How To Deal With Mould On Black Berkey Filters

Mould on gravity filter elements is not a fault in the filter. It is a storage problem, and it happens for one reason: water stood still in a warm place for too long with nothing in it to stop things growing.

The good news is that most of what people identify as mould is not mould. The less good news is that when it genuinely is on the elements themselves, the answer is usually to replace them.

First, is it actually mould?

Four things get called mould and only one of them is.

Black staining on the elements

Not mould. The elements are black to begin with, and they darken with use as they trap material from the water. Grey or black marks on the surface, and grey water running off when you first rinse them, are normal.

Black specks in the filtered water from brand new elements are carbon fines — loose dust from manufacture. They are harmless and they are exactly what the priming and first-flush instructions are for.

Pink or orange slime

Not mould either, and extremely common. This is almost always Serratia marcescens, an airborne bacterium that settles on any damp surface. It is the same thing that produces the pink ring in a shower tray or around a toilet bowl.

It grows on the chambers, the tap and the seals rather than inside the elements. It is easy to clean off and it comes back if the conditions do.

It is not something to ignore. In a healthy person it is a nuisance rather than a hazard, but it should not be in a drinking water system, and its presence means the system is being left damp and unused.

Green growth

Algae, and it needs light. If your filter stands on a sunny worktop or near a window, algae will grow in the chambers and on anything wet.

The fix is to move it out of direct sunlight. Clean it out and it will not return in a dark corner.

Actual mould

Fuzzy and raised rather than flat and slimy. It can be white, green, blue, black or grey, and it grows in circular colonies. It usually appears first around seals, in the tap, or in the bottom of a chamber that has stood with water in it.

This is the one that requires a decision about the elements.

Why it happens

Three conditions together, and a gravity filter creates all three if it is left standing.

Standing water. Water that sits still, at room temperature, for days.

No residual disinfectant. This is the part people miss. Mains water carries chlorine specifically so that nothing grows in it on the way to you. The filter removes that chlorine — which is the point — so everything downstream of the elements has no protection at all. Filtered water in a lower chamber is more prone to growth than tap water would be.

Warmth and damp. A kitchen worktop is a warm place, and the inside of a filter never fully dries while it is in use.

Add a fortnight away on holiday and you have close to ideal conditions. Almost every case of genuine mould in one of these systems traces back to a period of disuse.

What you can save

The chambers, tap and seals: yes

These are hard, non-porous surfaces. Whatever has grown on them is on the surface where you can reach it.

Dismantle everything. Take the tap off, remove the seals and washers, separate the chambers.
Wash with hot water and a little washing-up liquid, and rinse extremely thoroughly.
Sanitise with white vinegar, either neat or diluted, left for twenty minutes. Vinegar deals with mould and with mineral scale at the same time.
Rinse again, properly.
Dry everything completely before reassembly.

The tap is the priority. It is the most common place for growth and the most often skipped. It is a damp fitting that never dries, with internal surfaces nobody looks at. If your tap does not come apart, run vinegar through it, leave it, and flush thoroughly.

Check the seals while they are out. Perished or swollen washers should be replaced. A leaking element seal lets unfiltered water past into the lower chamber, which defeats the whole system and is worth catching.

The elements: probably not

This is the difficult judgement, and the reasoning matters more than the verdict.

Filter elements are porous by design. The whole structure is a network of fine channels running through the material. If mould has established on an element, spores and hyphae can be inside that network, and there is no way to reach them, no way to inspect them, and no way to confirm they are gone.

Scrubbing cleans the outer surface. It does not clean the inside.

You also cannot sanitise them chemically. Bleach destroys the carbon in them. Vinegar soaking does not penetrate and is not good for the media. Boiling damages them. There is no procedure that works.

The recommendation: if there is genuine fuzzy mould on the elements, replace them. Not because a small amount of mould is dangerous in itself, but because you cannot verify it is gone, and the whole point of the system is producing water you can rely on.

The exception: a small amount of pink or orange slime on the outside of an element, in a system otherwise in regular use, is surface bacterial growth rather than mould. Scrub it off, clean everything else thoroughly, re-prime, and return the system to daily use. Watch it for a fortnight.

If the elements have been standing wet and unused for weeks or months, replace them regardless of what you can see. That is the situation where growth has had time to get inside.

What to clean with

Vinegar. The right choice for chambers, taps and seals. It handles mould, bacterial slime and limescale, it rinses away cleanly, and it is safe on drinking water equipment.

Washing-up liquid. Fine on chambers and taps, with very thorough rinsing. Never on the elements — carbon adsorbs detergent and returns it to your water.

Homebrew sanitiser. Excellent for chambers, and formulated for exactly this kind of job. Follow the dilution instructions.

Bleach. Acceptable on chambers at weak dilution with very thorough rinsing, and never on elements. Bleach and activated carbon react and destroy each other, and residue in a filter element ends up in your glass.

Abrasive cleaners and scouring powder. No. They scratch the chambers, and every scratch is somewhere growth can shelter.

Stopping it happening again

All of these are habits rather than products.

Do not let water stand for more than about three days. If the lower chamber has water in it that has been there longer, tip it out and refill. Water that has stood a week is not what you want to drink even without visible growth.

Empty it completely before you go away. Both chambers, and take the elements out. This one habit prevents the great majority of cases.

Dry it out during any period of disuse. Elements out, chambers apart, everything air dried, stored somewhere dry.

Keep it out of direct sunlight. Light causes algae. A shaded corner solves it.

Keep it somewhere cool. Not next to the oven, the radiator or a sunny window.

Clean the tap regularly. Monthly, not annually. It is the part that grows things first.

Deep clean every few months. Combine it with cleaning the elements, which needs doing anyway — the guide to cleaning the elements covers that in detail.

Use it. A system in daily use, with water moving through it constantly, is far less prone to growth than one used occasionally. If your household only filters a couple of litres a day, use a smaller system rather than leaving a large one half full.

Is it dangerous?

Worth answering directly rather than leaving people to worry.

The common household moulds and the pink bacterium described above are not, for a healthy adult, a serious hazard in the quantities involved. People drink from taps with worse growing in the aerator without ever knowing. Nobody should panic about having found some.

Two groups should take it more seriously. Anyone immunocompromised — on chemotherapy, on immunosuppressant medication, or with a condition that affects immunity — has a genuinely higher risk from opportunistic organisms, and Serratia in particular is an established opportunistic pathogen in hospital settings. Very young children and frail elderly people are also less able to shrug it off.

People with mould allergies or asthma may react to spores regardless of whether the amounts would trouble anybody else.

The wider point is about trust rather than acute risk. The reason to have one of these systems is to know what is in your water. A system growing things is no longer telling you that, and the value of it is gone until it is cleaned or replaced.

So: do not panic, do not drink it, clean it properly, and replace the elements if you cannot verify them.

Sizing the system to how much you actually use

A prevention point that deserves its own heading, because it fixes the underlying cause rather than the symptom.

Growth needs standing water. A system that is emptied and refilled daily gives it very little chance. A system that holds twelve litres for a household that drinks three is holding water for four days at a time, every day, permanently.

People routinely buy the largest system they can afford on the reasoning that bigger is better. For this specific problem, bigger is worse. The right size is the one your household empties regularly.

Two people drinking and cooking with filtered water get through perhaps four to six litres a day. A system holding much more than that will always have old water in the bottom of it.

If you already have a large system and a small household, the practical answer is not to fill it to the top. Fill the upper chamber part way, draw it down, and refill. You lose nothing except the ability to leave it alone, which was the thing causing the trouble.

Coming back after time away

The specific case that causes most of the trouble, so it is worth a procedure.

Tip away everything in both chambers without tasting or using it.
Dismantle and clean the whole system as above, tap included.
Inspect the elements carefully in good light, all the way round.
If they are clean, scrub, re-prime and refit them.
Run at least two full chambers through and discard the water.
If there is any doubt at all, fit new elements.

Doing this every time you come back from a holiday takes half an hour and removes the question entirely.

When to give up on the elements

Replace rather than clean if any of these apply.

Visible fuzzy mould on the element itself.

They have stood wet and unused for weeks.

A smell that does not go after cleaning.

Filtered water that tastes or smells wrong once the system is back in use.

A failed dye test — add plain artificial red food colouring to the upper chamber and check whether colour reaches the lower one. Any colour means an element or a seal is letting water past.

They are near the end of their rated life anyway. If in doubt and they are two-thirds used, replace them and stop worrying.

Mistakes worth avoiding

Bleaching the elements. Destroys the carbon and contaminates them.

Soap on the elements. Adsorbed, then released into your drinking water.

Ignoring the tap. It is where growth starts and it is almost always skipped.

Assuming black marks are mould. They usually are not.

Assuming pink slime is mould. It is bacteria, and it is a signal that the system is standing damp.

Leaving it full while away. The single most common cause.

Scrubbing mouldy elements and hoping. You cannot verify the inside, and verification is the point.

The short version

Growth in a gravity filter is caused by standing water with no chlorine left in it, in a warm place, usually while nobody was using the system.

Most of what looks like mould is not: black marks are normal, pink slime is a common bathroom bacterium, green is algae from sunlight. Clean the chambers, tap and seals with vinegar and thorough rinsing, and dry everything.

If genuine fuzzy mould is on the elements, replace them. They are porous, you cannot clean inside them, and you cannot check.

Then prevent it: empty the system before you go away, never let water stand more than a few days, keep it out of the sun, and clean the tap monthly.