Natural methods such as lagoon plants are often a reasonable and economically interesting alternative to the technically intensified methods, particularly in rural areas with connection sizes up to a maximum of 5,000 PE. In the last 30 years, numerous analyses yielded information about the biological degradation in natural wastewater treatment plants, on the basis of which respective directives were created. Lately, there are reports that in rural areas of Germany there are a large number of older, mostly unaerated wastewater ponds which cause problems with the observation of the effluent values. This report will give some reasons for non-efficient purification performance (unsuitable design of the ponds, incomplete mixing of aerated ponds, insufficient maintenance, sludge removal etc.). After presenting some operational results several methods of optimisation will be discussed (efficient sludge removal, optimisation of the flow behaviour, combination with a technical step, etc.). Some examples like re-construction into a SBR system are presented at the end.

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