Slow Sand Filter

A slow sand filter cut open to show its layers: it only filters when water seeps through very slowly and the living film on the sand has ripened.

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Schmutzdecke: the living layer that does much of the treatment, thickening as the bed ripensCock quadrant: filtration rate, with the guide's 0.1–0.2 m/h band set offDay wheel: days since the bed was new or scraped, a major tick each weekDrag the cock's lever (or press Left and Right) to set the filtration rate from 0 to 0.4 m/h; drag the day wheel (or press Up and Down) to age the bed over 21 days; R scrapes the sand and starts the ripening again. The guide asks for at least 60 cm of graded sand and a rate of roughly 0.1–0.2 m/h, and says the Schmutzdecke develops over a week or two: faster flow is not filtering, a new or scraped bed is still ripening, and even the best state here only means 'still disinfect', because filtration alone may not meet a safe microbiological target. Figure details, not from the guide: the outlet is drawn as a vented gooseneck whose crown stands above the sand, a common arrangement that keeps the bed wet, and an overflow to the apron's drain. Illustrative, not from the guide: the bed's area (1 m by 0.9 m), the gravel depths, the cock's linear calibration and range, the one-minute sand-glass and the jug, the layer's thickness, which is drawn far thicker than life, the drifting particles and how many the layer catches, and all the motion, which is sped up (the one-minute glass runs in six seconds). The rate quadrant is a bench-model instrument. Treat a covered filter tank like the guide's cistern: a confined space and a drowning hazard; do not enter it without the controls the guide lists.

What you're looking at

This is a slow sand filter, a tank of graded sand that filters drinking water as it soaks downward. The front of the masonry tank is cut away, so you see it in layers: half-round drain tiles in coarse gravel at the bottom, then fine gravel, then 60 cm of sand marked on the depth staff at the front right corner, a dark green film on top of the sand, and standing water above it, kept at a steady level by the ball float at the back left. Filtered water leaves at the bottom, climbs a pipe on the right that loops above the sand so the bed never drains, and drips from a tap (the cock) into a measuring jug, timed by the sand-glass beside it. You drag the cock's lever on its curved rate scale, and you turn the day wheel on the front left of the base; the green film, the specks of dirt, the jug and the sand-glass all move in response.

What it means

The cleaning is done mostly by that green film, the Schmutzdecke (German for 'dirt layer'), a living layer of microbes that the guide says grows over a week or two on new or freshly scraped sand. The guide asks for at least 60 cm of sand and water moving down at roughly 0.1 to 0.2 metres per hour; any faster and it isn't filtering. So the figure shows the two things a builder controls: how fast you let water out, and how long the bed has been ripening. Even when both are right, the guide warns that filtering alone may not make water fit to drink, so even at the right rate on a ripe bed the readout still says to disinfect afterwards.

Try this

  1. Drag the cock's lever up its curved scale, or press Right (Shift+Right for bigger steps), or press End to open it fully. The stream into the jug thickens, the jug fills faster each turn of the sand-glass, the specks of dirt fall faster and slip through the green film, and the readout changes to 'above the band · too fast' past 0.20 m/h and to 'too fast · not filtering' from 0.25 m/h. Water that rushes through has no time to be filtered, so more flow means more water but not better water.
  2. Press R to scrape the bed back to day 0, or turn the day wheel back to zero. The green film on the sand thins away, more specks pass through it into the sand, and, at a rate within the band, the readout says 'ripening · not ready'. A new or scraped filter has lost the living layer that does most of the work, so it must ripen again before it can be trusted to filter.
  3. Turn the day wheel forward, or press Up (Shift+Up or Page Up for a whole week), and watch the readout at day 7 and day 14. The green film thickens and catches more of the falling specks; the readout reads 'ripening · week two' from day 7 and, at a rate between 0.10 and 0.20 m/h, 'design rate · still disinfect' from day 14. This is the guide's 'week or two' of ripening: the filter gets better with time as the film grows.
  4. With a ripe bed, press Left a few times (Shift+Left for bigger steps) to bring the rate below 0.10 m/h, then press Home to shut the cock. The readout says 'slow · still disinfect' and the jug fills only a little each turn of the glass; once shut it says 'cock shut · no flow' and the stream stops. Going slower than the band still filters, but gives you less water, so the band is a balance between cleaning and output.

Reading the readout

The first number is the filtration rate in metres per hour (m/h): how many metres the water level would sink in an hour as it seeps down through the sand, set by the cock. Next, 'day' is how many days since the sand was new or last scraped, set by the day wheel. The words at the end give the bed's state: 'cock shut', 'too fast · not filtering', 'above the band · too fast', 'ripening · not ready' (under 7 days), 'ripening · week two' (7 to 13 days), 'slow · still disinfect' or 'design rate · still disinfect', where 'still disinfect' means the water still needs boiling or other disinfection.

What the model uses

  • Slow sand filtration: a tank of graded sand, coarse gravel at the bottom and fine sand on top, at least 60 cm deep, with water passing downward very slowly, roughly 0.1–0.2 metres per hour; if it runs faster it isn't filtering. (WHO household water-treatment manual, which the guide's water-treatment details follow)
  • Over a week or two a biological layer, the Schmutzdecke, develops on the sand surface, and that living layer provides much of the treatment; a new or scraped filter needs a ripening period, and performance depends on design and operation. (WHO household water-treatment manual, which the guide's water-treatment details follow)
  • Filtration alone may not meet a safe microbiological target: disinfect afterward when safety is uncertain. Boiling: a rolling boil for at least 1 minute, or 3 minutes above about 2,000 m; cool it and store it covered in a clean vessel. (CDC emergency boiling guidance)
  • A sand filter that passes turbid or suspect water (excess flow, wrong media, a disturbed layer or breakthrough) must stop being used as safe water until it is corrected and re-ripened, and its output disinfected (the guide's failure-mode table).
  • A cistern is a confined space and a drowning hazard: screened inlets and overflows, a cover, and cleaning access from outside; do not enter without atmospheric testing, forced ventilation, an attendant, retrieval equipment and a trained rescue plan. (OSHA confined-space guidance)

The figure fixes the bed at 60 cm and lets the cock alone set the rate; a real filter's rate also depends on head loss as the layer clogs, its grading and depths come from a design, and its Schmutzdecke is far thinner than the band drawn here. This is a code-drawn explanatory model for orientation, not an equipment drawing or a safety procedure; follow the guide's warnings.