Description of biological wastewater treatment plants of city of Odesa as sources of marine environment nutrient pollution in current period

Abstract

Biological wastewater treatment plants (the BTPs) of city of Odesa (Northern and Southern Plants) are the most powerful permanent sources of nutrient pollution of coastal waters of Odesa Region of the north-western part of the Black Sea (the NWPBS) and the Khadzhybei Liman. The article includes a comparative analysis of changes, taking place since the beginning of the 21st century, related to the qualitative composition of return water of the Northern and Southern BTPs and the amount of nutrients reaching the marine environment together with such water, as well as the analysis of possible influence of such changes on eutrophication of coastal waters of Odesa Region of the NWPBS and the Khadzhybei Liman. It was established that despite significant decrease (by 1.7 - 2 times) of return water discharged from the BTPs the amount of nitrates and nitrites reaching the marine environment together with such water increased by 4-6 times. At the same time the reduced inflow of organic matter, ammonium nitrogen and phosphates can be observed.
 Treatment facilities of Odesa were put into operation back in the 1970s. They use a then typical technology of biological cleaning of wastewater from nutrient compounds in aeration tanks providing aerobic conditions. The technology is considered as incomplete in terms of nitrogen compounds treatment since its final product includes nitrate nitrogen in large quantities. When reaching the marine environment it is absorbed by algae at the stage of primary production of organic matter and thus is transformed in organic nitrogen again. It was noted that since the coastal waters of Odesa Region of the NWPBS and the Khadzhybei Lyman have a high level of trophicity and currently there is a disbalance between mineral nitrogen and phosphorus concentrations in the water towards insufficiency of mineral nitrogen reserves (as compared with the standard Redfield stoichiometric ratio), additional inflow of nitrates and nitrites in the marine environment together with return water from the BTPs would, in certain circumstances, lead to algal bloom and aggravation of negative eutrophication-related consequences.
 The research allowed making a conclusion that a modern flow sheet of advanced biological cleaning (treatment) of wastewater from nitrogen compounds should be implemented at the Northern and Southern BTPs in order to reduce the inflow of nitrates in the marine environment. Such flow sheet should ensure both nitrification and denitrification processes.