This article originally appeared in WWD June 2020 issue as "Troublesome Turbidity"
The Greer Commission of Public Works (CPW) water treatment plant in Greenville County, South Carolina, stands on top of the south bank of the South Tyger River, downstream from the Lake Cunningham recreation area. The natural beauty of the area combined with solid economic growth over recent years has placed increasing demands on CPW.
From 2000 to 2016, the population grew 58%. With the population now at more than 31,000, it is projected to reach 33,500 by 2024. For the Greer CPW plant, that means serving a total population of more than 50,000 with greater than 18,000 service connections and an average of 9 million gallons a day (mgd) of flow with seasonal fluctuations increasing demand.
Capacity Growth for CPW
In service since 1971, the plant initally had two basins for 8 mgd capacity. The plant added two basins to double capacity to 16 mgd. It later increased production from 16 mgd to 24 mgd through the use of “high-rate” filtration and upgrades in technology. Computerized controls using data from a full range of analyzers allowed conventional sand filters to operate at a throughput of 6 gallons per minute per square foot, 50% higher than industry standards.
The plant pulls water from Lake Cunningham, seeing variations in incoming water quality, especially during wet weather events. Using conventional treatment, the first step is oxidizing iron and manganese using potassium permangante. The water then goes to rapid mix with polymer based coagulant, along with caustic soda or chlorine as needed for floculation and sedimentation. The water is chlorinated at the prefilter stage and is followed by filtration through 12 rapid sand filters feeding a 5 million gallon clear well tank to add polyphosphate, fluoride, and caustic, before feeding a 3 million gallon storage tank.
Improving Turbidity Measurement Accuracy
In 2016, the plant evaluated operations to look for added efficiency. One of the most critical aspects of the operation of the filters was accurate turbidity measurement. Accurate turbidity measurement in turn rested on maintenance and verification of the analyzers in place.
The plant found that the turbidity analyzers initially installed required extensive cleaning and time consuming verification processes. The turbidimeters utilized submerged sensors that resulted in cleaning issues and reduced reliability. In addition, the units used incandescent bulbs that required periodic replacement.
At that point, the plant evaluated non- contact turbimeters offered by Swan Analytical USA. The non-contact sensor design eliminated coating issues and provided simple cleaning in place. In addition, these analyzers featured long-life LEDs with an expected life of 10 years.
Wet Weather Woes
While initially the new instruments were installed only in filters and settled water, plant management then reviewed its raw water system. Typically, raw water is fairly low in turbidity. After wet weather events, especially extended or severe rainstorms resulting in ground saturation, runoff would cause 40 to 60 NTUs and as high as 125 NTUs.
Previously, such events hit five times per year or fewer, but recently, they have been more prevalent. The raw water turbidity analyzer in place often did not match lab readings. While some variance is expected based on sampling and delays versus online measurements, the variances seen were enough to cause concern. Even with monthly cleaning, operators had to rely on lab readings.
Typically pH adjustment is not needed as alkalinity is between 12 and 16 mg/L. Wet weather events would cause the alkalinity to drop to 9 mg/L, requiring caustic feed to improve coagulation efficiency.
Raw water would occasionally clog the feed lines. The auto drain capabilities of the new instrument flushes the flow chamber daily, which reduced cleaning needs, even with weather events. Months can go by with no service requirement. One flush with iron-out to remove manganese every six months ensures that the analyzer remains clean.