Characterizing the free ammonia exposure to the nutrients removal in activated sludge systems

Abstract

As a by-product during liquid production, the liquor wastewater exhibits tremendous environmental risks and may cause undesirable effects to the biological systems due to the high concentration of ammonia. This study was carried out to characterize the effects of high concentration free ammonia (FA) exposure in liquor wastewater on the removal of nutrients (NH₄⁺–N, NO_x⁻–N, PO₄³⁻ and organics) in sequencing batch reactors (SBRs). Experimental data demonstrated that the short-term exposure of FA had little effect on the removal of typical pollutants. In the long-term, however, it was found that the removal of NH₄⁺–N was determined by the counteraction of negative effects caused by high concentration FA and the contribution brought by initial substrate concentration. Moreover, the high concentration FA had evident inhibitory effects on both the nitrification process and the phosphorus removal. The side-effect of high FA on organics removal was insignificant and the lower removal efficiency of apparent COD in high FA reactors was primarily attributed to the accumulation of NO₂–N, which was commonly neglected in previous studies. Mechanism explorations indicated that the microbes in the SBRs could excrete additional protein-dominated extracellular polymeric substances (EPS) to resist the side-effects caused by high FA. This made the sludge structure more compact, while it blocked the transport of substrates in the process of pollutants removal. Furthermore, FA would also influence the microbial diversity and reduce the abundance of key microorganisms (i.e. ammonia-oxidizing bacteria, nitrite-oxidizing bacteria and phosphate-accumulating organism) responsible for nutrients removal; thus the corresponding removal efficiency subsequently decreased in the presence of high FA.