Declining Flows: Managing the Consequences of Our Success

Emerging Issues, Resource Recovery, Technology and Innovation

Coverage of asset management topics is supported by MentorAPM


California’s wastewater systems were designed decades ago with flow rates predicted to be much higher than what we’re seeing today. Corrosion, odor, grit, nutrients and less water for reuse are all consequences of declining sewer flows. So now what?

To water systems expert Paul Brown, the challenges of declining flows are a consequence of our own success. We’re re-plumbing California’s cities into “one water” systems so challenges should be expected.

Mr. Brown has 40 years of experience in strategic and facilities planning and is the leader of Paul Redvers Brown Inc. His clients include large California water agencies. He frequently speaks about the future of water at conferences.

On the list of our priorities for water professionals, where do you place declining sewer flows?

I see declining wastewater flows as a serious operational and management challenge for utilities but not as an industry “crisis.” I think declining flows are a consequence of a lot of good things happening.

There are crisis-level priorities for the water sector – rising sea level, increasing temperatures, flooding and drought come to mind.

I think water conservation and increased water use efficiency are fundamentally good. They are positive outcomes. We have made ourselves more robust from a water management standpoint, now we have to deal with the consequences of our success.

Have other states or countries confronted the challenges caused by declining flows?

Every facility planner struggles with forecasting future wastewater flows – both increases and decreases. The challenge is not only projecting future flows but also the constituent loadings conveyed by those flows. Focusing on both the volume and the strength of wastewater.

We shouldn’t talk about declining flow without coupling it with the concept of increased loadings. When we pull water out of a wastewater stream, such as at a scalping plant, and return the biosolids to the sewer, the wastewater treatment plant at the end of the line receives lower flows and higher concentrations of waste. Discharging brine from advanced water treatment facilities to sewers has the same effect. This has been occurring at scalping plants for decades.

Are there positive outcomes from declining flows?

If we’re creative, the availability of unused facility capacity, both in pipes and treatment plants, could offer opportunities to repurpose infrastructure for other functions.

Perhaps we could do more with the capture and treatment of intermittent stormwater flows or retrofit existing clarifier capacity to introduce process upgrades at treatment plants.

Do you foresee a time when flows drop too low and the system fails or assets are abandoned?

In rare cases, possibly. But there is an opportunity to create new systems that incorporate and repurpose legacy infrastructure – avoiding, in part, the need for new infrastructure. Hopefully, some cost savings will result. For example, Orange County Sewer District and Orange County Water District have partnered to develop the groundwater replenishment system – creating a new source of water, reducing ocean discharges, and extending the life of their existing ocean outfalls.

Water agencies are going to be transformed by the pressures to change. At the end of that transformation there may be assets that haven’t been immediately reused, but they will remain valuable as additional redundancy given the uncertainties in front of us.

Do water and wastewater agencies need to work together more on declining flows?

All agencies need to recognize that a “one water” system is a “system of systems.” Declining flows is just one issue impacting those systems.

What does a “system of systems” mean? Individual water, wastewater, and stormwater management utilities operate under independent governance over their mission and goals. If we’re going to achieve the one water vision, we’ve got to recognize the need for increased collaboration, further integrating communication and operations among agencies. They need to cooperate on the implementation of a higher-level, closed loop system of systems.

We need to be aware of the interdependencies that result from that integration.

This one water system of systems is complex. We can’t predict everything that is going to happen as it evolves. It will always need adjustments, and we will always need to anticipate things that could go wrong, while minimizing the amount of time it takes to get back up and running. That’s resilience.

If the issue of declining wastewater flows stimulates thinking about all the other assumptions that could be different from what we are expecting, we’ll be better prepared to adapt and respond rapidly.

We’re in a transition from talking about one water solutions to actually implementing them, and we should expect to encounter circumstances we didn’t predict. Declining wastewater flows is one of those cases.

Be sure to join the Declining Flows session Wednesday, April 10th in Palm Springs. Curated by Aren Hansen from Brown and Caldwell.