Eel and Fish Screening

Rotorflush Filters Ltd have developed a range of our award-winning self-cleaning suction intake filters to comply with regulations for fish and eel screening. In Europe, most new and renewed abstraction licences require measures to be put in place for the Protection of European Eels, and screening for fish protection is becoming more prevalent in many countries.

What Do Eel Screens Do?

Eel and fish screens protect young eels - glass eels and elvers - and fish fry from becoming entrained in waterworks, cooling systems, irrigation systems and other raw water intakes. They also prevent eels and fish from impingement on screens enabling them to free themselves back into open water.

Entrainment means caught in the system, impingement means stuck on the intake screen.

Both possibilities are highly likely when you consider the huge volumes of water that are abstracted for industry, agriculture and human consumption each day, and that screening is required through 2 mm (or less) aperture mesh.  

The purpose of Eel and Fish Screens is to constrain the velocity at which water flows through a mesh so that the flow is slow enough for aquatic life can free itself; the mesh size of 2 mm or less is to ensure that young eels and fish cannot pass through.

A few years before the regulations were drawn up, Rotorflush Filters had already developed an innovative and reliable self-cleaning suction intake screen.  

Our Self-cleaning Filters are Eel & Fish Friendly

By happy chance, the design of Rotorflush self-cleaning suction intake filters lends itself to compliance with current Eel Regulations, and rules for fish friendly cooling water intakes in North America, due to the low intake velocities and backwash flow.

The self-cleaning intake makes possible the use of suction side 2 mm aperture screens when pumping raw water.  Without the self-cleaning mechanism, the 2 mm screens would rapidly clog with weed and other water-borne particulate. And this applies to young fish and elvers…

The regulations have come about due to a catastrophic decline in the eel population – succinctly described here in the Environment Agency Eel Manual Overview.

The regulations set criteria for the use of raw water and screening and pumping equipment. In simple terms, the regulations seek to ensure that Eels at all stages of life are protected, but particularly glass eels and elvers. Eels live in fresh water at this young stage of their lifecycle. 

The Eel regulations are concerned, in part therefore, with the intake velocity of pumps screens and strainers. For eel screening, the angle of approach, intake velocity and the size of mesh aperture are the determining factors.

Eel screens and Eel screening may be required if you are pumping and screening raw water. You may be affected by the current Eel Regulations designed to help the recovery of populations of European Eels. The relevant legislation in the UK is ‘The Eels (England and Wales) Regulations 2009 Statutory Instrument No. 3344’, and ‘The EU Water Framework Directive – integrated river basin management for Europe‘.

The installation of low intake velocity eel screens ahead of pumping equipment may be a condition of your water abstraction licence in England and Wales.

In the USA the Environmental Protection Agency (EPA) has put in place measures to protect aquatic wildlife – in particular the measures outlined in the Clean Water Act Section 316(b).  These regulations are similar to the European Eel Regulations in that they stipulate intake velocities and mesh size sat water intakes.  The relevant US legislation is the EPA’s ‘CWA 316(b) Cooling Water Intake Structures‘.

A few years before the regulations were drawn up, Rotorflush Filters had already developed an innovative and reliable self-cleaning suction intake screen.  The screens are cylindrical and designed to attach onto the suction pipe of pumps, like basket filters. They allow dirty water to be pumped without blocking the screen intake, the pump or any attached equipment such as irrigaton nozzles. The screens are water powered, the design is simple and can be scaled to meet flow rates up to 250 litres / second.

2 mm Aperture Eel Screens

The Eel regulations require a maximum screen aperture of 2 mm when drawing water in order to protect glass eels and elvers. Preventing 2 mm mesh from blocking while screening 1000’s of cubic metres of water a day is a challenge.

The self-cleaning intake makes possible the use of suction side 2 mm aperture screens when pumping raw water.  Without the self-cleaning mechanism, the 2 mm screens would rapidly clog with weed and other water-borne particulate. Our 2 mm screens scored highly in an Environment Agency trial of commercial eel screens carried out in the Somerset Levels. The trial evaluated four types of screen for use with large mobile pumps, and resulted in the purchase of 16 Rotorflush units.

Very Low Intake Velocities

Another challenge is that the Eels regulations stipulate intake velocities for screens.  The intake velocity for a particular installation of an eel screen is decided by the Environment Agency and is a condition of a water abstraction licence.  Typically, the intake velocity of eel screens for the protection of glass eels and elvers is 0.1 metres / second, although this can vary. In the US, fish screening for cooling water intakes requires a 0.5 ft per second intake velocity, which equates to 0.15 metres a second

South West Water have bought and successfully trialled 3 of our innovative self-cleaning eel screens in the River Avon at Matchams. They required an intake velocity of 0.15 metres / second through bespoke self-cleaning eel screens.  The screens remained clear while each delivered 750 litres / second through 2 mm mesh.

The design of Rotorflush self-cleaning lends itself to meeting low intake velocities.  The self-cleaning mechanism in these screens works on a simple principle.  The screen intake is high volume, low, velocity. The backwash that keeps the mesh clear is low volume, somewhat higher velocity.  Any debris, particulate or junior eels are gently pushed away from the mesh before they get a chance to adhere.

Construction projects on waterways requiring the removal or movement of water generally have to comply with Eel regulations.  The Environment Agency is making good use of Rotorflush self-cleaning eel screens on such sites where it is impractical to net the waterway.  The screens allow water to be pumped and screened without blinding, and protect wild life by keeping the intake velocity at 0.1 metres / second or less.

An intake velocity 0.1 metres / second is a very low for any pump.  The only way to lower the pump intake velocity for a given flow rate is to increase its open area. Increasing the open area of an intake screen increases its size. The available depth and space for installation for screening must be considered.

Smaller Screens – Bigger Flow Rates

For example, a cylindrical screen fitted with 2 mm aperture perforated mesh has approximately 33% open area. To ensure an intake velocity of 0.1 metres / second when pumping 100 litres / second, the screen would need to be 70 cm in diameter and 70 cm high. Woven stainless-steel mesh has a greater open area – approx. 60 %, but needs to be supported, so allowing for a supporting cage open area could be as high as 45% – reducing the screen size to 60 cm x 60 cm.

The completely submerged cylindrical Rotorflush screens take up less space for the same open area than travelling screens. To keep their self-cleaning eel screens as compact as possible while meeting Eels regulations, Rotorflush Filters Limited have introduced another innovation. The 2 mm mesh is tensioned to form the body of the screen, this maximises the open area of 2 mm mesh to as much as 60%. For 100 litres / second, the screen is reduced to around 50 x 50 cm.

Water companies have already invested significantly in addressing the requirements of the Eels Regulations and will continue to do so.

By adopting innovative approaches, the environmental impact of meeting Eels Regulations can be reduced while providing efficient, low maintenance eel screening.