Acidic Bioleaching of Nitrogen and Phosphorus from Sewage Sludge

Abstract

The study presented in this paper aimed to evaluate the impact of bio-acidification on the leaching of ammonia (NH4-N) and phosphorus (PO4-P) from various types of digested and undigested sludges. Bio-acidification produced a highly acidic and digestive environment which, accompanied by shifting microbial populations, accelerated the mineralisation of organic nitrogen and the leaching of phosphorus and heavy metals. The bio-acidification process was capable of leaching significant quantities (7–98%) of Ni, Zn, Pb, Cr, Cd, and Cu from sludge, however nitrogen and phosphorus were also leached simultaneously. It should be emphasised however that bio-acidification by itself did not reduce the TKN, total phosphorus, or heavy metal contents of the sludge. The separation of supernatant containing the leached products resulted in a significant loss in the nutrient value of the sludge. The measured organic nitrogen mineralisation results were in the range of 10–43%. Similarly for phosphorus, the leaching results were in the range of 16–65%. The phosphorus leaching results in individual sludge samples were significantly higher than the nitrogen leaching results. This is because the digestion of organic matter is the main cause of TKN mineralisation, while in addition, phosphorus leaching is possibly increased by the leaching of phosphorus from inorganic sources and polyphosphates stored in sludge. This study confirmed that while bio-acidification can be used to reduce the heavy metal contents in sludge, the process can also reduce the nitrogen and phosphorus values of the product.