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One alternative for treating wastewater is the use of biological systems. Thus it is possible to decrease the organic content of water, as well as remove pathogens and parasites. The biological water treatment can be achieved through aerobic and anaerobic processes.
Here are some:
Sludge ponds:
In this case, mechanisms are used entering air into the tanks. On contact with air, the organisms react to generate flocs, settles them in passing the so-called clarification tanks. To treat wastewater through the biological system, you need: an aeration tank and a clarification by the sludge being passed repeatedly.

The main objective of activated sludge system is the oxidation of biodegradable matter in the aeration tank, and flocculation, which enables the separation of biomass from treated effluent. Removal can be achieved up to 90% of the organic load. However, there are some drawbacks: it requires expensive facilities and investment in electromechanical equipment with a high energy cost. It also produces a significant amount of sludge that later require further treatment by anaerobic reactors and / or landfill disposal installed correctly.

Anaerobic treatment of wastewater:
This is a series of microbiological processes that occur within it an airtight container, for the digestion of organic matter to methane production. May intervene in the process different kinds of microorganisms, but usually is addressed primarily by bacteria. It presents some advantages over aerobic digestion means those facilities are less expensive, it is necessary to supply oxygen and therefore also reduce costs, and energy expenditure is lower.
Constructed wetlands:
This process is to reproduce in a controlled, natural conditions existing in shallow lake systems or lentic waters. These, naturally made the purification of wastewater. The purification process involves a mixture of bacteria, both aerobic and anaerobic, which take place in the environment of plant roots hydrophilic.

This system is very friendly from the environmental viewpoint, because it does not need complex installation, has a relatively low cost of maintenance and integrates with the natural landscape, including providing shelter for wildlife. Perhaps, the only drawback we can mention that you need a large area.