A pH-based control system for nitrogen recovery using hollow fibre membrane contactors

Nitrogen is accumulated in wastewater being the main sink of this nutrient. As consequence, wastewater appears as a relevant source to recover this nutrient. Current wastewater treatment are focused on nitrogen removal instead of recovering it. However, different technologies have been appeared in the last decade. Hollow fibre membrane contactor is one of them and its capacity has been demonstrated with different feedstock and conditions. In this work a pH- based control system has been developed using the pH slope as a variable to control the performance of these membranes operated for nitrogen recovery. This is a novelty low-cost control tool for recovery process automation and optimization which offers the possibility of process monitoring without the drawbacks of maintenance, costs and time of lab analysis and more complex sensors. During the nitrogen recovery process pH in the feed solution decreases because of free ammonia transfer across the membrane. pH measurements cannot be correlated with nitrogen concentration (usually pH is controlled to maintain the driving force of the recovery process). However, the pH slope varies with the ammonia transfer rate representing the ammonia nitrogen concentration remaining in the feed solution. Since the membrane contactors are operated in batch mode, the developed control system stops the recovery process when the pH slope is below a threshold value for four minutes. Three different threshold values were evaluated (−0.06, −0.03 and −0.015 pH·min−1) demonstrating that the nitrogen recovery efficiency desired can be achieved by selecting an appropriate threshold value for the pH slope.