Self-adaptive cycle-to-cycle control of in-line coagulant dosing in ultrafiltration for pre-treatment of reverse osmosis feed water

Abstract

Real-time self-adaptive approach to in-line UF coagulant dosing was developed and field demonstrated for integrated UF-RO seawater desalination. A novel coagulant dose controller was designed and successfully implemented in a pilot UF-RO seawater desalination plant. The coagulant controller, which tracks the UF resistance during filtration and backwash, adjusts coagulant dose to the UF feed with the objective of reducing the incremental cycle-to-cycle UF post-backwash (PB) resistance change (i.e., Δ n). Real-time tracking the above UF resistance metrics, as well as the rate of change of Δ n with coagulant dose, enabled the controller to quantify the progression of both irreversible fouling and UF backwash effectiveness. The above information was then utilized by the controller to make the appropriate coagulant adjustment. Field tests of the proposed self-adaptive coagulant dosing approach demonstrated measurable coagulant dose reduction while maintaining robust UF operation even during periods of both mild and severe water quality degradation. The approach to real-time coagulant dose control developed in the present work should be suitable for both seawater and brackish water UF treatment and has the potential of providing both effective UF operation as well as reduction in coagulant use.

A perspective on reverse osmosis water desalination: Quest for sustainability

“…Improvements in membrane element design could also be beneficial to reducing pressure losses while possibly enabling reduction in concentration polarization, thereby allowing higher recovery operation per element and thus decreasing the plant footprint and possibly plant components. Operational considerations include optimal plant control that will ensure energy optimal operation while meeting the target permeate productivity …”

“…Current practice of membrane fouling prevention emphasizes the removal of potential foulants, prior to any RO desalting unit operations, using conventional flocculation/coagulation, disinfection (e.g., via chlorine dosing or UV pretreatment), media filters or membrane filtration (microfiltration or ultrafiltration) processes for RO feed water pretreatment. 16,32,43,48,50,51 In some cases, sodium metabisulfite, sodium bisulfite, or activated carbon may also be used for dechlorination of the RO feed to avoid RO membranes degradation due to chlorine; 52,53 it is noted, however, that the use of chlorine in RO feed is now typically avoided in modern plants. 11 These feed pretreatment processes, while effective for minimizing membrane fouling due to particulate deposition, organic adsorption, and biological growth, do not remove mineral scale precursors.…”

“…Operational considerations include optimal plant control that will ensure energy optimal operation while meeting the target permeate productivity. 49,50 In recent years there have also been mounting efforts to develop effective methods of plant fault detection and isolation 49,91,98-100 so as to avoid plant failures and reduce maintenance costs. Effective self-adaptive RO feed pretreatment 48,51 that can handle temporal variability of raw feed water quality and production demand is also likely to lead to significant reduction in the use of feed treatment chemicals while reducing the frequency of membrane cleaning and replacement.…”

Sweating the assets – The role of instrumentation, control and automation in urban water systems

“…12 Coagulant dosing control in ultrafiltration (UF), which is often used prior to reverse osmosis for solids and organic matter removal, has also been studied recently, with the aim of reducing the UF membrane fouling. Gao et al (2017) presented a dosing controller that relied on real-time tracking of cycle-to-cycle UF resistance after each backwashing (the so-called PB resistance). The controller increased or decreased the coagulant dose depending on improvements in the cycle-to-cycle change in UF PB resistance.…”

Modeling of Chlorine and Coagulant Dose in a Water Treatment Plant by Artificial Neural Networks

“…An ANN has the ability to learn and model non-linear and complex relationships. Several studies have been carried out on the prediction of the coagulant dose for particular WTP [9][10][11][12][13][14][15]. RBFNNs and GRNNs have shown good performance capabilities for predicting residual chlorine in WTP [16].…”