In recent years, both drinking water standards and technology have changed to help ensure safe drinking water for public consumption. This paper describes particle counting as a relatively new tool used by water treatment plant operators to control drinking water quality. This technology is useful in the assessment of water quality, unit process performance and water treatment efficiency. As water treatment standards have become more stringent, the methods of analysis have become more sophisticated. Water clarity has been traditionally measured by turbidity (cloudiness). Turbidity is caused by particles in the water consisting of finely divided solids. Particles are an aesthetic concern, but can also be a health concern since they can carry and protect microbes. Turbidity is measured by passing light through the sample and seeing what is absorbed. The difference in the amount of light is the turbidity. The method gives a single number, but no definitive information on the cause of the turbidity. Particle counting provides another dimension. Through a light obscuring technique, the instrument can measure not only the number of particles, but also their sizes. Sizing particles is of significant interest to water treatment operators since certain size ranges can indicate different problems. By counting the size and number of particles, the operator can determine the effectiveness of chemical treatment and/or the efficiency of the individual filter beds. The size of particle can help identify potentially harmful organisms that may be passing through the filter bed media.

The measurement is in microns, which is one millionth of a meter or 1/1,000,000 meters. Bacteria (0.1 - 0.12 microns), viruses (0.5 - 100 microns), cysts and Giardia (7 to 12 microns) and Cryptosporidium (3 - 5 microns) can be identified by size using the particle counts as a guide. In many cases the operator can operate the filter in a manner that will prevent unwanted materials from passing through the bed and into the finished water that is delivered into the distribution system. The particle counting information is as important to the operator as the CAT scan is to the physician.

Salt Lake City’s water treatment facilities are equipped with particle counters on individual filter beds as well as the finished water. This process allows plant operators to see changes in water quality long before there are changes in turbidity. Filter aide polymers can be added to a filter to improve its ability to remove particles. Lester Anderson, City Creek Water Treatment operator believes that it provides the operator an added tool in predicting filter performance. He says, " Particle counters provide an early warning that a particular filter bed is being stressed and it can be washed before a significant turbidity break through occurs." "Using the particle counts," he adds, "allows us to provide quality drinking water to our customers."

Regulations are changing. The Interim Enhanced Surface Water Treatment Rule, finalized early in 1999, lowers the acceptable filter turbidity to 0.3 NTU. In 1994 Salt Lake City adopted a more stringent voluntary standard of 0.1 NTU as its commitment to provide the safest drinking water possible to its customers. To maintain this stringent turbidity level, the use of turbidimeters alone is insufficient. Particle counters, with their greater sensitivity, give the plant operators the added technology they need to perform their jobs.

The use of this technology also helps keep the cost of treatment at an optimum level. The operator is able to fine-tune the chemical additions to maximize treatment efficiency and keep the costs as low as possible.

According to Florence Reynolds, Water Treatment & Quality Administrator, "We carefully watch for the lowest particle size range between 2-5 microns (the size of Crytosporidium) and adjust the treatment processes to keep it within the average range. (The industry standard for a well operated granular media filter is the removal of particles 10 microns are greater). We usually have a low total number (<50) of particles. However, if the numbers begin to increase, we take action by either backwashing the filter or adding filter aides, depending on the head loss in the filter bed. There isn’t any regulatory level that we have to meet, but by correlating the turbidity and the particle counter data, we are able to maximize our operational results."

All of the surface water delivered into Salt Lake City's water distribution system is treated using this technology. City Creek, Parleys, Big Cottonwood, Metropolitan’s Little Cottonwood and the Jordan Valley water treatment plants all use particle counters in the treatment of water.

This is another example of Salt Lake City’s commitment to provide its customers the safest drinking water possible.

Questions regarding this article can be directed by e-mail to: florence.reynolds@ci.slc.ut.us