# Filter Media-Packed Bed Reactor Fortification with Biochar to Enhance Wastewater Quality
## 4 Filter Efficacy at P Removal  
The statistics showed that the mean influent PO4\({}^{3-}\) concentration (6.1 mg/L) was within a 95% confidence interval (CI) of 5.6-6.9 mg/L while those for the reference reactors, that is, Sa (3.9 mg/L), C (3.5 mg/L), and GC (0.2 mg/L), were within 3.6-4.3 mg/L, 3.1-3.8 mg/L, and 0.1-0.3 mg/L, respectively (Table 2). On the other hand, the mean PO4\({}^{3-}\) in the effluents of the fortified filters, that is, SaC (3.3 mg/L) and GCC (0.2 mg/L), were within CIs of 2.9-3.6 mg/L and 0.2-0.3 mg/L. As for P\({}_{\text{tot}}\), the influent and effluents of the reference Sa, C, and GC filters had mean concentrations of 10.0 mg/L and 7.6 mg/L, and 6.4 mg/L and 0.3 mg/L, respectively. Thus, it is clear from these findings that all reactors with GC were able to lower the influent P to <0.5 mg/L, thus satisfying the Swedish Environmental Protection Agency limit of <1 mg/L [7]. For the fortified filters, both Table 2 and Figures 5 and 6 show that while the GC filter seemed to be equally effective as its fortified counterpart, the SaC filter was a bit more effective than the reference Sa filter. Therefore, basing on this finding, it could be said that the C boosted the Sa in removing the PO4\({}^{3-}\) from the wastewater. In addition to statistical analysis of means and confidence intervals of the P in the influent and effluents, efficiencies in P reduction were also obtained (data not provided). By the end of the experiment, 25.14 L of wastewater had been treated through the different reactors. If we assume that each reactor received the same amount of PO4\({}^{3-}\) (6.1 mg/L/month), then the total amount of PO4\({}^{3-}\) loaded on each was about 152.7 mg. The analysis showed that Sa, GC, C, SaC, and GCC sorbed about 52.6, 147.1, 64.6, 69.9 and 147.2 mg of that amount, respectively. These corresponded to efficiencies of 35%, 96%, 42%, 45%, and 96%, respectively. This confirmed the conclusion that the GC filters were outstanding in P removal compared to those reactors with Sa and C.  
Several factors are believed to have contributed to the observed variations in efficiencies of the different filters. For instance, the varying particle distributions led to differences in porosity, HRT, and specific surface areas. [PERSON] and [PERSON] [28] observed that a 2-4-mm particle size was sufficient for GC to achieve up to 100% P removal efficiency within just 1 day. The GC they used achieved about 93-99% P removal from wastewater. Findings in this current study (96% PO4\({}^{3-}\) removal) seemed to agree with theirs. [PERSON] et al. [47] also found out that for biochar to achieve up to 100% efficiency, it should be graded to particle size of \(\leq\)2 mm and also allowed to be in contact with wastewater for 72 h. As the biochar in this study had a lot of course particles and shorter HRT, it was possible for this to have an effect on its performance. In addition to its negative charge, which would possibly repel inbound negatively charged substances such as PO4\({}^{3-}\), the hydrophobicity of the biochar possibly slowed part of the influent in penetrating into its particles. Further, since no insulation was provided, the low temperature condition in winter probably slowed down both chemical and biological processes by which the P was removed from the wastewater [43]. From the comparison of the reference and fortified filters, it was observed that while the GC was equally effective (96%) as its fortified pair (GCC), the SaC filter seemed to be more effective (43%) than the reference Sa filter (35%) in sorbing P. However, as was mentioned earlier, the efficiency of the reactors with Sa and C tended to reduce towards the end (Figures 5 and 6). This could be attributed to the effect of reduced effective filtration depth as well as limited intra-particular diffusion. On the other hand, the constantly high efficiency of the GC filters showed that there were still plenty of adsorbate-free sites and pores below the P mass transfer zone in them.