Pretreatment to remove suspended solids from raw makeup water is a requirement for the potable water industry, but it is also a critical application in many process industries, including steamgenerating electric utilities.
In the 1980s and 1990s, reverse osmosis exploded in popularity as a retrofit technique ahead of existing demineralizers at power stations. RO membranes, whose pore sizes are only angstroms in diameter, will remove most dissolved ions from water, thus greatly reducing the load on downstream ion exchange units.
At Kansas City Power & Light Co.’s (KCPLC) La Cygne, Kan., generating station, an RO system was placed in the Unit 1 (820 MW, supercritical boiler) makeup water system in the 1980s. As part of a major upgrade in the 1990s, an RO unit and downstream ion exchange system replaced the original flash evaporator in the Unit 2 (720 MW, drum boiler) makeup train. Both RO systems were designed for 75% recovery with a maximum product water flow rate of 200 gpm.
Even though both La Cygne makeup water systems were fitted with RO units, they continued to operate with the original clarifier/sand filters for suspended solids removal. By the early 2000s, combined chemical costs for the two clarifiers had easily exceeded $100,000 annually, with labor and routine equipment repair costs adding considerably to that amount.
When each clarifier operated properly, effluent turbidity could be lowered to around 0.3 nephelometric turbidity units (NTU). However, upsets in lake water chemistry or chemical feed equipment malfunctions periodically caused excursions in clarifier performance, such that effluent turbidities might exceed 1 NTU. In these cases, there would be quick fouling of RO prefilters and an increase in RO membrane differential pressures. The Unit 1 clarifier was particularly troublesome in this regard.
In autumn 2004, based on reliable information from colleagues within the power industry, KCPLC tested a Pall Aria 4-ft microfilter (MF) in the Unit 1 makeup water system to ascertain if it would produce cleaner water for RO feed, and how this would affect downstream equipment. Whereas most RO systems for power plant applications utilize spiral-wound membranes, the microfilter at La Cygne is of hollow-fiber configuration, in which each module contains thousands of spaghetti-sized hollow-fiber tubes. To produce the 300-gpm flow required by Unit 1 and auxiliary systems, 24-membrane modules were necessary.
The microfilter process, like RO, operates via cross-flow filtration, in which the raw water flows parallel to the membrane surface. Water that passes through the membranes and is purified is known as permeate. Not all water passes through each membrane, as a small portion at least must flow along the surface to carry away the suspended solids. This stream is known as the reject. The membranes in the unit KCPLC tested are configured such that the raw water flows from outside to in, with the reject flowing along the outside surface of the fibers.
No water lost
Raw water enters tank T-1 for feed to the membranes. A level control gauge in the tank modifies inlet valve operation so the tank maintains a constant level. Pump P-1 (rated at 20 hp) moves the raw water to the membranes. This pump is controlled by a variable frequency drive (VFD) to adjust the output based on the flow rate requested by the operator.The feed to the membranes passes through a basket strainer to remove any large solids that might otherwise foul the membrane surfaces. The permeate flows directly to an existing storage tank, while the reject flows back to tank T-1.
Thus, no water is lost during normal operation. The standard mode of operation for the system is 25 minutes of water production followed by a one-minute air scrub/reverse flush (AS/RF) to remove solids that collect on the membrane surfaces.
When the AS/RF sequence initiates, pump P-1 stops, and pump P-2 (also rated at 20 hp) feeds water from tank T-2. This tank contains previously filtered water to which sodium hypochlorite has been added via pump P-3, which takes simple feed from a drum of hypochlorite. Air valve 7 opens to allow air to scrub the membranes while the chlorinated water flows inside out through the membrane surfaces. Pump P-2 is also powered by a VFD to allow the operator to adjust reverse flush flow rate as necessary.
Once this process is complete, pump P-1 reactivates and flushes the system for a short period followed by a return to permeate production. At the beginning of the new production cycle, tank T-2 fills with clean water while pump P-3 injects fresh sodium hypochlorite to the tank. The controls also include a timer that periodically backwashes the inlet strainer with feed from tank T-1. The only significant cost to operate the unit is the electricity that powers the P-1 and P-2 pumps. This cost is negligible compared to what KCPLC spent on the clarifier and sand filters. The heart of the control system is a dedicated PLC mounted on the pump skid, which KCPLC controls from a personal computer in the Unit 1 laboratory.
KCPLC set the flow rate, AS/RF frequency, strainer backwash frequency and other parameters from this PC. The PLC acts upon any command changes instantly, and this provides excellent flexibility for adjusting water flow to meet plant requirements.
Results and lessons learned
Makeup water for the boilers is taken directly from Lake La Cygne, where the typical turbidity ranges from 5 to 15 NTU. KCPLC was given performance criteria that indicated the microfilter would remove particles down to 0.1 micron in size and produce an effluent turbidity of less than 0.1 NTU. Within an hour after system start-up, effluent turbidities had dropped to a range of 0.027 to 0.036 NTU, where they have consistently remained. KCPLC found that the cartridge pre-filters ahead of the Unit 1 RO, which normally have to be replaced every two to three weeks, did not have to be replaced once during the initial three-month test.
MF membrane pore sizes are larger than those of RO membranes, which require much less pressure to push water through the membranes. Typical membrane inlet pressures on the KCPLC system range from 10 to 20 psi. The minimal pressure requirement allows membrane construction of coarser but much more durable materials, in this case polyvinylidene fluoride. This aspect proved to be very important. KCPLC found early on during the test that even with regular AS/RF, membrane differential pressures (DP) would gradually increase from day to day.
As an experiment, in the spring of 2005, KCPLC began treating the raw water feed with a small but continuous dosage of sodium hypochlorite to maintain a 0.2 to 0.5 ppm chlorine residual in the membrane permeate. This did wonders for membrane cleanliness, and the gradual DP increases ceased, in fact dropped, to near start-up levels (8 psi at 300 gpm flow rate) for several months.
Subsequently, KCPLC has, at times, seen the membrane DP increase gradually in spring and summer. During these events, KCPLC increases the AS/RF frequency to better scrub the membranes. If the membrane DP climbs too high (30 to 35 psi), or the microfilter needs to be out of service for an extended period, KCPLC cleans the membranes with a dilute solution of sodium hydroxide followed by a rinse and then a cleaning with a citric acid solution followed by another rinse. KCPLC adds the chemicals manually to tank T-1 and then circulates the solution through the membranes using pump P-1.
From March to October 2005, KCPLC operated continuously before taking the unit off for cleaning. The DP did not recover to original values; although, KCPLC was able to continuously produce water until February 2006, at which time, with the aid of Pall personnel, KCPLC performed a more vigorous cleaning of the membranes. Subsequently, KCPLC set up a schedule to clean the membranes on a quarterly basis.
Other than a faulty inlet valve that the vendor replaced promptly, the reliability of the system has been superb. Results were so impressive that KCPLC purchased the unit and installed it in a permanent location in February 2005. Operation since has been very steady, and payback for the MF will be in less than three years.