Biological filtering

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Biological filtering is a comprehensive name for processes for the purification of waste water or other water polluted by organic pollutants through the use of heterotrophic microorganisms, in particular bacteria and fungi , which form a biofilm on the surface of a solid carrier material . The cleaning performance is not provided by the filter material itself, but by the organisms. This distinguishes them from membrane technology , in which semipermeable membranes clean themselves by filtering out harmful substances. Ultimately, the organic pollutants serve the organisms of the biofilm as nutrient substrates and are converted and fixed into biomass, which can then be separated.

Biological filtration is based on the same principles as self-cleaning in natural waters. Related processes, which are based on the same principles, are also the biological purification stage of sewage treatment plants or processes that work with aggregates of microorganisms (e.g. flakes). All of these processes work with two phases (solid and liquid phase: fixed bed reactors ), while many other technical bioreactors are based on suspended single cells. The main advantage of processes based on biofilms is that the retention of the pollutants is better because they cannot be flushed out so easily. Their area of ​​application is primarily at high flow speeds in connection with relatively low pollutant contents. In the case of highly concentrated water pollution, other methods are technically superior.

sewage

Biological filters belong to the fixed bed reactors . The breakdown of dissolved organic substances occurs through living microorganisms that settle as a biofilm on solid materials. Biofilms consist of the cells of the organisms themselves and of extracellular polymers secreted by them. As a rule, the substrate is specified in the process and the environmental conditions such. B. Oxygen content, pH value, flow (throughput rate) controlled. The community itself consists of organisms that colonize the filter independently, i.e. not specially bred or selected. Species-rich communities develop, which are often organized in layers due to the limited diffusion rate of substances into the biofilm. The trickling filters in biological sewage treatment plants, which have been used for over 50 years, belong here.

Technical filters (bioreactors)

In contrast to drip filters and other processes bound to solid carriers or membranes, technical bioreactors for wastewater treatment are almost always based on biofilms bound to carriers (particles or flakes) suspended in the water. The first technical application was the BFB (biofilm fluidized bed) process, which has been used for almost all wastewater treatment processes in sewage treatment plants since the early 1980s. It is also operated anaerobically for the denitrification of wastewater contaminated with nitrogen in sewage treatment plants of sugar factories. With the USB reactor (Upflow sludge blanket), wastewater is fed from below into an activated sludge basin, it flows through an aerobic sludge layer consisting of biofilm-coated granules about 1 to 4 millimeters in size. During microbial degradation, biogas is formed that is rich in methane . The particles are whirled up by adhering methane bubbles, and convection forms. The gas is collected and used at the top. The BAS reactor (Biofilm Airlift Suspension) is based on a two-chamber system. A gas is fed in at the bottom of a chamber (air in aerobic reactors). The reduced density creates an upward flow of water. The gas exits at the upper end, the denser water flows down into the second chamber and enters the first chamber from below. The particles that carry the biofilm are suspended and kept in suspension by the flow. The result is very good mixing and a particularly high active surface. In addition to these methods, others are in use.

Aquaristics

Hamburger mat filter , a simple biological filter in the freshwater aquarium

Biological filters clean the water with the help of bacteria that settle and multiply in the filter material. Since the bacteria consume oxygen in the process, this process is also known as aerobic degradation. For biological filters to work well, there must be a large number of bacteria and the water must be highly oxygenated . If the tank in salt water aquariums is equipped with enough living stones , a biological filter can be dispensed with in reef aquariums with a balanced population, but not in pure fish aquariums or large population.

literature

  • C. Nicolella, MCM van Loosdrecht, JJ Heijnen (2000): Wastewater treatment with particulate biofilm reactors. Journal of Biotechnology 80: 1-33.
  • Lutz G. Gohr: Sea water aquaristics. Franckh-Kosmos Verlag, 2000. ISBN 3440082504
  • Joachim Grosskopf: Coral reefs in the living room. Dähne-Verlag, 5th edition, 1999. ISBN 392168448X

Individual evidence

  1. ↑ Jürgen Bever, Andreas Stein, Hans Teichmann: Further wastewater treatment. Oldenbourg Scientific Publishing House, ISBN 978-3486265279 , p. 73.