Recovery Strategies for Heavy Metal-Inhibited Biological Nitrogen Removal from Wastewater Treatment Plants: A Review

Biological nutrient removal is an integral part of a wastewater treatment plant. However, the microorganism responsible for nutrient removal is susceptible to inhibition by external toxicants such as heavy metals which have the potential to completely inhibit biological nutrient removal. The inhibition is a result of the interaction between heavy metals with the cell membrane and the deoxyribonucleic acid (DNA) of the cell. Several attempts, such as the addition of pretreatment steps, have been made to prevent heavy metals from entering the biological wastewater systems. However, the unexpected introduction of heavy metals into wastewater treatment plants result in the inhibition of the biological wastewater treatment systems. This necessitates the recovery of the biological process. The biological processes may be recovered naturally. However, the natural recovery takes time; additionally, the biological process may not be fully recovered under natural conditions. Several methods have been explored to catalyze the recovery process of the biological wastewater treatment process. Four methods have been discussed in this paper. These include the application of physical methods, chelating agents, external field energy, and biological accelerants. These methods are compared for their ability to catalase the process, as well as their environmental friendliness. The application of bio-accelerant was shown to be superior to other recovery strategies that were also reviewed in this paper. Furthermore, the application of external field energy has also been shown to accelerate the recovery process. Although EDTA has been gaining popularity as an alternative recovery strategy, chelating agents have been shown to harm the metal acquisition of bacteria, thereby affecting other metabolic processes that require heavy metals in small amounts. It was then concluded that understanding the mechanism of inhibition by specific heavy metals, and understanding the key microorganism in the inhibited process, is key to developing an effective recovery strategy.