## 5 Approaches to Improve Biofilm-Based Wastewater Treatment
### Biofilm Reactors  
Different bioreactors have been used to promote biofilm-based wastewater treatment (Figure 3). These biofilm-based systems are especially important for processes that require different conditions, such as aerobic and anaerobic conditions. Moreover, the biofilm systems are important to prevent the washout of the bacteria used in the biological wastewater treatment. The washout is normally caused by the slow growth of microorganisms responsible for biological processes. Nitrifying bacteria are typical examples of microorganisms that are prone to washout. The biofilm systems prevent washout by maintaining the density of the bacteria in continuous systems [48]. The biofilms can provide aerobic and anaerobic conditions for different microorganisms in the bioreactor. In general, biofilm reactors are designed to address the issue of low biomass concentrations and long hydraulic retention times experienced during the sludge process. The microbes in biofilms tend to compete for substrates such as ammonia, organic carbon, and oxygen. Those microbes that consume organic carbon and oxygen, such as nitrifying bacteria, are normally found on the outer layers where the concentration of oxygen is high, while those bacteria that require low concentrations of oxygen, such as denitrifying bacteria and DPAO, exist in the inner layer of the biofilm. Hence, biofilm reactors are common for SND in WWTPs. Membrane-aerated biofilm reactors (MABRs) are well-studied biofilm reactors for wastewater treatment. In MABRs, the biofilm aggregates on the outer side of the membrane while it is aerated through the inner side of the membrane [49]. The operation of an MABR is flexible, especially when it comes to the diffusion of a gas or liquid through the membrane lumen. The ability to provide exogenous electron acceptors or donors through the membrane lumen makes such reactors suitable for treating difficult wastewater. Additionally, this type of reactor is suitable for the treatment of low carbon-to-nitrogen ratio (C/N) wastewater. This is done by supplying the methanol or hydrogen into the reactor through the membrane lumen, which further stimulates the biological wastewater treatment [50]. Other biofilm reactors such as packed bed and fluidized bed biofilm reactors have been investigated for the treatment of wastewater. Fluidized bed reactors have been shown to have high rates and low pressure drops as opposed to packed bed biofilm reactors. Moreover, the fluidized bed reactors have been reported to have no bed clogging and lower external transport resistance. These two reactors have been specially investigated for their applications in SND. These reactors support the existence of the aerobic and anoxic conditions that are required for SND. However, it has been noted that the existence of different microorganisms that compete for substrates necessitates the proper control of the ratios of organic carbon to ammonia in this type of bioreactor [48]. Other reactors that have been studied for SND include sequencing batch reactors (SBRs), moving bed biofilm reactors (MBBRs), sequencing batch biofilm reactors (SBBRs), moving biofilm reactors, and oxidation ditch reactors. SBBRs are highly recommended for SND over all other reactors mentioned, especially for the treatment of high-strength industrial wastewater. Furthermore, the SND-SBBR approach is ideal, since it saves on construction and energy costs, has low oxygen demands, and does not require expensive fillers and dosages. The reduced requirement for oxygen in SBBRs could potentially reduce the concentration of NO\({}_{\text{x}}\)\({}^{-}\) (NO\({}_{\text{2}}\)\({}^{-}\) and NO\({}_{\text{3}}\)\({}^{-}\)), thereby reducing the carbon required for denitifying bacteria. This could result in extra carbon sources being accessible for denitriving bacteria. This is why SND has been said to reduce 40% of the carbon needed for denitrification and to increase the rate of denitrification by up to 63% [51]. MBBRs have also been reported to be a suitable reactor for SND.