Solid-phase denitrification for tertiary nitrate removal from municipal wastewater

Elevated levels of nitrate in surface water and groundwater can lead to eutrophication, algal blooms, and depletion of dissolved oxygen, causing harm to aquatic ecosystems. Solid-phase denitrification (SPD) offers a promising approach for removing nitrate from wastewater sources using carbon-based materials to promote the activity of microorganisms that convert nitrate into nitrogen gas. This study evaluated the suitability of six common cheap and readily available materials as external carbon sources for denitrification in SPD systems. The materials tested were woodchips, barley straw, spruce wood bark, beech sawdust, and polycaprolactone (PCL). The study focused on the leaching rates of organic compounds and their impact on denitrification efficiency, with particular attention to the potential for secondary pollution. Results indicated that woodchips and wood bark exhibited a good balance between carbon content and secondary pollution risk, while barley straw posed the highest risk for secondary pollution and potential for the rapid depletion of organic compounds. PCL showed promise as a carbon source for nitrate removal and released minimal nitrate or ammonium-nitrogen. The low concentrations of metals found in the media suggest that they would not inhibit denitrification or pose significant health risks. The method was transferred to a pilot scale to evaluate the potential of woodchips in removing nitrate from wastewater sources and the performance of four columns for solid-phase denitrification using woodchip media, based on the results of the first part of the study, was studied for a period of 168 days. The nitrate removal efficiency was observed to be between 62-75% with the best performance seen in the hardwood seeded with sludge column. The addition of sludge from a wastewater treatment plant helped to speed up the start-up process. The results suggest that higher removal rates can be expected with higher influent nitrate concentrations. Over time, significant variability in the performance was observed. However, the variability in the results could not be clearly explained. In conclusion, all tested media have the potential to be used as supporting carbon sources for SPD, but their efficacy and potential for secondary pollution must be evaluated on a case-by-case basis.