The dirtier the membranes become, the harder they are to clean. Eventually, no matter how often the membranes are washed, some foulant remains. At this point, the membranes must be replaced to restore lost capacity. KWU has implemented a 500-hour run time between CIP for all membranes regardless of age, and expects to get five years of life between membrane changes. Also, following high-turbidity events, “deep cleans” are performed to further maintain filtration capacity.
Initially, KWU used proprietary chemicals supplied by the membrane manufacturer for the cleaning process. Process engineers from the manufacturer worked with plant staff to develop an effective cleaning regimen using locally purchased bulk chemicals, saving KWU a considerable annual cost. Commodity chemicals also cost less than replacement membranes. It is significantly less expensive to chemically clean the membranes regularly and often, thus extending plant capacity, than it is to forgo cleaning and replace membranes more often.STAFF TRAINING AND SUPPORT
Before the Kenosha plant went on line KWU staff visited the Marquette, Mich., plant. In addition, a membrane “buddy system” allows plant personnel at new membrane installations to visit plants already in operation. Kenosha has hosted personnel from Carmichael, Calif., and they meet regularly with neighboring plants like Manitowoc, Wis., to share information.
KWU is a lean operation with a single plant operator on duty per shift. An extensive and modern supervisory control and data acquisition (SCADA) system enables the operator to run both the membrane and conventional plant. KWU's six operators are cross-trained in operations and maintenance duties. Two teams of three work a 10-day rotation, during which operators work three maintenance days and seven operating days. Repairs are made on maintenance days, while membrane monitoring occurs on all days.
The membrane system is a highly automated process with advanced programming to help operators do their job. A two-level alarm system is built into the controls to protect the membranes and equipment from extended operation when something is amiss. The lower level sends a signal with a printout describing the problem in detail while the system continues to run. The higher level can shut down a skid if an emergency condition is detected. By paying close attention to all alarms and making timely corrections, operators can avoid an emergency shutdown condition.FINANCIAL PLANNING
KWU has a long-term agreement with the manufacturer that calls for membranes to be replaced on an as-needed basis, regularly scheduled service visits, and an annual process audit. The agreement helps KWU to budget its costs up front.
In the 10 years following the membrane plant's start-up, the manufacturer has developed an improved process using a different, chlorine-resistant membrane material. While the cost of replacement membranes is included in the long-term agreement, the cost to convert the existing system to operate under the new process is not included. Any investment in remodeling must be justified by operating cost savings, including longer membrane life, lower power consumption, fewer chemicals needed for cleaning, and less backwash wastewater generated for disposal.
Another development since the plant's start-up is that several of Kenosha's largest industrial customers have left the area, significantly reducing the demand for water. KWU has responded by operating the plant at a higher rate through the night, taking advantage of reduced electrical load charges while replenishing storage tanks within the distribution system, and then coasting during the day.
Today, KWU is at a fork in the road: to reload with another set of membranes and continue as before, or invest additional money to convert to the lower-cost operating option. If the membrane plant were producing near its rated capacity, the cost of converting would be more easily justified. The manufacturer, who has a partnership interest in the long-term success of the plant whichever path is chosen, is maintaining the data required to support Wisconsin DNR's permitting of the new process, should this be selected.
— Davis is technical sales manager for Memcor Products at Siemens Water Technologies. Lewis is superintendent of the O. Fred Nelson Water Treatment Plant in Kenosha, Wis.