Introduction

CPM Group in a joint venture with Hydro International has introduced a Stormwater treatment system – the Downstream Defender®.

The system is an advanced hydrodynamic separator which is incorporated into a reinforced concrete chamber; the system is designed to remove settleable solids, grits and silts, and oil and other floatables from Stormwater runoff.

The Downstream Defender® is suitable for:

• Highway runoff.
• SUDS projects.
• Wetland projects.
• Basin & pond protection.
• Vehicle maintenance washdown yards.
• Construction site runoff.
• Car parks.
• Park & Ride schemes.
• Industrial and commercial areas.
• Quarry settlement ponds.
• Vehicle wheel wash.

With increasing urbanisation, the problems of Stormwater runoff from impermeable surfaces are becoming more apparent. Stormwater runoff from road, car parking areas and other impermeable surfaces often carries a high sediment load, which along with other associated pollutants, such as oils and heavy metals can have a detrimental impact on receiving watercourses.

The Downstream Defender® system provides:

• High Removal efficiencies of greater than 80% depending on the performance specification.
• A wide range of sizes to cater for a broad band of design flows.
• A minimal area compared to lagoons and treatment ponds, typically a quarter of the footprint.
• An ideal solution to Sustainable Drainage Solutions (SUDS) for the prevention or reduction of solids and pollutants entering detention basins/ponds or to replace devices such as silt traps or lagoons.
• In line with the Pollution Prevention Guidelines, the system is ideal where Stormwater drainage is discharged into sensitive areas such as watercourses, rivers and ponds.
• Installation upstream of certain types of proprietary stormwater storage/infiltration devices that call for pre-treatment and/or silt removal.
• The Downstream Defender® has the added benefit that if installed at the beginning of the site construction phases it can be used to control water pollution from the site.

The Pollution Prevention Guidelines issued jointly by the Environment Agency, the Scottish Environment Protection Agency and the Environment and Heritage Services in Northern Ireland aim to prevent the discharge of water containing silts and oils into rivers, streams and surface water drains.

Numerous reports and studies have shown that chemical pollutants and heavy metals tend to be associated with the finer particles in Stormwater sediments. It is therefore essential to contain these pollutants within any removal device.

The design of the internal components prevents the collection material from being re-entrained into the flow during a storm event and more importantly during subsequent storm events, thus helping to retain the pollutants within the sediment storage or oil retention zones.

The advantages of the system include:

• No moving parts.
• No power required.
• Resistant to re-entrainment.
• Highly efficient with minimal head loss.
• Inlet and outlet pipes at same level & inline.
• Small footprint.
• No concrete backfill required.
• Low maintenance costs.
• Availability in a range of sizes.
• Delivered in easy to construct component system.
• 1000mm & 1200mm sizes delivered complete.

Components

The Downstream Defender® consists of either a single reinforced concrete chamber, or a series of pre-sealed reinforced sections (jointed utilising vertical tie bars and joint sealant), with specially designed internal components manufactured from co-polymer polypropylene material. Grade 304 or 316 stainless steel components are also available as specials.

How it works

As contaminated stormwater passes into the unit, it is directed downwards around the periphery of the chamber inducing a rotational flow. This flow, coupled with the design of the internal components, creates a unique flow pattern within the unit encouraging the solids separation.
The settleable solids are drawn down towards the centre into the sediment storage zone beneath the vortex at the base of the unit. The collected sediment is contained outside of the treatment flow path and protected by the centre cone and benching skirt, preventing re-entrainment.
Floatables are directed upwards to the top of the unit and into the oil retention zone (the inner space between the semi-annular ring and the centre shaft).

The treated stormwater is then released from the vortex to the outlet pipe near the top of the chamber through the centre shaft, which also provides access to the sediment storage zone for emptying.

Independent evaluation, Laboratory Tests on MK2 Downstream Defender® Professor CJ Pratt PhD, C Eng, FICE, FIWEM, Dean of The School of the Built Environment, Coventry University confirms the system’s ability to meet the removal requirements for a Class II oil interceptor, (free oil concentration of less than 100mg/l in the discharge) depending on the flow rates (see design flow table below).

Sizing

The table below shows the design flow rates at which typical removal efficiencies of greater than 80% are achievable for oil and sand. Under special circumstances upsizing is recommended. Extra storage capacity is available as specials.

The bulk of pollutant materials tends to be included in the first flush of the runoff during a storm event, as flow rates build up, therefore with a correctly sized unit, the vast majority of solids and hydrocarbons can be removed and retained within the unit.

Headloss at design flow is typically less than 150mm.

Maintenance

A simple gully sucker/vacuum tanker procedure is used periodically to remove the collected oils, floatables and sediments via the access point on the surface.
Hydro International also offers a maintenance contract for the regular inspection and cleansing of the Downstream Defender®. Please contact Hydro International direct for further information.

Design flow rates

Diameter of unit	Design flow	Capacity flow	Design Inlet/outlet pipe diameter	Oil storage capacity	Sediment storage capacity
DN	l/s	l/s	mm	litres	m3
1000	15	24	225	310	0.74
1200	21	35	300	530	0.86
1470	32	53	375	980	2.53
1800	51	104	450	1790	2.95
2000	66	139	525	2460	2.58
2550	115	266	600	5100	3.86
3000	160	410	750	8310	5.79

Precast concrete chambers

Reinforced precast concrete chamber units are manufactured using a combined Portland cement and pfa manufactured to meet Design Class 4 as defined in BRE Special Digest ‘Concrete in Aggressive Ground’ Part 4 Design for Specific Precast Products. The 1000mm & 1200mm units are supplied as a complete one piece unit. The 1470mm, 1800mm, 2000mm units are supplied as a two piece unit with a sealing joint. The 2550mm and 3000mm units are supplied in three main sections with joint seals. No stub pipes are pre-fitted on the 2000, 2550 and 3000mm models, but can be provided loose as an optional extra. The 1000mm unit has a seal for a PVC 250mm OD pipe; the 1200mm unit has a seal for a PVC 315mm OD pipe. The 1470 and 1800mm have plain stub concrete pipes are pre-fitted as standard and are suitable for connections utilising band seal couplings. Please check before ordering for the latest inlet/outlet arrangements. The design of the chamber is such that each unit can be adapted to incorporate extra sediment storage capacity as an additional extra.

General Arrangement
Section

Plan

The drawings show a typical arrangement for the CPM Group/Hydro Downstream Defender®, with polypropylene internal fittings. Drawings for all sizes of the Downstream Defender® are available on request.

Please note stub pipes are pre-fitted on the 1470mm & 1800mm units only.
(Please check when ordering as this maybe subject to change)

Item	Component description	Material
1	Support Frame	Steel reinforced copolymer polypropylene
2	Outlet and by-pass weir	copolymer polypropylene
3	Centre shaft and cones	copolymer polypropylene
4	Semi-annular rings	copolymer polypropylene
5	Benching skirt	copolymer polypropylene
6	Weir plate	copolymer polypropylene
7	Outer annular ring	copolymer polypropylene
8	Base cone	copolymer polypropylene

Unit Weights and Dimensions

Downstream Defender	Depth		Approx Weight	Height	Outside diameter of unit **	Nominal Working Volume
	A	B
	m	m	tonnes	mm	m	m3
1000 Unit (single unit including HD cover slab 600mm sq access)
Type 2 cover slab 600 x 600mm access			0.360	150
Concrete unit (including Vortex and benching skirt)			3.078	2615
Totals	1.864	2.615	3.468	2765*	1.2	1.42
1200 Unit (single unit including HD cover slab 600mm sq access)
Type 2 cover slab 600 x 600mm access			0.390	150
Concrete unit (including Vortex and benching skirt)			3.844	2765
Totals	1.864	2.615	4.234	2765*	1.43	2.02
1470 Unit (two piece unit plus HD cover slab)
Type 2 cover slab 750 x 600mm access			0.787	150
Concrete upper section (including Vortex and benching skirt)			5.179	2615
Concrete base sump section			3.160	1250
Totals	3.114	3.750	9.126	3900*	2.22	4.66
1800 Unit (two piece unit plus HD cover slab)
Type 2 cover slab 750 x 600mm access			1.632	185
Concrete upper section (including Vortex and benching skirt)			7.505	2615
Concrete base sump section			4.620	1250
Total	3.114	3.750	13.356	3900*	2.70	6.87
2000 Unit (two piece unit plus HD cover slab)
Type 2 cover slab 750 x 600mm access			1.906	185
Concrete upper section (including Vortex and benching skirt)			5.282	2500
Concrete base sump section			3.786	1000
Totals	2.740	3.500	10.954	3685*	2.26	7.98
2550 Unit (four piece unit plus HD cover slab)
Type 2 cover slab 900 x 600mm access			2.766	185
Concrete upper section			4.280	1500
Concrete middle section (includes benching skirt)			5.848	2000
Concrete base section			4.725	750
Main vortex section			0.300	2953
Total	3.102	4.25	17.919	4.435*	2.83	15.31
3000 Unit (four piece unit plus HD cover slab)
Type 2 cover slab 900 x 600mm access			4.59	210
Concrete upper section			7.984	2000
Concrete middle section (includes benching skirt)			8.646	2000
Concrete base section			8.240	1000
Main vortex section			0.480	3540
Total	3.480	5.000	29.884	5210*	3.35	24.02

General Handling & Storage Instructions

1. The Downstream Defender® internal components are manufactured utilising highly durable co-polymer polypropylene. Improper handling can result in damage to the components and the precast concrete chamber. Failure to comply with the handling, storage and installation instructions voids warranties.
2. Upon delivery of the Downstream Defender® components, inspect immediately for delivery damage. If any discrepancies or product problems are found notify the supplier prior to unloading to initiate corrective action. Unloading of a damaged unit without notifying the supplier voids all warranties and releases liability of costs of repair or replacement from Hydro International or CPM Group.
3. At all times during unloading and installation, avoid any unnecessary and extreme impacts to the units. Do not allow components to be dropped, rolled or pushed. All components shall be lifted and transported using the designated lifting points where supplied giving firm and complete support to the unit. At no time shall anyone step, stand or otherwise place unnecessary load on the components.
4. The Downstream Defender® shall be installed as soon as possible. Pending installation any loose components including universal head-links, sealant or seals shall be temporarily stored in an area protected from dirt & impact.

     

 

General Installation Instruction:

1. The chamber excavation shall be properly prepared in advance so that the unit may be installed as soon as practical upon delivery. The chamber excavation shall meet all minimum Health and Safety standards for construction.
2. The area of excavation should be carefully checked for services to at least 800mm greater than the external diameter of the chamber (see Information Table Page 9) and any services should be excluded from the area.
3. A sub-base of compacted stone or concrete (depending on site conditions) must be level and at the correct elevation prior to placing the Downstream Defender® concrete chamber in to the excavation.
4. There are thee lifting arrangements for the Downstream Defender® depending on size range.
   • The Cover slab requires a three legged chain (to be supplied by the customer).
   • The 2m to 3m unit requires the use of a spreader beam (Spreader beam is supplied by CPM Group).
   • The 1000 to 1800mm unit uses a pair of universal head-links which hook under the head of the cast-in lifting anchor into; these are located opposite each other on the outside surface of the unit. (Two legged chain is required to be supplied by the customer).

     Details of how to engage the universal head-links are shown below.

   
             
   
5. The inlet/outlet connection arrangements are different on each size range, therefore please contact the supplier for up to date advice.

1000mm & 1200mm single piece Installation Instructions

1. Offload the ready to install precast concrete chamber containing the Downstream Defender® and install in the prepared excavation.
2. Ensure the inlet and outlet is correctly orientated and the unit has been checked for line and level.
3. Make sure all the internal equipment is in place and no tools or loose items have been left in the unit.
4. Place the Butyl sealant around the top surface of chamber before installing the cover slab. The orientation of the access is across the flow, as shown in the 2000mm installation section.
5. The unit can now be carefully back filled with suitable material.
6. The inlet/outlet pipes can now be push fitted into place.
7. Setting-to-work – Fill the chamber with clean water until the discharge from the outlet commences.

1470mm & 1800mm two piece unit Installation Instructions

1. These units are delivered to site in two main components, the sump base chamber which is 1.25m high and the main top chamber which contains the benching skirt and the vortex.
2. Install the sump base chamber into the prepared excavation ensuring to check line and level.
   
       
   The above shows the starting and finishing position of the seal.
   
   
3. Ensure the sealing ring is in the correct position as shown in above diagram. The inner surface of the steel band and the seal must be coated with a jointing lubricant (not supplied by CPM Group) before jointing commences.
4. Carefully lower and guide the main top chamber onto the sump base. Ensure the inlet and outlet stub pipes are correctly orientated.
5. Make sure all the internal equipment is in place and no tools or loose items have been left in the unit before the cover slab is put in place.
6. Place the Butyl sealant around the top surface of chamber before installing the cover slab. The orientation of the access is across the flow as shown below in the 2000mm section.
7. The unit can now be carefully back filled with suitable material.
8. The inlet/outlet stub pipes can now be connected with the use of a band seal coupling (not supplied by CPM Group).
9. Setting-to-work – Fill the chamber with clean water until the discharge from the outlet commences.

2000mm two piece unit Installation Instructions

1. The 2000mm unit is delivered to site in two main components, the sump chamber which is 1m in height and the main chamber which has the vortex and benching skirt pre-fitted (effective height 2.75m). Note: The stub pipes are not fitted on this size of unit, please contact your supplier for up to date connection advice on the inlet/outlet arrangements.
2. Install the bottom sump into the prepared excavation ensuring to check line and level. The unit has alignment marks along the direction of the flow to aid installation.
3. Screw in the four locating nuts and ensure the sealing ring is in the correct position as shown below.
   
   
   
4. Carefully lower and guide the main top chamber onto the sump chamber and the four locating nuts. Ensure the inlet and outlet holes are correctly orientated. Check for line and level.
   
   
   
5. Make sure all the internal equipment is in place and no tools or loose items have been left in the unit before the cover slab is put in place.
6. Place the Butyl sealant around the top surface of chamber before installing the cover slab. The orientation of the access is across the flow as shown below.
   
   
   
7. Setting-to-work – Fill the chamber with clean water until the discharge from the outlet commences.

2550mm and 3000mm four piece units Installation Instructions

1. The 2550mm and 3000mm units are delivered to site in four main components, the base sump chamber which is 1m in height, the middle section which contains the benching skirt, the top section which has the inlet/outlet holes (height of unit 2.55m is 1.5m & 3m is 2m). The main vortex is delivered in a carrying cradle as shown below. Note: The stub pipes are not fitted on these sizes, please contact your supplier for up to date connection advice on the inlet/outlet arrangements.
   
   
2. Install the bottom sump into the prepared excavation ensuring to check line and level. The unit has alignment marks along the direction of the flow to aid installation.
3. Screw in the four locating nuts and ensure the sealing ring is in the correct position as shown in the 2000mm installation.
   
   
   
4. Carefully lower and guide the middle chamber onto the base sump chamber and the four locating nuts. There are alignment marks on the unit to aid installation.
5. Follow the same procedure for the top section ensuring the inlet and outlet holes are correctly orientated. Check for line and level.
6. Unscrew the securing bolts from the carrying cradle to release the main vortex, and carefully lift the vortex out of the cradle. Carefully lower the vortex into the chamber section ensuring the correct orientation of the inlet/outlet holes, there are 10 cut-outs located in the top inside surface of the chamber that the vortex sits into.
   
   
   
7. Place the sealing strip around the top of the chamber, placing centrally over the gap between the concrete and the vortex. Make sure all the internal equipment is in place and no tools or loose items have been left in the unit before the cover slab is put in place. The cover slab can now be installed, orientation as in the 2000mm instructions.
8. Setting-to-work – Fill the chamber with clean water until the discharge from the outlet commences.

Maintenance instructions
Maintenance programme

The purpose of an effective maintenance programme is to ensure the stormwater is being collected and directed through the Downstream Defender® and to ensure that pollutant accumulations do not exceed pollutant storage capacities. The following half yearly inspections, monitoring and clean-out procedures are recommended (note – may require more frequent inspection/cleanout depending on the quality and quantity of the flow):

1. Half yearly inspection and annual clean-out of the unit to monitor and remove pollutants (sediments, oils and floatables) and ensure proper operation and free flow of stormwater through the unit. Removed pollutants should be disposed of in accordance with Local Authority environmental regulations. More frequent clean-outs maybe required in busy commercial areas such as shopping streets and market places.
2. An annual maintenance report should be completed for each unit summarising the monitoring logs that document observations as well as the volume of sediment, oils and floatables that have been removed.

Half yearly inspections procedures

The following procedures outline the work to be completed as part of the maintenance programme:

1. Coordinated inspections with the property owner, facility manager, or assigned contact person to ensure that access to the immediate upstream and downstream structures and the unit is possible.
2. Opening to the immediate upstream and downstream structures from the Downstream Defender® should be inspected to ensure debris is not preventing stormwater from being collected in the drainage system and directed into the unit. Any debris should be removed and properly disposed of.
3. The Downstream Defender® should be inspected for floatables and sediment accumulation. Maintenance logs for the treatment should be completed for each site visit.

Annual clean-out procedures

1. Whilst the Downstream Defender® internal components are manufactured utilising highly durable co-polymer polypropylene, care should be taken during the clean-out operations. Improper handling can result in damage to components and accessories.
2. For most installations, sediment and associated pollutants are considered hazardous waste and therefore, under section 62 of the Special Waste Regulations 1996, a pre-notification notice must be completed prior to the removal or disposal of any waste. For higher risk sites where the potential for spills or accumulation of extra hazardous materials exists, a chemical analysis of floatables, oils and sediment should be carried out prior to removal and disposal.
3. Removal of floatables and sediments from the Downstream Defender® using a vacuum tanker or similar equipment.
4. Evacuating of water and oil should be performed by fully trained and authorised personnel only, and the equipment used, such as cleansing hoses must be equipped with protective nozzles to avoid damage to the Downstream Defender® components.
5. Every time the unit is emptied the internal components should be checked and any damage repaired. Any damage liable to affect the units operation should be reported immediately to the supplier.
6. After emptying, the unit should be thoroughly cleansed using clean water at sufficient pressure. The unit must be then filled with clean water (not black water) until discharge from the outlet commences. Then all the covers should be replaced securely.

Maintenance contract

General
Hydro International can offer annual maintenance contracts that include inspection, monitoring and clean-out procedures all as previously described.

Annual maintenance costs
Maintenance costs for the Downstream Defender® will vary depending on the volumes of sediment and floatables that are disposed of as well as transportation costs for disposal. Typical annual costs for maintenance contracts are available upon request.

Terms and conditions
Upon request of a signed maintenance contract, an inspection will be completed to determine the current accumulation of pollutants. An annual clean-out of the Downstream Defender® will be scheduled along with subsequent half yearly disposal. Payments are due with the maintenance contract.