Multivariable modeling and control of an activated sludge process

This paper contains two contributions. First it is shown, in a simulation study using the IAWQ model, that a linear multivariable time-invariant state-space model can be used to predict the ammonium and nitrate concentration in the last aerated zone in a pre-denitrifying activated sludge process. Secondly, using the estimated linear model, a multivariable linear quadratic (LQ) controller is designed and used to control the ammonium and nitrate concentration.     

Filter clogging in coarse pore filtration activated sludge process under high MLSS concentration.

Coarse pore filtration activated sludge process is a type of hybrid process in which the secondary settling tank of the conventional activated sludge process is replaced by non- woven and coarse pore filter modules. The filter has pores, which are irregular in shape, and much bigger than micro-filtration membrane pores in size. The objective of the study is to find out the effect of the microbial community structure on filter clogging in the coarse pore filtration activated sludge process under high MLSS concentration in aerobic and anoxic/aerobic (A/A) conditions. Filter clogging started from day 65 and 70 in the A/A and aerobic process, respectively, but it was more severe in the A/A process compared to that in the aerobic process. EPS contents of sludge did not change significantly during the operation in both processes, and did not have a crucial effect on the observed filter clogging. There was no strong evidence for direct effect of the type and number of metazoa on filter clogging. The main difference between aerobic sludge and A/A sludge during the filter clogging period was the relative abundance of filamentous bacteria. According to the obtained results, it can be concluded that a higher presence of filamentous bacteria could reduce the severity of filter clogging in a coarse pore filtration activated sludge process.     

Improvement of ammonia removal in activated sludge process with feedforward-feedback aeration controllers.

In the paper three linear aeration controllers that can be easily implemented are presented and evaluated on the activated sludge process pilot plant. Controllers differ according to the information that is used about the process, which can be oxygen in the last aerobic reactor, ammonia in the last aerobic reactor and ammonia in the influent. The aeration controllers that are addressed are: oxygen cascade PI controller, ammonia cascade PI controller and ammonia feedforward-cascade PI controller. Experiments show that, in comparison with the oxygen cascade PI controller, the ammonia cascade PI controller allows better control of effluent ammonia and airflow savings of around 23%, while the ammonia feedforward-cascade PI controller gives the best reduction of ammonia peaks and can save up to 45% of the airflow.     

投加短管填料对活性污泥工艺性能的改善

在传统活性污泥工艺中投加聚氯乙烯短管填料,研究投加短管填料对活性污泥工艺性能的影响,将该系统应用于城市污水处理试验研究,并与单一活性污泥系统进行对比研究。结果表明,投加短管填料可以改善活性污泥工艺性能,系统对有机物及氮、磷去除效果显著,可在同一反应器内实现脱氮除磷。与活性污泥系统相比,投加短管填料反应器具有较强的抗冲击负荷能力。     

The application of model predictive control of ammonia nitrogen in an activated sludge process

In this paper a model predictive controller (MPC) for ammonia nitrogen is presented and evaluated in a real activated sludge process. A reduced nonlinear mathematical model based on mass balances is used to model the ammonia nitrogen in the activated sludge plant. An MPC algorithm that minimises only the control error at the end of the prediction interval is applied. The results of the ammonia MPC were compared with the results of the ammonia feedforward-PI and ammonia PI controllers from our previous study. The ammonia MPC and ammonia feedforward-PI controller give better results in terms of ammonia removal and aeration energy consumption than the ammonia PI controller because of the measurable disturbances used. On the other hand, with the ammonia MPC, comparable or even slightly poorer results than with the ammonia feedforward-PI controller are obtained. Further improvements to the MPC could be possible with an improved accuracy of the nonlinear reduced model of the ammonia nitrogen, more sophisticated control criteria used inside the controller and the extension of the problem from univariable ammonia to multivariable total nitrogen control.     

A comparison between model and rule based control of a periodic activated sludge process

Two strategies for control of nitrogen removal in an alternating activated sludge plant are compared. One is based on simple model predictions determining the cycle length at the beginning of each cycle. The other is based on simple rules relating present ammonia and nitrate concentrations. Both strategies are close in efficiency measured as effluent total (inorganic) nitrogen and both perform better than using fixed phase lengths for a test scenario describing a typical dry weather diurnal variation. After modifying the criterion functions of the rule based strategy the two strategies are found to produce equivalent controls, which means that the optimal criteria of the model based strategy can be fulfilled without the need for model predictions.     

Achieving nitrification in pure oxygen activated sludge by seeding

An innovative “seeding” approach was used to increase the nitrification capacity of a pure oxygen activated sludge plant. In this approach, waste activated sludge from a parallel air activated sludge system was used to seed nitrifiers to the pure oxygen plant to increase the apparent sludge age and achieve nitrification under the high loading in the pure oxygen system. Calculations indicate that nitrification can be achieved at a traditional sludge age of 3 days with a seeding rate of 20 percent. Full-scale tests showed that nitrification can be achieved at a seed rate of 35 percent and a traditional sludge age of 4.6 days, producing effluent ammonia below 5 mg/l. Secondary clarification remained good during the operation of the system even at the higher solids loading rates, with effluent TSS concentrations at 6 mg/l.     

Phosphorus and polyhydroxyalkanoates variation in a combined process with activated sludge and biofilm

This study investigates variations of phosphorus and polyhydroxyalkanoates (PHAs) in a combined activated sludge - biofilm process, operating under various sludge retention times (5, 10 and 15 days) and different dissolved oxygen conditions (0.1, 0.5, 1.0 and 2.0 mg/l in aerobic stage). Experimental results indicate that phosphorus uptake closely corresponds to utilization of PHAs during anoxic and aerobic stages. Moreover, the sludge in the anoxic stage exhibits a higher PHAs utilization efficiency with respect to phosphorus uptake than sludge in the aerobic stage, when it is under low COD-SS loading conditions. The values of r_P/PHA, representing sludge capacity on phosphorus uptake, range from 0.1–1.0 mg P/mg PHAs. In addition, analyzing the distribution of 3-hydroxybutyrate (3HB), 3-hydroxy-2-methylbutyrate (3H2MB), 3hydroxyvalerate (3HV) and 3-hydroxy-2-methylvalerate (3H2MV) reveals that 3HB and 3HV are the major components of PHAs. The values of 3HB/PHAs and 3HV/PHAs vary with COD-SS loading of the process. When F/M ratio increases, 3HV/PHAs value increases and 3HB/PHAs value decreases simultaneously. This phenomenon implies that more bacteria accumulated 3HV as storage matter under high COD-SS loading conditions. The kind of bacteria population shift would intensify the competition of “G bacteria” with polyphosphate accumulating organisms, possibly causing process deterioration during phosphorus removal.     

Behaviour of arsenic species in batch activated sludge process: biotransformation and removal.

The behaviour of As(III), As(V), MMA(v) and DMA(v) in batch activated sludge process were investigated. Experiments were carried out by using aerobic and anoxic reactors with an initial As concentration of 100 microjg I(-1). Under aerobic condition, As(III) was oxidized to As(V) within 9 hours, some part of MMA(v) was methylated to DMA(v) and some other part was demethylated to As(III), which in turn was immediately oxidized to As(V). Under anoxic condition, As(V) was reduced to As(III) within the same time-course. No significant transformation occurred during experiments conducted with DMA(v). It was found that all reactions were biologically mediated. The overall As removal was low (< 20%) during the experiments. Although a relationship seems to exist between the sludge concentration and As removal, it is concluded, under the conditions of our study, that the activated sludge process cannot remove arsenicals efficiently. However, it can control their transformations well. Thus, if associated with an appropriate technology, the activated sludge can be used for As pre-oxidation to treat As contaminated wastewaters. Finally, care must be taken on possible presence of MMA(v) in the influent of any wastewater treatment plant as it can be easily oxidized by the activated sludge.     

循环式活性污泥法工程设计方法探讨

分析了CASS与CAST工艺的异同点,指出SBR工艺可以模拟多种不同的连续处理工艺,而CAST可以根据连续流A2/O工艺的设计方法进行设计.通过定义(进水 反应)阶段占每个运行周期的比例(§),引入有效污泥龄与有效水力停留时间的概念,分析了CAST同步硝化反硝化功能对主反应区DO的控制要求及对好氧SRT的影响,推荐采用CAST处理城市污水时使用经验值θc厌∶θc缺∶θc好=1∶2∶17;强调系统生物量的衰减由系统总SRT决定,调整(§)即可调整总污泥龄,并影响总产率系数.最后,给出了设计步骤和计算实例.当采用平均日流量设计时,建议采用较长的运行周期(6 h),便于在峰值流量时增加周期数,以保持CAST运行的灵活性.     

Automatic control for do and pH in the activated sludge process in a coke wastewater treatment plant

The objective of this study is to develop an automatic control system for dissolved oxygen (DO) and pH of the activated sludge process in a coke wastewater treatment plant. A discrete type autotuned proportional-integral (PI) controller using an auto-regressive exogenous (ARX) model as a process model was developed to maintain the DO concentration in aerators by controlling the speed of surface aerators. Also a nonlinear pH controller using the titration curve was used to control the pH of influent wastewater. This control system was tested in a pilot scale plant. During this pilot plant experiment, there was small deviation of pH and the electric power consumption of surface aerators was reduced up to 70% with respect to the full operation when the DO set point was 2 mg/1. For real plant operation with this system, the discrete PI controller showed good tracking for set point change. The electricity saving was more than 40% of the electricity consumption when considering surface aerators. As a result of maintaining the DO constantly at the set point by the automatic control system, the fluctuation of effluent quality was decreased and overall improvement of the effluent water quality was achieved.     

Reactive tracers reveal hydraulic and control instabilities in full-scale activated sludge plant.

The hydraulic characteristics of aeration tanks in WWTPs have a major impact on the degradation of pollutants, as well as on the control of the aeration. In particular in long reactors, which are not separated by baffles, hydraulic shortcuts or large scale recirculation can lead to a loss of performance. This work demonstrates that reactive tracers such as ammonium and oxygen can be used to investigate the hydraulics of aeration tanks in detail. With the use of electrochemical sensors it is possible to investigate effects in a broad range of time scales.In the present case study a slow oscillation of the aeration control loop was investigated. Large scale recirculation in the aeration tank and fast fluctuations of the ammonium concentrations close to the oxygen sensor were identified as the cause of these oscillations. Both, the recirculation as well as the fluctuation of the ammonium have a substantial influence on the performance of the aeration tank and the aeration control loop.     

Performance analysis of anoxic selector in upgrading activated sludge process in tropical climate.

The paper describes and analyses the performance of anoxic selectors in upgrading activated sludge process in a municipal wastewater treatment plant under tropical climate, where poor sludge settleability is a problem due to elevated temperature. Site monitoring and laboratory experiment were conducted to study the denitrification, sludge settleability, kinetics, mass balance, pH and alkalinity variation in the system. The sludge settleability measured in Sludge Volume Index (SVI) was observed to improve with the increasing degree of denitrification in the anoxic selector. Under well-developed stable state, an average SVI value of 136 ml/g was achieved, which was significantly lower than the value of 250 ml/g before the application of anoxic selector. The specific reaction rates for denitrification and nitrification at 30 degrees C were 0.06 mg NO3-N/mg MLSS day and 0.08 mg NH4-N/mg MLSS day, respectively. The overall efficiencies of nitrification and denitrification were 86 percent and 55.4 percent, respectively, with an alkalinity recovery ratio of 15.4 percent. 32 percent of total COD removed was used up as electron donor in the denitrification process. However, due to absence of the internal Mixed Liquor Recirculation (MLR), a higher degree of denitrification occurred in the secondary sedimentation tank than in the anoxic zone. Issues for further studies are also discussed.     

碳源不足的城市污水Highsludge(R)高浓度活性污泥法研究

Highsludge高浓度活性污泥工艺采用兼氧-好氧组合流程,最显著的特点是活性污泥浓度高和好氧段溶解氧低.对该工艺处理碳源不足的城市污水进行了中试研究,结果表明:High-sludge高浓度活性污泥工艺可以在进水碳源不足的条件下,保证很好的TN去除效果,在进水TN不高于59.2 mg/L的条件下,出水TN低于15 mg/L,满足<城镇污水处理厂污染物排放标准>(GB18918-2002)一级A标准.试验分析了TN去除情况与各工艺运行参数的相关关系,发现MLSS和DO是控制出水TN最关键的参数.同时试验发现,高浓度活性污泥条件下,普通沉淀池可以正常运行,剩余污泥产量较少但会产生较多的生物泡沫.     

Partial ozonation of activated sludge to reduce excess sludge, improve denitrification and control scumming and bulking.

Disposal of sewage sludge is forbidden and agricultural use of stabilized sludge will be banned in 2005 in Switzerland. The sludge has to be dewatered, dried, incinerated and the ashes disposed in landfills. These processes are cost intensive and lead also to the loss of valuable phosphate resources incorporated in the sludge ash. The implementation of processes that could reduce excess sludge production and recycle phosphate is therefore recommended. Partial ozonation of the return sludge of an activated sludge system reduces significantly excess sludge production, improves settling properties of the sludge and reduces bulking and scumming. The solubilized COD will also improve denitrification if the treated sludge is recycled to the anoxic zone. But ozonation will partly inhibit and kill nitrifiers and might therefore lead to a decrease of the effective solid retention time of the nitrifier, which reduces the safety of the nitrification. This paper discusses the effect of ozonation on sludge reduction, the operation stability of nitrification, improvement of denitrification and gives also an energy and cost evaluation.     

Co-conditioning and dewatering of alum sludge and waste activated sludge.

Co-conditioning and dewatering behaviors of alum sludge and waste activated sludge were investigated. Two different sludges were mixed at various ratios (2:1; 1:1; 1:2; 1:4) for study. Capillary suction time (CST) and specific resistance to filtration (SRF) were utilized to assess sludge dewaterability. Relatively speaking, waste activated sludge, though of higher solid content, was more difficult to be dewatered than alum sludge. It was found that sludge dewaterability and settlability became better with increasing fraction of alum sludge in the mixed sludge. Dosage required of the cationic polyelectrolyte (KP-201C) for dewatering was reduced as well. It is proposed that alum sludge acts as skeleton builder in the mixed sludge, and renders the mixed sludge more incompressible which is beneficial for sludge dewatering. Implications of the results of the study to the sludge management plan for Taipei City that generates both alum sludge and waste activated sludge at significant amount are also discussed. The current sludge treatment and disposal plan in Metropolitan Taipei could be made more cost-effective.     

活性污泥工艺模型在计算机模拟中的建立与应用

利用计算机技术模拟活性污泥生化过程是分析研究污水处理工艺的有效手段之一,模拟的有效性取决于采用的动力学模型、工艺模型、沉淀模拟以及模拟计算中的数学分析方法。其中工艺模型反映了工艺特性,是真实工艺在模拟过程中的再现。根据水力学及运行特征,可将现有主要工艺分为单池间歇式、多池间歇式、单向连续式和交替连续式。运用ASM2D动力学模型,对某污水处理厂的UNITANK工艺进行模拟,结果表明,选择的动力学模型和工艺模型是合适的。     

Pre-ozonation in the activated sludge process: fate of nitrogen species

The objective of this research was to include ozonation prior to an activated sludge treatment and investigate the effect on the nitrogen species, their fate and the consequences of this oxidation upon the biomass. Three parallel treatment systems were used: the base system, where feed went directly to the activated sludge reactor, and two others, where the influent was ozonated at two different dosages, 15 and 25 mg/L of influent, prior to the biological reactors. The results from the ozonation chamber show a high oxidation capacity of the entering ammonia and organic nitrogen, proportional to the ozone dose. The oxidation product was nitrate. No de-nitrification was expected because a high oxygen concentration (4 mg/L) was maintained in the reactors. The reactors receiving ozonated influent showed a lower assimilation of nitrogen by the biomass. The sludge nitrogen content resulted in 11, 9.3 and 7.4% dry-weight corresponding to no-ozone, low ozone and high ozone dosages, respectively. In spite of the lower ammonia available in the ozonated flows, the corresponding reactors showed a higher specific nitrification rate. The ozonated system also performed better in terms of chemical oxygen demand (COD) and biochemical oxygen demand (BOD5) removals, besides showing a higher true biomass yield coefficient.     

Fundamental nonlinearities of the reactor-settler interaction in the activated sludge process

The activated sludge process can be modelled by ordinary and partial differential equations for the biological reactors and secondary settlers, respectively. Because of the complexity of such a system, simulation models are most often used to investigate them. However, simulation models cannot give general rules on how to control a complex nonlinear process. For a reduced-order model with only two components, soluble substrate and particulate biomass, general results on steady-state solutions have recently been obtained, such as existence, uniqueness and stability of solutions. The aim of the present paper is to utilize those results to formulate some implications of practical importance. In particular, strategies are described for the manual control of the effluent substrate concentration subject to the constraint that the settler is maintained in normal operation (with a sludge blanket in the thickening zone) in steady state. Such strategies contain how the two control parameters, the recycle and waste volumetric flow ratios, should be chosen for any (steady-state) values of the input variables.