## 3 Role of Biofilms in the Biological Wastewater Treatment System
The formation of the biofilm is the result of biotic and abiotic factors that include (1) the diversity of the microbial community, (2) the physical properties of the surface carrier, which are linked to the interaction between electrostatic and surface energies in the microbial adhesion to the carrier surface, (3) the roughness of the surface, (4) the chemical properties of the surface carrier, and (5) factors such as the temperature and pH. The most crucial factors in the  
Figure 2: Formation and maturation of a biofilm from the planktonic state. formation of biofilms have been said to be the physical and chemical properties of the carrier surface. The use of inorganic materials as carriers has been shown to cause large resistance, poor permeability, and low formation of biofilms as compared to carriers that are modified, e.g., modified zeolites or light porous ceramic carriers. Organic-based carriers are better options for biofilm formation. Moreover, the carriers that are efficient in promoting biofilm formation are those that can also promote microbial growth by serving as electron donors. The hydrophilicity and electronegativity of the carrier have also been noted to play a crucial role in the formation of biofilms [32]. Factors Hindering the Application of Biological Processes and the Role of Biofilms in Mitigating Them  
The ammonium-oxidizing bacteria (AOB) are sensitive, and they have been reported to be prone to inhibition by various compounds present in wastewater, thereby inhibiting nitrification [33]. High concentrations of ammonium and phenolic compounds have been known to have an inhibitory effect on SND. These compounds are normally found in wastewater effluents from many industrial processes, such as petrochemical manufacturing, coke production, landfill leachate, and petroleum refinement [34]. [PERSON] et al. [35] reported the inhibition of nitrification using 200 mg/L of thiocyanate, 0.2 mg/L of free cyanide, 200 mg/L of phenol, and 100 mg/L of p-cresol. There can also be selective inhibition, such as the inhibition caused by free-ammonia or toxic compounds. This type of inhibition only inhibits nitrite-oxidizing bacteria (NOB) but has a minimal inhibition activity towards AOB. Additionally, saline water also has a high inhibitory effect on nitrite-oxidizing bacteria (NOB); it can cause plasmolysis and the loss of activity of NOB [36]. The presence of free ammonia and free nitrous acid can also inhibit SND. The first step of SND is known to be sensitive to high concentrations of free nitrous acid (FNA), while the second step is sensitive to high free ammonia (FA) concentrations. It has been reported that Nitroster species are inhibited by 0.1-1.0 mg/L of ammonia, while ammonia-oxidizing species are inhibited by 10-150 mg/L of ammonia [37; 38]. [PERSON] et al. [39] reported that NOB decreased by 29.2% while AOB decreased by 15.9% in the presence of FA. Additionally, [PERSON] et al. [40] also reported the inhibition of NOB by 36.06-50.66 mg/L of free ammonia. These results are comparable to the results obtained by [PERSON] et al. [41], who reported the inhibition of NOB by high FA concentrations. However, [PERSON] and [PERSON] [42] claimed that 40 mg/L of free ammonia did not inhibit the nitrite oxidation when a pre-acclimatized biomass was used, while 3.5 mg/L of free ammonia completely inhibited nitrite oxidation when a non-acclimatized biomass was used. SND is beneficial over traditional nitrification and subsequent aerobic denitrification because it addresses some of the drawbacks of the traditional nitrification subsequent aerobic denitrification process, such as the treatment of low-C/N wastewater. Municipal wastewater tends to have a low carbon-to-nitrogen ratio (C/N), which may necessitate the addition of a carbon source to provide electron donors for denitrification thus, preventing a declines in the nitrogen removal efficiency [17]. Some heavy metals are important in trace amounts; however, they can be toxic when they are present in large concentrations.