论文总字数：47401字
毕业设计（论文）报告
院（系） 土木工程学院
专 业 给水排水工程
设计（论文）题目 Calibration of E.coli Removal Model in Stormwater Biofilter
学 生 姓 名 姚叶鹏 学 号 05512120
起 止 日 期 2015.02.22—2015.06.10
设 计 地 点 澳大利亚 莫那什大学
指 导 教 师 David McCarthy
顾 问 教 师 马金霞
教研室主任
教学院长（教学系主任） 邓 琳
Calibration of E. coli Removal Model in Stormwater Biofilter
Abstract Stormwater is normally considered to contain various pollutants and unable to be utilized. In particular, faecal microorganisms are believed to be extremely dangerous for human heath. However, the newly developed Water sensitive urban design (WSUD) systems can clean the pollutants in stormwater so that it can be utilized for low standard purposes. Among all the WSUD systems, stormwater biofilter shows the most promising results. While many studied have been conducted to optimize different design factors of stormwater biofilter, some operational factors haven’t been paid enough attention for the difficulties in experiments. However, using modeling as a powerful tool, it is feasible to investigate the processes and their corresponding factors in stormwater biofilter in a more detailed approach. The presented study utilized an 8 months’ data collected from field experiment to calibrate an E. coli removal biofilter model, to bring a insight view of the processes happen in stormwater biofilter and the influence of their corresponding factors.
Key words: stormwater, biofilter, E.coli, modelling, calibration
Contents
1.Introduction 1
2.Literature Review 2
2.1 Water Sensitive Urban Design (WSUD) systems for stormwater treatment 2
2.1.1 Indicator of faecal microorganism in stormwater 2
2.1.2 Comparison between different Water Sensitive Urban Design (WSUD) systems 2
2.1.3 Introduction of stormwater biofilters 3
2.2 the governing processes of E. coli removal in stormwater biofilters 5
2.2.1 Adsorption 6
2.2.2 Desorption 6
2.2.3 Straining 6
2.2.4 Die-off 7
2.3 Predictive models for faecal microbe removal in urban stormwater biofilters 7
2.4 Summary and conclusion 8
3. Model Description 9
3.1 Description of water quantity module 9
3.2 Description of water quality module 14
4. Data collected for calibration 15
5. Calibration of water quantity module 18
5.1 Introduction 18
5.2 Calibration method 19
5.3 results and discussion 20
6. Calibration of water quality module 22
6.1 Introduction 22
6.2 Calibration method 23
6.3 Results and discussion 24
7. Conclusions 27
8. Acknowledgement 28
Appendix 29
References 35
1.Introduction
As the side effect of fast development of human civilization, urbanization has increased the amount of stormwater in urban area, which may cause flooding, degradation of urban streams and public health problems (Wong, 2006). However, with proper treatment, stormwater could also be transformed into an alternative water resource. According to Wood et al. (2002), more than half of the supplied high quality water is used for lower quality purposes like washing; on the other hand, the amount of stormwater runoff from Australian cities is rough the same to the amount of drinking water use. But the high uncertainties in both quantity and quality remain a problem, which impede the reclamation of stormwater. If not be treated sufficiently, reused stormwater may cause some public health safety issues, which can lead to serious consequences. Therefore, several types of stormwater treatment systems have been developed.
Water sensitive urban design (WSUD) systems have been widely employed to deal with the challenges and opportunities associated with both stormwater quantity and quality (Wong, 2006). Among them, stormwater biofilters show especially promising results and have been commonly adopted (FAWB, 2009). Numerous studies have been conducted to evaluate and optimize the performance of stormwater biofilters in reducing different kinds of pollutants, such as sediments, nutrients, heavy metals, micro pollutants, and faecal microorganisms (Zhang et al., 2014, Hatt et al., 2009, Bratieres et al., 2008, Feng et al., 2012). Most of the studies focused on the overall removal capability of stormwater biofilters on these pollutants, the detailed influences different factors (e.g. type of plants, depth of submerged zone) have on the processes happened in biofilter is not clear enough due to the limitation of experimental capabilities. However, with the help of modeling, it is feasible to acquire an insight understanding of removal processes in biofilters. The presented study used an E .coli removal model developed in Monash University to investigate the governing processes and corresponding factors in stormwater biofilters.
2.Literature Review
2.1 Water Sensitive Urban Design (WSUD) systems for stormwater treatment
2.1.1 Indicator of faecal microorganism in stormwater
Microorganisms in stormwater can endanger human health and lead to public health problems (NHMRC, 2004). In particular, faecal microorganisms, which are sourced from the faeces of animals and humans, are considered as the cause of waterborne diseases and present a higher degree of risk to end-uses compared to non-faecal microorganisms (i.e. the microorganisms not from the intestines of animals or human) (NHMRC, 2004, Makepeace et al., 1995). In this study, E. coli (Escherichia coli) is selected as the indicator of faecal microorganism for some certain advantages:(a) exclusively associated with a faecal source; (b) inability to multiply in different environments; (c) easy and cheap to be detected with enzymatic methods (Edberg et al., 2000).
2.1.2 Comparison between different Water Sensitive Urban Design (WSUD) systems
To remove pollutants and reuse the stormwater, several kinds of Water Sensitive Urban Design (WSUD) systems have been developed. WSUD is a concept that integrates holistic management of urban water cycle (potable water, wastewater, and stormwater) into design, and one of its aims is to treat urban stormwater to meet the water quality requirements for stormwater harvesting and water body protection (Wong, 2006). The most promising WSUD systems include stormwater biofilters and wetlands. Table 2.1 compared several common WSUD systems on their performance of E. coli removal:
According to Table 2.1, different WSUD systems show different results of faecal microorganism removal, among which biofilter have the most promising outcome. In addition, the outstanding performance biofilters have been demonstrated both in laboratory studies and field studies. Therefore, the following literature reviews will mainly focus on stormwater biofilters.
2.1.3 Introduction of stormwater biofilters
Stormwater biofilters, which also named as biofiltration systems, biorentention systems or rain gardens, are one of the most widely used WUSD systems in Australia for stormwater management and treatment. Biofilters are soil-plant based systems with the enhancement of infiltration and evapotranspiration of stormwater. Therefore, they are a low energy treatment technology with the potential to provide both water quality and quantity benefits (FAWB, 2009).
The treatment of stormwater in biofilters is a combination of physical, (e.g. mechanical straining), chemical (e.g. adsorption) and biological (e.g. plant uptake) processes (FAWB, 2009). A typical biofilter (Figure 2.1) consists of a filter media with generally high sand content, a gravel layer at the bottom, and a sand transition layer in between (FAWB, 2009). Plants are cultivated on the top of filter media, and sometimes a small portion of water is kept in the gravel layer by elevating the outflow pipe to create a submerged zone (SZ).
The importance of SZ was revealed by many studies, and the establishment of SZ is highly recommended under any possible situation (Chandrasena et al., 2014a, FAWB, 2009). The increased removal performance could be explained by at least three functions of SZ: (1) Hydraulic gradients and retention times; (2) Contribution made by the submerged zone volume in the final composite outflow; (3) Increase the plants durability for extended dry weather periods by providing sufficient water (FAWB, 2009, Payne et al., 2014).
Biofilters have been proved to have satisfying removal rates of different pollutants: 57 - 93 % for total suspended solids (TSS), 30 - 55 % total nitrogen (TN) and 5 - 80 % for total phosphorous (TP) (CWSC, 2010). Biofilters also perform very well in microbial removal: different laboratory studies reported that the removal rates for E. coli were between 70% and 99% (Chandrasena et al., 2014a, Li et al., 2012, Zhang et al., 2010, Zhang et al., 2011).
2.2 the governing processes of E. coli removal in stormwater biofilters
The incoming stormwater contains certain amount of faecal microorganism, through the infiltration process; pollutants are effectively removed through a combination of several processes. It is believed that adsorption, desorption, straining and die-off are the major processes for faecal microbial removal in biofilters (Bradford et al., 2006, Hathaway et al., 2011, Li et al., 2012, Zhang et al., 2011). In the meantime, these processes are governed by many design and operational factors. The major processes and the corresponding governing factors for microbial removal in stormwater biofilters is summarized in Figure 2.2.
According to figure 2.2, these four dominating processes in biofilter are influenced by their corresponding factors such as media type, plants type and so on. Among these processes, adsorption and desorption are the governing processes in wet period (i.e. period when there are incoming stormwater), while die-off is more dominating in the dry period (i.e. period when there are no stormwater).
2.2.1 Adsorption
Adsorption is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface. In this case, adsorption refers to the processes that E. coli in the stormwater are adhering to the filter media. It is the dominant mechanism in porous media when the pores are larger than the bacteria (Stevik et al., 2004). During the rain event, the incoming stormwater contains relative higher concentration of E. coli, leading to the adsorption process where E. coli is adhering to the surface of particles in biofilter. It’s the major process that keeps the E. coli retained in biofilter for further degradation. The adsorption process is affected by many factors, where media type, plants characteristics and solution and flow conditions are believed to be the most influential.
2.2.2 Desorption
Desorption is a phenomenon whereby a substance is released from or through a surface. The process is the opposite of adsorption. This occurs in a system being in the state of sorption equilibrium between bulk phase (fluid, i.e. gas or liquid solution) and an adsorbing surface (solid or boundary separating two fluids). When the concentration of substance in the bulk phase is lowered, some of the absorbed substance changes to the bulk state. So in this case, when the concentration of E. coli become lower in liquid phase due to adsorption, desorption occurs and the adhered E. coli is released from the biofilter system. Adsorption and desorption together construct the E. coli equilibrium between stormwater and the biofilter system.
2.2.3 Straining
Straining is a physical process that the bacteria are trapped by pore sizes that are smaller than the size of microbes. The incoming E. coli is constrained due to physical clogging, the size and shape of microbes and the size and shape of filter media (Stevik et al., 2004, Ferguson et al., 2003). Straining becomes more important when the average cell size of bacteria is greater than the size of 5% of filter media. (Stevik et al., 2004).
2.2.4 Die-off
Die-off is the process that E. coli both in the stormwater and constrained in the solid phase gradually degrade through out time. While adsorption, desorption and straining are the main processes for microbial capture during wet weather, die-off is the process that takes place in both wet and dry weather (Chandrasena et al., 2013, Chandrasena et al., 2014b) And it’s the only process that happens throughout the biofilter system. Die-off is affected by many factors as seen from figure 2.2, among which temperature is most changeable in a biofilter system and needs special attention. During dry period when adsorption and desorption rare is relatively low, die-off become the dominating process. It is found that extended dry periods have a great effect on the performance of biofilter for the retained E. coli need certain time to degrade (Chandrasena et al., 2014b).
2.3 Predictive models for faecal microbe removal in urban stormwater biofilters
Although there are only few models have been developed for the faecal microorganism removal in biofilters, some models developed for other filtration/infiltration applications (e.g. wastewater treatment and aquifer recharge) are also valuable as references. Therefore, though this kind of model might not be directly transferable because of the differences in inflow and system design, common approaches used in modelling microbial behavior in porous media will also be discussed below.
The most widely used approach for microbial transport in porous media modelling is the adsorption-dispersion equation (Foppen et al., 2007, Tufenkji, 2007). This equation models the movement of microorganisms through advection, dispersion and retention-related processes like adsorption, desorption and straining (Bradford et al., 2006, Gargiulo et al., 2008). In addition, the governing processes, adsorption, desorption, staining and die-off, are generally modelled by first order kinetics (Crane and Moore, 1986); however, some studies aimed at small laboratory columns disregarded the die-off process due to shorter simulation periods (Bradford et al., 2006, Foppen et al., 2007).
All of the above processes can be summarized in three generic equations that simulate the microbial concentration in the liquid phase (Equation 2.1) and the solid phase (Equation 2.2 and Equation 2.3) within a porous medium at any given time, as shown below.
Where; C is the microbial concentration in the liquid phase (cells/mL), S1 is the microbial concentration in the solid phase due to kinetic attachment (cells/g), S2 is the microbial concentration in the solid phase due to straining (cells/g), t is the time (s), x is the down gradient distance from the inlet (cm), D is the hydrodynamic dispersion coefficient (cm2/s), v is the pore water velocity (cm/s), ρ is the bulk density of the porous medium (g/cm3), is the porosity (-), katt, kdet and kstr, are the attachment, detachment and straining rates respectively (s-1), μl and μs, are the inactivation or die-off rates for liquid and solid phases respectively (s-1), and φ is the fitting/reduction coefficient for straining (-). The approach discussed can help to develop a predictive E. coli removal model that can be used for the presented study.
2.4 Summary and conclusion
Biofilter is an effective Water Sensitive Urban Design (WSUD) system. While its outstanding function is proved by many study, the insight view of different factor’s influence on specific processes in biofilter haven’t been paid much attention due to the limitation of operational condition. However, with the powerful tool of modelling, it is now capable to investigate the governing processes and corresponding factors in stormwater biofilters. The presented study utilize data collected from a field study to calibrate a E. coli removal model and discuss the processes happen in stormwater biofilter and the influence of different factors.
3. Model Description
As mentioned earlier, a specific E. coli removal model is required for further investigation. Here in Monash University, a predictive biofilter E. coli removal model is developed by Shen (2016), thus the following study become practical. The utilized model can be divided into two parts, water quantity module and water quality module.
3.1 Description of water quantity module
剩余内容已隐藏，请支付后下载全文，论文总字数：47401字
该课题毕业论文、开题报告、外文翻译、程序设计、图纸设计等资料可联系客服协助查找;
