## 1 Introduction  
The biological treatment is one of the fundamental processes for the treatment of contaminated water in WWTPs, as it mainly involves the removal of toxic contaminants such as inorganic nitrogen and phosphorus. It is a preferred method for the removal of inorganic impurities in various municipal and industrial WWTPs. Biological treatment is ideal for inorganic contaminants due to the benefit it offers over physical and chemical methods. Among the many benefits of biological treatment, its economic advantages and environmental friendliness are the main attractive benefits of this process. The use of activated sludge has been a popular biological treatment process for centuries in WWTPs [1, 2]. Although the sludge process is an efficient process for the treatment of dissolved contaminants, including total nitrogen and phosphorus compounds, it has its drawbacks, such as the production of excess sludge. The post-treatment and disposal of the sludge are other challenges when using sludge processes. Declines in available land space, coupled with increasingly stringent regulations governing the design and operation of new landfills, have caused the costs of siting, building, and operating new landfills to rise sharply, thereby increasing the post-treatment costs of sludge. As a matter of fact, it has been noted that the treatment of sewage sludge accounts for 60% of the overall cost of wastewater treatment. Although it has been suggested that sludge must be recycled for land applications, it is restricted owing to the health risk associated with it. This means that the majority of the produced sludge ends up being incinerated. The incineration process generates ash, which tends to go into landfills as it cannot be disposed of elsewhere due to the high heavy metal content and general toxicity. The rising costs and public sensitivity to sewage sludge disposal have provided considerable motivation to explore and develop strategies and technologies for the minimization of sludge production [3]. This has resulted in many scholars investigating other interesting alternative technologies such as adsorption [4; 5] and biofilm processes. The biofilm process has emerged as a cost-effective and environmentally friendly option to replace the sludge process. Thus, biofilm systems have been increasingly used in WWTPs, and this approach is said to address some of the drawbacks of using the sludge processes. Moreover, the use of biofilms has been noted to be an essential method for microorganisms to prevent external stress caused by chemical compounds that may result in the inhibition of biological processes. Additionally, the long start-up time for biofilm technology has also been reported as a major problem. Many mechanisms that biofilms use to protect themselves against chemical destruction have been reported, such as extracellular polymeric substances (EPSs), SOS, and quorum sensing [6]. These defense mechanisms of biofilms make them ideal for WWTPs, since wastewater is composed of different chemical compounds that are toxic to biological processes. Biofilms are especially essential for sensitive processes such as SND. Many strategies have been investigated to accelerate the biofilm's adhesion to the surface, thereby reducing the start-up time. Different studies have investigated various reactors for their ability to enhance biofilm formation, although several bioreactors have been shown to efficiently enhance biofilms in WWTPs, thereby enhancing the removal of inorganic materials. Their application is still limited in large-scale applications owing to the large capital costs associated with commissioning these reactors. Moreover, different chemicals such as nanoparticles have been investigated for their ability to enhance the biofilm's attachment, e.g., ferric citrate [7] and ferrous oxalate [8]; however, the application of these methods is still limited in wastewater treatment, owing to high costs of applying them in WWTPs.