Pharmaceuticals removal from wastewater: Ultrasound technology and its potential amalgamation with membrane processes

Pharmaceuticals residual contamination is among the top global challenges on par with greenhouse gas emissions and other issues. The danger of pharmaceuticals lies in their stable structure and the development of antimicrobial resistance genes (ARGs). These have serious health and economic consequences that have led to the loss of millions of lives and cost the world economy billions of dollars. An important strategy to mitigate this risk is upgrading wastewater treatment plants with powerful treatment techniques such as ultrasound technology (US). A comprehensive literature data analysis was conducted to detect common trends for the impact of system design and operational parameters on removal efficiency and energy consumption. It was found that most of the studies applied inefficient parameters for pharmaceuticals degradation resulting in significantly higher energy consumption than other AOPs, which in turn led to a downtrend in the interest in developing this technology further. Based on the literature data, direct contact transducers with a frequency range of 500–800 kHz and pulse mode were found to be the best settings for pharmaceuticals removal. Ultrasound was found to be ineffective with some classes of pharmaceuticals such as hormones and antipyretics probably due to their high pKa. Guidelines for the design and operation of ultrasonic systems were provided. The opportunity of combining ultrasound with different membrane technologies within the wastewater treatment environment was discussed and conceptual designs were proposed. The best combination is determined by the treatment target. If the goal of the treatment is to produce clean water and a nonhazardous reject stream, then US and nanofiltration (NF) or reverse osmosis (RO) is recommended. However, if improving pharmaceutical degradation by the biological processes and reducing the risk of ARGs development within the treatment train is the desired outcome, then US combination with forward osmosis (FO) and membrane bioreactor is recommended.