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NUTRIENTS REMOVAL The necessity of removing the nutrients, nitrogen and phosphorus, depends mainly on the final use of treated wastewaters. If the wastewater is reused for agricultural purposes, the nutrients have a beneficial action for the growth of the crops, allowing a saving of chemical fertilizers. A nutrients excess can however cause the growth of algae in the storage reservoirs and in the distribution pipelines, with consequent increase of clogging problems. The nitrogen in a raw wastewater is mainly present as ammonia. The biological conversion of ammonia to nitrates (nitrification) occur by specific aerobic bacteria (nitrosomonas and nitrobacter). The nitrates can be converted to molecular nitrogen through a denitrification process, in oxygen absence. The combined effect of nitrification and denitrification allows the removal of the nitrogen from wastewaters. The biological nitrogen removal can be conducted both in suspended biomass reactors or attached biomass reactors. An appropriate design of the treatment unity allows to obtain values of few mg/l as residual concentrations in the final effluent. These treatment processes can be realised in combination with the oxidation phase or in separate units from it. Alternatives for the nitrogen removal are stripping, chemical oxidation or ionic exchange. Cationic exchange can be used for the ion ammonium removal (NH4+) while the anionic exchange can be used for the nitrate (NO3-) and the nitrite (NO2) removal. The removal of phosphorus is obtained by converting the soluble phosphorus to particulate phosphorus, which can be removed through sedimentation and/or filtration. The particulate phosphorus can be formed, as calcium phosphate, through chemical precipitation using lime treatment or, as iron or aluminium phosphate precipitates, using iron or aluminium salts. The biological phosphorus removal can be accomplished by operating a biological treatment system with alternating aerobic and anaerobic conditions that promote uptake of orthophosphates, polyphosphate, and organic phosphates by microbial cells. In many cases the biological removal of the nitrogen and phosphorus are coupled in a treatment systems for the biological nutrient removal.In the
following Tables are listed the phosphorus and nitrogen removal percentages with
different alternatives of treatment. Nitrogen
removal percentages with different alternatives of treatment
Phosphorus
removal percentages with different alternatives of treatment
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