For the first time ever I have looked properly at my cold water tank. It has one inch of brown sludge at the bottom, what should I do. I assume this is the tank that provides the water hot/cold to everything except the main kitchen taps. (But is not involved in the radiators, that is in another tank in the loft ?). The water itself appears clear but it cannot be right for then to be brown sludge.

(I have also checked the feed/expansion tank used by the central heating, and this is looks murkier, but doesnt have an inch of sludge at the bottom)

I assume I have it the right way round, the cold-water tank has alot more connectins than the feed-expansion tank.

Where would It come from and how can I get rid of it ?

2 Answers 2


It is hard to say what the sludge is without examining it. If light is able to get in and if the water stagnates alot instead of flowing, it could be organic. If that is the case, I would boost the chlorine level. Also, a water softener would reduce the mineral content of the water, which would reduce all sludge, organic or not. Also, check the pH of your water to make sure it is not too acidic or basic. If so report to your water supplier utility. My understanding is they are supposed to provide neutral pH water. If they don't remedy, add appropriate chemicals to neutralize pH.

If you can detach and remove tank, do so. Once detached, If you can get a filler brush in there go ahead and brush off the bottom. Get some vinegar and baking soda or any commercial grade non-toxic solvent (Puriclean or Milton) in there and agitate the tank. Back wash several times with hot water and repeat procedure until all sludge is gone.

For in situ situations, here are how different people did it at this forum:

If you can get the end of a hosepipe through the inlet, with a high pressure spray fitting on the end, just wiggle and poke it around and then mop out the residue with a rag/hand pump.


a stick with a ball of cloth on the end then lots of rinsing.


by putting a little sharp sand in plus some water and sloshing it all about

  • But where does the sludge come from, there are an awful lot of connectors so I don't want to try and detach, can I clean in situ Dec 3, 2017 at 22:21
  • I read the link, but someone's suggests using bleach, this tank feeds the taps that I brush my teeth with, so surely that is dangerous. Dec 4, 2017 at 8:13

The sludge comes from the fresh water supply. Well supplied systems an inch is very normal with regular flushing at least 1 time a year for the pressure tank and hot water tank. A city water system this amount will usually take several years unless you are at a low point. Most homes on city water don't have pressure tanks but some do. In all cases it is a good idea to flush any storage tanks to get rid of the sediment. New wells are the worst and wells that are pumped to there max flow rate are the next. This is normal , it is a deep hole in the ground that some dirt and minerals enter so flushing at least 1 time a year is a good idea.

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