Install/commission a water filter like an EXPERT.
I’ve seen poorly designed filters… Badly installed filters… and shockingly operated filters!
Follow this 3 part series where I take you through some key elements of 1) Design 2) Installation 3) Operation of water filters. Follow us on LinkedIn to catch the next posts.
Installation - Pipework
Get the pipe sizing right. Its going to help with pressure drop issues, noise, cavitation and air entrainment. It depends on pipe materials, temperatures and more. Remember to size for the largest flow (usually backwashing) where pipes have dual purpose.
Ideally use pipe modelling software like https://revalizesoftware.com/pipe-flo/ or https://fluidflowinfo.com/ if you can.
Otherwise, for some rule of thumb design we like to use:
Suction pipework ideal flow velocity: <0.5m/s Max <1m/s (i.e. between a tank and pump)
Discharge pipework ideal flow velocity: <2.5m/s Max <3m/s (i.e. from the pump, through the filter, to the discharge point)
Make sure your vessel isn’t full of air. You are not going to see this issue, but it will cause all sorts of problems. Even small bubbles in the feed water during long filtration cycles could build up and create issues within your vessels.
Install combination (air release and vacuum breaker) valves.
Size are release valves appropriately and start filtration slow enough for the air to escape
Placing the ARV on the vessel means less clogging/grit leaking. It means complete air removal as installing these on the pipe requires air pockets to fight against incoming water flow. High velocities across a combination valve can in fact entrain air INTO the system via a venturi effect.
It is sometimes not possible due to provided FRP filter connections, in that case aim for position A and keep pipe size maximal between the vessel and ARV.
Preferrential positioning of an Air Release / Vacuum breaker (combination) Valve on a water filter
Installation - Distributors
Top distributors are not strainers. A top distributor can prevent massive media loss in a pinch, sure… but it is not its purpose. If you got your design and control right, it will never be needed to block media loss in a backwash. It will only fulfill its purpose of distributing the influent flow so that the filter bed surface is not disturbed and flow is even accross the entire surface area.
I’m always shocked how wildly undersized top distributors are. Ask for the manufacturer flow curve and compare it to your backwash flow rates. You will find you may have 2-3bar differential pressure loss accross this unit alone! Then add tiny particles of media getting stuck in the tiny slots and you will see the tell-tale sign of a top-distributor issue:
You start the backwash and initially get your desired flow rate e.g. 20L/s
After 20-30 seconds the filter bed has expanded and some particles start blocking your slots. The flow rate creeps down.
After 2 minutes your low-flow alarms go off (hopefully you have them) or your backwash completes without you realising there was no flow.
The solution is to buy suitable top distributors if you can find some… or customise your own (thats what I do).
Bottom distributors are delecate. I’ve seen experienced engineers and companies try and load GAC or media through the distributor layer with no gravel. Don’t.
Firstly, gravel ensures the distributor slots/holes remain clear for reduced pressure drop and even backwashing flows. Sand/GAC/resin will cover those slots.
Secondly, whilst you may get some ‘EBCT’ or contact zone for some extra adsorption/exchange… the results will be temporary until this media is spent, but more importantly, unless you do a full cleanout on your next media change, you are going to run the risk of ‘dumping’ or ‘leaching’ contaminants back into your polished effluent.
Thirdly, when you do a media changeout, your operator may be a bit rougher or ‘enthusiastic’ than necessary and could damage the distributor leading to a whole world of hurt when you realise you are losing media into your effluent stream and clogging downstream distributors or worse. A good buffer of gravel, or inert glass will protect your distributors and improve your backwash.
Example of top and bottom distributors alongside their flow vs pressure drop curves. Notice how much less flow is achieved on the single top style.
Comissioning - Loading and wetting
Loading is generally done well. Most companies get this right. Fill the vessel 1/3 or 1/2 with water. Then fill the media. This protects your bottom latterals or plenum floor. At Guardian Water we do this via custom designed hoppers for speed and safety or via hydraulic conveying system for difficult to reach places, or when a crane may not be suitable.
Accurate measuring equipment and recording is key to ensure all vessels which will be run in parallel are equally filled.
Wetting is usually insufficient. During filling into a water filled vessel, most media will have air bubbles stuck to their surface or deep within pores and adsorption sites.
Mistake 1 - If you are loading GAC for example, it will float. You may need to release some water to fit all the GAC in.
Mistake 2 - You need to fill the vessel and let the media ‘wet’. For sand only an hour or two. For GAC 24 and like 48 hours. You may need to top the water in the vessel up over the 48 hours as the water level will drop during this time. Backwashing the filter without proper wetting will result in media loss.
Comissioning - Backwashing
Step 1 ensure the media was ‘wetted’. Insufficient wetting will backfire in this step and cause incorrect conclusions and settings. Media with air attached will have a lower density and will backash/expand at lower flux or flow rates. An engineer seeing media in the discharge may lower backwash settings and set the chain of events for massive and potentially irreversible filter blockage.
Step 2 the greater of design or practice. I like to test TWO backwash rates.
Firstly, the design rate, which “should” expand the bed 30-60% as required by the media selection.
Secondly, the rate at which media loss becomes significant.
Knowing these conditions allows you to set the highest possible backwash rate without significant loss. Note however that during initial backwashes the finest media particles may still be lost, this is not cause for reducing the backwash rate lower than design rates. Contact your engineer before ever backwashing less than this rate. Also note, that some media loss during every backwash is normal. Losing 200g/backwash in a 2 tonne filter would mean 10% media loss after 1000 backwashes. At 1 backwash a day that could be almost 3 years.
Just a fun photo of running a backwash with the top distributor removed
Comissioning - Record the ‘clean state’
Dont forget! this is your one chance to record initial clean pressure and flow data. These are the values which confirm your backwashing is returning the system to a clean state.
If you would like some help contact us for an obligation free chat. Follow us on LinkedIn to be notified of the next parts in this series.