Influence of dissolved organic nitrogen on surface waters

SummaryThe aim of this study was to determine the susceptibility of dissolved organic nitrogen (DON) contained in biologically treated wastewater disposed from municipal wastewater treatment plants (WWTPs) to biodegradability and bioavailability in a water environment. Additionally an evaluation was performed of the participation of this organic nitrogen fraction, including bioavailable DON (bDON), in the nitrogen balance for the Baltic Sea.Based on the samples of secondary effluent taken from two large municipal WWTPs located in Northern Poland DON bioavailability and biodegradability tests were carried out. It was concluded that DON concentration in the tested samples was on average from 1.5 to 2.0 g N m−3. This fraction constituted as much as 50% of organic nitrogen and 15–18% of total nitrogen contained in treated wastewater.The participation of biodegradable DON (brDON) in activated sludge tests was on average 24–35%. In the bioavailability tests Selenastrum capricornutum were able to use from 19 to 26% of DON, however taking into account the results of the control test, these values are reduced to 3–4%. On the other hand, taking into account the combined effect of bacteria and algae it was possible to reduce the DON concentration by nearly 40%.The estimated annual bDON load introduced to Baltic Sea waters from Poland through disposal of treated biological wastewater in 2010 reached up to 1.7 thousand tons of N year−1.