Technique And Theory Investigation Of A Fluidized-pellet-bed Bioreactor For Wastewater Treatment

Contaminates in wastewater could be classified as particular and soluble components in terms of their size or organic and inorganic components in terms of their chemical property. Therefore, one of the main tasks in wastewater treatment is to remove particular and organic components by means of biological method such as activated sludge and solid/liquid separation such as sedimentation. Researches in wastewater treatment thus focus on improvements of solid/liquid separation and biological degradation efficiencies. A notable technology for improving solid/liquid separation is the fluidized pellet reactor with high performance for particular components removal because of the big size and high density pellets formation by means of optimal coagualant dosage and suitable mixing. On the other hand, granules in biological reactors could effectively improve biodegradation efficiency because of its high biomass concentration, rich divergence of species and outstanding settleabliliy that could also improve the solid/liquid separation in the secondary settling tank. However, cultivation of granular sludge is time-comsuming and hard to be well controlled, which limited its practical application.Based on the above-mentioned technological advancement, this study proposed a new technology name as fluidized pellet bioreactor (FPB), which integrated the granular sludge and pellet solid/liquid separation technologies in one reactor, by using fluidized pellets technology to convert the loose flocs into dense granules quickly and then providing appropriate biological environment such as oxygen to inspire the biological degradation. This study set the scope on the characteristics of fluidized pellet bioreactor for domestic wastewater treatment and its mechanisms for contaminates removal with the support of the National Natural Science Foundataion of China (NSFC). Results and findings in the study include the following perspectives:(1)Pilot-scale study on the performance of FPBA pilot-scale FPB system was established and a long-term operation was carried out by treatment the domestic wastewater from the inlent of a wastewater treatment plant in Xi'an city, and reasonable oxygen supply approaches and optimized operational conditions were determined. Results demonstrate that the FPB system could simultaneously achiev biodegrading and solid/liquid performances in a single reactor for treating domestic wastewater. The optimized operational conditions were determined as: PAC dosage 50 mg/L, PAM dosage 5 mg/L, upflow velocity 1.32 mm/s, mixing speed 10 rpm, recycle ratio 50%, cycle length of sludge removal 6h and the blanket height 110cm¹60cm. Under the above conditions, the removal efficiency for SS, COD, BOD, NH3-N, TN, TP, color and turbidity was 95%, 90%, 91%, 38%, 42%, 95%, 84%and 77%, respectively.(2)Physical property of pellets in FPBThe formation process of pellets, the morphological characteristics and size distribution of the pellets in the reactor were examined, and the following results could be obtained: The pellets in the reactor were sphere and their size decreased along with the height of reactor, namely, the average size of pellets from the bottom, middle and upper part of the reactor were in the range of 3mm-4mm, 2mm-3mm, and 1mm-2mm, respectively. Additionally, the relationship between diameter and density were determined by floc-settling in static water. It was found that the effective density of pellets slightly decreased with the increased of diameter, suggesting the compact structure. The effective density was in the magnitude of 10^-2 g/cm³, which was one magnitude higher than the inorganic pellets and also higher than the conventional activated sludge. It contributes the high performance of solid/liquid separation.(3)Biological property of pellets in FPBThe characteristics of microbial community and predominated bacteria species as well as their metabolic features were investigated.A large amount of bacteria could be observed in the entire pellet, using microbial and molecular biological technologies. SEM photo shows that bacteria were predominated by bacillus and coccilus. The former were mainly on the surface of pellets especially for those pellets near the bottom, and latter were mainly inside the pellets or on the surface for those granules from the upper side. The number of bacteria was in the of 40⁹0 milion/g and the aerobic bacteria accounted for more than 97% and the anaerobic was less than 3%. Along with height of the reactor, aerobic bacteria decreased due the oxygen deficiency, anaerobic bacteria however increased. The DGGE results show that the diversity of microbe in pellets was rich, and there were more than 20 strips predominated, indicating that the environmental in the reactor was suitable for the proliferation of microbes. From bottom to top, there were a large number of common strips, and the comparability of the microbial communities from bottom to top was 83.1%, indicating a very stable structure of the microbial communities.(4)Mechanisms of contaminate removal in FPBThe distribution of oxygen, organic matters, nitrogen and phosphorus were analyzed along the height of the reactor to identify how contaminates were removed. Two processes, transfer and degradation, were happened in the reactor for contaminate removal. Contaminates were firstly transferred on the surface of pellets by coagulation, flocculation and adsorption and then be biodegraded by microbes in the pellets which was similar with that happened in conventional sludge systems. However, the removal efficiency of soluble contaminates was very low indicating the biodegradation was slight, therefore transferring of contaminates was the main mechanisms.The aerobic and anaerobic zones in the pellets could provide the condition for nitrification and denitrification. However, the phosphorus was removed mainly by chemical precipitation.(5)Evaluation of FPB in wastewater treatmentThe long-term pilot operation shows that, the COD, TP and SS in the effluent of FPB system treating domestic wastewater could meet the standard GB18918 level I class A and BOD could meet that standard GB181918 level I class B, and TN was close to GB18918 level I class A. NH3-N could only meet the standard GB181918 level II. However the system could achieve pellet formation, biodegradatia and solid\liquid separation in a single unit and the total hydraulic retention time(HRT) was less than 1 hour, and the sludge loading rate was 4 kgBOD/kgMLSS·d. Therefore the FPB reactor was a high performance reactor. The MLSS and MLVSS remained in the range of 10000¹5000 mg/L and 5000⁸000 mg/L, even in the upper side of the reactor, It was much higher than the conventional sludge system, which was the main reason of high performance of FPB.