Effect Of Nitrite And Ammonia On The Granule-Based Enhanced Biological Phosphorus Removal System

Given its stable and efficient phosphorus removal characteristics, the granule-based enhanced biological phosphorus removal (EBPR) technology has been recognized as the advancing front of research in wastewater treatment worldwide. However, the mixed urban sewage in China includes a large number of toxic and hazardous substances which presents new challenges to the EBPR operation. The wastewater contains high levels of nitrogen and phosphorus but limited carbon source. High concentrations of nitrite and ammonia in the sewage have serious inhibition on the microorganisms. The mature granules were taken as the seed sludge in this research. The effect mechanism and recoverability of nitrite and ammonia in different concentrations to the granule-based EBPR system were studied from macro-to micro-scale. The inhibition study of nitrite and ammonia lasted44d and51d, respectively, and divided into two phases:inhibition and recovery test. Settleability of sludge, size and morphology of granules, P removal efficiency, P release and uptake rates, polyhydroxyalkanoates (PHAs) transformation, composition of extracellular polymeric substances (EPS), poly-phosphate accumulating organisms (PAOs) and glycogen accumulating organisms (GAOs) competition, microbial community structure, biochemical reaction kinetic parameters and other indicators of significant changes have occurred.Initially, the mature granules possessed a regular, smooth and elliptical outer shape with compact structure and particle size about800μm. PAOs were accounting for84.3±4.2%of all bacteria in the sludge. The EBPR system showed100%P removal performance and operated stably for more than three months.After the treatment of different levels of nitrite and ammonia, sludge bulking had occurred in the experimental group. But the difference was that nitrite triggered filamentous bulking and high ammonia caused non-filamentous bulking, the so-called viscous bulking. Disintegration of the granules accompanied by sludge bulking was also observed. The size of the granules were reduced to less than400μm and even reached200μm. The ratios of MLVSS to MLSS were increased by the addition of inhibitors. This was probably due to the adsorption of organic substance in sewage, EPS secretion and intracellular inorganic matter (such as poly-P) exhaustion.It was discovered that nitrite and ammonia severely affected the metabolic process of PAOs. PAOs and heterotrophic denitrifying bacteria on the competition of carbon source always make PAOs at a competitive disadvantage which caused by nitrite in whatever concentrations. At the same time, the traditional heterotrophic bacteria were also proliferating in the systems which were eventually leading to EBPR failure. If the concentration of ammonia nitrogen converted to free ammonia (FA), the threshold concentration for PAOs was about17.76mg N L-1. The reactor performance became completely deteriorated when exposed to that higher than this concentration. Acclimation took place by PAOs so that being able to partially overcome the inhibition for FA concentration of8.88mg N L-1.The polysaccharides content in EPS were largely reduced by the presence of nitrite and high ammonia, which closely associated with the granule disintegration, while the ratios of proteins to polysaccharides (PN/PS) exhibited the opposite trend obviously. Granules with lower PN/PS had better structure and function in this study.The molecular analysis revealed that nitrite and high ammonia had a significant influence on microbial communities in the long-term. PAOs were replaced by other microorganisms, such as GAOs, heterotrophic denitrifying bacteria, denitrifying phosphorus accumulating bacteria (DPB) and traditional heterotrophic bacteria. PAOs diminution and the dominant population shifts were the main cause of EBPR deterioration. Moreover, nitrite and ammonia inhibition may provide a competitive advantage to GAOs over PAOs. The community composition was not reversible by recovery (Dice coefficients,33.0%and36.3%), although good EBPR performance was re-achieved only by propagating other types of PAOs and DPB.From the perspective of biochemical reaction kinetic parameters, the microbial activity decreased in each experimental group due to the nitrite and high ammonia inhibition. The ability of organic matter degradation also became slower than the control. When nitrite concentration was greater than15mg N L"! or ammonia level exceeded250mg N L-1, the sludge began to appear negative growth, which means the decline rate is higher than growth rate.