Composition, Structure And Performance Of Biological Carbon And Nitrogen Removal Sludge

Biotechnology is integral part of biological wastewater treatment and it offers an effective way to control water pollution. Microorganisms play a key role in the biotechnology for wastewater treatment. In a highly efficient biological wastewater treatment system, functional microorganisms mainly exist in the form of granular sludge. It is worthwhile to study the composition, structure and properties of granular sludge in biological wastewater treatment system so as to provide some guidelines on cultivating granular sludge of high quality, optimizing ecosystem of functional microorganisms, and improving the efficiency of biotechnology.Carbon and nitrogen are the main pollutants in wastewater and three kinds of granular sludge for efficient biological carbon and nitrogen removal have been chosen as the model to investigate their composition, structure and properties systemically by employing techniques such as polymerase chain reaction (PCR), denaturing gradient gel electrophoresis (DGGE), electron microscope observation, enzyme activity assay and fluorescence in situ hybridization (FISH).The main results of the study were as follows:1) The microbial consortium and its spatial distribution in anaerobic digestion granular sludge were revealed.The dominant archaea were Methanobacterium, Methanosaeta and Methanospirillum, and the dominant bacteria were Firmicutes, Deltaproteobacteria, Spirochaetes, Actinobacteria and Gammaproteobacteria. The methanogenic archaea were found to dominate in the upper part of reactor, the hydrogen-producing acetogenic bacteria and methanogenic archaea in the middle part, and the hydrolytic fermentative bacteria in the bottom part.Furthermore, a stratified structure of microorganisms was found in the granular sludge, where the hydrolytic fermentative bacteria were present in the outer shell, the hydrogen-producing acetogenic bacteria in the middle layer, and the methanogenic archaea in the core. The stratified structure of microorganisms was relatively simple in the granular sludge in bottom part, but it was complex in the granular sludge in middle and upper parts.The microbial consortia and their spatial distribution were in accordance with the organic loading rate and chemical components in the three compartments of reactor.2) The particle size, microbial consortia, and properties of autotrophic denitrification granular sludge were determined.The medium granular sludge was observed to dominate at the high volumetric nitrogen loading rate and to offer a strong support to the good performance. Its indispensable contribution was found to originate from the better settling velocity (0.84±0.10 cm·s-1), higher SOUR-A (specific oxygen uptake rate for ammonia oxidizing bacteria,25.93 mgO2·(gMLVSS·h)-1), lower SOUR-N (specific oxygen uptake rate for nitrite oxidizing bacteria,3.39 mgO2(gMLVSS·h)-1), higher functional microorganism proportion (54%) and more reasonable microbial spatial distribution. The medium granular sludge was revealed to be sustained by the cycle of large particle disassembly and small particle reassembly.3) The microbial consortia, enzyme activity, and properties of denitrification granular sludge were determined.Under high loading rate conditions, the organic matter was vulnerable to be limiting substrate in denitrification process. Set the nitrate loading rate in the range from 5.80 to 24.75 kg·m-3·d-1, nitrate removal rate has been maintained at above 99%, but COD removal rate dropped to 85% at high nitrate loading rate. There were linear relationships between the enzyme activity (alkaline phosphatase activity and dehydrogenase activity) and the nitrate loading rate with correlation coefficient R2 being 0.98 and 0.96 respectively. The predominant bacteria and archaea at high organic loading rate were identified as Thauera and Methanomethylovorans respectively.Suffering from a 95 min shock load, the denitrification performance deteriorated, leading to a high nitrite accumulation (207.99 mg·L-1).The alkaline phosphatase activity and dehydrogenase activity were in accordance with the performace of denitrifying reactor. The activity of two enzymes can be used as an early warning index. However, high nitrite concentration (60-210 mg·L-1)inhibited the alkaline phosphatase activity and dehydrogenase activity and interfered with the response of alkaline phosphatase activity and dehydrogenase activity to shock load.4) The structural characteristics of granular sludge were revealed.The microorganisms in the granular sludge were organized in the different microbial structures, which include individual microorganism (micron), zoogloea (several microns to tens microns), zoogloea complexes (several tens micron to two hundred microns), and granular sludge (two hundred microns to several millimeters). The microbial structures were joined by cohesive body which contained extracellular polymers and filamentous bacteria. As the microbial structures go larger, the cohesion between them became weaker, and the microbial structures were prone to disassembly. There were small and micro pores in the microbial structures, and they were the channels for substrate tranportation.