Water reuse and potable reuse

Every city on a river downstream of another city is already drinking treated wastewater. This is unplanned indirect potable reuse, it is universal, and it works. Planned reuse differs only in being deliberate, engineered and monitored — which makes it considerably safer than the unplanned version.

The types

Non-potable reuse

Treated effluent used for irrigation, industrial cooling, toilet flushing or street cleaning. The largest category globally by volume and the least controversial.

Unplanned indirect potable reuse

Effluent discharged to a river, abstracted downstream and treated for supply. Happens everywhere, is not usually described as reuse, and involves no additional treatment beyond the normal.

Planned indirect potable reuse

Advanced treated effluent deliberately returned to a reservoir or aquifer, then abstracted and treated conventionally. The environmental buffer provides time, dilution and a psychological boundary.

Direct potable reuse

Advanced treated effluent piped directly into the supply network. Technically feasible and increasingly permitted; the smallest category, and the one requiring the greatest public confidence.

The treatment train

Potable reuse schemes use multiple advanced barriers beyond conventional wastewater treatment: microfiltration or ultrafiltration, then reverse osmosis, then an advanced oxidation process — usually ultraviolet with hydrogen peroxide — to destroy anything that passes the membrane.

The output routinely exceeds drinking water standards, and often exceeds the quality of the conventional supply it supplements. Singapore’s NEWater is largely sent to industry precisely because its purity is a technical advantage there.

Why acceptance is the hard part

The engineering is settled. The obstacle is public confidence, and it has defeated schemes that were technically sound — Toowoomba in Australia rejected a reuse proposal by referendum in 2006.

The schemes that succeeded, in Singapore and Orange County California, invested as heavily in visitor centres, education and visible political commitment as in treatment plant. Both also emphasised that the product goes through an environmental buffer, which appears to matter more to people than any technical argument about barrier count.