How to Size a Hydrodynamic Separator

Hydrodynamic Separators (HDS) have been a staple in stormwater management for over 20 years. These systems are effective at removing Total Suspended Solids (TSS), hydrocarbons, trash, and debris from stormwater runoff. HDS units are often used as standalone treatment solutions or as pretreatment before filtration, detention, infiltration, or rainwater harvesting systems. With their varied applications, HDS units come in a wide range of sizing options.

Choosing the right HDS unit for your project can be a challenging task, but understanding key design considerations can help guide you through the process. Below are several important factors to keep in mind when sizing your HDS unit.

Check Local Regulations First

Before starting your design, always check the local or state regulations. In many cases, regulations will dictate the sizing of HDS units and may even provide a list of approved products. For example, both Washington and New Jersey require specific removal targets based on percentage and particle size, and each state provides a list of products that meet those targets, simplifying the selection process.

What if There Are No Regulatory Requirements?

If there are no explicit regulations, here are some key steps to follow:

• Define Removal Targets: Identify the pollutants you need to remove (e.g., TSS, trash, hydrocarbons) and the required removal percentages. Not all HDS units perform equally across different pollutants, so select one that is optimized for your target pollutant.

• Assess Site Limitations: Consider any physical constraints, such as available footprint and depth for installation. If space is limited, a smaller footprint manhole system may be more appropriate. In cases where utilities, bedrock, or groundwater are factors, a shallower vault system might be more suitable.

• Determine Treatment and Peak Flow Rates: First, identify the required treatment flow rate or annual removal target to help determine the appropriate HDS unit size. Then, evaluate the maximum (peak) flow rate the system will need to handle. Some HDS units include internal bypass features to handle flows above the treatment rate, but for units without a bypass, it may be necessary to take the system offline during peak flow events.

• Evaluate Product Features: Look for HDS units that include features that align with your project goals. For example, systems with screens can help capture trash, while baffles can target oil and grease. Strategic placement of multiple inlet HDS units or grate inlet systems can minimize the number of structures needed, saving time and money. An internal bypass system, for example, can eliminate the need for a separate bypass structure.

• Consider Maintenance Access: Ensure that maintenance access is accounted for in your design. Not only should the unit be accessible for maintenance equipment like vactor trucks, but it should also allow for easy access to remove pollutants effectively. Without proper access, ongoing maintenance could become expensive or even impossible.

Focus on Performance

Ultimately, the right choice will come down to performance. When comparing different HDS units, review third-party testing data and product features to ensure you select the best option for your specific site. Be sure to compare units with equivalent removal requirements to ensure a fair assessment.

By considering these factors, you can confidently select the right Hydrodynamic Separator to meet your project's needs.