Dealing With Declining Wastewater Flows

Collection Systems, Resource Recovery, Technology and Innovation

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CUWA Survey Reveals Seriousness of Issues

If your water or wastewater utility hasn’t already experienced impacts from declining flows, you may want to prepare for them in the future. 

That recommendation comes from the California Urban Water Agencies (CUWA) following a survey of water management agencies it conducted around the state. Of 270 respondents, approximately 50 percent of them reported impacts from lower flows during the six-year drought and the state’s 2015 emergency conservation mandate on potable water consumption. 

“Declining flows are definitely having an impact on drinking water systems, wastewater conveyance and treatment operations, as well as water recycling,” explains Melanie Holmer, water reuse leader with Brown and Caldwell (Sacramento) and deputy executive director of CUWA. “Municipalities must take a holistic, ‘one-water’ approach to their water infrastructure in the future to optimize operations,” she says. 

The impact on wastewater collections systems is already being felt at the Eastern Municipal Water District (EMWD) in Riverside County. Assistant General Manager for Operations and Maintenance  Jeff Wall says his utility has increased spending on chemicals to control odors and arrest corrosion caused by lower flows, high salinity and hydrogen sulfide in local sewers. 

In Santa Barbara, Thomas Welche, Chief Plant Operator at the El-Estero Wastewater Treatment Plant, says his staff is making several process changes to deal with lower flows and higher waste concentrations which affect the quality of the plant’s effluent and recycled water. One modification is to reroute secondary treated effluent back to the head of the plant to “freshen” influent. 

CUWA White Paper

The drought and tightening statewide water conservation targets are impacting all segments of the municipal water and wastewater infrastructure, according to CUWA. 

In a white paper on declining flows, CUWA points out that drinking water can reside in supply pipelines for longer than normal times. Increased residence time can lead to issues with chlorine residual and disinfection byproducts (DBP).  “Some utilities are experiencing nitrification in their distribution systems” Holmer says.  

Water systems are employing aggressive nitrogen, chlorine residual and DBP control practices, improving water chemistry, increasing flushing and improving storage facility hydraulics. Holmer noted one utility in San Diego mitigated these impacts by increasing flushing operations, which raised flushing costs 10-fold, from $200,000 to $2 million per year. 

On the wastewater side, low flows can cause septicity and salinity in sewers, increasing hydrogen sulfide which can cause odors and corrosion of pipes and manholes. Lower flows can also lead to more blockages. 

At the wastewater treatment plant, the CUWA report observes, declining flows can impact the characteristics of influent wastewater, necessitating operational adjustments and potentially even plant upgrades to deal with higher concentrations of constituents such as biochemical demand and ammonia that exceed the plant’s original design specifications.  

These changes can also impact the quantity and quality of the effluent water available for reuse. As a result, utilities may not be able to meet customer recycled water demands or may require facility upgrades to meet water quality goals. 

Salinity In Sewers

Salinity in the sewers is a serious headache for the EMWD in Riverside County. Jeff Wall says salinity in the district’s water sources, coupled with low flows, have produced abnormal levels of salinity in the collections system. The resulting hydrogen sulfide is causing increased odors and corrosion, and the District is spending more money to address those concerns. 

“Our household water usage rates have dropped from 200 gal/day to 127 gal/day over the past decade.  In the last year alone, and our plant flows have dropped from 45.5 mgd to 43 mgd. 

Wall says his agency has more than doubled its odor control budget for additional bio-filters and chemical scrubbing, and is planning to spend “millions” on rehabbing manholes and sewers. “Rather than kick the can down the road, we’re increasing spending on maintenance now,” he says.  

“EMWD is producing less and selling less recycled water. “We had 39,000 acre feet available last year, but only 36,000 acre feet this year. Declining flows are definitely not great for recycled water systems.” He says some growers have complained about the quality of the water they are purchasing. 

A bright spot may be the construction of plants to desalinate brackish groundwater. Wall says his agency is building a third desal plant, bringing the total desal capacity to 13.5 mgd.  Salinity removed from brackish groundwater, in the form of brine is pumped to Orange County Sanitation District which has an ocean outfall.  Upon completion of the third plant, EMWD will be a net exporter of salt.  

“Our board and our public have been very supportive of the desal projects,” he says, noting the cost thus far has been $64 million for the wells, plants, and brine discharge facilities. 

“Where we live, it’s important that we adopt a long-term water-wise outlook,” he concludes. 

Lower Flows at the Plant

At the El-Estero plant, Thomas Welche reports that the average daily flow has dropped nearly 25 percent–from 7.96 mgd in 2008 to 6.38 mgd last year. On the other hand, average daily TSS is up from 342 to 445 mg/L in the same time frame.  

“While the city’s collections system is monitoring odors and corrosion caused by hydrogen sulfide, as well as increased root intrusion, the impact at the treatment plant relates directly to its recycled water supply, according to Welche. “This is an ocean discharge plant so meeting our permit is no problem. 

“Rather we’re having difficulty meeting the high quality water required for recycling. That’s because of the septicity in the sewers and nutrient levels that aren’t really textbook.” 

Despite increased rainfall in early 2017, El-Estero is planning for continued extreme conditions, which he says “put a strain on the plant.” About $30 million is being spent to modify the plant’s activated sludge system by making improvements to the aeration system blowers and diffusers as well as the clarifiers in order to produce a higher quality secondary effluent. 

In addition to contemplating returning secondary effluent to the head of the plant to dilute the influent, changes are also being made to sludge age and nutrient loadings. 

And the drought is having another unintended effect on treatment operations as the city seeks additional water sources. “We are starting up our desalination plants,” says Welche. “They produce water that is softer than our normally hard water, and contains less total dissolved solids than normal.” 

Those changes could have an impact at the treatment plant, making it harder to achieve biological nutrient removal in the future, according to Welche. 

Be Prepared

With climate change and potential upcoming changes to water use efficiency standards in California, agencies should be prepared for possible impacts from declining flows, says Holmer. 

Financial sustainability in the midst of drought is challenging enough.  Declining flows adds another layer of complexity utilities need to be aware of. Considering potential impacts of declining flows in a holistic, comprehensive manner will allow utilities to better manage their water systems. 

They need to be aware of the coming changes and the associated costs, and both capital and maintenance expenses may increase. “With increased cost of facility improvements or operational adaptations to deal with these impacts, there may be an impact on water rates,” Holmer says. 

Maintaining safe drinking water must be the highest priority, and utilities may have to employ better monitoring to assure quality, she says. 

CUWA recommends that—as various state water managers confront declining flows—others pay attention to successful mitigation programs and build a tool box of solutions. 

More than likely, they’re going to need it. “This is a new era in water management,” says Holmer.