Bleach by the truckloads

Wastewater gets a final disinfection before returning clean to the environment

Most of us buy liquid bleach by the gallon jug – handy for the average household. But can you imagine having bleach delivered in a tank truck at about 4,800 gallons per load? Moreover, needing that much at a time?

Plant operator monitoring pums that convey the bleach to the wastewater

Metro Plant operator Nate Kotfis monitors pumps that convey the bleach to the wastewater.

This staggering volume is routine for the Metropolitan Council’s Metro Wastewater Treatment Plant in St. Paul, the flagship of the eight regional plants in the seven-county region. Routine, considering that for the warmest seven months of the year, the Metro Plant takes this size delivery of bleach about four times a week.

The plant uses bleach (sodium hypochlorite) in the wastewater treatment process. First, pollutants in the wastewater – often more than 200 million gallons per day – are removed by a variety of physical processes (screening, skimming and settling). Then the remaining pollutants are consumed by bacteria cultivated in the biological process at the plant, which also settle out of the wastewater. Most of the pollutants, at least, and this is where bleach comes in.

Bleach cleans water and controls odors

In the final stage of treatment, about 2,200 gallons of bleach each day are mixed in to disinfect the wastewater. The bleach has 12.5 percent available chlorine, compared with a 4-5 percent level in household bleach. Another 850 gallons of bleach is used daily to control odors at the plant.

“Adding chlorine in the form of bleach inactivates the vast majority of pathogenic bacteria – enough so that the now-clean water meets federal standards and is safe enough to discharge into the Mississippi River,” said Dave Simons, an assistant manager in the plant’s Liquids Business Unit.

Before the water flows down a side channel to the river, however, it gets a daily dose of about 425 gallons of liquid sodium bisulfite. This neutralizes any remaining residual chlorine in the water, which otherwise could adversely impact aquatic organisms in the river.

New, safer facilities

The Metro Plant has been chlorinating the wastewater since the 1970s, and added the dechlorination process in the mid-1980s. Construction of new facilities in 2005-2006 ushered in new – and safer – methods.

In the late 1990s, Metropolitan Council Environmental Services (MCES) staff began studying the need to replace the aging disinfection system. They also analyzed several new technologies along with the existing method that used gaseous forms of chlorine and sulfur dioxide (the latter for neutralizing residual chlorine).

Soon after the wastewater is mixed with bleach, it enters the effluent channel, which on most winter days is teeming with waterfowl. The chlorine needs approximately 30 minutes to kill the bacteria, then it is neutralized farther down the channel. The resulting clean water -- or "effluent" -- flows into the Mississippi River just beyond the trees in the upper middle of this photo.

When judged purely on cost, rebuilding with the existing method was the lowest-cost alternative. But it also had higher safety and security risks. When a technology decision was made in 2002, the additional costs for the liquid bleach system were offset by lower overall risks.

“We felt it was prudent to go with the safer, liquid bleach system, and last fall we completed the first season of using these new disinfection facilities,” said Bill Moore, MCES general manager. “With the diligence of our engineering, construction, and operations employees, the new system was built on time, it ran safer and more efficiently than the old system, and our clean water discharged to the river was well within the limits set for bacteria. We had previously replaced the old disinfection systems at some of our smaller plants, and are looking to do the same at our two medium-sized plants in the next several years.”

In addition to storage, pumping, mixing and spill-containment facilities, the new system includes more sophisticated electrical and computer monitoring equipment. Plant operators use the equipment to constantly monitor the process and adjust the amount of chemicals needed to get the job done. This already has decreased the overall chemical use when compared with the old system, while still meeting permit standards, Simons said. While the system is off-line this winter, further improvements will make the monitoring and mixing equipment more reliable and efficient.

Ready for more staggering numbers to consider? The Council’s eight plants combined treat up to about 300 million gallons of wastewater a day, 365 days a year. Sound like a lot? It would be enough to fill the Metrodome in less than two days.