英文英语文章-Comparative evaluation of mass transfer of oxygen

Comparative evaluation of mass transfer of oxygen in three activated sludge processes operating under uniform conditionsVenkatram Mahendraker, Donald S. Mavinic, and Kenneth J. HallAbstract: This investigation was conducted in three laboratory-scale activated sludge processes, namely, the conventional completely mixed activated sludge process (CMAS), the modified Ludzack-Ettinger process (MLE), and the University of Cape Town (UCT) process. These systems were operated under controlled conditions by feeding a uniform influent at a single 10-d solids retention time (SRT). The oxygen transfer parameters (KLaf, OTRf, and OTEf) determined indicated that the alpha () and oxygen transfer efficiency (OTEf) in processes where the enhanced biological phosphorus removal (EBPR) mechanism was active were higher than in the conventional completely mixed activated sludge (CMAS) process, where such a mechanism was absent. Thus, presenting a clear evidence of improved oxygen utilization in activated sludge processes contain aerobic, anoxic, and anaerobic selector zones. Key words: biological nutrient removal, completely mixed activated sludge process, oxygen uptake rate, oxygen transfer efficiencyMaterial and methodsFig. 1.Schemes for (a) completely mixed activated sludge (CMAS) process, (b) modifiedLudzack Ettinger (MLE) process and (c) University of Cape Town (UCT) process.The CMAS, MLE, and UCT process schemes employed in this study are shown in Fig.1. In these processes, the aerobic, anoxic, and anaerobic reactor volumes were 16 L, 8.55 L, and 3.85 L, respectively, whereas, the clarifier volume was equal to 5.25 L. The percentage of anoxic reactor volume was equal to 35 of the total process volume (excluding the clarifier ) in the MLE process, whereas, the percentages of anoxic and anaerobic volumes were equal to 30 and 13.5(excluding the clarifier ), respectively, in the UCT process. The influent and biomass recirculation flow rates (including return activated sludge) were constant at 3L/h, in all systems. Each of the reactors had a mixer operating at 60 rpm for mixing the reactor contents, and the clarifier had an internal scraper rotating at 4 rpm to dislodge the attached biomass. The entire setup was located within a temperaturecontrolled room set at 200.5 and the systems were operated at a 10d SRT. The aerobic reactor had a finepore ceramic diffuser ( Model Number AS2, Aquatic Ecosystems, Florida) made from heatbondedsilica with a maximum pore size of 140 ?m, producing air bubbles of 1 mm to 3 mm radius in clean water. The air to the diffuser was supplied via an air pressure regulator, Pressure gauges, and a high precision flow meter (accuracy 2; flow range 90 to 2520 mLmin-1, LABCOR Catalogue #P32044-18, Tube # 023-92ST) and controlled manually to maintain a DO concentration of 2 to 3 mgL-1 in the aerobic reactor.At least twice a week during “normal operational days”, and daily during “oxygen transfer test days”, grab samples from the feed tank, aerobic, anoxic, and anaerobic reactors, as well as the effluent were collected for analyses. The parameters analyzed included mixed liquor suspended solids(MLSS)(Standard Method 2540D), mixed liquor volatile suspended solids(MLVSS) (Standard Method 2540E),chemical oxygen demand(COD) (Standard Method 5220D),total organic carbon(TOC) (Standard Method 5310B), ammonianitrogen(NH3N), nitrite plus nitrate nitrogen species (NOXN),orthophosphate(PO4P),total phosphorus(TP), total Kjeldahl nitrogen(TKN), conductivity and sludge volume index(SVI),generally following Standard Methods(APHA et al.1992). All inorganic analysis was done on a Lachat QuickChem Automated 表一 综合废水进水特性（以mgL-1计）注：SD，标准；N，数据点的数量；TN，总氮。Analyzer following Methods (QuickChem 1990). Dissolved oxygen (DO), oxidationreduction potential (ORP), and ph in the aerobic reactor were continuously monitored and recorded by an online data logging system. Additional details of the UCT system and its operation were given in Mahendraker et al. (2002), which are also generally applicable to the operation of the CMAS and the MLE processes. The oxygen transfer testing was conducted as per the steady state OUR given in the guidelines(ASCE 1997).Oxygen uptake rates were determined ex situ, in a leak proof respirometer mounted with a DO probe (YSI model number 5739) that was connected to the date logging system. A 350mL volume (equal to 2.19 of the aerobic reactor volume) of mixed liquor sample was used in the respiration tests. Furthermore, the clean water testing of the diffusers was conducted in a batch reactor that was similar to the aerobic reactor, following the standard (ASCE 1992), to obtain clean water mass transfer rates. The DO data during the clean water tests were logged at the rate of 2 dissolved oxygen (DO) measurements/minute, and the data were analyzed using the DOPAR nonlinear least squares approximation program, developed by Prof. Michael K. Stenstrom and his group at the University of California at Los Angeles (Lee et al. 2000). These clean water test results were used in the calculation of value. The synthetic wastewater was made up from glucose, yeast extract, peptone, sodium, acetate, ammonium chloride, potassium phosphate monobasic, sodium bicarbonate, magnesium chloride, calcium chloride, potassium chloride, manganous sulfate, and zinc sulfate in tap water. The average influent wastewater characteristics presented in Table 1, show a low level of variation and good control of conditions in these experiments.Results and discussionProcess performance The online DO, ORP, and pH data summarized in Table 2 indicate uniform environmental conditions in the aerobic reactor during each test. This is a basic requirement for a successful implementation of the steadystate OUR method (ASCE 1997). Only in the case of ORP, can small changes be observed, as the testing progressed; this was mainly due to a draft in the ORP probe. Often, a 20 mV to 30 mV reductions in the ORP signal was recorded over a testing period (10 to 12 d) when it was calibrated before and after each run. The cause for greater than 50 mV changes in ORP in the UCT process run could not be clearly established, especially given that there was no significant variation in the process performance during the testing (see Fig.2 and Table 3). Table 2 also provides the data on air flow rate (AFR) supplied to maintain the aerobic reactor DO concentration, within the desired level of 2 to 3 mgL-1 in the systems. As an example, Fig. 2 illustrates a broad set of data (except solids) collected for the UCT process during the entire experimental run, including the testing period. This figure consists of (1) COD and TOC removal by the system, as well as COD and TOC uptake in the anaerobic reactor in Fig.2a; (2) effluent NH3N, NOXN, PO4P concentrations in Fig.2b; and (3) PO4P and NH3N concentrations measured in the anaerobic reactor in Fig.2c. Table 3 present a summary of the process performances in the three processes, during the testing period only. The overall COD and TOC percentage reductions in all the processes were similar, although the average reduction in the CMAS process was 2% less when compared to the UCT and MLE processes. The MLE process performed like a pseudo UCT process (see Table 3), probably, due to the selected volume of the anoxic reactor. The original intent, during the experimental design stage in choosing the 8.55L anoxic reactor, was to prevent entry of nitrates into the anaerobic reactor in the UCT configuration. This was successfully achieved based on less than 0.10 mg/L of NOXN measured in the anaerobic reactor (date not shown). This UCT processlike behavior by using the same anoxic reactor design in the MLE process configuration was not anticipated. The fraction of unaerated volume of 35% for the anoxic zone is within the maximum recommended criteria of 50% (WRC 1984) and close to value of 38% reported elsewhere (Randall et al. 1992a.). A low biomass recycle ratio of one (low nitrate loading from aerobic reactor), below the ratio of 2 to 3 typically used in practice (Grady et al. 1999), perhaps led to unutilized denitrification capability and anaerobiclike conditions in the anoxic reactor. In the MLE process, the combined COD reduction due to storage by phosphorus accumulating organisms (PAO) and simultaneous COD utilization by the denitrifying microbial population resulted in COD reduction in the anoxic reactor. This percentage is close to the sum of COD reduction due to uptake in the anaerobic reactor (65%) by PAO activity and the uptake plus utilization in the anoxic reactor (25%) by both the portion of PAOs that denitrify (Kuba et al. 1997) and other denitrifies (i.e., those do not participate in carbon storage) in the UCT process. The difference as far as COD was concerned, in the UCT process, was the anaerobic zone providing an advantage to the PAO to store carbon, without the need to simultaneously 表2 好氧池测试日日常的pH, ORP和DO注：SD，标准偏差；N，数据点数量普遍是氧化还原电位，pH,和溶解氧；AFR，空气流量。compete with denitrifies. The mean TN and TP removed in the CMAS system were 38% and 57% lower than in the UCT and MLE processes; whereas, the TN and TP removed in the UCT and MLE processes were virtually the same (within a 1% to 2% difference), as shown in Table 3. The biomass phosphorus content in the BNR (UCT and MLE) processes was between 3.90% and 4.90%, whereas, it was only 1.43% in the biomass from the CMAS process, clearly establishing the difference in quality of the biomass as well as the population makeup under these situations. The TKN content was within a close range of 11% and 12.8% in all the experimental runs. The mean solids concentration in the aerobic reactor in the UCT and MLE process was at 3100 mg/L of MLVSS. However, it was about 30% lower in the CMAS process. In all experiments, the effluent suspended solids (SS) were low (less than 15 mg/L), which provided a good level of SRT control. The SVI date (not shown) resulted in the highest mean SVI value of 273 mL/g in the CMAS process, while it was approximately the same at 160 mL/g, in both the UCT and MLE processes, suggesting that the biomass from the CMAS process had a relatively poor settling quality.Fig 2. UCT process performance: (a) COD and TOC uptake in the anaerobic reactor and removal by the process, (b) concentration NH3N, NOXN, and PO4P in the effluent ,and (c) anaerobic reactor NH3N and PO4P concentration.Table 3. Process performance in each of the experimental runs during oxygen transfer test days.Note: SD, standard deviation; N, number of data points; TN, total nitrogen; TKN and TP include the contribution from the SS present in the effluent. units in mg/L, except as mention otherwise.Oxygen transfer parametersThe oxygen uptake rates obtained in the three processes are presented in Table 4. the linear regression of the DO data, with time, resulted in excellent correlation coefficients (R20.98), implying a high degree of linearity in the respiration rates. The respiration rates gradually increased as the environment in the process train became progressively aerobic, under the same influent characteristics and SRT (10d). The mean OUR in the UCT process was lowest at 23.4mg L1h1 of DO, which changed to a slightly higher value of 28.0 mg L-1h-1 of DO in the MLE process, and further increased to 34.0 mg L1h1 of DO in the CMAS process. The CMAS process had greater variation in respiration rates (SD equal to 5) compared the UCT and MLE process (SD less than 2) for a similar data set (N=10 to12). Conclusions The data collected in this work showed that BNR processes resulted in a higher and oxygen transfer efficiency, compared to the CMAS process, under comparable influent and operating conditions. The air flow supply needed in the CMAS was between 2 and 3 times higher than in the UCT and MLE processes. The MLE process behaved like the UCT process, most probably due to higher anoxic reactor volume and low nitrate loading. These results do not represent a true case of the MLE process, where EBPR process is not particularly active and the process is primarily designed to remove nitrogen from wastewater.统一条件下运行的三个活性污泥工艺的氧转移的质量比较评价Venkatram Mahendraker, Donald S. Mavinic, and Kenneth J. Hall摘要：本研究是三个实验室规模的活性污泥法，即传统的完全混合活性污泥法（CMAS），改进后的Ludzack-Ettinger型工艺（MLE）和开普敦大学（UCT）工艺。这些工艺全是控制运行在一个单一培养的10天进水固体停留时间（SRT）的条件下。氧的传递参数（KLaf，OTRf和OTEf）测定表明，强化生物除磷（EBPR）机制活跃的UCT和MLE工艺在值和活性氧转移效率（OTEf）方面比在常规完全混合活性污泥（CMAS）工艺更高，而常规完全混合活性污泥（CMAS）工艺则缺乏这种机制。因此，提出了明确的证据，在活性污泥过程中包含好氧，缺氧和厌氧区可提高了氧的利用率。关键词：生物脱氮除磷，完全混合活性污泥法，氧摄取率，氧传质的效率材料与方法图1（a）完全混合曝气活性污泥（CMAS）工艺流程，(b) 改进后的Ludzack Ettinger (MLE) 型工艺流程，（c）开普敦大学(UCT)工艺流程研究中CMAS，MLE和UCT的采用的流程如图1所示。在这些过程中，好氧，缺氧和厌氧池容积分别为16 L，8.55 L和3.85 L，而澄清量的容积相当于5.25 L。在MLE工艺中缺氧池体积相当于总流程池子容积的35（不包括澄清池），而在UCT工艺中缺氧和厌氧池的百分比分别为30和13.5（不包括澄清池）。在所有的系统中进水流量和生物量再循环（包括回流活性污泥）恒定在3升/小时。每个池子都在60 转的搅拌下混合池子内容物，而澄清池内部有一个4转的旋转刮刀刮走所附的沉淀物。整个设备位于内温度控制在20 0.5的房间，系统全都是在10天的SRT下操作。好氧反应器中有一个细孔陶瓷扩散器（型号AS2，水生生态系统，佛罗里达州）制成的最大孔径大小为140微米热粘合硅胶，在净水中产生气泡的半径为1毫米至3毫米。扩散空气的供给通过空气压力调节器，压力表，以及高精密流量计（精度 2;流量范围90至2520毫升每分钟，LABCOR目录P 32044-18，管023 - 92ST）和手动控制，以维持好氧反应器中溶解氧浓度为2到3 毫克每升。 至少有一周两次在“正常运作天”，每日在“氧传递调测天”，从进料箱，好氧，缺氧和厌氧反应器中提取样品，以及收集出水用于分析。参数分析包括混合液悬浮固体（MLSS）（标准方法2540D），混合液挥发性悬浮固体（MLVSS）（标准方法2540E），化学需氧量（COD）（标准方法5220D），总有机碳（TOC）的（标准方法5310B），氨氮（NH3N），硝酸盐氮（NOXN），磷酸盐（PO4P），总磷（TP），总凯氏氮（TKN），电导率和污泥体积指数（SVI），一般都遵循标准方法（美国公共卫生协会等联盟 1992）。所有的无机分析是Lachat QuickChem的自动分析仪遵循QuickChem方法（QuickChem 1990）。溶解氧（DO），氧化还原电位（ORP）和pH值在好氧反应器连续在线监测和记录在一个数据记录系统。其他UCT工艺详细情况和操作在Mahendraker 等人的著作中给出（2002），这也普遍适用于MLE和CMAS的操作流程。 氧传递的测量是按准则中给出的稳定状态OUR（ASCE 1997）进行的。氧摄取率的测定是易地在安装有连接到数据记录系统的溶解氧探头（美国YSI型号5739）的防漏呼吸中进行。在一个容积为350毫升（相当于好氧反应器体积的2.19）装有混合液样品中进行了呼吸测试。此外，清水氧传质的测试是在一批反应器中进行的，这批反应器类似于好氧反应器，测试遵循下面的标准（ASCE 1992），以便获得清水水中氧传递速率。在清水中DO数据进行测试的记录在2溶解氧（DO）测量/分钟的速度，对资料进行分析使用相仿的非线性最小二乘法近似方案，该方案由洛杉矶加州大学教授迈克尔和他的小组研发（李 等人2000）。这些清洁水试验结果用于计算值。 合成废水，由自来水添加葡萄糖，酵母膏，蛋白胨，钠，醋酸，氯化铵，磷酸二氢钾，碳酸氢钠，氯化镁，氯化钙，氯化钾，硫酸锰和硫酸锌构成。表1呈现了平均进水废水的特点，展现在这些实验中其低的变化水平和较好的控制条件。 结果与讨论 工艺性能 在线溶解氧，氧化还原电位，pH值数据列于表2，表明每次测试中好氧反应器具有统一的环境条件。这是一个稳态方法（ASCE 1997）成功实施的基本要求。只有在ORP的情况下，可观察到的微小变化，作为测试的进展，这主要是由于在氧化还原电位探头灵敏度。通常，当它每次运行前后校正时，在约一个信号测试期间（10至12天）记录一个20 mV到30 mV的氧化还原电位。在开普敦工艺运行中对于大于50 mV氧化还原电位的变化的原因不明确，特别是对于在测试中（见图2和表3）工艺性能没有明显的变化。表2也提供了有关空燃比（AFR）的数据，供给维持好氧反应器溶解氧浓度，系统的理想水平为2至3mg/L。作为一个例子，图2说明了在整个开普敦工艺实验运行（除固体）中收集的一套广泛的数据，包括测试期。这一数据包括：（1）系统中COD和TOC的去除，以及在图2a的厌氧反应器中的COD和TOC的摄取；（2）图2b中出水氨氮，氮氧化物氮，磷酸盐磷浓度;（3）图2c中厌氧反应器的磷酸盐磷和氮氨的测量浓度。表3呈现出仅仅在测试期间三个工艺过程的工艺性能的一个总结。在所有过程中总的COD和TOC下降的百分数是相似的，虽然CMAS工艺的平均降幅与UCT工艺和MLE工艺相比少2。也许，MLE工艺运行就像一个伪开普敦工艺（见表3），由于选定的缺氧反应器的体积。原先在实验设计阶段选择缺氧反应器的体积为8.55L的目的是为了防止将在开普敦工艺中全部的硝酸盐进入厌氧反应器。在厌氧反应器（不显示日期）中成功的获得了基于少于0.10毫克/升的氮