The on-line measurement of greenhouse gases emitted by a reactor for nitrogen removal from digested zootechnical slurry

摘要

Anaerobic digestion can be considered the choice treatment for most organic waste but digested bulk contains a little percentage of carbon-based compounds and a big load of nitrogen-based compounds. This imbalance is particularly evident for digested zootechnical slurry that originally contains high amounts of nitrogen so that its use as fertilizer had to be limited, nitrogen removal is necessary to achieve a good quality fertilizer so to avoid its stockpiling or misuse. The choice treatment for nitrogen removal is obtained though reactors that apply the so called "Alternate Oxic/Anoxic" treatments. These treatments produce, especially during oxidation (i.e. Oxic phase), emissions of airborne substances including pollutants and greenhouse gases. We found that the literature on this topic is limited and there are no uniformly accepted values for pollutant emissions. Also it is hard to estimate the emissions due to the particular variety of slurry with high nitrogen content and the countless types of treatment plants. The aim of our work was evaluating and estimating the emissions of greenhouse gases (C02, CH4, N20) and nitrogen oxides (NOX) from such nitrogen removal reactors. We monitored a mid-size pilot reactor that performs a continuous alternate Oxic/Anoxic biological process to perform an on-line measurement of emissions produced. The reactor was designed and used to reduce the nutrient loads from the supernatants of anaerobically digested swine and cattle flows. An ad hoc monitoring station has been designed and assembled. Its measurement ranges have been specified mainly on the basis of the few values reported in literature. The air supply of platform is intermittent since aeration occurs only in Oxic phase. Thus we have tested several caps for the tanks and different methods to perform off-gas analysis. Here we present the preliminary results of the system start-up that included a measurement campaign of six months, and a tentative calculation method to evaluate pollutant emissions as a function of slurry composition and digestion.