City water and sewerage

Physically, absolutely, and with a margin. Take water from above the settlement; discharge below every household, and below your neighbours, not just below your own boundary.

Define a protection zone around the intake in writing: no latrines, no burials, no livestock, no workshops and no waste within it, enforced by the health authority. Broad Street in 1854 is the canonical lesson — one pump, contaminated by one cesspit a metre away, killed hundreds in ten days. The failure is almost never the treatment works; it is a pipe or a pit somebody put in the wrong place.

Slow sand filtration takes out turbidity and most of the pathogen load: 0.15–0.35 mm effective grain size, a bed of at least 60 cm, 0.1–0.4 m³ per m² per hour, ripened 2–3 weeks. See the slow sand filter.

Then chlorinate, which is the step that closes the gap on viruses. Aim for a free chlorine residual of at least 0.5 mg/l after 30 minutes of contact, at pH below 8 where chlorine is far more effective, and at least 0.2 mg/l at the furthest tap as the evidence that nothing has been added along the way. Turbidity must be below 1 NTU first — chlorine spends itself on suspended matter.

An intermittent, unpressurised supply sucks groundwater in at every joint — and in a settlement with any sanitation problem, that groundwater is sewage. A pipe is only a barrier while the water in it is pushing outward.

Hold a minimum of about 15 m of head (1.5 bar) at the highest tap, from a service reservoir or a tower sized for at least a day's demand so pumps stopping does not mean pressure stopping. Chlorinate to leave a residual through the whole network. Fix leaks as contamination events rather than as water losses, and never let a repaired main back into service without flushing and disinfecting it.

A sewer that silts becomes a blocked sewer becomes sewage in the street. The design rule is velocity, not size: at least 0.6–0.75 m/s at the daily flow, which for a small foul sewer means a steady fall — typically between 1 in 60 and 1 in 150 for a 150 mm pipe, depending on flow.

Bigger is worse: an oversized pipe runs shallow and slow. Egg-shaped brick sections hold velocity at low flow, which is why the Victorian interceptors were built that way. Put access chambers at every junction, every change of direction and every 50–100 m, and ventilate the system — sewer gas is explosive and, in a confined chamber, lethal.

Untreated discharge exports the epidemic downstream to someone who did nothing to earn it.

1. Screening and grit removal first, or everything downstream fills with rag and sand.
2. Primary settlement — 2 hours or so of quiet retention removes perhaps a third of the oxygen demand as sludge.
3. Biological treatment — a trickling filter of 40–60 mm stone with rotating distributors, or activated sludge with air blown in. This is where most of the work happens, and it is alive: it needs feeding steadily and it dies if shock-loaded or poisoned with trade waste.
4. Final settlement, then discharge to water with enough flow to dilute what remains.

Judge it by measurement: falling oxygen demand and no sewage fungus or dead fish below the outfall.

Treated sludge returns phosphate and nitrogen to farmland, which matters enormously before synthetic nitrogen — and phosphate never stops mattering, because there is no synthetic route to it at all.

But treat it first: digest the sludge anaerobically at about 35 °C for 15–30 days, which cuts pathogens sharply and yields methane worth burning, or compost it hot at 55 °C for several days. Keep industrial waste out of the sewers that feed it — heavy metals from plating and tanning make sludge permanently unusable on land, and that decision is irreversible. Apply to arable, not to salad crops, and observe a waiting period before harvest.

This never happens voluntarily. Every historical case required law, taxation and enforcement, because the household that must pay to connect gets a benefit that is mostly enjoyed by its neighbours.

So: a duty on the authority to provide, a duty on the householder to connect within a stated period, a charge levied on all rather than on users, and a power to do the work and recover the cost where someone refuses. Fund maintenance in the same law as construction — the commonest failure in the record is a works built by a grant with no revenue behind its operating cost, silted up within a decade. See the first public good.

Most failures give no sign until there is an outbreak. A schedule, written and audited:

• Daily — chlorine residual at several points, turbidity, pressure.
• Weekly — bacteriological samples from the network. The standard is no E. coli in 100 ml, and a single positive is an incident, not a statistic.
• Continuously — flow and level alarms at the works.
• After every repair or burst — flush, disinfect, and sample before restoring supply.

Publish the results, including the failures, and keep them for years so trends are visible. Test the water; never assume it.