## 3 Role of Biofilms in the Biological Wastewater Treatment System  
The formation of a biofilm is a complex process that is influenced by the conditions of the surroundings, such as the properties of the surface and the deposition of the organic materials in the surface carrier. Before the microorganism attaches to the surface for biofilm formation, the surface will initially be acclimatized to adsorb organic molecules present in the surrounding environment. This step is followed by the attachment, adhesion, retention, and proliferation of the microorganisms to the surface (Figure 2). The motile bacteria, which have flagella and pili, are responsible for initiating biofilm adhesion to the carrier material. The flagella is essential in the movement of bacteria from the planktonic state to the surface. Type IV pili are filamentous protein complexes that have been known to be responsible for the initial adhesion of the cells to the surface. They have also been said to be vital for the maturation of biofilms [6]. The chemical properties of the surface affect the absorption and distribution of the surrounding organic material into the carrier surface. Moreover, the type of organic materials absorbed by the surface may also change the properties of the surface carrier. Biofilms usually grow in a consortium instead of a monoculture. During the biofilm formation, the microorganisms of the same family communicate by transmitting chemical compounds that signal to the planktonic species to migrate to the biofilm state. This microbial communication is known as quorum sensing [28]. As microorganisms form biofilms, they produce EPSs, which act as a protective layer for the biomass and helps themto attach to the surfaces. Biofilms are generally resistant to killing by external contaminants, as opposed to planktonic bacterial cells. The resistance of a biofilm towards external stress is associated with several factors, such as the changes in gene expression in response to biofilm growth and the external stresses and intrinsic properties of the biofilm's structures [4]. Biofilms in wastewater treatment are often exposed to inhibitory chemicals. It has been hypothesized that the exposure of a biofilm to inhibitory chemicals may induce biofilm formation. This hypothesis has been tested in various studies, where _Pseudomonas aeruginosa_ and _Escherichia coli_ (_E coli_) were found to be induced by aminoglycoside antibiotics and a _Mycobacterium avium_ biofilm was induced by hydrogen peroxide [29]. The products of this phenomenon were named stress-inducible biofilms. Biofilms play a significant role in the removal of chemical and microbiological contaminants in WWTPs. They are used in various reactor setups, such as moving bed reactors (MBRs) [30], trickling filters, and rotating contactors [31]. The use of biologically active carbon (BAC) is one method used for the generation of naturally occurring biofilms. Generally, granular activated carbon is used in WWTPs as a filter to remove undesirable microbes and organic and inorganic contaminants. However, the microbes tend to colonize the rough porous surfaces of the carbon, thereby growing into biofilms, which can then filter the contaminants in wastewater. The biofilm-based wastewater treatment systems are preferred over sludge processes owing to their benefits, which include a minimum requirement for space, low hydraulic retention time, high concentrations of active biomass, and low production of sludge [32]. Additionally, the microorganisms in the biofilm system tend to be more diverse as compared to those present in the sludge system, making biofilms capable of treating a diverse range of organic pollutants. The performance of the biofilm greatly depends on its formation.