Self-cleaning intake screens and disc filters: reduce the dirt load before it reaches your plant
By Dan Norman, Sales Manager, Water Process Systems, PureTec Separations
Every stage of dirt you remove at the intake makes the water worth more and costs the downstream plant less. The kit to do it now sits in the river all year round, without a diver.
Intake water is the point where the water in your process starts its journey. That journey could take it once through your power station as cooling water, maybe all the way into a finished product such as beer.
No matter what the final point in its journey, the water comes from a source as diverse as a sea, a river, a borehole or the town's water main.
Before the water even enters your process, we could start to reduce the dirt load in it.
The traditional intake train
A traditional water treatment process for a boiler feed would see a fixed coarse screen positioned in the river. Normally sized around 5mm, this screen is a simple barrier to larger items. Planned and unplanned maintenance of these screens isn't simple, usually involving a diver or a crane, sometimes both.
Next in line would be a lamella clarifier using flocculation chemicals to take out the silt and clay fractions then onto ultrafiltration (UF) and reverse osmosis (RO) plants. Along the way there would be a screen filter, sometimes called a police filter, to add peace of mind to the whole filtration train, with a resin bed or two to make sure the final water was within boiler specification.
What replaces the fixed screen and the clarifier
What about updating some of this tech?
The idea with intake filtration is always centred around gradually reducing solids. Putting a 0.1 micron filter straight into work would see it blind off immediately, requiring manual intervention and a very unhappy client.
Right at the start, this boiler feed could have a self-cleaning intake screen to keep large items out of the intake line such as twigs, branches, plastic bags and native wildlife. The European eel is protected under the Eels (England and Wales) Regulations 2009, and a well-designed intake screen should have a sufficiently low intake velocity to prevent elvers from being trapped by suction forces.
Self-cleaning intake screens use a small return line from the intake pump to continually jet the dirt away from where it gathers on the outside of the screen. This means that blockages are extremely rare. Combined with the common practice of sizing at 2 x 100%, two screens each rated for the full flow, they'll happily sit in the river all year round.
Then the water could move through a series of disc filters. Typically a two-stage process is selected with common filtration values of 130 and 40 microns.
Disc filters are a better choice here than screens as they offer a form of depth filtration rather than the simpler one-shot chance to capture dirt particles. They also use much less water than a media filter and take much less time to backwash themselves.
These disc filters can be containerised or sit inside existing infrastructure depending on customer choice. They require very little power to operate and offer the choice of pneumatically or hydraulically driven valves.
Stopping zebra and quagga mussels at the intake
Zebra and quagga mussels are invasive species from the Black and Caspian Sea region. They have been spread via international shipping around the world and are now prevalent globally. They're freshwater mussels that tolerate brackish water, so estuaries and rivers are both at risk. In the UK they're well established across southern and eastern England and still spreading. They are as far upstream in the Thames as Oxford and they've been recorded in the Severn at Worcester.
They attach themselves to any available surface where running water carries nutrients to them and multiply ferociously. They will obstruct pipework, foul heat exchangers and cost money in lost flow as well as additional cleaning.
A 40 micron disc, or anything finer, does a second job as well as cutting the dirt load. It stops the larval form of mussels, their veligers, from entering your pipe network and infesting your plant.
Read that again. If you have a mussel infestation problem, a 40 micron disc system will stop it in its tracks.
From the discs, you can then proceed straight into a UF plant as the dirt load in the water is now sufficiently low to bypass the need for a lamella clarifier.
If you are retro-fitting discs into your system and want to retain your lamella clarifier, you will see a sharp drop in solids presenting at the lamella and associated chemical usage will drop considerably at this point.
Intake filtration at any flow rate
I know what you’re thinking: the flow at my power station is way too big for the technology you’re proposing.
All of the technology I described above is scalable.
If you have a factory using 80 m³/h or a power station drawing 3,500 m³/h, the same technology applies. The only thing that changes is the footprint.
If any of this applies to your intake, book a discovery call with us and we'll be happy to chat.
Dan Norman is Sales Manager, Water Process Systems at PureTec Separations, specialists in bespoke water and wastewater treatment from the intake through to ultra-pure water.
