# Shifting from Conventional to Organic Filter Media in Wastewater Biofiltration Treatment: A Review
## 1 Introduction  
Lately, the presence of excess nitrogen in wastewater receives attention as it promotes the advancement of eutrophication and other hazardous effects on bodies of water [1]. Likewise, the continually diminishing freshwater resources worldwide coupled with depleting accessibility to clean water has inflicted water scarcity issues in both developed and developing countries [2, 3]. To prevent further supply deterioration of this necessity, various techniques to remedy wastewater have been introduced either via physicochemical or biological treatments. Conventional physicochemical treatments include aeration, chemical oxidation, coagulation-flocculation, filtration, and ion exchange while biological methods are comprised of activated sludge with various modifications [4, 5, 6], aerobic digestion, lagoons, and biofiltration. Despite their wide variety, these physical-chemical treatments have many drawbacks, such as chemical consumption, higher sludge production, higher investment, and capital cost, as well as a high energy requirement [7]. Therefore, studies that investigate the use of biological processes in water and wastewater treatment are conducted due to their advantages such as energy-saving, operation flexibility, lower cost, environmentally friendly, and higher retention capacity [7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17].  
Amongst the existing biological treatments, the process using biofiltation has been broadly favored by various researchers in the past few years due to its promising potential. This system is normally used as a tertiary treatment process in the removal of nutrients, toxins, and recalcitrant compounds [18]. Biofilm, which is an attached growth system, is different from the suspended treatment process as there is a clear separation between the treated effluent and the microbial biomass present in the biofilm. The microbial biomass present in the biofilter is immobilized to the filter bed while the effluent flows through the filter media, the event of which creates a separation between the microbial biomass and the effluent [19]. Biofilters are widely applied in the treatment of nitrogenous and organic pollutants in municipal wastewater treatment plants due to their great efficiency in handling various kinds of water, for instance, oil and gas produced water [20], river water [21], raw sewage [22], groundwater [23], and domestic wastewater [24,25]. Studies have also been focused on operating biofilters under different conditions such as filter media, temperatures, backwash regimes, and dissolved oxygen concentrations to achieve remarkable pollutant removal performance [26, 27, 28]. Even so, the cleaning mechanism of the biofilter is somewhat complex, thereby requiring further investigation before an optimally advanced biofiltation system can be developed.  
Other than that, there exists an increasing interest in exploring suitable alternative filter materials such as wood chips, wheat straws, and natural fibers [29, 30, 31] to replace conventional filter media such as sand, gravel, and anthracite in order to reduce the operational and investment costs of the biofiltation treatment system. This is further motivated by the local availability of these proposed organic materials and their similar physicochemical properties compared to the conventional synthetic media such as polystyrene.  
Although there are review papers [32, 33, 34, 35] discussing some aspects of biofilters, a comprehensive review on the challenges and prospects from the perspective of shifting the use of conventional filter media to that of organic is missing. Therefore, this review paper fills the intellectual gap by addressing the factors affecting the biofiltation process, the potential use of organic material such as from those of agro-waste as the filtration media, and the prospects of wastewater treatment by biofilters.