Build a gravity bucket filter

A squeeze filter works, but somebody has to stand there squeezing it, one bottle at a time, and in a household that gets old within a day. Stack the same filter element between two buckets and gravity does the work while you do something else.

This is a thirty minute build and it changes the daily water routine more than anything else here.

Why gravity beats squeezing

• It runs unattended. Fill the top bucket in the morning, come back to filtered water.
• It gives you volume. Ten or twenty litres a day rather than a bottle at a time.
• It is gentler on the filter. No hand pressure forcing water through, which means less stress on the fibres.
• A child can use it. Turning a tap is within reach of anyone; squeezing a filter bag properly is not.
• It separates the two buckets clearly. Dirty on top, clean below, which is the single most important habit in any water system.

What you need

Two elements rather than one roughly doubles the flow rate, which is worth doing if you are supplying more than two people.

Ceramic candle elements have one real advantage for this job: they can be cleaned and reused many times by scrubbing the surface, and unlike hollow fibre they are not destroyed by freezing. They are more fragile to knocks.

Building it

1. Drill the element hole in the centre of the bottom of the top bucket. Deburr the edge so it does not cut the washer.
2. Fit the element through the hole, gasket above, washer and wing nut below, snug but not overtightened. Ceramic cracks if you force it.
3. Drill the tap hole near the bottom of the lower bucket, high enough that the tap body clears the base and you can get a jug under it. About 5 cm up is usually right.
4. Fit the tap with a washer inside and out, PTFE tape on the thread.
5. Stack them. Top bucket sits inside or on top of the bottom one. The element must hang free in the air inside the lower bucket, not sit in the filtered water.
6. Lid on top. Always.
7. Prime and test. Fill the top with clean water and let it run through. Check for leaks around the tap and the element. Discard the first full batch.

Sizing it for a household

The page has said this gives you volume, and volume is the point, so it is worth putting numbers on what a household actually needs.

A household of four needs somewhere around 10 to 20 litres a day of genuinely drinkable water, and considerably more than that if washing and cooking are coming from the same source.

Flow is the constraint, not capacity. A single element runs at a few litres an hour when the top bucket is full and slows markedly as the head of water above it drops. So:

• Two elements roughly doubles the rate, and four is not unusual on a household system. This is the cheapest way to get flow.
• Keep the top bucket topped up rather than letting it run down to nothing. A full bucket has more head and runs faster.
• Size the lower bucket to a day's draw, so it is emptied daily and never gets near the element.
• Run it overnight. Fill in the evening, draw in the morning, which turns a slow system into an invisible one.
• A larger top bucket is worth more than a larger bottom one, because it holds the head up for longer.

Have a second, separate route to clean water. A gravity system is a single point of failure: one cracked ceramic and the household has none. A squeeze filter, a stock of chlorine, and the ability to boil are all cheap and all independent of this. See look after a squeeze filter.

Running it

The daily routine:

• Fill the top bucket with the least dirty water you have.
• Let it run through. Expect a few hours for a full bucket.
• Draw off what you need from the tap.
• Do not let the lower bucket fill to the element.

Pre-filter anything cloudy. Let silty water stand overnight and pour off the top, or strain it through a cloth first. Silt clogs an element quickly and it is a waste of a filter that should last years. See judging raw water for what to look for.

Clean the element when flow slows. For ceramic, scrub the outside gently with a clean brush or a scouring pad under running water, which removes the loaded surface layer. For hollow fibre, backflush as described on squeeze filter care.

Keep the two buckets apart in your head and in the cupboard. Mark them, clearly, DIRTY and CLEAN, in permanent marker on the outside. Never put the dirty lid on the clean bucket, never dip a jug that has touched raw water into the clean one, never swap them round. Cross-contamination is the way this system fails, and it fails silently.