Water Quality, El Niño and Drought: Oh My!

Posted by Peter Fiske on 1/26/16 8:00 AM

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Water treatment plant operators have a challenging job under normal circumstances. They must optimize treatment strategies for varying water demand and cope with unpredictable (but inevitable) emergencies such as infrastructure repairs and emergency response.  Now, large-scale weather patterns such as drought and El Niño are making a tough job even more challenging.

Challenges Due to Drought

Organics and algae: Drought can create water quality challenges in several ways. First, lower water levels in surface water sources can concentrate organics, leading to higher total organic carbon (TOC) and a greater treatment challenge. And, as drought also coincides with warmer-than-average temperatures and more sun, algae growth in surface waters can be greatly accelerated – further adding to TOC and also potentially creating taste and odor problems.

The bromide "Gotcha!": Water systems that rely on brackish water sources face an additional unwanted surprise during droughts: higher bromide levels. Bromide is a naturally-occurring compound usually associated with salt water. During droughts, the bromide levels in brackish water can increase because the proportion of fresh water decreases. Increased bromide levels means increased trihalomethanes (THMs) in treated water – the most common disinfection by-product (DBP). Even water systems that rely on ground water can be bitten by the bromide “gotcha”: if the ground water comes from geologic layers of marine sediment, there may be bromide in those layers that comes out with the water.

Water conservation = water age: The final challenge for water operators dealing with the consequences of conservation. During drought, utilities (rightly) encourage customers to conserve water use. But the consequence of conservation is higher water age and longer detention times in the distribution system. High water age can result in disinfectant residual loss and increased THM levels. And, during a drought, conventional water quality practices such as flushing old lines and mains may create confusion or frustrate the public. Nothing brings the news trucks faster than thousands of gallons of water gushing out of a hydrant during the height of a drought.

Challenges Due to El Niño

With the arrival of rain to much of the western U.S. this winter, most people would think that a water utility’s problems would be over. But that’s wrong. Heavy rains bring a new set of challenges and problems to municipal water operations.

The Big Muddy: Heavy rain storms create increased levels of suspended sediment and organics in surface waters. In some cases, organics (and pathogenic bacteria) can accumulate in a watershed for months, and then be suddenly washed into rivers and lakes during a single storm. Water plants have to quickly adjust treatment strategies to deal with increased TOC which may be in the form of both particulate matter (requiring increased filter maintenance) and increased dissolved organic content (DOC) which may necessitate more intense chemical treatment. Water with increased organic levels not only demands more intense primary treatment, but can also exhibit higher secondary disinfectant demand.

High Water Age: Residential and commercial irrigation systems are wisely turned off during the rainy season. But this can result in higher water age and water quality problems such as disinfectant residual loss and elevated DBP levels. Some utilities that use chloramines as their secondary disinfectant report that their worst season for residual loss and nitrification is during the wet months when water use drops, especially in warm southern climates where temperatures are still warm during winter.

Tools to Tackle Water Quality Challenges During Extreme Weather

Most of the water quality challenges associated with drought or El Niño can be greatly reduced by better distribution system water quality management. Actively mixing water storage tanks can reduce disinfectant residual loss during periods of high water age and can eliminate thermal stratification during periods of high temperature. Active disinfectant control systems in tanks can combine with mixers to ensure that disinfectant levels remain stable and healthy. And in-tank aeration systems can strip out THMs in the distribution system, allowing utilities to comply with all their regulatory goals under even the most extreme weather circumstances.


 

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Topics: drinking water quality

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