Arcata Marsh and Wildlife Sanctuary — Constructed Wetlands Treating Municipal Wastewater (USA)

Arcata, on California's north coast, treats its municipal sewage with a conventional plant followed by constructed wetlands that double as a public wildlife sanctuary. Primary treatment dates from 1949, secondary from 1957, disinfection from 1966 and the marsh-based tertiary stage from 1986 (Wikipedia). The sanctuary covers 307 acres of freshwater marsh, salt marsh, tidal slough, mudflat and upland with about 5 miles of paths (City of Arcata); the plant handles roughly 2.3 million US gallons a day, with winter peaks up to 16.5 million (Wikipedia).
A Humboldt State University thesis analysing BOD5 and TSS time series found satisfactory and improving water-quality enhancement across seasons, identified eleven beneficial uses, and reported that the natural process is more energy-efficient than the average conventional plant in the region. More than 300 bird species are documented (the Wikipedia figure is 327), and the site draws thousands of birding visitors yearly. It received the Ford Foundation/Harvard Kennedy School Innovations in Government award.
Caveats: the sources reviewed give no numeric effluent or cost-comparison figures, and the evidence is a single-site case study rather than a controlled evaluation; transfer depends on available land, climate and regulatory acceptance of wetland treatment.

Read the full analysis: https://www.cityofarcata.org/340/Arcata-Marsh-Wildlife-Sanctuary