Pretreatment That Pays Off: Protecting Underground Detention Systems with Hydrodynamic Separators

How Hydrodynamic Separators Extend the Life and Lower the Cost of Underground Detention Systems

Pretreatment That Pays Off: Protecting Underground Detention Systems with Hydrodynamic Separators
Underground detention systems have become a go-to solution for stormwater management in space-constrained developments. They’re compact, versatile, and allow engineers to meet detention volume requirements beneath parking lots, roadways, and plazas.

But detention alone doesn’t treat runoff, and without proper pretreatment, these systems can quickly fill with sediment, debris, and floatables. The result? Lost storage volume, clogged orifices, and higher maintenance costs. That’s where hydrodynamic separators (HDS) come in. Properly sizing an HDS as a pretreatment device is one of the most cost-effective ways to extend the service life and performance of underground storage systems.

The Problem: Sediment Loading and Lost Capacity

Every storm event carries suspended solids and litter into drainage systems. When runoff enters a subsurface detention facility directly, these pollutants settle within the chambers or pipes, gradually reducing effective volume and impeding flow.

Even small sediment accumulations can cause major operational issues:

• Reduced storage volume over time, compromising detention performance.
• Blocked outlet controls and flow restrictors, leading to unpredictable discharge rates.

Without upstream capture, many detention systems require regular cleaning to maintain design capacity, an expensive and disruptive process for property owners.

The Solution: Hydrodynamic Pretreatment

Hydrodynamic separators like the CDS® provide an efficient way to intercept pollutants before they reach underground storage. Using swirling or vortex flow, these systems separate and store coarse sediment, trash, and floatables in a confined sump area or inner cylinder while allowing clean water to pass through.
When installed upstream of a detention system, an HDS unit acts as the first line of defense, keeping the larger system cleaner, more efficient, and easier to maintain.

Quantifying the Payoff

Many HDS/pretreatment devices are documented to remove 50-80% of TSS under field conditions; in select cases, higher removal rates have been observed. In practice, that translates to:

• Maintenance interval extension, detention systems can go 3–5 years between cleanouts instead of every year.
• Reduced vacuum time and disposal costs, as most debris is captured in a single, easy-to-service device that can be accessed and maintained from a single point at the surface.
• Preserved hydraulic performance, ensuring outlet controls operate as designed.

The cost of adding an HDS unit is typically recovered quickly through lower O&M expenses and reduced system downtime.

Design Integration Tips

For civil engineers, integrating pretreatment is straightforward:

• Locate the HDS structure upstream of the storage system to intercept flow before it enters the chambers or pipes.
• Verify sizing using the water-quality design flow rate (often the 80th percentile storm).
• Provide vactor truck access for annual inspection and maintenance.
• Coordinate elevations to maintain hydraulic grade line (HGL) consistency.
• Include inspection ports or sumps in the detention design for long-term maintenance access.

The Bottom Line

Hydrodynamic separators don’t just protect water quality, they protect your investment in underground infrastructure. By capturing sediment and debris before it reaches a detention facility, an HDS unit safeguards system performance, reduces maintenance burdens, and ensures long-term compliance.
In short: pretreatment pays off, and for underground detention, it’s the simplest way to keep your system working as well in year ten as it did on day one.