Reuse, Recovery & Stormwater
Hydrodynamic Separator Companies
Vortex and swirl-concentrator manufacturers removing TSS, oil, and floatables from stormwater runoff.
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Hydrodynamic Vortex Separator Design: Treatment Flow Rate, Bypass, and Pollutant Removal Efficiency
Hydrodynamic separators use centrifugal vortex action to remove settleable solids and free-phase hydrocarbons from stormwater and highway runoff. Design parameters include: treatment flow rate (Qtr, typically the 1-year return period 15-minute flow for the catchment), bypass flow rate (flows above Qtr are bypassed untreated via overflow weir to prevent re-suspension of captured material), and capture efficiency (typically 80 to 95 percent of particles above 50 to 125 microns at Qtr). Product certification in the UK uses BBA (British Board of Agrément) assessment; in the US, the NJCAT (New Jersey Corporation for Advancement of Technology) verification protocol tests removal of ISO 11 medium test dust to specified particle size distributions.
Separator types include: dual-vortex separators (primary upflow vortex chamber for floatables, secondary downflow cyclone for settleable solids), screened hydrodynamic separators (perforated internal screen retains coarse debris), and CDS (Continuous Deflective Separation) units using a screen cartridge and cyclonic flow. Flow-through velocity must be maintained below the scour velocity of captured sediment (typically 0.15 to 0.30 m per s in the settlement zone) to prevent re-suspension during peak flows; the integral bypass weir or sluice achieves this. Sediment storage capacity is designed for 6 to 12 months' accumulation between cleanouts based on catchment-specific sediment generation rates (0.5 to 2.0 tonne per ha per year for urban roads).
Regulatory context in the UK: hydrodynamic separators are listed in CIRIA C697 (the SuDS Manual) as a flow-treatment SuDS component providing water quality benefit. The SuDS Non-Statutory Technical Standards (DEFRA 2015) require that highway runoff not enter surface water without appropriate treatment. Environment Agency PPG3 (now replaced by relevant guidance in site condition reports) specifies Class 1 separator performance (EN 858-1) for high-risk spill locations. US highway authorities (FHWA) include hydrodynamic separators in stormwater best management practice (BMP) manuals. Annual maintenance cost (vacuum tanker cleanout): $500 to $2,000 per unit. Capital cost: $5,000 to $50,000 depending on flow rate and device type.
Frequently Asked Questions
What is a hydrodynamic separator and what does it remove?
A hydrodynamic separator is an underground or surface stormwater treatment device that uses centrifugal vortex flow to separate settleable solids (sand, grit, fine sediment), free-phase hydrocarbons (oil and fuel from road runoff), and floating debris (litter, leaf matter) from stormwater. At treatment flow rate, devices certified to NJCAT or BBA protocols typically remove 80 percent or more of total suspended solids (particles above 125 microns), 90 percent or more of free-phase oil, and capture gross pollutants above 5 mm. They do not remove dissolved pollutants (dissolved metals, chlorides, dissolved hydrocarbons) or colloidal particles below 20 to 50 microns. For dissolved contaminants from highway runoff (zinc, copper, polycyclic aromatic hydrocarbons), biofiltration or infiltration SuDS components are required downstream.
How often does a hydrodynamic separator need to be emptied?
Cleanout frequency depends on catchment sediment load: highway catchments generate 1 to 5 tonnes of sediment per hectare per year; commercial areas generate 0.5 to 2 tonnes per hectare per year. Separator sediment storage capacity is typically 0.3 to 3 m3 depending on device size. Most installations require cleanout every 6 to 24 months. Annual inspection is mandatory to assess sediment depth (typically trigger cleanout when sediment depth reaches 50 to 70 percent of storage capacity to prevent bypass of accumulated material in storm events). Cleanout requires a vacuum tanker service (environmental contractor with waste carrier licence, cost $500 to $2,000 per visit). Floatable hydrocarbons are removed by vacuum along with the water above the sediment layer. Waste is disposed of as liquid waste or, if soils-only, potentially as controlled waste at lower cost.
What is the difference between a hydrodynamic separator and a Class 1 oil separator?
A hydrodynamic separator is primarily designed for stormwater management - handling peak flow events with a built-in bypass for flows above the treatment rate. A Class 1 oil separator (EN 858-1) is designed for continuous low-flow process drainage (forecourt drainage, vehicle wash, workshop floor drainage) and must achieve effluent free oil content below 5 mg per L at its nominal flow rate with no bypass. Hydrodynamic separators provide variable performance depending on flow relative to treatment rate; EN 858-1 separators provide specified performance at all flows within design range. For highway drainage, a hydrodynamic separator is typically specified. For fuel station forecourts or workshop areas where hydrocarbon spills are a primary risk, a Class 1 separator to EN 858-1 is required by the Environment Agency.
Can a hydrodynamic separator handle spills as well as routine runoff?
Hydrodynamic separators provide limited spill containment: the free-phase oil storage capacity is typically 400 to 2,000 litres depending on unit size, accumulating between maintenance visits. A significant fuel spill (200 to 500 litres from a vehicle accident) would partially be captured but could exceed the floating oil storage capacity, particularly if a storm event flushes the separator during or after the spill. For high-risk spill locations (fuel depots, chemical storage, motorway maintenance depots), dedicated spill containment with bunded storage, interceptors rated to EN 858-1 Class 1, and automatic inlet shut-off (activated on oil detection) is required. Hydrodynamic separators should not be the sole protection measure for sites with foreseeable high-volume spill risk; they are appropriate for routine diffuse pollution management.
A highways authority in the East Midlands needed to retrofit stormwater treatment to 18 km of an A-road dual carriageway draining untreated to a chalk stream SSSI. Annual TSS load from the carriageway was estimated at 45 tonnes, with free-phase hydrocarbons from road runoff a particular concern given the chalk stream's designation as a special area of conservation.
Specified dual-vortex hydrodynamic separators sized to the 1-year return period 15-minute treatment flow rate for each drainage catchment (units ranging from 150 to 450 mm diameter). BBA-certified units with NJCAT-verified 80 percent TSS removal at Qtr were selected. Integral bypass weirs provided hydraulic protection at flows above Qtr. A 2-year maintenance contract with a licensed waste carrier was established for annual vacuum tanker cleanout of all 22 units.
Post-installation monitoring over 12 months showed a 74 percent reduction in TSS load entering the chalk stream and 91 percent reduction in free-phase hydrocarbon events detected by Environment Agency automated samplers. All 22 units were maintained within the 6 to 12 month cleanout schedule. The authority subsequently used the same specification for 6 additional highway improvement schemes, creating a standardised procurement approach.
Questions to Ask Shortlisted Providers
- 1
What certification standard does the device hold: BBA, NJCAT, or both, and at what particle size distribution was the removal efficiency tested?
UK specifiers typically require BBA certification. NJCAT is the US standard and may not be accepted by the Environment Agency as evidence of performance. The particle size used in testing (ISO 11 medium test dust d50 125 microns is the NJCAT standard) significantly affects quoted removal efficiencies: efficiency measured at coarser particle sizes looks better but is not conservative for urban highway catchments where fine particles dominate.
- 2
What is the treatment flow rate (Qtr) for my catchment, and how was it calculated?
Qtr determines the hydraulic size of the unit. Under-sizing (using a lower flow rate to reduce cost) means a greater proportion of storm events bypass treatment; over-sizing wastes capital. Qtr is typically the 1-year return period 15-minute rainfall intensity applied to the contributing impermeable area. Confirm the rainfall data source (FEH statistical method) and the impermeable area calculation used to derive the design flow.
- 3
What is the sediment storage capacity and the recommended cleanout trigger depth, and how do you expect sediment accumulation to differ for my catchment?
Sediment accumulation rates vary significantly: highway catchments generate 1 to 5 tonnes per hectare per year; car parks and industrial areas generate 0.5 to 2 tonnes per hectare per year. A unit sized for low-generation rates on a high-generation catchment will fill to trigger depth in 3 months rather than 12, resulting in bypass of accumulated sediment in storm events if cleanout is delayed.
- 4
Is the unit compatible with the local groundwater depth and seasonal high water table, and how is flotation of the unit prevented?
Hydrodynamic separators are typically installed as in-ground precast concrete or GRP units. If groundwater is within 1 to 2 m of the invert, upward hydrostatic pressure during the non-operational season can cause flotation of an empty unit. Anti-flotation collars or ballasting must be specified and their adequacy checked against site investigation data for the highest recorded groundwater level.
- 5
What waste classification will the extracted sediment attract, and who bears responsibility for waste carrier licensing and disposal?
Sediment from highway drainage separators often contains petroleum hydrocarbons, lead, zinc, and PAHs at concentrations that classify the material as hazardous waste under the Hazardous Waste Regulations 2005. Disposal cost for hazardous liquid waste runs to 150 to 400 GBP per tonne versus 50 to 100 GBP per tonne for non-hazardous. Confirm whether the maintenance contract includes waste characterisation, whether the contractor holds a waste carrier licence, and how disposal costs are structured.
What Drives Cost in This Category
A 150 mm diameter hydrodynamic separator handling Qtr of 5 to 10 L per s costs 1,500 to 4,000 GBP in materials. A 1,200 mm diameter unit for Qtr of 150 to 300 L per s costs 15,000 to 35,000 GBP. Installation cost (excavation, bedding, backfill, connection to existing drainage) adds 3,000 to 15,000 GBP per unit depending on depth and proximity to carriageway.
Vacuum tanker cleanout of a single unit costs 300 to 800 GBP per visit, plus waste disposal. If sediment is classified as hazardous, disposal adds 150 to 400 GBP per tonne. For a programme of 20 units on a highway scheme, annual maintenance costs run 15,000 to 40,000 GBP. Maintenance costs over a 25-year design life typically equal or exceed the installed capital cost of the devices.
High-generation catchments (motorway service areas, industrial estates, fuel depots) may require 4 to 6 cleanouts per year per unit versus 1 per year for a suburban road. This increases annual maintenance cost by 3 to 5 times, making larger units with greater storage capacity (and higher capital cost) more economical over the asset life.
Hydrodynamic separators in medians, verges, or areas without road access require traffic management (lane closure, stop-and-go signing) for cleanout. Traffic management for a single cleanout visit adds 2,000 to 8,000 GBP per visit on a principal road. For difficult-access installations, the lifetime maintenance cost uplift may justify a higher-capacity unit with less frequent cleanout.
Key Regulations & Standards
CIRIA C697 (the SuDS Manual, 2007, updated 2015) classifies hydrodynamic separators as a flow-treatment SuDS component providing water quality benefit for removal of suspended solids, hydrocarbons, and gross pollutants. The Non-Statutory Technical Standards for SuDS in England (DEFRA 2015) require that highway and surface water drainage not discharge untreated to surface water without appropriate quality management. Hydrodynamic separators are an accepted compliance measure for standard 'source control' water quality management.
EN 858-1 classifies oil separators as Class 1 (below 5 mg per L residual oil) or Class 2 (below 100 mg per L residual oil) under full-flow conditions. Hydrodynamic separators are not rated to EN 858-1, as they have a bypass for flows above Qtr and do not achieve a specified effluent oil concentration. The Environment Agency requires EN 858-1 Class 1 separators for fuel station forecourts, vehicle washes, and areas with foreseeable bulk hydrocarbon spill risk. Hydrodynamic separators are appropriate for diffuse highway runoff, not high-risk spill sites.
Environment Agency Pollution Prevention Guidance Note 3 (now superseded by sector guidance but still referenced in planning conditions) specifies that surface water drainage from high-risk sites (fuel stations, vehicle service areas) must pass through a Class 1 separator before discharge to surface water or ground. For lower-risk highway drainage, the equivalent guidance is to apply appropriate SuDS treatment. Planning authorities routinely apply these requirements as planning conditions enforced under the Environmental Permitting Regulations 2016.
DMRB Volume 11, Section 3, Part 10 (HA 103/06) provides design guidance for stormwater management on trunk roads and motorways in England. It specifies that highway drainage to sensitive watercourses must include appropriate treatment to prevent pollution. Hydrodynamic separators meeting BBA certification are an accepted proprietary treatment measure under DMRB for diffuse runoff management. National Highways (formerly Highways England) uses DMRB requirements as the design standard for all trunk road and motorway schemes.
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