What this step adds; in part B, the clean waterParts already in placeA household biosand filter of the concrete kind, cut through its middle: a box 40 cm square and 1 m tall holding 5 cm of drainage gravel, 5 cm of coarse sand and 50 cm of fine sand. The outlet tube climbs inside the wall, so its lip, not the user, sets the water level: 5 cm always stands on the sand and the living layer at its top never dries. Each 20 L bucket fills the basin 22 cm deep. Field filters remove about 1 log of bacteria (90%), 0.5 log of viruses (68%) and 2 logs of protozoa (99%); a well-run one reaches 3, 3 and 4. In trials, filtering at home about halved diarrhoea, where chlorination cut it by about a quarter; unsafe water, sanitation and hygiene killed about 1.4 million people in 2019. The flow is this model's assumption: 0.6 L/min with the basin full, falling in proportion to the head above the lip (Darcy's law), so the basin drains as e^(−t/33 min) and 95% of a batch is through in about 100 minutes; the drained litres in the last step take sand as 40% pore space. Drawn larger than most guides in this set, since the whole stand is under a metre long. This is a guide to how the parts go together, not engineering drawings. A new filter needs 2–4 weeks before its biolayer works, and it never removes chemicals such as arsenic or fluoride: until it has ripened, and whenever in doubt, boil the water or chlorinate it.