Improved understanding of the interactions and complexities of biological nitrogen and phosphorus removal processes

Nitrogen and phosphorus removal from wastewater is now considered essential for the protection of our waterways. Biological nutrient removal processes are generally the most efficient and cost-effective solution to achieve this. While the principles of these processes are well known, intriguing and useful details are being discovered with the recent advances in bio-process engineering and microbial sciences. Phosphorus accumulating organisms have only been identified in recent years, and there are now competing glycogen accumulating organisms being found in biological phosphorus removal systems. These can possibly explain the reasons for the variable phosphorus removal performance of certain systems, and their control can help in the development of more stable and better performing processes. Detailed investigations of the traditional nitrification-denitrification systems, but also of novel developments for nitrogen removal, reveal a more complex and diverse range of processes involved in these transformations. Increasingly, linked phosphorus and nitrogen removal processes are being developed, creating further opportunities to optimise the technologies. However, this might also bring certain risks such as the potential to produce the greenhouse-gas nitrous oxide (N₂O) rather than nitrogen gas as the final denitrification product. A range of recent developments in these areas is covered in this paper.