Principles of Water and Wastewater Treatment Processes
224
Principles of Water and Wastewater Treatment Processes
224Paperback
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Overview
The book outlines the principle unit operations that are involved in the separation, degradation and utilisation of organic and inorganic matter during water and wastewater treatment. The module builds on the subjects of chemistry, biology and engineering covered in Process Science and Engineering for Water and Wastewater Treatment (Module 1) and provides a descriptive introduction to unit operations that are further described with design and operational details in later books in the series.
The text of Principles of Water and Wastewater Treatment Processes has been divided into the following Units:
- Water Quality
- Process Flowsheeting
- Physical Processes
- Chemical Processes
- Sorption Processes
- Biological Processes
- Membrane Processes
- Sludge Treatment Utilisation
- Odour Management
These units have has been designed for individual self-paced study that includes photographs, illustrations and tables and describe the form, function and application of unit operations for the treatment of water and wastewater. Each section of the text gives step-by-step learning in a particular subject, that includes an approximation of how long you will need to spend on that section and provides key points that highlight the principles of the different sections. Each unit includes exercises to help understand the material in the text, self-assessment questions to test your understanding and text references.
Product Details
| ISBN-13: | 9781843390268 |
|---|---|
| Publisher: | IWA Publishing |
| Publication date: | 09/30/2009 |
| Series: | Water and Wastewater Process Technologies Series |
| Pages: | 224 |
| Product dimensions: | 6.12(w) x 9.25(h) x 0.75(d) |
Read an Excerpt
CHAPTER 1
Water Quality
Contents
Aims & Objectives Essential Prerequisites
1.1 Introduction
1.2 Monitoring
1.3 Measurement of Quality Parameters
1.4 Parameter Types
1.5 Key Water Quality Parameters
1.7 Solutions to Exercises
Bibliography
Aims and Objectives
This Unit covers the principles of water quality from water and wastewater treatment works:
After studying these notes you should be able to:
1. explain in your own words the following terms and concepts:
compliance monitoring
performance monitoring
process control
2. describe the different quality parameters used to monitor water quality from water and wastewater treatment works.
It is important that you are able to complete all the self assessment questions at the end of this Unit.
Essential Prerequisites
It is not necessary to have a completed any other Units before undertaking this Unit.
Additional, information on the fundamentals for this Unit, refer to the following Units in Process Science and Engineering for Water and Wastewater Treatment:
Unit 1 Fundamentals of Water Chemistry
Unit 4 Fundamentals of Microbiology
Unit 7 Fundamentals of Process Engineering
1.1 Introduction
Water and wastewater treatment plants are designed to provide water and wastewater to an acceptable quality, however changes to the quality and quantity of raw water and wastewater as it arrives at a treatment works as well as operational conditions, can cause changes to the quality of the water and wastewater as it is discharged from a treatment plant. These changes in water and wastewater quality can have an impact on its receptors i.e. the consumer in the case of water and the environment in the case of wastewater. Depending on the intensity of this impact, water utilities may receive different levels of response ranging from increases in the consumer complaints to prosecution by environment regulators.
To prevent the production of sub standard water and wastewater from treatment processes, water quality parameters need to be defined based on physical, organoleptic, chemical and biological measurements. As well as defining the different types of water quality parameters it is also essential to define the reasons why it is necessary to monitor for particular parameters. Different measurement parameters will provide different information on the ability of a process to handle extreme variations and still produce an acceptable product quality.
1.2 Monitoring
1.2.1 Compliance Monitoring
Compliance monitoring involves monitoring for a particular parameter in order to ensure that the water quality complies with a particular standard (often a legislative standard) such as potable water quality, industrial water quality or effluent discharge consent. If the water fails to meet the standard for a particular parameter we will need to know how often the limit is exceeded and by how much. Typical examples are flow, pH and the concentrations of regulated substances.
Compliance monitoring may be by on-line measurements with automatic data logging, but more often compliance monitoring involves sample collection and retrospective analysis in the laboratory, for example 90% of the UK water quality monitoring is laboratory based. Often under-rated is the fact that the logging of results provides evidence of historical patterns of compliance which, in itself, can be a valuable aid to a discharger's public relations.
1.2.2 Treatment Plant Design
The routine analysis of a water or wastewater influent to a treatment works provides an understanding of the nature of the process stream and how its quality varies seasonally or diurnally, which allows us to assess the best approach to treatment. This is particularly important in the case of the treatment of moorland surface waters, whose organic content can vary dramatically from season to season, and in industrial wastewaters from batch processes such as those used in the food and drink and pharmaceuticals industries which vary in flow and composition from hour to hour.
1.2.3 Performance Monitoring
Performance monitoring enables a water or wastewater treatment operator to determine the effectiveness or efficiency of a treatment process, in most case using indicator measurements. These measurements allow the operator to know how well a treatment process is performing in order to make any necessary adjustments to the operating conditions. Typical examples include nitrate concentrations, to check on the efficiency of nitrification in activated sludge plants and total heavy metals concentration to check on the efficiency of clarification after precipitation. These parameters are usually measured by field (Figure 1.1) or laboratory (Figure 1.2) analysis.
1.2.4 Process Control
In order to improve water and wastewater treatment efficiency, on-line monitoring of water quality parameters is often used to enable either feed forward or feedback process control. Typical examples, include raw water quality monitoring (Figure 1.3), dissolved oxygen monitoring for activated sludge aeration control, ammonia monitoring of wastewater nitrification control (Figure 1.4) and redox potential monitoring to control bisulphite addition for chromate reduction.
1.3 Measurement of Quality Parameters
1.3.1 Measured Parameters
It is important to know what is meant by a measured value of a parameter and how it should be interpreted. In some cases this is fairly obvious. For example, dissolved oxygen concentration is simply the concentration of oxygen dissolved in the liquid. However, it is also important to know how the determination was carried out because some tests are more sensitive than others and, in the case of laboratory determinations, may depend on the skill of the analyst. In addition many on-line measuring cells are temperature dependent and we will need to know whether this has been taken into consideration in the measurement – most instruments provide automatic temperature compensation. Of paramount importance is the statement of the units in which the parameter was measured. For example a calcium concentration of 130 mg/l expressed "as calcium" becomes 325 mg/l when expressed "as calcium carbonate" (Process Sciences and Engineering for Water and Wastewater Treatment – Unit 2).
Where there is a time delay between sampling and analysis – for example when a sample is sent to a laboratory – it is essential that that the sample is properly stored. Viable bacteria may deplete the dissolved oxygen (DO) concentration so that the DO value measured in the laboratory bears no relationship to that actually present on site.
In many instances the measured value has only a passing relationship with the parameter whose value we really need to know. We frequently need to know how "oxidisable" a waste stream is because we need to provide oxidation either by biological or chemical means. The available tests – Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Total Oxygen Demand (TOD) and Total Organic Carbon (TOC) will all give different values because of the method of analysis. When operating a biological treatment plant BOD and COD are the most important parameters but they are also the most time consuming to determine. TOD, on the other hand, is quick, simple and can be measured on-line. By carrying out parallel monitoring of COD and TOD over a period of time it may be possible to build up a relationship between these two measurements, which can allow monitoring of TOD to be used for process control instead of COD. Again absolute values are not necessary for process control purposes.
1.3.2 Accuracy and Precision
In water quality monitoring, very few measurements are direct comparisons of the measured parameter against a standard as is the case in, for example, the measurement of length. Most measurements are inferential, that is some characteristic is measured and the required parameter computed. Therefore the importance of accuracy and precision is essential when discussing the measurement of parameters for water quality monitoring (Process Sciences and Engineering for Water and Wastewater Treatment – Unit 7). Accuracy tells us how close a set of measurements are to the real value; precision tells us how close together a set of measurements are to each other.
In the case of compliance monitoring, where analyses are essential evidence in legal proceedings, it is clear that high levels of both precision and accuracy are of vital importance. For example, wastewater flows are commonly measured by V notch weir (Process Sciences and Engineering for Water and Wastewater Treatment – Unit 11). In this case the height of water above the weir is measured and the flow calculated (often automatically by an electronic circuit) from this measurement. Clearly if the dimensions of the V notch are not true – for example if it has become enlarged by erosion or if it is partially blocked by weeds – then no matter how accurately the height is measured the flow computation will be incorrect.
Such an inaccuracy may be a major problem. If we are being charged a rate per unit volume of discharge then we need to know, in absolute terms, how much volume we are discharging. However, we do not always need to know the absolute value of a parameter. In performance monitoring we are usually more interested in trends, that is the value of the parameter increasing, decreasing, remaining constant or fluctuating. In this case precision is of rather more importance than accuracy. An error in measurement is acceptable provided that it is consistent in all readings. For example, an instrument that gives readings which are consistently 10% high but are within a 1% tolerance, will still be a good indicator of trends and will tell us if the plant is performing as well today as it was yesterday. An instrument which has a 5% random error and is, therefore, more accurate, will not be such a valuable tool for performance monitoring.
In most automatic process control applications, accuracy is of more importance than precision. Most automatic control loops operate by comparing the measured value with a desired value and generating a control signal which is a function of the error between the two. A large error in the measurement will, therefore, result in poor control, frequently manifested as an offset from the desired value.
1.3.3 Economics of Monitoring
Clearly it is important to carry out compliance monitoring to ensure that legislative standards are not contravened, the consequence of which might be prosecution or closure of the works. Such extreme consequences have a commercial value which is off-set by the cost of monitoring. However, there are other economic considerations for monitoring. Performance monitoring of treatment plant can lead to optimisation of operating conditions and this, in turn, allows operating costs to be minimised. Analysis of trends in waste stream quality and quantity allow water utilities to predict when new plant or processes may need to be introduced and, thus, is a useful tool in capital budgeting.
1.4 Parameter Types
1.4.1 Physical Parameters
Physical parameters relate to measurements which are essentially physical in nature – temperature, flow, electrical conductivity and gravimetric measurements such as the mass of suspended solids per unit volume.
1.4.2 Organoleptic Parameters
Organoleptic parameters are those measurements that are detect with our senses, that is sight – whether water is clear or turbid (hazy), colourless or coloured – and smell. Taste is also used in the case of potable waters where tainting compounds can cause so called off-flavours in drinking water supplies resulting in customer complaints.
1.4.3 Chemical Parameters
Chemical parameters are those which relate to the concentrations of chemical species in water. Some of these parameters, such as cyanide, arsenic, lead, mercury and cadmium can be toxic. Others are health related: nitrates have been linked to infantile methaemoglobinaemia, nitrites to bowel cancers, sodium to heart disease. Some are measures of chemicals deliberately added such as acids and alkalis for pH control, aluminium and iron for coagulation, chlorine and ozone for disinfection, fluoride for dental health while others such as alkalinity, hardness and Total Dissolved Solids (TDS) may indicate problems if the water is used in industry for cooling or steam raising. It is necessary to understand the chemistry of water and wastewater to be able to predict how it will behave when we add or remove chemical species (Process Sciences and Engineering for Water and Wastewater Treatment – Unit 1).
1.4.4 Biological Parameters
Water systems include potable water, lakes, rivers and leisure pools such as swimming pools and jacuzzis. For water to be used for human consumption, it must be free from organisms that are capable of causing disease. In the late 19th century, acute waterborne diseases, such as cholera and typhoid fever were common. Since then, a whole range of infectious diseases, mainly bacterial, viral and protozoan, have been found to be water borne. It is important to remember that before a disease can occur, the host must have been in contact with an infectious dose of the disease-causing agent. This can be defined as the number of a particular pathogenic organism required to cause a disease in man and can differ for different types of organisms.
The removal of waterborne diseases is often focused on from a water treatment point of view, where reduction is through source control, filtration and disinfection, for example, chlorination. However, wastewater treatment processes can also remove a significant numbers of pathogenic organisms (Table 1.1).
In recent years, the number of reported illnesses relating to waterborne diseases has increased. This is mainly a result of improved diagnostic techniques for the detection of these organisms as well as an increased understanding and awareness of waterborne diseases. The range of organisms causing water-related diseases is not just limited to bacteria. Other agents include protozoa, viruses, fungi and algae.
Bacteria are isolated from water by filtration, incubation on nutrient media at 22–C and 37–C, identification by staining and enumeration by optical microscopy. Other organisms are filtered, stained where appropriate and enumerated by optical microscopy (Process Sciences and Engineering for Water and Wastewater Treatment – Unit 4).
The isolation, identification and enumeration of pathogenic organisms is difficult not only due to the relatively low numbers of these organisms in water but also because of difficulties in growing these organisms under laboratory conditions. Other organisms are then used as indicators of the presence of the pathogenic organisms in the water. It is critical that the indicator organisms must always be present when the pathogen is present and absent when the pathogen is absent. However, there are other criteria which also must be met:
• the micro-organism should only originate in the digestion tract of warm-blooded animals, especially humans;
• identification and enumeration of the micro-organism should be easy, rapid and reliable;
• the analysis should be cost effective;
• the indicator organism should survive longer than pathogens in the environment;
• the indicator organism should be present in high numbers; and
• the indicator should not be pathogenic.
While no organism can meet all these conditions consistently, Escherichia coli (a faecal coliforms) fulfils most of these requirements with other coliform organisms, faecal streptococci and Clostridium perfingens also widely used. Two techniques are principally used to enumerate indicator organisms, membrane filtration and multiple tube methods. More recently new enzyme based techniques have emerged for microbial water testing. These tests can be used to give most probable number (MPN) values or simply indicate the presence or absence of coliforms.
(Continues…)
Excerpted from "Principles of Water and Wastewater Treatment Processes"
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Copyright © 2009 Cranfield University.
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Table of Contents
Introduction to the Water and Wastewater Treatment Technologies Series, v,
Water Science at Cranfield University, vi,
Editors, vii,
How to Use This Book, viii,
Introduction to Principles of Water and Wastewater Treatment Processes, ix,
Unit 1 Water Quality, 1,
Unit 2 Flowsheeting, 23,
Unit 3 Physical Processes, 45,
Unit 4 Chemical Processes, 73,
Unit 5 Sorption Processes, 101,
Unit 6 Biological Processes, 117,
Unit 7 Membrane Processes, 147,
Unit 8 Sludge Treatment and Utilisation, 165,
Unit 9 Odour Management, 187,