Silica discovered to be the limiting system recovery factor - Case Study
Introduction
A consulting engineering firm selected to design and oversee the construction of a 5MGD RO system for a Southern California military facility handed over the system to an operations management firm. Upon handover, the original OEM of the system recommended that the management firm contact Avista Technologies for support with analytical membrane services, antiscalant recommendations and onsite technical training for the operators.
Avista received the call and was onsite the same day to evaluate the system.
Background
Once onsite, an Avista application expert provided a technical audit of the system evaluating operating conditions including pressure flows and conductivities. The system standard operating procedures and historical operating data were reviewed as well as chemical membrane compatibility standards. As a part of the system audit, membranes from first and second stage were pulled and evaluated under manufacturer specs, using The Avista Off-site Cleaning and Restoration (OSCAR) system. This in-depth audit identified that the system had minor membrane fouling. A complete membrane autopsy was recommended which identified the specific foulant and appropriate cleaning chemicals and procedures.
The lag element contained colloidal silica, calcium carbonate crystals and microorganism shells.
Membrane Autopsy
Membrane elements were selected based on the audit results to be taken to Avista for individual element testing. This detailed analytical evaluation concluded that there was minor fouling with colloidal silica and calcium carbonate scale. RoClean L403 and L211 were recommended as the formulation to be used in the next clean.
The lead element revealed organic material across the membrane surface, with calcium carbonate crystals isolated to the feed spacer contact points.
Antiscalant Dosing and Product Selection Water samples were collected, and a detailed system-specific water analysis was performed to be used in the Avista Advisor dosing software to establish product selection and accurate dose rate. To operate at the maximum recovery a scale inhibitor that targeted silica was recommended. Vitec 4000 was selected for this system based on high dissolved silica levels in the feedwater to the RO and high recoveries required under the operating contract.
Onsite Application and Training Avista provided comprehensive on-site operator training covering all aspects of membrane operation, record keeping, operation, and maintenance. Avista was onsite for the bulk delivery assuring a smooth transition and covering all of the required sitespecific safety training and documentation.
Conclusion
Through an in-depth system audit and analytical evaluation, silica was discovered to be the limiting system recovery factor. The antiscalant was changed to an antiscalant that controls silica at the higher recoveries required under the contract. The membrane surface foulants were identified, and a membrane cleaning protocol was established to ensure effective cleaning and maximum recoveries during operation. Operators were trained detailing the proper system recoveries and troubleshooting procedures. The system is currently being operated using good engineering practices following the membrane manufacturer guidelines.
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