The third stage of wastewater treatment

Biologically treated water contains considerable quantities of mineral nutrients which, because of ever increasing eutrophization will have to be disposed in the future as well. At present, the third stage is not much applied (particularly because the costs increases by 50%), but from the experimental viewpoint it has already been quite well elaborated. From industrial waters, first of all phosphorus has to be eliminated, and then, to a lesser extent, the nitrogenous compounds [1-3]. Physico-chemical methods of treatment have been successfully developed, for example, by adsorption, precipitation or the use of ion exchangers studies involving the biological elimination of nutrients are so far rather scarce. 

Phosphates are eliminated primarily by precipitation with the salts of iron or aluminium, calcium or magnesium chloride. For biological removal of phosphorus, intensive growth of algae in assimilation ponds can be used. The effect is as much as 80%, but because intensive photosynthesis is required, it depends considerable on the temperature and amount of sunlight. The efficiency of elimination of the biogenous elements is rather poor in cold, cloudy or rainy weather. 

Algae must be removed from water so as to avoid the return of nutrients into the cycle. One of the possible solutions might consist in the utilization of biologically treated water for growing the algae which could be used, for example, as fodder. 

Nitrogenous substances could be removed by microbial denitrification into gaseous nitrogen or its oxides following the nitrification of ammonia 

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