Experimental Study On The Inhibition Of Humic Acid On The Process Of Anaerobic Digestion

The goal of “Carbon Neutral” operation of WWTPs is stimulating in-depth study on energy conversion of excess sludge through anaerobic digestion. However, the energy conversion efficiency of traditional anaerobic digestion has been restricted by the rate limiting step- hydrolysis. The slow hydrolysis rate is attributed to the large amount of refractory organic matters in the excess sludge, which includes lignocellulose, humic acid and macromolecular. In addition, the presence humus and hydrolysis products(such as long chain fatty acid) may also inhibit the activity of extracellular enzymes dramatically.Among them, humus shows relatively high inhibitory effect on the hydrolysis of organic matters in sludge. This is closely related to the molecular structure of humic substances: the oxo-functional groups(carboxyl) in their molecular structure has a strong binding affinity for the active points- amino on the extracellular hydrolytic enzymes, thus reducing the opportunity for the active points to contact with the organic matter and resulting in poor hydrolysis efficiency. Previous studies have shown that inorganic cation can effectively combine with the carboxyl group in humic acid, thereby avoiding masking effect of humus on the activity point in hydrolase and reducing their inhibition on the hydrolytic enzymes. Therefore, alleviating the inhibition of humus to hydrolysis by adding metal cations is a method deserving further experimental and theoretical research.In this study, laboratory scale anaerobic digestion testes were carried out with synthetic real excess sludge as the substrate respectively, to investigate the inhibition of humus on anaerobic digestion of excess sludge. Then, adding cations(Ca2+、Mg2+ and Fe3+) was attempted to alleviate the inhibition,and its effectiveness was evaluated by the final CH4 production.The experiments with synthetic sludge demonstrated that humic acid could indeed inhibit the hydrolysis process. The CH4 production decreased by 9.2% compared with the control(without humic acid input) when humic acid was added to the level of 250mg/g VSS. The presence of divalent cations could dramatically release this inhibition effect. When the Ca2+ content reached 500mg/L in the bulk liquid of anaerobic reactors, the inhibition was completely mitigated. Trivalent cations(Fe3+) could also completely mask the inhibitory effect of humic acid on hydrolysis.In the experiment with the real excess sludge, it was found that the Ca2+ and Mg2+ contents in the sludge had exceeded the level for completely masking the humus inhibition. This could be probably attributed to the fact that the water source in the north is mainly groundwater-based. Therefore, adding cations further will not help to suppress the hydrolysis inhibition, but could induce flocculation and precipitation of the soluble polysaccharide and proteins and consequently reduce the CH4 production.