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Very roughly, undesirable/excess wastewater constituents may be present in either or both of two forms, namely soluble and/or particulate, a cutting point being about 0.45 µm. To a certain extent, nonsoluble fractions can or ought to be removed by appealing to essentially physical pretreatment means, e.g. settling, flotation, screening. However, this first step leaves us with an at times sizable amount of material in soluble form that will still have to be dealt with. Because further appeals to physical processes will be of no avail (also law of diminishing returns), present environmental engineering practice brings in and banks on the phenomenal power of natural/biological processes. Wastewater treatment plants essentially replicate in somewhat controlled mode what has been going on in nature for ages: biological processes. In this way, "troublesome" soluble components are gladly gobbled up by living matter in specially conditioned "microorganism farms." Typical embodiments include attached-biomass type biotowers and suspended-growth activated sludge basins. Common to all approaches is growth and development of large microorganism inventories that will pick up soluble stream components, be it organic matter, nutrients like N and P, which in turn can and will be subsequently removed from said stream by ... physical means, e.g. settling, flotation, screening/membranes.
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