## 5 Approaches to Improve Biofilm-Based Wastewater Treatment
### Modification of Carriers to Promote Biofilm Adhesion  
Methods to enhance the biofilm's attachment and stability have gained in popularity, to the extent that there has been a rise in studies investigating good carriers that support biofilm growth. The type of carrier used has been said to play a major role in the bioreactor's performance, especially in MBBR systems. The common materials used for biocarriers include high-density polyethylene, polypropylene, and polyethylene/polymeric-reticulated sponge or foam. The most commonly used biocarriers in MBBRs are small cylindrical-shaped polystyrene carriers, LEVAPOR carriers, polyethylene cylinders, and polymeric carriers (polyhydroxybutyrate/polycaprolactone) (Table 2). These biocarriers can be found in different sizes, with densities ranging 0.94-0.98 g/cm\({}^{3}\)[59]. Some researchers have been investing time in investigating carriers that will support different biofilm thicknesses. The type of substrate used has also been reported to play an important role in the attachment of bacteria to the surface. The perfect carrier must cause turbulence to transport dissolved oxygen and nutrients to the bacteria that are attached. Additionally, a good carrier is said to possess a positive surface charge, since the bacteria have a negative surface charge (Figure 4). The use of inappropriate carriers may disturb the microbial growth, resulting in unsteady bioremediation [60]. Hence, the effect of the surface chemistry for MBBR carriers in biofilm formation has been intensely investigated. There has been an increase in studies investigating ways to promote the attachment of bacteria to the substratum. The modification of carriers or carrier materials is a popular method of improving the biofilm attachment, as well as maintaining a great thickness of the biofilm. Some of the methods used for surface modification include pre-seeded biofilms, film coatings, and physical or chemical changes to the surfaces. Different carriers such as granular-activated carbon, polyurethane materials, polyethylene plastics, polyurethane foam, polymeric gels, and polyurethane sponges have been shown to enhance the biofilm attachment, e.g. polyurethane sponge carriers have been reported to enhance the immobilization as a result of their high porosity [61]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Reactor** & **Contaminant** & **Carrier Types** & \begin{tabular}{c} **Removal** \\ **Efficiency** \\ \end{tabular} & **Influent** & **Effluent** & **Ref** \\ \hline hybrid membrane & Total nitrogen & nylon biomass & 96.61\% for & & & \\ aerated biofilm & and ammonium & carrier & NH\({}_{4}\)\({}^{*}\)-N & - & 2 mg/L for & \\ reactor & & 72.23\% for TN & & & NH\({}_{4}\)\({}^{*}\)+N, & [PERSON] et al. [53] \\ \hline packed bed & Total nitrogen & K1 carriers & 84.8 \(\pm\) 4.3\% & \begin{tabular}{c} 207.6 \(\pm\) 29.9 \\ mg N/L \\ \end{tabular} & \begin{tabular}{c} 876.8 mg \\ N/(L-d) \\ \end{tabular} & [PERSON] et al. [54] \\ \hline aerobic fluidized bed & Total nitrogen & modified & \begin{tabular}{c} TN, \\ polyurethane \\ sponge \\ \end{tabular} & \begin{tabular}{c} \\ 66.98 \(\pm\) 4.23\% \\ NH\({}_{4}\)\({}^{*}\)-N, \\ 74.70 \(\pm\) 2.30\% \\ \end{tabular} & & [PERSON] et al. [55] \\ \hline sequencing & \begin{tabular}{c} TN removal \\ batch biofilm \\ reactor \\ \end{tabular} & Resin fillers & 93\% & 68 mg/L & - & [PERSON] et al. [56] \\ \hline moving bed & Total nitrogen & noxKaldness-K3 & 96.2\% & 64.3 to 68 & & Almomani and \\  & & & mg-N/L & & [PERSON]. [57] \\ \hline sequencing & \begin{tabular}{c} Total nitrogen \\ batch \\ \end{tabular} & \begin{tabular}{c} polypropylene \\ and ammonium \\ \end{tabular} & \begin{tabular}{c} NH\({}_{4}\)\({}^{*}\)-N (95\%) \\ hollow balls \\ \end{tabular} & \
\begin{tabular}{c} 50 mg/L \\ NH\({}_{4}\)\({}^{*}\)-N (88\%) \\ \end{tabular} &
\begin{tabular}{c} \\ 6 mg/L for \\ NH\({}_{4}\)\({}^{*}\)-N \\ \end{tabular} & [PERSON] et al. [58] \\ \hline \hline \end{tabular}
\end{table}