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Time To Replace Membranes or New System?

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Hi, I've been cleaning WFP for over a year now but only been producing my own water for last 3 months. I got given one of the old Merlins (3 filters in line) by a mate when he left cleaning. From reading around the forum I get the idea they're not the best to begin with, but this one of late seems to shoot up on TDS more frequently. I use DI to finish off but even with refilling the vessel the last week in September I checked and today its up at 008 again! So I'm looking for advice on whether any feel it's time to replace the membranes - my mate said they were put in last year - or switch to a 4040? And if I go 4040 what sort of pre-DI TDS can i expect?A few stats are: I use about 900-1200 liters a week, tap TDS is around 300-315ppm, pre-DI is about 34-45ppm. Not sure what water from outside tap is but it is about 60l per minute. Any suggestions on how to proceed are more than welcome. Apologies if a bit wordy, I'm new to all this! Thanks.

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My opinion @DJTheWindowCleaner would be to go the 4040 route and put the money you will spend on membranes and prefilter towards the new r/o.


The Merlin and the later PRF were primary designed as under counter kitchen reverse osmosis systems in American kitchens. So the pure would be connected to the faucet and when the faucet is switch off the resultant back pressure switches the flow off to the r/o. This design works well when using an IBC tank for water storage. When the tank is full a ball valve switches the pure off and the r/o stops producing pure.


If you need the same automatic control with a 4040 then you will need a float switch and a normally off solenoid valve. PureFreedom sell them.


A Merlin was never a very efficient r/o. We've got a local lad who is on his second Merlin and both started off new at around 90% efficiency. Another cleaner got shot of his for the same reason as well as the large amount of waste he had. Being on a water meter meant high water bills and low efficiency meant higher resin costs.

Yours is running at 89% efficiency. So it isn't bad but at your rejection rate your resin is going to be an expense.


A flow of 60liters a minute from the tap is very high - that's 2 & 1/2 x 25 liter plastic water containers. What is more important is the pressure you have at the tap.

We have 50 psi and I fitted an HF5 membrane. It works better than the HF4 with lower water pressure. The HF5 is more expensive.


My 4040 works at an efficiency of 98% plus and is over 4 years old. It helps that our tap water tds is between 90 and 120ppm. At the moment its around 99ppm.

Before di the pure is 2 so our resin cost are negligible. Our pure to waste ratio is 1 to 1 which is better than the old Merlin which was 5 waste to 1 pure.


Most cleaners who have 4040's seem to get a rejection rate of around 97 - 98%. If this is the same with you, then with 315ppm tap water, the resultant pure should be between 7 and 10ppm.

At 45ppm your resin costs are going to be in the region of £32 per month. At 10ppm it will be about £7 per month. That's £300 per year saving on resin alone. In 2 years the resin savings will pay for a basic 4040.

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Thanks spruce. Big help with deciding what to do.

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