Variations in dissolved inorganic carbon species in effluents from large-scale municipal wastewater treatment plants (Qingdao, China) and their potential impacts on coastal acidification.

ABSTRACT

With rapid development of global wastewater treatment plants (WWTPs), acidification and enhanced CO2 release in receiving waters caused by high-CO2 treated wastewater input have raised concerns. Insights into the variations in dissolved inorganic carbon (DIC) species in treated wastewater contribute to understanding the mechanisms of the acidification process. Here, we investigated three large-scale municipal WWTPs that discharged into the coast of Qingdao, China, for variations in effluent DIC species and their control mechanisms. The results showed that the effluent DIC concentrations, with a range of 2554-5718 µmol/L, significantly exceeded the concentration in seawater and mainly increased from winter to spring and decreased from summer to autumn. The effluent DIC and its δ13CDIC showed a good negative correlation. The ratios of effluent DIC to total alkalinity (DIC/TAlk) ranged from 1.00 to 1.24, and the proportions of CO2 in DIC ranged from 0.9 to 19.7%; both sets of values significantly exceeded those in seawater. The proportions of CO32- in DIC were only ~ 0.4%. These features determined that the CO2 concentrations in effluents fluctuated from 3 to 80 times the concentration in seawater, whereas the CO32- concentrations were less than 1/15 of those in seawater. Organic matter degradation and nitrogen removal processes made important contributions to the high effluent CO2 concentrations. The increase in solubility induced by decreased temperature may be the main cause for the higher effluent CO2 concentrations during winter as well as spring months with low effluent temperatures. Correspondingly, the effluent pH values were significantly lower than the seawater pH values and showed a good negative logarithmic correlation with the DIC/TAlk values, reflecting the control of DIC species on the pH values in treated wastewater. Variations in DIC species in treated wastewater can cause changes in the affected region and the degree of the induced acidification in receiving waters.