Where Did the Flow Go? A Discussion on Declining Sewer Flows and System Impacts

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Where did the flow go? (photo by Jim Aanderud, Innerline Engineering)

The recent drought in California, combined with water conservation measures, has resulted in lower flows entering the sewer system and going to wastewater treatment plants. These lower flows have impacted operations, maintenance and how we plan system upgrades for the future. Where are flow rates headed? Join the conversation as we start to get our sewer systems and resource recovery plants ready for the “new normal”.

This story was written in advance of CWEA 2016 Annual Conference.

Jon Wells, Principal Engineer – West Yost Associates

Jon Wells, Principal Engineer at West Yost Associates, will be presenting “From Drought to El Niño – Managing Climate Impacts on Collection Systems from Low Flows and Water Restrictions to Inflow/ Infiltration” on Thursday morning, April 28th.

Q: What is one finding from your research you will be presenting?

Dry weather, per capita flows have gone down 15-20% since the start of the drought/start of the economic downturn, which roughly coincide in California. In many urban areas that are close to full build-out conditions, this per capita decrease is enough to cancel and reverse any growth that has been seen or projected for this time, so there has been a net decrease of dry weather flow at the plant/system outfall. Many capacity improvement projects that were identified just prior to the drought have been un-needed, and capital planning has been difficult.

In addition to the difficulty in capital planning because of the reduced flows, we’re seeing two other drought impacts in collection systems: low water conditions are increasing the number and intensity of root intrusions, as plants search for a water source. Also, collections systems crews are feeling pressure to be very water conscious as they hydro-jet lines to clean them, and in some cases have even experienced some pushback to increasing gravity main cleaning frequency.

Q: When will flows go back up?

That’s the million dollar question, in combination with the related question of “How much will they go back up?” Nobody is expecting per capita flows to stay this low permanently, but nobody is expecting them to return completely to pre-drought values as well. Some of the water demand reductions that have lowered the per capita sewer flows are based upon new high efficiency appurtenances and standards and are therefore basically permanent. Some are based upon changed behavioral patterns that are likely not permanent. We’re relying on our colleagues in the water resources world to help estimate the answers to this question.

Q: What happens if flows continue to fall?

I believe if flows continue to fall, the biggest impact will be that flow-based revenues continue to fall, and overall revenue generation for collection systems will need to be examined and maybe reworked. Because the need to maintain, replace, and reduce inflow and infiltration (I&I) in aging systems will not be decreasing, even as flow based revenues decrease.

Seppi Henneman, PE, Brown and Caldwell

Seppi Henneman, PE, Brown and Caldwell

Seppi Henneman, PE, Brown and Caldwell and Lisa Arroyo, City of Santa Barbara will be presenting “Where’s the Rain? Impacts from the Drought on Wastewater Treatment” on Friday morning, April 29th.

Q: What is one finding from your research you will be presenting?

We found due to extreme water conservation the influent flow rate to the plant has decreased significantly (~30% reduction), resulting in corresponding increases in pollutant loading, with increases of nitrogen being the issue at this facility. However, most of the alkalinity in this area comes from drinking water, so the alkalinity load actually decreased due to less potable water entering the system.

While this was happening, the El Estero Wastewater Treatment Plant (EEWWTP) was in the middle of designing upgrades to the plant which would increase the solids residence time (SRT) to 7 days. This change would result in growing a population of biomass which would oxidize the ammonia into nitrate, something that is not currently done at the EEWWTP. This oxidation process consumes alkalinity. In fact, approximately 7 pounds of alkalinity are consumed for every pound of ammonia oxidized.

The original basis of design showed that there was sufficient alkalinity to account for the amount of ammonia being oxidized. In addition, due to the on-going drought the City decided to evaluate turning on an existing desalination plant to make up approximately 30 percent of the City’s water supply.

Unfortunately, the desalination water only has 40 mg/L of calcium carbonate (alkalinity), compared to 350 mg/L of alkalinity in the existing potable water supply. This means that the alkalinity loading will decrease even further once the desalination plant is online. The average Alkalinity:ammonia ratio would decrease from approximately 7.9 down to approximately 5.8 after turning on the desalination plant.

Because of this, the City has to design a supplemental alkalinity system to make up the difference in alkalinity that they are losing due to the combination of water conservation and desalination. BioWinTM modeling showed that if they would violate their instantaneous pH limit if they did not add supplemental alkalinity. The City is currently evaluating the use of either caustic soda or magnesium hydroxide.

Q: When will flows go back up?

Although potable water flows may go back up somewhat during non-drought years, if California is lucky enough to get several consecutive wet seasons, it will not go back up to where it was before. A lot of people and businesses have installed low flow appliances which lower water use long term. If the drought is declared over in the future, people are not going to remove their low flow appliances to instead have high flow appliances.

Most of the increase in potable water flow will likely be due to resuming irrigation of household lawns, which will have minimal impact on wastewater flows. It’s the indoor water use that directly impacts wastewater flows. Due to climate predictions in California, it would be wise to plan for the future for these lower (and more concentrated) dry weather wastewater flows.

The more concentrated wastewater that we are seeing today will likely to be the new normal moving into the future, and it’s important that we plan accordingly.

Q: What happens if flows continue to fall?

If flows continue to fall due to perpetual drought, the issues associated with lower than design flows will continue to get worse, resulting in the need for more maintenance and potentially a change in management practices. Agencies have typically designed their infrastructure based on flow more than anything else.

Depending on the wastewater treatment process, an agency may have been able to treat 10 MGD 10 years ago, but may not be able to treat 10 MGD today, since pollutant concentrations have gone up significantly.

Collection systems experiencing lower than design flows will have more odor issues, as the velocities through the sewer pipes decrease, resulting in more material settling in the pipeline as well as more fermentation occurring, and thus more hydrogen sulfide generation. This settled material will build up in the collection system until either a manual cleaning occurs or storm flow flushes it all out at once, which may have detrimental impacts at the downstream treatment plant.

In addition, issues with tree roots in the collection system will increase as trees look for water in sanitary sewers since they may not be irrigated, leading to the need for more frequent line cleaning.

Treatment plants that nitrify will need to evaluate their need of using supplemental alkalinity (i.e. caustic soda, magnesium hydroxide, etc) to prevent violations related to pH and/or significant reductions in nitrification rates, as we have found at the EEWWTP. Plants that also have to denitrify will find out that their effluent total nitrogen will increase with increasing influent nitrogen concentrations. Plants designed to achieve 10 mg-N/L without the use of chemicals may find that they now need chemicals (i.e. methanol) to achieve this same concentration. At plants that do not nitrify, increasing ammonia concentrations may cause risk of ammonia toxicity violations.

I’m sure there are other issues that we haven’t thought of and we’re hoping to hear from other people during our discussion on Friday at 11:10 in the “Adapting to the New Normal” track.



About the Author

Megan Barillo

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