A treatment plant for domestic wastewater should consist of a fine screen, two aerated
lagoons in series and at least one polishing lagoon.
First, a fine screen removes coarse matter. A paved inlet zone in the first lagoon allows
settlement of heavy sludge and grit. A scum baffle separates inlet zone and lagoon and
prevents floating matter from entering the lagoon. Floating matter has to be removed
manually once or twice a week with a rake.
According to our experience, sludge in the inlet zone of the first aerated wastewater
lagoon has to be removed at regular intervals of several years. Liquid manure-vacuumtankers
are used to pump off the sludge.
Dissolved and suspended organic pollutants are distributed in the whole first lagoon.
Decomposable organic matter should mainly be stabilized aerobically to avoid odours or
anaerobic sludge rising to the water surface.
The treatment process in aerated lagoons is similar to the natural process in flowing
water bodies. Biological degradation of pollutants is based on attached growth. Unlike
Aerated lagoons /3
the “activated sludge process”, the active biomass builds a biofilm attached to the
lagoon bottom and slope.
The biofilm needs continuous supply with oxygen and organic pollutants. In addition to
sufficient oxygen transfer, circulating flow in the whole water body and effective mixing is
required. The circulation of wastewater and dissolved oxygen ensures optimal conditions
for aerobic growth at the lagoon bottom. Thus, organic pollution is highly reduced and
dead zones can be avoided.
Polishing lagoons further improve the water quality. In particular, suspended light flocs of
biomass settle down.