Wetlands and peatlands

Wetlands hold about 11,470 cubic kilometres of water — under a thousandth of one percent of the global total. Their significance is entirely disproportionate to that, because of what they do rather than how much they store.

What they do for water

• Remove nitrogen through denitrification, converting nitrate to inert nitrogen gas — a process that costs a wastewater works energy and chemicals and that a wetland does for nothing
• Trap sediment and the phosphorus bound to it
• Attenuate flood peaks by storing water and releasing it slowly
• Sustain river baseflow through dry periods
• Recharge groundwater where they sit over permeable strata
• Support disproportionate biodiversity — freshwater habitats hold a share of species far above their share of area

Peatlands specifically

Peat forms where waterlogging prevents plant material from decomposing fully. UK upland peat catchments supply a large share of drinking water in the north of England, Scotland and Wales — and their condition directly determines the treatment cost downstream.

Degraded peat — drained, burned or eroding — releases dissolved organic carbon, which colours the water brown. That colour has to be removed at the treatment works by enhanced coagulation, and the organic carbon reacts with chlorine to form disinfection by-products. Several UK water companies now invest in peatland restoration on the explicit basis that it is cheaper than the treatment it avoids.

Constructed wetlands

The same processes can be engineered. Constructed wetlands are used as tertiary treatment at small wastewater works, for polishing after conventional treatment, for treating mine water, and for intercepting agricultural runoff. They are slow and land-hungry, and they need almost no energy or chemicals — which makes them well suited to small rural works where operating cost dominates.