A/O工艺城市污水生化处理过程建模与优化控制研究

可持续发展的要求增强了人类有效利用资源和环境保护的意识，而城市污水处理过程是水环境保护的一项重要内容。本文针对我国城市污水处理系统能耗高、出水水质不稳定以及工业毒水经常入侵的现状，以A/O工艺城市污水生化处理过程为研究对象，研究通过建模、控制和优化手段提高污水处理系统运行性能和降低运行降耗的理论和方法。论文工作是在国际水协会发布的典型工艺与污水处理厂实际运行环境相结合的基础上完成，内容主要涉及活性污泥模型简化、基于简化模型的工艺改进与过程模拟、仿真平台的建立、关键控制回路设计、工业毒水诊断与应对系统开发以及优化控制方法与应用研究。具体内容包括： (1) 基于国际评价基准benchmark实现了污水生化处理过程的稳态模拟和动态模拟，对A2/O工艺的改进进行了仿真研究和分析，结果表明在保证出水水质稳定达标的前提下，改进工艺由于取消了内回流，大大节约了回流泵的能耗。 (2) 在国际水协会活性污泥模型ASM1模型的基础上，结合我国污水处理过程特点，并充分考虑我国污水处理厂现场测量信息严重不足的现状，对ASM1模型的组分和反应过程进行简化，建立了简化的活性污泥模型，研究了难以测量的模型组分浓度与易测的常规水质指标的转换方法，针对温度对反应速率的影响，研究了模型参数的校正方法。在此基础上，开发了A/O工艺污水生化处理过程模拟与仿真平台，并对辽宁某污水处理厂的实际运行过程进行了模拟，取得了较好的模拟结果。 (3) 建立了完整的城市污水处理过程控制系统设计框架，引入入水有机负荷和比耗氧速率分别表征微生物（活性污泥）的“食物”（有机物）数量和微生物活性，进行了系统动态特性分析，在此基础上完成了入水流量串级控制、污泥浓度前馈－反馈控制、溶解氧前馈－串级控制三个控制回路的设计。针对溶解氧控制过程采用了仿人智能控制方法，在实际应用中取得了较好的控制效果。 (4) 针对城市污水处理厂经常遇到的工业毒水侵入的问题，开发了基于专家经验的工业毒水诊断与应对系统，编写了较为完备的专家规则，系统能够在毒水侵入时及时发出警报，并采取相应的应对措施，最大程度的降低污水处理厂的损失，该系统在辽宁某污水处理厂取得了较好的应用效果。 (5) 在优化控制方面，针对污水处理过程能耗过高的问题，设计了A/O工艺城市污水处理过程的优化控制策略。建立了只考虑底物与微生物两种组分的简化活性污泥模型，以溶解氧浓度和污泥排放量为决策变量，以污水处理厂日运行费用为性能指标，以物料平衡、水质排放标准等限制为约束条件，建立了完整的优化控制模型，实现了污水处理过程的最优控制，采用基于最优步长参数动态搜索的改进型梯度法进行最佳运行工况寻优。通过对辽宁某城市污水处理厂的优化，获得了当前溶解氧浓度设定值和污泥排放量的最优值，在保证出水水质稳定达标的前提下，污水处理厂日运行费用显著降低，为污水处理厂的实际操作提供了指导。

Uso di modelli matematici per l'ottimizzazione dei processi e dell'efficienza energetica di un impianto di depurazione a fanghi attivi.

La tesi analizza la risposta di modello matematico biologico (ASM1) applicato ad un impianto di depurazione in scala pilota al fine di conoscere la risposta del sistema in seguito alla variazione delle condizioni iniziali e, in funzione dei risultati ottenuti, ipotizzare ed applicare diverse strategie di controllo tramite le quali ottimizzare l’efficienza dell’impianto, riducendo i costi in termini economici ed energetici e migliorando la qualità dell’effluente.

Predicting the effect of mixing on oxygen transfer and nutrient removal in activated sludge basins

Activated sludge basins (ASBs) are a key-step in wastewater treatment processes that are used to eliminate biodegradable pollution from the water discharged to the natural environment. Bacteria found in the activated sludge consume and assimilate nutrients such as carbon, nitrogen and phosphorous under specific environmental conditions. However, applying the appropriate agitation and aeration regimes to supply the environmental conditions to promote the growth of the bacteria is not easy. The agitation and aeration regimes that are applied to activated sludge basins have a strong influence on the efficacy of wastewater treatment processes. The major aims of agitation by submersible mixers are to improve the contact between biomass and wastewater and the prevention of biomass settling. They induce a horizontal flow in the oxidation ditch, which can be quantified by the mean horizontal velocity. Mean values of 0.3-0.35 m s-1 are recommended as a design criteria to ensure best conditions for mixing and aeration (Da Silva, 1994). To give circulation velocities of this order of magnitude, the positioning and types of mixers are chosen from the plant constructors' experience and the suppliers' data for the impellers. Some case studies of existing plants have shown that measured velocities were not in the range that was specified in the plant design. This illustrates that there is still a need for design and diagnosis approach to improve process reliability by eliminating or reducing the number of short circuits, dead zones, zones of inefficient mixing and poor aeration. The objective of the aeration is to facilitate the quick degradation of pollutants by bacterial growth. To achieve these objectives a wastewater treatment plant must be adequately aerated; thus resulting in 60-80% of all energetic consummation being dedicated to the aeration alone (Juspin and Vasel, 2000). An earlier study (Gillot et al., 1997) has illustrated the influence that hydrodynamics have on the aeration performance as measure by the oxygen transfer coefficient. Therefore, optimising the agitation and aeration systems can enhance the oxygen transfer coefficient and consequently reduce the operating costs of the wastewater treatment plant. It is critically important to correctly estimate the mass transfer coefficient as any errors could result in the simulations of biological activity not being physically representative. Therefore, the transfer process was rigorously examined in several different types of process equipment to determine the impact that different hydrodynamic regimes and liquid-side film transfer coefficients have on the gas phase and the mass transfer of oxygen. To model the biological activity occurring in ASBs, several generic biochemical reaction models have been developed to characterise different biochemical reaction processes that are known as Activated Sludge Models, ASM (Henze et al., 2000). The ASM1 protocol was selected to characterise the impact of aeration on the bacteria consuming and assimilating ammonia and nitrate in the wastewater. However, one drawback of ASM protocols is that the hydrodynamics are assumed to be uniform by the use of perfectly mixed, plug flow reactors or as a number of perfectly mixed reactors in series. This makes it very difficult to identify the influence of mixing and aeration on oxygen mass transfer and biological activity. Therefore, to account for the impact of local gas-liquid mixing regime on the biochemical activity Computational Fluid Dynamics (CFD) was used by applying the individual ASM1 reaction equations as the source terms to a number of scalar equations. Thus, the application of ASM1 to CFD (FLUENT) enabled the investigation of the oxygen transfer efficiency and the carbon & nitrogen biological removal in pilot (7.5 cubic metres) and plant scale (6000 cubic metres) ASBs. Both studies have been used to validate the effect that the hydrodynamic regime has on oxygen mass transfer (the circulation velocity and mass transfer coefficient) and the effect that this had on the biological activity on pollutants such as ammonia and nitrate (Cartland Glover et al., 2005). The work presented here is one part to of an overall approach for improving the understanding of ASBs and the impact that they have in terms of the hydraulic and biological performance on the overall wastewater treatment process. References CARTLAND GLOVER G., PRINTEMPS C., ESSEMIANI K., MEINHOLD J., (2005) Modelling of wastewater treatment plants ? How far shall we go with sophisticated modelling tools? 3rd IWA Leading-Edge Conference & Exhibition on Water and Wastewater Treatment Technologies, 6-8 June 2005, Sapporo, Japan DA SILVA G. (1994). Eléments d'optimisation du transfert d'oxygène par fines bulles et agitateur séparé en chenal d'oxydation. PhD Thesis. CEMAGREF Antony ? France. GILLOT S., DERONZIER G., HEDUIT A. (1997). Oxygen transfer under process conditions in an oxidation ditch equipped with fine bubble diffusers and slow speed mixers. WEFTEC, Chicago, USA. HENZE M., GUJER W., MINO T., van LOOSDRECHT M., (2000). Activated Sludge Models ASM1, ASM2, ASM2D and ASM3, Scientific and Technical Report No. 9. IWA Publishing, London, UK. JUSPIN H., VASEL J.-L. (2000). Influence of hydrodynamics on oxygen transfer in the activated sludge process. IWA, Paris - France.