Fine tuning wastewater treatment

How we manage the wastewater as it goes through the treatment plant can enhance the cleaning process and make it more efficient. To do this, operations, maintenance, laboratory, and process staff closely monitor the water we clean as it goes through our treatment plants and can make adjustments.

Over the last year and a half, staff at our Brightwater Treatment Plant in Woodinville optimized the treatment process in a way that improved staff safety and the performance of the membrane filters.

A worker in a safety vest, hardhat, and mask, inspects the magnesium hydroxide tank at the Brightwater treatment plant. The person is standing on blue scaffolding above the big, white tank
A worker inspects the magnesium hydroxide tank at the Brightwater treatment plant

Not just pipes and tanks – microorganisms and chemicals too

Wastewater treatment plants are often large, industrial facilities. So, it makes sense that people might think that the pipes, pumps, and tanks are the most important part of a treatment plant. But the microorganisms (bacteria) and chemicals used to clean the water are just as important as the infrastructure.

Treatment plants use processes similar to how nature cleans water. Mechanical systems remove solids and floating material. Bacteria help break down organic matter in the wastewater.

Like other living creatures, the bacteria we use to clean wastewater need the right combination of air,  food and heat to thrive. Chemicals are used at key steps in the treatment process to help maintain favorable conditions for these bacteria to survive and treat the wastewater. The chemicals and the bacteria are closely monitored by operations and process staff.

Many treatment plants, including Brightwater, use a chemical known as caustic soda (or NaOH) to maintain a neutral pH in the water. Beneficial microorganisms tend to thrive and perform more effectively under these conditions. Caustic soda is also used to ensure that the final treated water meets the standards set by our environmental permit before it is discharged into the Puget Sound.

This year, staff tested a switch to magnesium hydroxide (Mg(OH)2) instead of caustic soda with great success.

The main reasons to switch to magnesium hydroxide were to see if it helped the performance of the membrane system, save ratepayer money, and minimize using caustic soda, which is corrosive and hazardous. While caustic soda is good at keeping the pH neutral, it can be a problem for humans or bacteria at large concentrations – and maintaining the pH with it can be difficult.

Switching the chemicals wasn’t just a simple substitution of one chemical for the other. It took a team to select compatible equipment and tanks, develop a test plan and monitor how it affected the treatment process. But it was worth the effort because the benefits include:

  • Improving safety by reducing the risk of exposing staff to hazardous chemicals,
  • Improving energy efficiency and the filter performance of the membrane system,
  • cost savings for chemical usage,
  • better control of pH permit compliance,
  • a “friendlier” environment for the microorganisms, and
  • reducing the salt (Na) in the recycled water Brightwater produces for places like Willows Run Golf Course!

The successful chemical switch was first tried at our Carnation Treatment Plant. Carnation is a smaller plant but, like Brightwater, it has a membrane system. The success there helped us figure out what it would take to do a full-scale test at Brightwater. Work is being done to install a permanent magnesium hydroxide system at Carnation.

Thanks and kudos to all the staff who made this improvement possible!