Hydraulic behaviour of sand-biochar mixtures in water and wastewater treatment applications

Interest in using biochar in environmental engineering applications has substantially increased with strong evidence emerging for its applicability in water and wastewater treatment, the containment of hazardous gases, carbon dioxide sequestration and the filtration of microplastics. Understanding the hydraulic behaviour of sand-biochar mixtures is critical for designing biochar-based water and wastewater filter systems. We present an investigation on the impact of the biochar microstructure on the hydraulic conductivity of sand-biochar mixtures, focusing on where there are gaps in existing knowledge on how the biochar affects the hydraulic properties. A dual-porosity concept for sand-biochar mixtures is introduced and included in four existing constitutive models for the hydraulic conductivity of soils.We have experimentally investigated the hydraulic properties of sand-biochar mixtures for uniformly graded sand with three representative grain sizes, and one type of hardwood biochar in the UK market. Based on a series of constant-head permeability tests, the hydraulic conductivity of the mixtures with biochar content from 0 to 50% has been measured. We used randomly generated spherical 3D models representing the same particle distribution of sand biochar systems to characterise the biochar's intraparticle porosity and tortuosity. By employing the dual-porosity concept and effective porosity instead of the total porosity, the accuracy of the four models' predictions was considerably improved when compared with the experimental measurements. We show that by excluding biochar's intraparticle porosity, the hydraulic conductivity prediction accuracy can be significantly improved, thus providing a more accurate prediction of the hydraulic behaviour of the soil-biochar filters.