Sweden are getting old and many of them lack sufficient phosphorus, nitrogen and organic carbon reduction. Biochar is a material that has been suggested as an alternative to the common sand or soil used in onsite wastewater treatment systems. The objective of this study was to compare the phosphorus removal capacity between three different modified biochars and one untreated biochar in a batch adsorption and column filter experiment. The modifications included impregnation of ferric chloride (FeCl3), calcium oxide (CaO) and untreated biochar mixed with the commercial phosphorus removal product Polonite. To further study nitrogen removal a filter with one vertical unsaturated section followed by one saturated horizontal flow section was installed. The batch adsorption experiment showed that CaO impregnated biochar had the highest phosphorus adsorption, i.e. of 0.30 ± 0.03 mg/g in a 3.3 mg/L phosphorus solution. However, the maximum adsorption capacity was calculated to be higher for the FeCl3 impregnated biochar (3.21 ± 0.01 mg/g) than the other biochar types. The pseudo 2nd order kinetic model proved better fit than the pseudo 1st order model for all biochars which suggest that chemical adsorption was important. Phosphorus adsorption to the untreated and FeCl3 impregnated biochar fitted the Langmuir adsorption isotherm model best. This indicates that the adsorption can be modeled as a homogenous monolayer process. The CaO impregnated and Polonite mixed biochars fitted the Freundlich adsorption model best which is an indicative of heterogenic adsorption. CaO and FeCl3 impregnated biochars had the highest total phosphorus (Tot-P) reduction of 90 ± 8 % and 92 ± 4 % respectively. The Polonite mixed biochar had a Tot-P reduction of 65 ± 14 % and the untreated biochar had a reduction of 43 ± 24 %. However, the effluent of the CaO impregnated biochar filter acquired a red-brown tint and a precipitation that might be an indication of incomplete impregnation of the biochar. The FeCl3 effluent had a very low pH. This can be a problem if the material is to be used in full-scale treatment system together with biological treatment for nitrogen that require a higher pH. The nitrogen removal filter showed a total nitrogen removal of 62 ± 16 % which is high compared to conventional onsite wastewater treatment systems. Batch adsorption and filter experiment confirms impregnated biochar as a promising replacement or addition to onsite wastewater treatment systems for phosphorus removal. However the removal of organic carbon (as chemical oxygen demand COD) in the filters was lower than expected and further investigation of organic carbon removal needs to be studied to see if these four biochars are suitable in real onsite wastewater treatment systems.