When chlorine reacts with natural organic material commonly found in surface water sources like lakes, rivers and streams the reaction forms compounds called “disinfection byproducts." Two groups of disinfection byproduct compounds, trihalomethanes and haloacetic acids, have drinking water standards. Byproduct levels found in water depend primarily on:

 

• The amount of natural organic material in the water;
• The amount of chlorine used to treat the water;
• The amount of time (water age) it takes for water to reach the customer.

Levels vary throughout the year and often become highest during the warmest months typically when the highest levels of natural organic material can be found in the water supply. Current drinking water rules require samples of these byproducts be collected quarterly. The results of samples taken from all four quarters of the previous year get averaged to determine if our utility meets drinking water standards.

 

New drinking water rules require water systems to take a close look at areas where byproduct levels can be the worst—generally in older water. The current rule allows results from all system sample sites to be averaged. New rules will soon require water systems to meet similar drinking water standards at new individual sample sites. Tacoma Water is currently evaluating new sample sites and has found some individual samples at these sites with elevated levels.

 

Our staff works to minimize byproduct levels and has already made adjustments to portions of our system operations. The utility is currently designing new covered storage tanks to replace the open McMillin Reservoirs. This will be a major future improvement for water quality and will help reduce disinfection byproducts.