| The pollution problem of water environment has been increasing along with the rapid development of urbanization and intensive, large-scale modern agriculture, which seriously affects the safety of drinking water. Because the limited operation performance of conventional processes including coagulation, sedimentation, filtration and disinfection in treating raw water with higher organic matter and ammonia nitrogen, the biofilm pretreatment technology has been gradually used to treat polluted raw water. In this study, the typical polluted raw water with higher organic matter and ammonia nitrogen levels obtained from river network region in eastern China was used as the influent, and the measures for accelerating the growth of microorganisms and the formation of biofilm were used to strengthen the startup and operation performance. Based on the analysis of mathematical models and high-throughput sequencing techniques, the performance strengthening mechanisms were systematically explored. In addition, the technology application with further enhancement of operation performance was studied in pilot scale reactors. The main results are presented as follows:1. In order to enhance the growth of microorganisms and the formation of biofilm, the effects of nutrients addition, inoculation source enhancement and hydraulic mixing on the startup performance of biofilm pretreatment processes were studied. Results showed that nutrients addition strategy could effectively enhance the formation of biofilm and improve NH4+-N oxidation rate (x, mg L-1 d-1) and TOC removal rate (ORR). AOR(mg L-1 d-1) and initial NH4+-N concentration had an exponential increasing mode (AOR=0.4558e0.0450x, R2=0.9999), and ORR (mg L-1 d-1) was positively related to the initial TOC concentration (x, mg L-1)with an equation of ORR=0.32548x-1.016 (R2=0.9995). In this study, the maximum AOR and ORR reached to 6.43 and 28.50mg L-1 d-1, respectively. The AOR and ORR were limited due to the lower temperature, but the operation performance were enhanced via hydraulic mixing.Analysis of the microorganisms community in biofilm under different starting up conditions for performance improvement, nutrients addition, inoculation of river sediment and feeding effluent of a biofilm pretreatment process improved community diversity in biofilm, and Shannon diversity index H reached to 3.15,3.31 and 2.87, which were 1.44,1.51 and 1.31 times higher than that observed in control reactor, respectively. Nutrients addition and inoculation of river sediment enhanced biofilm formation, however, the biofilm formed under high nutrient levels had higher Ks values and inoculation of sediment increased effluent UV254 level and reduced organics in biofilm. Comparably, Accelerating the flux of nutrients need for microorganisms via shortening HRT could effectively enhance the start-up performance.2. The biofilm reactors filled with elastic stereo carrier, combined carrier and polyurethane carrier and fed real polluted raw water were started up with low concentration nutrition addition and completely effluent circulation strategy, and the operation stability under continuous flow mode and the biofilm characteristics were investigated. Experimental results showed that the reactors filled with different carriers were successfully started up within 20d using the strategy of low concentration nutrition addition and completely effluent circulation. The NH4+-N removal efficiency (ARE) of biofilm reactors with elastic stereo carrier, combined carrier and polyurethane carrier were 84.0±7.54%,80.1±11.1% and 79.62±10.70%, while TOC removal efficiency were 8.75±5.40%,18.07±8.39% and 4.41±5.46%, respectively. It is obviously shown that the NH4+-N removal performance of biofilm reactor filled with elastic stereo carrier was highest, and the biofilm reactor with combined carrier had the highest TOC removal performance. It is worth noting that low nutrient addition strategy still has the operation risk of increasing the affinity constant Ks and decreasing the biomass of biofilm. Fortunately, the risk could be relived by the measure of NH4+-N loading control and long-term acclimatization to oligotrophic conditions. When ALR was higher than 6.29 mg L-1 d-1 in biofilm reactor filled with elastic stereo carrier, the increasing of Ks could not reduce AOR. The Ks value could be effectively reduced after long-term acclimatization to oligotrophic conditions, which could further increase the biomass, adenosine triphosphate (ATP) and extracellular polymeric substances (EPS) in biofilm, and maintain long-term operation stability of biofilm systems.Based on the high porous adsorption properties of diatomite, a novel biofilm process, i.e.,Bio-diatomite biofilm process (BDBP) was developed to enhance the performance of biofilm pretreatment systems. After successfully started up using low concentration nutrition addition with completely effluent circulation strategy, the AOR and ORR of BDBP were 9.33 and 5.13mg L-1 d-1,respectively. Under continuous flow operation conditions, the removal rate of TOC, NH4+-N and TN were 7.87,14.1 and 1.93mg L-1 d-1, which were obviously greater than the control values of 7.49,11.3 and 0.73mg L-1 d-1, respectively. At the same time, the average THMs removal performance was enhanced with a removal efficiency increased by 6.29%. Observation of electron microscope and analysis of biofilm fraction showed that both diatomite and biofilm are attached to the carrier with the maximum biomass of 3.75 mg DOC g-1 carrier, which is 1.44 times of that obtained in control reactor and the maximum ATP content reached to 251.6 mu g g’1 TOC, which showed that diatomite addition is advantage to the enrichment of microorganisms and increasing in biological activity.3. The operation of biofilm reactors treating raw water had the problems of fast growth of algae and nitrogen removal performance need further improvement, the influence algae on pollutants removal performance with its control strategy and the performance enhancement were studied. Results showed that the effluent average NH4+-N concentration in reactor with precoated biofilm carrier was stable at 0.30 mg L-1 with reduced effluent NO2--N concentration. However, hlorophyll a levels accumulated more than 200 mg m-3 during the algae outbreak operation time, and the the effluent NH4+-N concentrations were 0.78-4.33 greater than the influent level and nitrite was obviously accumulated when algae was damaged. Fortunately, the excessive reproduction of algae and operation risk could be effectively controlled via artificial algae removal and light shading methods, and the effluent ammonia nitrogen concentration is lower than 0.5 mg L-1 without any nitrite accumulation.The effects of aeration and temperature on the removal performance of pollutants showed that the optimal DO concentration and temperature ranges in biofilm pretreatment systems treating raw water were 1.0-2.6 mg L-1 and 21-22℃, respectively. Under this operation conditions, the maximum NH4+-N removal efficiency increased from 70.0% to 91.1%. The average DOC removal efficiency reached 25.4±19.9% and the UV254 removal performance increased from nearly no removal to a removal efficiency of 13.2±11.3% with the removal of potential THMs production decreased by 9.2-21.8%.4. Based on the preliminary laboratory study, two pilot scale biofilm reactors with effective volume of 3.6 m3 each were constructed in Haining second waterworks Co. Ltd., Zhejiang Province, and the enhancement of operation performance in the two reactors were studied. Experimental results showed that the optimum operation performance was obtained in reactor filled with combined carrier of elastic filler-suspended ball carrier-elastic filler under aeration mode of none aeration-aeration-none aeration (NA-A-NA), and the average removal efficiencies of NH+-N, CODmn and turbidity reached to 89.4±9.9%,32.4±9.2% and 76.7±7.9%, respectively, which were more than those obtained in reactor filled with suspended ball carrier only of 88.7±9.9%,26.1±11.1% and 17.2±12.6%, respectively. At the same time, the TN removal performance was enhanced and more than 50% energy consumption of aeration with the construction costs of aeration device was reduced. In addition, the consumed coagulant of 47.0%-76.7% turbidity removal was saved due to the the enhanced turbidity removal performance, which suggested obvious economic benefits.Analysis shows that the nutrients releasing of sediment, microorganism enrichment due to the similar effects of diatomite induced by turbidity, and the functional partition due to the DO distribution are keys to improve the pollutants removal performance.5. The bacterial community of biofilm in lab-, pilot- and full-scale biofilm reactors treating raw water were analyzed using high-throughput sequencing method Illumina MiSeq. Results showed that the structure of bacterial community was affected by many operation factors including operation time, carrier, substrates and DO and lower microbial diversity index were observed at the initial operation time. Results showed that the main bacteria of Proteobacteria, Bacteroidetes, Planctomycetes, Firmicutes, Actinobacteria, Verrucomicrobia, Acidobacteria and Gemmatimonadetes were common co-existed in lab-, pilot-or full-scale reactors, and Proteobacteria was basically dominant in all reactors. The removal of organic matter, nitrogen and other pollutants were conducted by these co-existed functional bacteria. It is worth noting that biofilm desquamation may increase Cyanobacteria and some pathogenic bacteria in effluent. However, this risk could be effectively reduced by light shading, aeration or increasing the operation scale which reduced the relative abundance of Cyanobacteria and pathogenic bacteria in biofilm. |
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