Mar 21, 2007

Cold Weather Problematic for Colorado Sewer Plant

Cold weather has been causing problems with a new ultraviolet disinfection system at the wastewater-treatment plant last month in Longmont, Colo.

Officials have reported to the Longmont FYI that the weather has been causing the release of higher-than-permitted levels of fecal coliform bacteria and suspended solids into the St. Vrain River.

Last month two separate occurrences caused problems in the plant. Cold temperatures reduced the plant’s ability to “settle” out solid matter from treated water, and dirty covers on the UV system affected its ability to cure bacteria in the water.

Plant superintendent John Anderson told the Longmont FYI that he hadn’t seen water this cold in his ten years on the job. Last month the water, which had been exposed to the air, dropped to 10 degrees Celsius. The water usually stays above 15, and Anderson reported he had never seen it go below 14.

Since cold water is denser than warm water, the organic and other particles remaining in the water after being digested by bacteria were not able to settle out properly. Then, the cloudy water was not adequately treated by the UV system, which inactivates bacteria. This compounded with the fact that the UV already had the problem of dirty glass, made the problem even bigger.

The UV system needs clear water and clear glass, but the system was automatically cleaning the glass tubes housing the bulbs only every 24 hours. Technicians decided to increase that to every four hours to remedy the problem and avoid growing slime on the glass covers of the bulbs that are submerged in water.

However, Anderson reported that even if the bulbs had been clean, the system would still not have properly disinfected the cloudy water. Therefore, plant workers have made other changes to the way water flows through the clarifying filters, such as slowing it down and giving the solids more time to settle.

Officials reported to the Longmont FYI that in February, the plant’s monthly discharge rates for suspended solids were slightly higher than permitted and the fecal coliform levels were an average 374 organisms per 100 milliliters for the month, compared to a permitted level of 251.

The UV system had been installed in the city after an installation by a competing vendor failed to work properly despite years of upgrades and design changes.

In 2001, taxpayers paid for a $246,000 ultraviolet treatment system as part of a $23 million upgrade and expansion of Longmont’s wastewater-treatment plant. The system was installed in 2002 and never worked properly.

A new $700,000 system was installed last fall, and it has been working reliably until the problem last month.

Longmont could encounter fines from the state and federal governments for violating clean-water laws; however, since the problem was discovered and remedied quickly, that should not be the case. The plant also met all other permit standards for that month.