Biochar and activated carbon filters for greywater treatment : comparison of organic matter and nutrients removal

Abstract: Activated carbon has long been used for the treatment of water. The industrial activation processes increase the environmental footprint and cost of this product. Biochar, which is organic material pyrolyzed/charred, often by means of simple and low cost techniques, might be an interesting alternative to replace the industrial activated carbon for greywater cleaning. The objective of this study is to evaluate and compare biochar and activated carbon by assessing reductions of COD, MBAS, Tot-N and Tot-P as well as the transformation of nitrogen in the greywater-infiltrated filters. The activated carbon and biochar were packed to a depth of 50 cm into columns with a diameter of 4.3 cm. The columns were fed with artificial greywater at a hydraulic loading rate of 0.043 m3/m2/day under vertical flow. The bulk density, particle density, total porosity and residence time of the filters were initially measured. Then their water cleaning capacity was assessed over a period of ten weeks based on nine chemical parameters (pH, EC, NH4-N, NO3-N, Tot-N, PO4-P, Tot-P, MBAS and COD). The residence time in the activated carbon filters was 119 hours and in the biochar filters 108 hours. Both materials cleaned the greywater well from organic matter, with 99% efficiency for COD and MBAS. It is remarkable that biochar removed Tot-P and PO4 –P more effectively than activated carbon, on average to 89% in case of Tot-P and 86% in case of PO4–P. The efficiency of biochar in Tot-N and NH4-N was not stable, whereas activated carbon had stable efficiency levels of 97% and 98% for Tot-N and NH4–N. Further investigations are recommended in regards to how long the cleaning capacity of biochar lasts and how the performance of the filter changes under an increased load of greywater. The performance of biochar from different parent materials and recycling options for the used filter materials are also interesting aspects for future research.