I make a litre of coldbrew coffee every few days, its 72g of grounds in 1.2 litres of water which sits overnight and then needs filtering. The problem is the finer particles block the filter paper, it usually takes two or three papers to filter the whole lot which is a pain in the **** I'd like to avoid. I assume a blocked filter is a common problem in other areas so maybe someone can help?

I'll borrow an Aeropress soon and try forcing it through that but any ideas are appreciated. I've got a big Erlenmeyer flask somewhere, maybe I'll try vacuum filtration.

It gets to this stage and stops dripping so I tip the unfiltered stuff into a new filter paper. Maybe the burrs that ground it are worn, it does look like an awful lot of fines.

I put it in a cream whipper and serve it on nitrogen though, I think the fines would stop the seal from sealing and leak my shockingly expensive gas.

If you have a clogging there's only two things to do.
Coarser filter to let it slip thru or larger filter area.
Basic filtration rules.

Then you can go into multiple stage filtering but the final filter still needs to be large enough for the amount of particles to be filtered and it's much overkill for coffee making

cut a hole in the bottom?

Thats a VIZ top tip

I've finally found Chemex filters for a price I'm willing to pay so I'll try those next time, they're about four times the size of a Hario filter but quite a bit thicker too so it might not help.

With regards to multi-stage filtering, I tried cafetiere followed by paper filter but it didn't make much difference. I wondered if the bigger particles sitting in thr filter actually helped as they might stop the fines blocking the pores in the paper?

Is the last 3mm above the grounds that tast the best

Work out a way of "back flushing" it?

Syphon most of it from the top then do the rest, less solids at first

More to the point of your original post what I would go for would be Whatman #1 filters in 500mm diameter with a fluted fold to increase surface area and in the biggest funnel the catering suppliers can sell. Might be a bit spendy though. I can send 1 or 2 from work if you want to try them.

Thanks the the offer, Chemex filters are about 500mm squares, I'll pleat one when they arrive. Didn't think of that.

You are only using a tiny portion of that filter paper in the above setup. A flat bottom grill feeding into the funnel will spread the flitration.

My guess is all the blockage will be in a 10mm diameter circle in the centre above the funnel outlet.

Make up a small circular basket that fits onto the funnel with a coarse wire mesh and a flat bottom. Probably find something suitable in a kitchen shop.

The funnel is lined with ridges which in theory negate that, but you're right, it definitely doesn't work.

How about the suds going thru a fine flour seive first....?