# Filter Media-Packed Bed Reactor Fortification with Biochar to Enhance Wastewater Quality  
[PERSON]  
1 Department of Civil & Environmental Engineering, Botswana International University of Science & Technology, Private Bag 16, Palapye, Botswana; [EMAIL_ADDRESS]
2 Division of Water and Environmental Engineering, KTH Royal Institute of Technology, 100 44 Stockholm, Sweden; [EMAIL_ADDRESS]  
[PERSON]  
inert fluviatile sand and gravel used for constructing STUs and soak-aways, are generally less capable of attenuating much of their concentrations. The nutrients, that is phosphorus (P) and nitrogen (N), nourish aquatic plants and thus fuel algae blooms. This often leads to eutrophication and hypoxia conditions that are detrimental to aquatic biota. In centralized treatment systems, high P removal is mostly accomplished by processes which employ mechanisms that convert soluble reactive P (PO4\({}^{3-}\)) to particulate form before filtration, and microbial assimilation [8]. DOC is most commonly removed by use of granulated activated carbon (GAC). The drawback of all these technologies is that they are costly (for example purchasing mixing tanks, chemicals, feeding systems, etc.) [9]. They also produce large amounts of sludge requiring special handling. For example, precipitating PO4\({}^{3-}\) in 1 m\({}^{3}\) of wastewater to calcium hydroxyapatite (Ca5(PO4)3 OH) using slaked lime (Ca(OH)2) requires about 0.3-0.6 kg of Ca(OH)2. This in turn produces about 4-10 L of sludge [10]. In general, these are unaffordable to most rural communities. A few supposedly feasible innovations are available for improving on-site wastewater treatment (OWT) systems. These include sequential batch reactors (SBR) or package treatment plants (PTP) [11] for substituting septic systems, multi-chamber tanks or anaerobic baffled reactors [12] for replacing the two-chamber septic tanks and hybrid constructed wetlands for post-secondary treatment. Being that advanced requires expert knowledge for their operation and routine maintenance [13]. Therefore, they may be unsuitable for some would-be users, especially single households. However, as efforts to acquire more efficient technologies continue to be made, an increasing interest seems to be shown in the use of so-called reactive filter (RF) media as retrofits for septic systems. The most common of these materials are the granulated mineral-based media with ionic surface composites, which can react with and chemically or physically bind some wastewater constituents [14]. For example, those used for P removal comprise ionic compounds of calcium (Ca), iron (Fe), aluminum (Al), or magnesium (Mg). Soluble reactive phosphorus (PO4) has a high affinity for these cations [15]. In principle, as wastewater infiltrates a granulated RF, the targeted wastewater constituent may react with or be exchanged with exposed ligands on surfaces of its particles and thus become part of either surface complexes or precipitates formed. Another important property of RF media responsible for their high sorption capacity is high porosity. This contributes to both high pore volume for holding more water and physical straining of larger diameter particles in wastewater. Numerous RF media were tested in field-scale studies for their capability of removing nutrients from wastewater [14]. Examples are lava sand [16], Polonite\({}^{\textregisteredregistered}\)[17], shell sand [18], Filtralite\({}^{\textregisteredregistered}\)[13], and slags [19]. Since bad color and odor are common in STEs, using RF media that are versatile at removing not only P but also DOC may prove to be promising. Biochar, a biomass-derived material, has been found to be versatile in various applications. Examples include carbon dioxide sequestration and reduction of greenhouse gas emission in climate change mitigation [20], soil amelioration, and contaminant immobilization in agriculture [21]. Learning from this, a number of studies tested its capacity in removing nutrients [22], metals [23], organic compounds [24], and pathogens [25] from wastewater.