Case Study: CA 71 Ammonia Measurement In Reclaim Water

Many utilities are finding new markets for treated wastewater, also known as "reclaim water". Reclaim water has been used in the past, mostly for golf courses and agriculture. Now, with drinking water being a precious commodity, more utilities are switching to this new source in order to conserve the potable water for the public.

A major power utility in Colorado made a decision to use reclaim water from its Las Vegas wastewater treatment plant as cooling water for the power plant. Because 90 to 95% of the water is used for cooling, this saved an average of 30 million gallons per year of drinking water.

16" pipes from the treatment plant to the power plant carry the reclaimed water. The water is circulated through plant pipe works which cool bearings, generators and other moving parts. The heated water is then sent to the cooling towers for heat transfer.

Reclaim water is used in power plants and presents a special problem because the water needs to be treated at a higher level to remove minerals and solids that could destroy power plant machinery and pipes.

The ammonium content of the water had to be near zero for use in the cooling towers and heat exchangers. Water containing ammonia, particularly in the presence of oxygen, readily attacks copper and copper bearing alloys. This is commonly referred to as "yellow metal attack" by power plant personnel.

Ammonium can also effect the disinfection of cooling water in a negative way. Free ammonium in the tower will combine with the chlorine based disinfection chemicals added to form chloramines. This type of chlorine is much less effective as a biocide than the free chlorine formed in the absence of ammonium.

To accomplish ammonium removal, the water treatment plant uses a feed forward control scheme using the CA71 analyzer to control dosing for "break point chlorination". By maintaining the chlorine to ammonia content at an 8:1 ratio, the ammonium is eliminated, along with the negative effects on the plant cooling water system.