Sand Filter

A sand filter is a barrel or tank packed with graded sand over gravel, through which water moves slowly down. The sand is not really what cleans it. Within one to three weeks of starting up, a thin living film of bacteria and other small organisms forms across the sand surface - often called the schmutzdecke - and that film is what strains and digests what the water carries. It runs on gravity and needs no power. A drum of about 208 litres (55 gallons) will deliver roughly 40 litres (10 gallons) of drinking water a day.

Because the film is alive it has to breathe, and it takes its oxygen from the water passing through. Let the filter run dry or work in fits and starts and the film dies with no outward sign, while clear untreated water keeps coming out. Grading takes real care: no more than about a tenth of the sand finer than 1 mm, most grains between 1 and 4 mm, over graded gravel. A filter in good order takes out particles down to about 2 microns, all the algae and organic matter, some four fifths of the ammonia, and between 75 and 98 percent of bacteria along with parasites.

Read that last figure carefully. Removing 98 percent of bacteria is not disinfection, so a sand filter can never be the final stage before a drinking tap and something that disinfects has to sit behind it. Nor can it cope with muddy water, since fine clay is too small for any filter to catch, so settling has to happen ahead of it. Water also leaves with less dissolved oxygen than it arrived with, worth restoring by cascading before it reaches a pond or fish. A carbon stage downstream handles taste and smell, and at larger scale the same design is a cheap way to clean recirculated irrigation water. Clean it when flow drops by about a fifth rather than on a date: rake or skim the surface, an hour or two of work. Scrubbing the bed strips the film and sets the filter back to the start.