First-of-its-kind, pump-and-treat sand filters are reducing phosphorus in stormwater.
by Alex Schmidt, Water Resources Engineer
Does incorporating pumping into stormwater treatment projects make it expensive, infeasible, or unreliable? Four stormwater sites with pumping and automation are proving otherwise.
It’s true, water flows downhill. When it comes to stormwater treatment, passive (or gravity-fed) treatment systems are the simplest option over a system that includes pumping. Letting nature do the work to fill an infiltration or filtration system should be considered first for cost-effectiveness and simplicity.
But sometimes relying on gravity to direct stormwater into an infiltration basin just doesn’t work. Stormwater pipes are underground, making it difficult to drain to a treatment feature on the surface given the limited amount of open space or ideal topography. The common solution is to move the stormwater treatment underground. But underground treatment comes with a high price tag and its own difficulties, especially where infiltration is not possible.
When infiltration isn’t possible, soil amendments such as iron are now being used in filtration systems to better treat the phosphorus polluting stormwater. These iron-enhanced sand filters (IESF) require cycling of drying times to maintain their phosphorus capturing processes. This creates an issue for gravity-fed systems. If there are continuous flows for long periods after a rainfall, a gravity-fed IESF would be inundated for too long creating anoxic conditions, and actually, releasing phosphorus rather than capturing it.
The good news is, pumps can be controlled and set to cycle times that optimize the IESF treatment system. Further, at locations where gravity is possible, using automated valves to control the flows can also provide the necessary cycle times for IESF treatment.
Utilizing pumps can overcome these issues, but often stormwater managers see pumps as expensive, making the project seem unrealistic. More often than not, pumps are not even considered as an option.
But by paving the way for this type of treatment, the
Rice Creek Watershed District worked with Houston Engineering, Inc. (HEI) to develop the concept for pump-and-treat stormwater treatment projects, with the first project successfully implemented in
Hansen Park.
Together, we have created a design that can overcome the limitations of gravity-fed systems.
Read more about the Hansen Park IESF story embedded below.
With pump-and-treat IESF, phosphorus can be removed from stormwater 24 hours a day, 7 days a week. You are probably thinking to yourself that this must be terribly complex and expensive, right? You might be surprised to find the projects are quite cost-effective (
see FAQ below).
When it comes to maintenance, yes, there are additional steps — but as we are quickly learning in the stormwater treatment area, every type of treatment practice requires maintenance.
In addition to the
Hansen Park example discussed above, HEI has designed and implemented three additional pump-and-treat IESF projects to date:
- Oasis Pond
- Lebanon Hills
- Bald Eagle Lake
Benefits of Pump-And-Treat IESF
In summary, what are the benefits of a pump-and-treat IESF?
- Pumping systems can treat phosphorus 24/7 with a design that includes multiple filter beds
- A smaller footprint is required compared to passive gravity systems
- Cost-effective phosphorus treatment
- Treatment targeting dissolved phosphorus, which is difficult to remove from stormwater
- More accessible maintenance compared to underground systems
- Flexible site configurations
- More potential sites are viable without being reliant on the topography of the site
HEI is proud to have created this first-of-its-kind treatment and successfully installed it at five sites. Our team is eager to help you consider if a pump-and-treat filter is right for your project. Contact Alex Schmidt if you have a project in mind or want to learn more.
alschmidt@houstoneng.com | 763.493.6663
Learn more about our Urban Stormwater services.
Frequently Asked Questions (FAQ)
Q: How much will a pumping system cost?
A: The cost might be less than you would think. Over the five sites we have implemented, the average construction cost was under $280,000 (ranging from $184,000 to $478,000). Cost can vary based on treatment goals. For example, higher levels of treatment will require higher pumping rates and typically three filter beds. In contrast, if less treatment is desired the site may only require one filter bed and less complicated automation and valves.
Q: How much phosphorus can be removed?
A: Removals are variable for each site, our sites are ranging from 7 to 44 pounds of total phosphorus (TP) removed annually (50-90%), with an average of 25 pounds/year. The result is an average cost effectiveness of $720 per pound of TP.
Q: What sort of maintenance is involved?
A: The filter beds require regular tilling, algae removal, and weeding. Pumped systems will typically see less accumulation of muck and debris when compared to pond bench filters because pretreatment is provided in a sedimentation pond and intake screen before entering the pump and filter. Staff will also need to learn how to monitor the operation of the system and troubleshoot as necessary. The automated system can be remotely monitored and controlled.
Q: What are good sites for implementation?
A: Ideal sites are locations adjacent to either an existing pond or along an intermittent or perennial stream; however, it is possible to install alongside a storm sewer. This type of project can be installed on lots as small as a quarter to a half-acre in size. Large drainage areas for regional treatment are more cost-effective than small or local site drainage areas.
