# Shifting from Conventional to Organic Filter Media in Wastewater Biofiltration Treatment: A Review
## 2 Factors Affecting the Efficiency of Biofiltation  
Biofiltation is a biological treatment process that involves a series of complex cleaning mechanisms and operating conditions. The performance of biofiltation is largely influenced by various factors such as dissolved oxygen concentration, organic loading rate, hydraulic retention time, temperature, and filter media (Figure 1). Therefore, it is important to capture the influence of these factors in the development of efficient biofilters. Maintaining enough dissolved oxygen concentration is important in the establishment of biological activity to promote biodegradation in a biofilter. The amount of dissolved  
Figure 1: Factors affecting the efficiency of a biofiltration treatment system. oxygen largely affects the growth of the microbial community in the biofilm and the overall efficiency of the biological process. For example, aerobic microorganisms need sufficient dissolved oxygen from an electron acceptor to promote the growth of a biofilm and biodegradation of the pollutant [36]. [PERSON] et al. [37] observed that adequate oxygen of 2-3 mg/L promotes the growth of nitrifying biofilms and the removal of ammonia in a biological aerated filter. In contrast, a study by [PERSON] et al. [38] reported that a low dissolved oxygen concentration (<0.9 mg/L) leads to a lower nitrification efficiency but higher denitrification performance, thus promoting higher total nitrogen (TN) removal. This is because the denitrification process in the biofilter is carried out by heterotrophic denitrifies, where high organic carbon sources and low oxygen concentration favor the growth of this bacteria [39]. Increasing the organic loading rate (OLR) enhances the growth of a biofilm and water holding capacity. This can be explained by the stimulation of microorganisms to promote biological activity under sufficient flux of organic matter to the biofilm [40]. [PERSON] et al. [41] reported the biofilm formation, biomass concentration, and denitrification process all increase with the OLR. However, the nitrification efficiency decreases with an increase in the OLR from 1.0 to 4.0 kg COD/m\({}^{3}\) per day. [PERSON] et al. [42] also reported that the treated wastewater effluent meets almost all the discharge regulations under low OLR while increasing the OLR causes foaming issues, which results in an unstable removal performance. As a result of extremely high organic loads that restrict the movement of the substrate into the interior of the biofilm due to the formation of a dense biofilm, the microbial community structure in the biofilm is disrupted [40; 43]. Moreover, high dissolved organic matter tends to compete for the adsorption sites with the bacteria in the biofilm, therefore reducing the overall number of available adsorption sites [44]. Hydraulic retention time (HRT) is also one of the important operation factors in preserving the long-term performance of the biofilter. An optimum hydraulic retention time affects the efficiency of the biofilter in terms of cost, as it is directly related to the capacity of the substrate that can be handled per unit time and to the effective contact between the substrate and the microorganism [45; 46]. According to [PERSON] et al. [47], a fast-growing heterotroph tends to grow in the suspension, so the formation of a biofilm by slow-growing nitrifiers would occur under a comparatively longer HRT. A longer HRT minimizes the competition of dissolved oxygen between the heterotroph and nitrifiers; however, a longer HRT shows disadvantages such as longer treatment time and cost consumption in maintaining the long HRT operational conditions [48]. Besides that, temperature plays a crucial role in controlling and regulating the performance of the biofilter by affecting the growth of the microorganisms in the biofilm. A lower temperature tends to reduce the overall biological process due to lower microbial metabolism and nutrient utilization [49]. [PERSON] et al.