Valorisation of anaerobic digestate to nutrients and humic substances

Abstract

Nutrient-rich product fractions were produced from abundant, yet currently chemically under-utilized nutrients-containing feedstock, residual digestate formed during anaerobic digestion (AD). The objective of this research was to experiment individually three sub-processes, i.e., precipitation of organic humic substances and phosphorus from the digestate reject water, liberation of reject water nitrogen as ammonia gas during the lime treatment and recovering it with membrane contactor (MC), and finally novel utilization of ammonia for leaching nitrogen-enriched organic substances from the digestate residue. With calcium precipitation, the main part of the phosphorus and significant part of organic material could be precipitated, and simultaneously ammonium could be liberated with good yield as ammonia gas, so that it could be recovered by MC. On the other hand, ammonia could be used with promising results as an extraction media, by which the solubility of the organic matter and the content of nitrogen attached to the soluble organic fraction could be significantly increased. Hence, all sub-processes were found to achieve their goals and digestate could be successfully utilized as a feedstock for manufacture of varying nutrient-rich products. Combining these three subprocesses together enables the development of novel cascading process concept, in which treated product stream can be used in the next process step and in which each subprocess step benefits the next.