Novel design and operational control of integrated ultrafiltration — Reverse osmosis system with RO concentrate backwash

Abstract

A novel design for a reverse osmosis (RO) desalination system directly integrated with an ultrafiltration (UF) pre-treatment unit was developed. The integration involves direct RO feed from the UF filtrate and UF backwash using the RO concentrate. This alignment reduces overall plant footprint, while the use of RO concentrate for UF backwash allows 100% UF recovery and implementation of flexible backwash strategies. The present system design utilizes a control scheme, whereby RO productivity can be prescribed independently of the UF system which selfadjusts to provide the RO system with its required feed flow rate at the specified RO pump inlet pressure. UF backwash, achieved via direct RO concentrate flow from the RO system provided a continuous flow for sequential UF backwash which was additionally integrated with pulse backwash using a hydraulic accumulator. Seawater desalination field studies with a UF-RO pilot system of 12,000 gallons/day permeate production capacity successfully demonstrated the advantage of RO concentrate UF backwash that was triggered based on a membrane resistance threshold. The above self-adaptive UF backwash strategy significantly extended the projected UF operation period (by a factor of nine) to the threshold of required chemical cleaning.

A perspective on reverse osmosis water desalination: Quest for sustainability

“…47 Right: a small mobile RO desalination plant (capacity of 45.4 m 3 /day). 48,49 The same size membrane elements (8 00 ) are used in both plant demonstrating the scalability of RO desalination.…”

“…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.…”

Ultrafiltration with self-generated RO concentrate pulse backwash in a novel integrated seawater desalination UF-RO system

“…Field studies of UF pulsed backwash were conducted using a seawater UF-RO desalination pilot plant consisted of UF and RO skids integrated as shown schematically in Fig. 1 and described in detail elsewhere [34,42,43]. Briefly, the plant was designed with water feed capacity of up to 129.1 m 3 /day (i.e., 34,116 gal/day) and permeate product water production of up to 45.…”

“…Low frequency backwash is the preferred approach in UF/MF pretreatment of RO feedwater [3,8,9,20], and the addition of low frequency (~2-5 backwash cycles/hr) pulse backwash using hydraulic accumulators has been proposed for improvement of backwash efficiency [34]. It is noted that UF and MF filtration with pulse backwash, actuated with hydraulic accumulators, has been described in the patent literature [35,36].…”

“…Such hydraulic accumulators can in principle be utilized to enhance backwash flux of UF and MF membranes used for RO feedwater pretreatment. Indeed, in a recent seawater desalination study [34], it was shown that UF pretreatment of RO feedwater was improved with the use of pulsed UF backwash. The above was demonstrated in a UF-RO system in which the RO concentrate stream was used directly for UF backwash, thereby eliminating the need for both intermediate storage tanks (for both RO feed and backwash) and UF backwash pump.…”