Membrane treatment for U.S. EPA’s surface water treatment rule compliance has become recognized in recent years as being technically superior and cost effective compared to other treatment, easy to own and operate, and “influent flexible.”
Success of membranes for drinking and wastewater applications can be assumed, however, working with the right partners (suppliers & consultants) is crucial to making this assumption a reality. California Water Service Co., (CWS) realized this at their 20 MGD drinking water treatment plant in Bakersfield, Calif.
Diligence in design
CWS hired Black & Veatch (B&V) to evaluate a treatment system to process 20 MGD of water from the Kern River for the city of Bakersfield. B&V selected a treatment system that uses coagulation and sedimentation with ferric chloride as coagulant.
A microfiltration system from Pall Corp. was selected and pilot tested to prove the concept and to verify the sizing of the full-scale plant.
The treatment plant was commissioned in July 2003. The plant operated well, until late in 2003 when it was discovered that the transmembrane pressure (TMP) of the system after routine flux maintenance procedures was rising more rapidly than expected. This unforeseen rise in TMP if left unchecked would result in lower water throughput and more cost in the form of increased power usage for pumping and more frequent membrane cleanings.
Partners reunite
CWS contacted B&V and Pall Corp., to assist in solving the problem. The pilot test data was reviewed but did not reveal any indications of the realized problem. It was decided that the best way to proceed was to examine the full-scale treatment scheme in an attempt to determine any inconsistencies between the pilot test and the full-scale process. The main difference between pilot plant and full-scale plant is in the recycling of decanted sludge from the plate settlers. This recycled water was being fed to the plant at a ratio of 1/100th that of the raw water being fed to the system, so it was initially discounted as being a significant factor in the unexpected results.
Scientists and engineers from Pall Corp., and B&V teamed to conduct several coordinated laboratory and field tests. Analysis of a used membrane by Pall’s Scientific and Laboratory Services organization revealed that an iron-organic complex had fouled the membrane. This complex foulant was determined to be forming as a result of organic matter in the decanted wastewater combining with free iron from the ferric chloride coagulant that was present due to the low alkalinity of the water. Studies were performed to verify that the standard cleaning regime of caustic and chlorine and citric acid was not effective to completely break down this material and remove it from the membrane.
After finding the cause of the problem, potential solutions included:
- Incorporating a cleaning regime more effective on the iron-organic compound; and
- Eliminating the iron or organics present in the feed stream.
The Pall Scientific and Laboratory Services team evaluated enhanced cleaning procedures to determine if an effective alternative could be found. It was determined that addition of EDTA into the base cleaning solution of NaOH and NaOCl helped to accelerate the break down on this complex fouling compound. The citric acid cleaning solution was also evaluated. It was determined that the membranes could be restored through the use of a stronger acid to fully clean the fibers.
Several alternate coagulants were also evaluated. It was determined that for this low alkalinity feed water, polyaluminum chloride (PACl) was a cost effective coagulant that prevented the formation of the iron-organic complex by eliminating free iron fed to the membrane system. PACl was also effective in allowing the membrane to remove total organic carbon to meet regulations for disinfection by-products.
Real world results
Changes were implemented in late February 2004 and system performance dramatically improved. Ferric-based coagulant was replaced with PACl and a thorough membrane cleaning using EDTA with the basic (caustic and chlorine) cleaning solution followed by a solution of FDA approved hydrochloric acid was performed.
Today, CWS is operating the plant at just below the design capacity of 18 MGD, and has been certified to run at up to a flux of 80 gallons per square foot (of membrane area) per day equating to a total capacity of 25 MGD. The system is performing Enhanced Flux Mainten-ance on the membrane modules using a caustic and chlorine solution every 3-4 days. CIP procedures are performed using a standard regime of NaOH with NaOCl and citric acid at intervals greater than 30 days. This operation is in line with the design conditions, and the low rate of TMP rise that is being experienced signals that the plant efficiency being realized in terms of power cost and chemical usage will be better than predicted by the original pilot test.
Water quality to distribution has been excellent with 100% of samples registering a turbidity reading below 0.1 NTU with an average reading of 0.04 NTU. Total organic carbon in the water is being reduced by about 42% and disinfection by products (TTHM and HAA5) in the effluent water typically read 16 to 19 micrograms per liter.
Gary Witcher, treatment plant manager of CWS stated “We are pleased with the level of knowledge and commitment that the Pall management and scientific teams brought to this project. Without any doubt, we feel that we selected the right process and the right filter for this particular application.”
Final analysis
The Bakersfield plant success after experiencing the complex and unforeseen process issues described earlier demonstrates that scientific resources including experienced technical professionals, equipment and methodologies are essential to ensuring success with water treatment installations. These resources and a dedicated partnership between supplier, consultant, and system owner were the keys to success, in this case, and are repeatable.
This combination of scientific app-roach, technical resources, and cooperation should be a part of the evaluation when selecting partners for any water treatment plant.
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