April 1, 2010

Sub-Sea Water Treatment System Provides Reliable Supply for the Huarun Power Plant

Echo Zhao, Dow Water & Process Solutions; Yu Fangbing, Huarun Thermal Power Plant; and Yasushi Maeda, Dow Water & Process Solutions

Operational Results Are Excellent

Operational data of the first-pass RO were recorded from February 2007 to July 2007 and normalized by Dow’s FTNORM program to evaluate the performance of the Filmtec BW30 elements (see sidebar). During the test period, the raw water makeup varied widely, from 92 mg/l to as high as 2,617 mg/l; approximately one-third of the time the water TDS ranged from 520 mg/l to 2,617 mg/l (Table 4). According to the definition of sub-sea water, Xiao HuLi’s water became the sub-sea water type (TDS level of 2,000 mg/l to 8000 mg/l) on the days of seawater refilling during March 16 to 28, April 14 to 20, and April 22 to 29. At the same time, the temperature changed from 19C to 33C and the COD level ranged from 7 mg/l to 32 mg/l.

Table 4. Raw water data record, February to July 2007. Source: Dow Water & Process Solutions

Three key results were observed during this testing:

• The normalized permeate flow was very consistent, even with such high variation in the TDS feed conditions (Figure 9). Upstream, the water feed TDS fluctuated, as did the RO 1 driving pressure from the end of February to the end of April because of seawater refilling. After April, we observed the temperature of the feedwater increased from 25C to 33C, the seawater refilling became weak, and the permeate flow of both trains became stable. The RO 1 system operated at 78% to 80% water recovery, meaning that 80 gallons of good product water were produced for every gallon of raw water supplied.

• Pressure drop (PD) is the signal that indicates how much contamination is present in the RO system. Low PD tells the operator that the feed space is smooth or all the contamination was backwashed out by concentrate water. High PD indicates that there is some fouling present in the RO system that must be removed. Chemical cleaning is recommended to remove the fouling contamination if the PD level is 10% higher than normal value. The normalized pressure drop of RO 1 was measured and recorded during the test period (Figure 9). The initial PD of RO 1 was 1.3 bar. Even with the increase in water temperature and operating hours, RO 1 PD remained stable in the 1.3 bar to 1.4 bar pressure drop range during the test period.

• The performance of normalized salt passage (SP) and salt rejection (SR) are plotted in Figure 10. The SR remained a conservative 97% during the test period. Both trains’ SR basically met the required 97%, except the raw water was of the sub-sea water type from March 16 to 28, April 14 to 20, and April 22 to 29.

9. Normal system performance. Normalized net driving pressure and pressure drop of the RO 1 system confirmed that the system was performing as expected. Note the variation of the total dissolved solids in the incoming water. Source: Dow Water & Process Solutions


10.    Salt adverse system. Normalized salt rejection of RO 1 is a measure of the reverse osmosis system performance. Source: Dow Water & Process Solutions

During the periods of seawater refilling (March 16 to 28, April 14 to 20, and April 22 to 29), the sub-sea water’s TDS was in the range of 2,023 mg/l to 2,617 mg/l, and the SR of each of the two trains was lower than in other periods. Hence, similar to the trend of the permeate flow rate, both first-pass RO SRs kept fluctuating until May.

The initial SR of RO 1 was 98.32%. After contamination of the sub-sea water, the TDS during the period (2,500 ppm to 2,700 ppm from March to April) in the RO 1’s SR declined. After May, the two RO trains’ SR returned to higher than 97.5% when the raw water TDS decreased. By the end of July, the average normalized SR for RO 1 was 98.4%.
The integrated membrane process water system at the Huarun Power Plant continues to reliably produce boiler-quality water from river and sub-sea raw makeup water. Lesser-quality water sources will only become more common as high-quality water resources around the world are stressed to supply the needs of the world’s population. The Huarun Power Plant water treatment design illustrates that desalination is a viable option for industrial water supplies in the future.

—Echo Zhao (ezhao@dow.com) is regional applications development leader, Asia Pacific, Dow Water & Process Solutions. Yu Fangbing is water treatment plant manager, Huarun Thermal Power Plant. Yasushi Maeda (ymaeda@dow.com) is an applications specialist for Ultrapure Water, Dow Water & Process Solutions.