Active Mixing & Ventilation Provide Efficient THM Removal for California Utility

Aug. 3, 2018
THM levels in tanks lowered by up to 65%

In the summer of 2014, Placer County Water Agency (PCWA) in California was facing a trihalomethane (THM) crisis. Drought across the western United States had led to increased levels of organic matter in the source water and water conservation measures had led to lower water usage, resulting in higher water age in its distribution systems. Higher organic levels required more chlorine disinfectant, which increased THM levels during treatment. Higher water age allowed THM to grow further in the distribution system. After registering an operational exceedance for THMs in the early summer, PCWA staff wanted to act fast.

The Problem

Relying on surface water captured by the Sierra Nevada mountain range, PCWA provides drinking water for more than 200,000 people in cities, towns and communities along I-80 in the heart of California’s “Gold Country.” PCWA serves an area of over 1,500 sq miles—from the foothill cities of Roseville to Lake Tahoe—and it runs multiple water treatment plants for large populations, such as its Foothill/Sunset system (population 83,394), and smaller systems for cities such as Colfax (population 3,079).

Operators saw THM levels spike in February 2014 and again in May of the same year.

“Telling our customers that we were exceeding the [maximum contaminant level] wouldn’t be fun—we needed to act fast,” said PCWA Drinking Water Operations Manager Andy Hamilton.

System flushing is one of the most common operational methods for reducing water age and lowering THMs. However, during a drought, flushing becomes problematic.

“Flushing was always an option, but we would be sending mixed messages by telling people to save water while we dumped a bunch of water on the ground,” Hamilton said.

PCWA staff challenged themselves to quickly identify and address locations where THMs were on the rise.

“We have always been concerned about THMs, but we didn’t have a way of assessing THM levels beyond the quarterly [compliance] sampling,” Hamilton said. “Between shipping water samples and waiting for results, the delay was a big issue for me. I had to wait two weeks before I found out what was going on, and THMs aren’t exactly cheap to analyze.”

The 600,000-gal ballpark tank had a PAX Water Mixer and PAX Powervent. 

The Solution

At the request of Brad Wilkins, water quality supervisor, PCWA decided to trial a Parker Hannifin desktop THM analyzer to gain insight into the specific locations where THMs were the greatest problem and understand how THM analyses could be used to better optimize treatment and distribution. The small desktop unit uses helium and an advanced set of sensors to provide highly accurate species-specific THM analyses in drinking water in 30 minutes.

Using the Parker Hannifin unit, PCWA operators made an important discovery: THMs were forming quickly during primary treatment, and THM levels in the clearwell tanks at the Colfax system were particularly high. Treatment plant operators experimented with lowering the chlorine pre-feed into the treatment plant and saw a substantial reduction in THMs.

With the insights gained from the Parker Hannifin THM analyzer, PCWA operators explored options for further reducing THMs in the clearwells and in distribution system tanks. PCWA staff had attended webinar training sessions by PAX Water Technologies and had learned how the addition of full aeration systems inside water storage tanks could strip THMs from finished drinking water. But under a tight deadline and with limited emergency funds, PCWA wanted to explore how much THM reduction it could achieve with active mixing and active ventilation alone.

PAX Water Mixers continuously push THM-rich water up to the surface, where the THMs can evaporate out of the water into the headspace of the tank. The PAX Powervent is a high-efficiency ventilation system designed specifically for water storage tanks that will exhaust the THM-rich air out of the tank.

Targets for improvement included a pair of welded steel tanks (1 million gal and 300,000 gal) that made up the clearwell and a 600,000-gal “ballpark tank” in the Colfax distribution system. PAX recommended the installation of a powerful PAX Water Mixer (PWM400) and a Powervent (PPV400) in the 1-million-gal tank and 600,000-gal ballpark tank, as well as a smaller PAX Water Mixer (PWM200) and Powervent (PPV200) for the 300,000-gal tank. Combined, the three powerful active mixers and three energy-efficient Powervents used only 2,800 W.

THM reductions measured on three different days in the Colfax water system. Average THM removal rates were 45% for the 1-million-gal and 300,000-gal clearwell tanks and 65% for the 600,000-gal ballpark tank.

The Results

Initial results were very encouraging. Operators saw an immediate and substantial reduction in THMs leaving the clearwell tanks, and the ballpark tank also saw substantially lower THM levels compared to historical values.

“One of my initial concerns was chlorine loss,” Hamilton said.

However, after installation and operation of the PAX Water Mixers, Hamilton and his team observed that chlorine levels were actually quite stable.

“We have been able to maintain residuals just fine,” Hamilton said.

Before installing the mixers, Hamilton’s team observed that as the Colfax tanks drained, they saw residual levels drop. But after installation of the PAX Water Mixers, this was no longer observed.

“That’s another big benefit: The tanks are homogenized,” Hamilton said.

After running the mixers and Powervents for a year, utility managers were eager to quantify exactly how much THM reduction they were achieving with the new technology. The Parker Hannifin THM analyzer was suitable for this study. Water quality experts from PAX Water Technologies developed a sampling protocol that would allow PCWA to not only quantify the level of THM reduction, but also determine the level of THM formation in the absence of a mixer and Powervent.

The results were impressive. The PAX Water Mixer and Powervent were able to remove 45% of the THMs in each of the two clearwell tanks, as well as 65% of the THMs in the 600,000-gal ballpark tank.

[Visit PAX's W&WD Storefront]

“I think the magnitude of THM reduction from just the PAX Water Mixer and the PAX Powervent surprised everyone,” said PAX research and development scientist Robin Giguere. “We knew the PAX Water Mixer was powerful, but we did not anticipate the efficiency with which it and the Powervent could reduce THMs without any additional aeration equipment.”