The Mechanism And Process Study On Biological Nutrient Removal Driven By Intracellular Polymers

In wastewater treatment plants(WWTPs), the microbial uptake of phosphorus needs to consume a large amount of organic carbon source, and a certain concentration of organic matter is required to serve as electron donor for denitrification process. However, the influent organic concentra tion is extremely low in the south of China. Consequently, the addition of organic carbon source is needed during the anoxic stage to meet the demand of denitrification, resulting in a high treatment cost. This not only limits the operation and development of the WWTPs, but also restricts the promotion and application of biological nutrient removal(BNR) process. Therefore, a cost-effective and reliable BNR process is essential to solve the growing problem of eutrophication.Recently, studies have shown that microbial intracellular polymer accumulation can be induced in feast-famine activated sludge systems. Therefore, this study aims to couple this metabolism of microorganisms with the nutrient removal from wastewater, so that organic matter in wastewater c an be mainly removed through the excessive accumulation of intracellular polymers, and then the accumulated intracellular polymers can be further used as electron donor for the denitrification process. This process reduces the cost of BNR, and thereby exhibits a broad application prospect in the wastewater treatment plant.In this research, BNR induced by intracellular polymers was realized in the feast-famine activated sludge system. The polymer storage with different substances was investigated, the control conditions for intracellular polymer accumulation were optimized, and the microbial community structure characteristics with different substrates were analyzed. Furthermore, the BNR performance induced by intracellular polymers was compared with the con ventional biological phosphorus removal process and the oxic/extended-idle process. The cyclic variations of nitrogen and phosphorus as well as intracellular polymers were analysized to explore the mechanisms for biological nutrient removal driven by intra cellular polymers in this study. Finally, the feasibility and stability of the BNR induced by intracellular polymers was examined with domestic sewage.The experimental results showed that excellent BNR could be achieved in an oxic/anoxic/extended-idle sequencing batch reactor, the phosphorus and ammonia nitrogen removal efficiencies reached 98% and 99%, respectively, and the total nitrogen removal efficiency was over 80%. Post anoxic denitrification in feast-famine process depended strongly on the types of carbon sources, and high BNR efficiencies were obtained when volatile fatty acid(VFA) substrates were used as carbon source. Different carbon sources affected the transformation of poly-β-hydroxyalkanoates(PHA) and glycogen. The biomass cultured with ac etate accumulated more poly-3-hydroxybutyrate(PHB), which could drive a high post anoxic denitrification and BNR performance.Carbon source concentration, aeration rate, p H value and idle duration showed great influence on intracellular polymer accumulation. Carbon source concentration determined the accumulation of PHA in microorganisms. Low carbon concentrations usually led to a low PHA synthesis which cannot meet the carbon demand of denitrification, thereby resulting in a low rate of denitrification. H owever, when the carbon concentration was too high, denitrifying bacteria and other heterotrophic bacteria could be accumulated feeding with the excess carbon source, which results in low phosphorus removal. When the carbon source concentration was control led at 300 mg·L-1, the microorganisms showed a high phosphorus removal ability, thus a high phosphorus removal efficiency of 98% was obtained in the system. Moreover, a considerable amount of PHA accumulation was detected in the reactor, which could raise high denitrification rates and the total nitrogen removal efficiency reached 89%. A low aeration rate could cause insufficient oxygen and a low PHA accumulation in the reactor. However, the excess aeration will consume the PHA synthesized. When the aeration rate was controlled at 2 L·min-1, a high PHA accumulation and nutrient removal efficiency was oberved. The microorganisms were sensitive to p H changes in the reactor due to low p H restrained the nitrification process. However, high p H value inhibited the denitrification process and deteriorated the phosphorus removal system. It has been demonstrated that the p H of 7.5 could enhance the phosphorus removal and the process of nitrification and denitrification, thereby resulting in high nutrient removal performance. The aerobic polymer synthesis and phosphorus uptake could be enhanced with the presence of extended-idle phase. When the idle duration ranged from 2 to 4 h, excellent BNR could be achieved in the system. The nitrogen removal efficiency experienced a small decrease with the idle time was extended to 8h. Nevertheless, after the idle phase was removed, biological nitrogen and phosphorus removal efficiencies decreased rapidly in the system.Microbial community structure analysis showed that a rich micro bial community structure was achieved in the system. When acetate and propionate serve d as carbon source, the dominant bacteria in the system was Proteobacteria. Proteobacteria can absorb organic carbon for PHA synthesis, which is beneficial to the BNR performance in the system. In the systems feeding with methanol and ethanol, the efficiencies of BPR dereased because Betaproteobacteria was no longer the predominant bacteria. However, in the glucose system, filamentous bacteria became the dominant microorganisms. As a result, low BNR efficiencies were observed. The main energy storage microorganisms in the system might be Zoogloea bacteria by coupling the denatured gradient gel electrophoresis profiles with the phosphorus removal performance of the system.The underlying mechanisms for the BNR induced by intracellular polymers were explored by analyzing the cyclic variations of nitrogen and phosphorus as well as intracellular polymers. The process can be divided into three stages, namely inner carbon accumulation phase, inner carbon consumption phase, and polyphosphate hydrolysis phase. In the oxic stage, external carbon source was sufficient, and the microorganisms could absorb the carbon source to synthesis PHA and store it. During the anoxic phase, the external carbon had been exhausted, thus the microorganisms consumed the stored PHA for the growth and the process of denitrification and phosphorus removal. And in the idle phase, both of the external carbon source and the accumulated PHA had been exhausted, the microorganisms mainly obtained energy from polyphosphate hydrolysis to maintain the growth. This enhanced the polyphosphate metabolism of the microorganisms.The BNR performance induced by intracellular polymers was higher than the performance of traditional anearobic/anoxic/oxic and the oxic/extended-idle process. This system provided favorable conditions for the growth and metabolism of phosphate accumulating organisms(PAO), thus achieved excellent BPR performance. Meanwhile, the absorption of organi c carbon source and the synthesis of intracellular polymers in the oxic period ensured the sufficient polymers during the anoxic stage, which was favorable for the denitrification process and drove a high denitrification.It was demonstrated that BNR from domestic wastewater could be well achieved in this process. The average effluent nitrogen and phosphorus concentrations were as low as 6.5 and 0.29 mg·L-1, and the average total nitrogen and phosphorus removal efficiencies reached 80% and 97%, respectively, which showed a high feasibility and stability.In this study, the polyphosphate metabolism of the microorganisms and the process of denitrification induced by intracellular polymers were interconnected. The control conditions for intracellular polymers a ccumulation were optimized, and the mechanisms for BNR induced by intracellular polymers were explored. The experimental results enrich the existing theory of nitrogen and phosphorus removal, and provide a new method for solving the problem of carbon sourc e shortage in the denitrification process. Furthermore, the results of this research can also provide a scientific basis for the engineering application of this technology in the future.