(454b) Optimization of UF Backwash with Real-Time Control of Coagulant Dosing

Authors: 
Gao, L., UCLA
Gu, H., University of California, Los Angeles
Rahardianto, A., University of California, Los Angeles
Christofides, P. D., University of California, Los Angeles
Cohen, Y., University of California, Los Angeles
Over the past two decades, reverse osmosis (RO) has emerged as the leading technology for seawater and brackish water desalination. However, membrane fouling remains a major challenge for robust RO operation. Hence, effective RO feed pre-treatment is vital to long-term operation of RO desalination plants. Amongst existing RO pre-treatment technologies, ultrafiltration (UF) has proven to be effective in producing consistently high quality filtrate for RO feed. One of the major advantages of UF membrane pre-filtration is the ability to reverse flow through an UF module (a process known as backwash) and thereby removing the foulant layer. After UF backwash, a portion of the foulant may remain on the membrane (i.e., not removed by backwash) as irreversible foulant layer. Irreversible fouling is typically removed through chemical washing which can significantly increase RO feed pretreatment. UF filtration performance can be increased via coagulant dosing of the raw feed water. Studies have also shown that UF backwash effectiveness improves as coagulant dose increases up to a certain threshold, beyond which UF backwash effectiveness no longer improves via additional coagulant dosing. Optimization of coagulant dosing is therefore critical in order to reach both optimal UF and backwash effectiveness while reducing coagulant consumption. However, real-time coagulant dose optimization is a major challenge when feed water quality is temporally variable both in the short term as well as seasonally. Under such conditions coagulant dose optimization via static jar tests is inappropriate. Therefore, in the present work, a novel method is introduced to allow real-time adjustment of coagulant dose so as to ensure effective UF backwash even under conditions of changing water feed quality. In the present method, the change in initial UF filtration resistance (post backwash) is used as a metric of UF backwash effectiveness. The above information is utilized by a coagulant dose controller designed to utilize the above metric to determine whether the coagulant is being underdosed or overdosed, and correspondingly adjust the coagulant dose to the UF feed. The controller was implemented in an integrated UF-RO seawater desalination pilot plant (capable of 18,000 GPD production of fresh water). The controller performance was evaluated, in a field study in California, with respect to its performance, including during a storm event in which raw seawater feed quality rapidly worsened, and its ability to reduce coagulant consumption. Results of the study demonstrated that the present approach enabled system operation that was able to adapt to worsening feed water quality conditions while reducing coagulant dose by up to ~29% relative to operation without real-time coagulant dose optimization.