Efficient denitrification of liquid digestate with its indigenous microflora

Abstract

It has been a common way to recycle digestate after biogas production as a biofertilizer. However, the huge generation of digestate especially its liquid part surpasses the market consumption quantity, which in turn hinders the sustainable operation of a biogas plant. Biotreatment of liquid digestate (LD) with microorganisms is one way to reduce chemical oxygen demand (COD) and nitrogen content. This study enriched three microflorae from environmental water, liquid and solid digestate. After community structure analysis, the microflora from LD itself exhibits superior nitrogen removal function to the other two microflorae, and its predominant microbial strains are from Aliidiomarina, followed by Parapedobacter. The microflorae can grow at pH range from 7 to 10. pH and ammonium concentration show synergistic effect on the growth of microflorae. The microflora from LD can completely realize nitrogen removal in man-made medium within 48 h, and remove 93.7 % ammonia nitrogen (NH3-N) and 88.1 % COD from LD at optimum C/N ratio of 9. Sodium acetate and the fermented mash from grain ethanol production process are the preferable carbon sources to improve the removal of nitrogen and COD. The re-inoculation of indigenous microflora into LD can realize 2.9∼10.9 % increase of NH3-N removal with or without addition of carbon sources, and 0.1∼11.7 % improvement on COD removal with addition of carbon sources except glycerol. It means that the enrichment of the indigenous microflora combined with the addition of proper carbon source is a potential way to enhance the nitrogen and COD removal from LD.