The City of Clifton, Colorado, designed and built a 2.4 million gallons per day nanofiltration plant to treat Colorado river water and supply the city and nearby areas with drinking water. The plant was partially started in September 1997 with NF Skid 1. It was shutdown in the Spring of 1998 due to low river TDS. The plant was re-started in August 1998. Shortly after, Skid 1 was cleaned after about 1,400 hours of operation to control the rising membrane pressure drops and permeate flux decrease due to suspected biofouling in the system. Two other cleaning cycles followed in November and December 1998. Operating data for the entire period since startup in 1997 till middle of December 1998 were provided to MASAR Technologies, Inc. for performance and fouling evaluation.
MASAR®'s unique analysis of the Fouling Monitor (FM) history showed that biofouling actually started to develop in Skid 1 as early as beginning of December 1997 (after only 200 hours of operation) where the FM started to rise from an average of 0.5 % at 50 hours to 3.6% at 290 hours, while the ASTM-normalized flux continued to show a improvement (as shown in the top graph). When Skid 1 was cleaned for the first time, the FM was 4.0%, indicating some type of moderate to significant fouling already in progress. During the same period, the measured membrane DP showed a very slight, hardly noticeable and certainly not alarming, rising trend while Osmonics' calculated Fouling Factor showed a very stable performance with no apparent change from baseline value of 1.2 (as shown in the bottom graph). Both indicators took a sudden turn for the worse in late August 1998 prompting the need for cleaning in September.
Had the plant been using MASAR® to monitor its performance, the operators would have been able to realize the development of a serious fouling problem very early within the first month before significant deterioration in pressure drops and permeate flux started to occur. Corrective measure would have then been taken and plant performance restored and maintained at startup levels.
We wish to thank Mr. Dave Payne, Water Treatment Plant Supervisor at Clifton Water District and Mr. Peter Eriksson, Senior Applications Development Engineer at Osmonics, Crossflow Filtration, for providing plant's operating data and input on its performance history. (Ref. - Small Systems: Nanofiltration as Salt Reducer, Behind the Clifton, Colorado, Project, Peter K. Eriksson, Water Conditioning & Purification, pp. 60-64, April 2001, Publicom, Inc., Tucson, Arizona.)