Biological Wastewater Treatment in Warm Climate Regions
857
Biological Wastewater Treatment in Warm Climate Regions
857Hardcover
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Product Details
| ISBN-13: | 9781843390022 |
|---|---|
| Publisher: | IWA Publishing |
| Publication date: | 09/30/2005 |
| Pages: | 857 |
| Product dimensions: | 6.12(w) x 9.25(h) x 0.75(d) |
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CHAPTER 1
Introduction to water quality and water pollution
1.1 INTRODUCTION
Water, because of its properties as a solvent and its capacity to transport particles, incorporates in itself various impurities that characterise the water quality.
Water quality is a result of natural phenomena and the acts of human beings. Generally it can be said that water quality is a function of land use in the catchment area. This is due to the following factors:
• Natural conditions: even with the catchment area preserved in its natural condition, the surface water quality is affected by run off and infiltration resulting from rainfall. The impact of these is dependent on the contact of the water with particles, substances and impurities in the soil. Therefore, the incorporation of suspended solids (e.g. soil particles) or dissolved solids (e.g. ions originating from the dissolution of rocks) occurs even when the catchment area is totally preserved in its natural condition (e.g. occupation of the land with woods and forests). In this case, the soil protection and composition have a great influence.
• Interference of human beings: the interference of man manifests itself either in a concentrated form, such as in the discharge of domestic or industrial wastewater, or in a diffused form, such as in the application of fertilisers or pesticides onto the soil. Both contribute to the introduction of compounds into the water, thus affecting its quality. Therefore, the form in which human beings use and occupy the land has a direct implication in the water quality.
Figure 1.1 presents an example of possible interactions between land use and the presence of factors that modify the water quality in rivers and lakes. Water quality control is associated with a global planning at the whole catchment area level, and not individually, for each impacting source.
Separate from the above concept of existing water quality, there is the concept of the desired water quality. The desired quality for a water is a function of its intended use. There are various possible intended uses for a particular water, which are listed in Section 1.2. In summary:
• Existing water quality: function of the land use in the catchment area
• Desired water quality: function of the intended uses for the water
Within the focus of this book, the study of water quality is essential, not only to characterise the consequences of a certain polluting activity, but also to allow the selection of processes and methods that will allow compliance with the desired water uses.
1.2 USES OF WATER
The main water uses are:
• domestic supply
• industrial supply
• irrigation
• animal supply
• preservation of aquatic life
• breeding of aquatic species
• generation of electricity
• navigation
• landscape harmony
• dilution and transport of wastes
• recreation and leisure
In general terms, only the first two uses (domestic supply and industrial supply) are frequently associated with a prior water treatment, in view of their more demanding quality requirements.
There is a direct relation between water use and its required quality. In the above list, the most demanding use can be considered domestic water supply, which requires the satisfaction of various quality criteria. Conversely, the less demanding uses are simple dilution and transportation of wastes, which do not have any specific requirements in terms of quality. However, it must be remembered that multiple uses are usually assigned to water bodies, resulting in the necessity of satisfying diverse quality criteria. Such is the case, for example, of reservoirs constructed for water supply, electricity generation, recreation, irrigation and others.
Besides the cycle of water on Earth (hydrological cycle), there are internal cycles, in which water remains in the liquid state, but has its characteristics modified as a result of its use. Figure 1.2 shows an example of typical routes of water use, composing partial cycles. In these cycles, the water quality is modified at each stage of its journey.
The management of these internal cycles is an essential role in environmental engineering, and includes the planning, design, construction and control of the works necessary for the maintenance of the desired water quality as a function of its intended uses. Therefore, the engineer or scientist must know how to ask for and interpret the results of water quality samples in the various points of the cycle. This book focuses mainly on the aspect of wastewater treatment, and the impact of the discharge of wastewater to receiving bodies is covered in Chapter 3.
1.3 WATER QUALITY REQUIREMENTS
Table 1.1 presents in a simplified way the association between the main quality requirements and the corresponding water uses. In cases of water bodies with multiple uses, the water quality must comply with the requirements of the various intended uses. The expression "free" in the table is different from "absolutely free". Zero levels of many contaminants cannot be guaranteed and in most cases are not necessary. The acceptable concentrations are based on risk analysis, a tool that is used for deriving quality guidelines and standards.
• Raw water. Initially, water is abstracted from the river, lake or water table, and has a certain quality.
• Treated water. After abstraction, water undergoes transformations during its treatment to be able to comply with its intended uses (e.g. public or industrial water supply).
• Raw wastewater. The water, after being used, undergoes new transformations in its quality and becomes a liquid waste.
• Treated wastewater. Aiming at removing its main pollutants, wastewater undergoes treatment before being discharged into the receiving body. Wastewater treatment is responsible for the new modification in the quality of the liquid.
• Stormwater. Rain water flows on the ground, incorporates some pollutants, and is collected at stormwater systems before being discharged into the receiving body.
• Receiving body. Stormwater and the effluent from the wastewater treatment plant reach the receiving body where water quality undergoes new modifications, as a result of dilution and self-purification mechanisms.
1.4 WATER POLLUTION
Water pollution is the addition of substances or energy forms that directly or indirectly alter the nature of the water body in such a manner that negatively affects its legitimate uses.
This definition is essentially practical and, as a consequence, potentially controversial, because of the fact that it associates pollution with negative alterations and with water body uses, concepts that are attributed by human beings. However, this practical view is important, principally when analysing the control measures for pollution reduction
Table 1.2 lists the main pollutants and their source, together with the most representative effects. Chapter 2 covers in detail the main parameters, which characterise the quality of a wastewater (second column in the table). For domestic sewage, which is the main focus of this book, the main pollutants are: suspended solids, biodegradable organic matter, nutrients and pathogenic organisms. Their impact in the water body is analysed in detail in Chapter 3.
The solution to most of these problems, especially biodegradable organic matter and pathogens, has been reached in many developed regions, which are now concentrated on the removal of nutrients and micro-pollutants, together with substantial attention to the pollution caused by storm-water drainage. In developing regions, the basic pollution problems still need to be dealt with, and the whole array of pollutants needs to be tackled. However, because of scarcity of financial resources in these regions, priorities need to be set (as they have been, in the past, and continue to be, in the developed regions), and the gross pollution by organic matter and contamination by pathogens are likely to deserve higher attention. Naturally, each region has its own specificities, and these need to be taken into account when setting up priorities.
Also from the table, it is seen that it is very difficult to generalise industrial wastewater, because of its variability from process to process and from industry to industry.
In the table, it is also seen that there are two ways in which the pollutant could reach the receiving body (see Figure 1.3):
• point-source pollution
• diffuse pollution
In point-source pollution, the pollutants reach the water body in points concentrated in the space. Usually the discharge of domestic and industrial wastewater generates point-source pollution, since the discharges are through outfalls.
In diffuse pollution, the pollutants enter the water body distributed at various locations along its length. This is the typical case of storm water drainage, either in rural areas (no pipelines) or in urban areas (storm water collection system, with multiple discharges into the water body).
The focus of this book is the control of point-source pollution by means of wastewater treatment. In the developing regions, there is practically everything still to be done in terms of the control of point-source pollution originating from cities and industries.
CHAPTER 2
Wastewater characteristics
2.1 WASTEWATER FLOWRATES
2.1.1 Introduction
Wastewater sewerage (collection, treatment and disposal) is accomplished by the following main alternatives (Figure 2.1):
• Off-site sewerage
• Separate sewerage system
• Combined sewerage system
• On-site sewerage
In various countries a separate sewerage system is adopted, which separates storm water from sewage, both being transported by independent pipeline systems. In this case, in principle, storm water does not contribute to the wastewater treatment plant (WWTP). In other countries, however, a combined (unitary) sewerage system is adopted, which directs sewage and storm water together into the same system (see Figure 2.1). In this case, the pipelines have a larger diameter, to transport not only the sewage flow, but mainly rainwater, and the design of the WWTP has to take into consideration the corresponding fraction of rainwater that is allowed to enter the treatment works. In countries with a warm climate, during the dry season, sewage flows slowly in these large diameter pipes, leading to long detention times which allow decomposition and generation of malodours. In this book, focus is given to the separate sewerage system, analysing only the three components listed above. However, the principles for the design of a combined sewerage system, based on dry-weather flow, are the same.
Similarly, the book concentrates on off-site collection systems (with a water-borne sewerage collection and transportation network) and does not cover the on-site systems (e.g. latrines and septic tanks). These are of great importance and in many cases the best alternative in various regions, being more applicable in locations with a low population density, like rural areas (even though they are also applied in various densely occupied locations, but frequently presenting problems of infiltration in the soil and the resulting contamination of the water table).
Urban wastewater that flows in an off-site sewerage system and contributes to a WWTP is originated from the following three main sources:
• Domestic sewage (including residences, institutions and commerce)
• Infiltration
• Industrial effluents (various origins and types of industry)
For the characterisation of both quantity and quality of the influent to the WWTP, it is necessary to separately analyse each of the three items.
2.1.2 Domestic wastewater flow
2.1.2.1 Preliminaries
The concept of domestic flow encompasses the sewage originating from homes, as well as commercial activities and institutions that are normally components of the locality. More expressive values originating from significant point sources must be computed separately and added to the global value.
Normally domestic sewage flow is calculated based on the water consumption in the respective locality. The water consumption is usually calculated as a function of the design population and of a value attributed for the average daily per capita water consumption.
It is important to observe that for the design and operation of the sewage treatment works it is not sufficient to consider only the average flow. It is also necessary to quantify the minimum and maximum flowrates, because of hydraulic and process reasons.
This Section describes the population-forecast studies, the estimates of water consumption and the production of wastewater, together with the variations in flow (minimum and maximum flow).
2.1.2.2 Population forecast
The population that contributes to the treatment plant is that situated inside the design area served by the sewerage system. However, the design population is only a certain fraction of the total population in this area, because maybe not all the population is connected to the sewerage system. This ratio (population served/total population) is called the coverage index. This index can be determined (current conditions) or estimated (future conditions), such as to allow the calculation of the design flow. In the final years of the planning horizon, it is expected that the coverage will be close to 100%, reflecting the improvement and expansion in the collection network. The coverage index is a function of the following aspects:
• Physical, geographical or topographical conditions of the locality. It is not always possible to serve all households with the sewerage system. Those not served must adopt other solutions besides the off-site water-borne sewerage system.
• Adhesion index. This is the ratio between the population actually connected to the system and the population potentially served by the sewerage system in the streets (not all households are connected to the available system, that is to say, not all adhere to the sewerage system). In some communities, it is compulsory to connect to the collection system, in case it passes in front of the house; in other communities, this is optional.
• Implementation stages of the sewerage system. In the initial operating years of the WWTP, maybe not all of the designed collection and transport system has been actually installed, and this affects the initial flow.
For the design of a sewage treatment works it is necessary to know the final population (population at the end of the planning horizon – see Chapter 6 for the concept of planning horizon) as well as the initial population and its evolution with time, in order to allow the definition of implementation stages.
The main methods or models used for population forecasts are (Fair et al, 1973; CETESB, 1978; Barnes et al, 1981; Qasim, 1985; Metcalf & Eddy, 1991):
• linear (arithmetic) growth
• geometric growth
• multiplicative regression
• decreasing growth rate
• logistic growth
• graphical comparison between similar communities
• method of ratio and correlation
• prediction based on employment forecast or other utilities forecast
Tables 2.1 and 2.2 list the main characteristics of the various methods. All of the methods presented in Table 2.1 can also be solved through statistical regression analysis (linear or non-linear). Such methods are found in many commercially available computer programs. Whenever possible it is always best to adopt a regression analysis that allows the incorporation of a largest historical data series instead of two or three 3 points, such as the algebraic methods presented in Table 2.1.
The results of the population forecast must be coherent with the population density in the area under analysis. The population density data are also useful in the computation of flows and loads resulting from a certain area or basin within the town. Typical population density values are presented in Table 2.3. Table 2.4 presents typical saturation population densities, in highly occupied metropolitan areas.
(Continues…)
Excerpted from "Biological Wastewater Treatment in Warm Climate Regions Volume One"
by .
Copyright © 2005 IWA Publishing.
Excerpted by permission of IWA Publishing.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents
Preface, xi,
Dedication, xv,
The authors, xvii,
List of authors by chapter, xix,
PART ONE INTRODUCTION TO WASTEWATER CHARACTERISTICS, TREATMENT AND DISPOSAL,
1 Introduction to water quality and water pollution, 3,
2 Wastewater characteristics, 11,
3 Impact of wastewater discharges to water bodies, 79,
4 Overview of wastewater treatment systems, 165,
5 Overview of sludge treatment and disposal, 244,
6 Complementary items in planning studies, 279,
PART TWO BASIC PRINCIPLES OF WASTEWATER TREATMENT,
7 Microbiology and ecology of wastewater treatment, 297,
8 Reaction kinetics and reactor hydraulics, 319,
9 Conversion processes of organic and inorganic matter, 367,
10 Sedimentation, 421,
11 Aeration, 457,
PART THREE STABILISATION PONDS,
12 Overview of stabilisation ponds, 495,
13 Facultative ponds, 502,
14 System of anaerobic ponds followed by facultative ponds, 540,
15 Facultative aerated lagoons, 552,
16 Complete-mix aerated lagoons followed by sedimentation ponds, 564,
17 Removal of pathogenic organisms, 578,
18 Nutrient removal in ponds, 610,
19 Ponds for the post-treatment of the effluent from anaerobic reactors, 617,
20 Construction of stabilisation ponds, 621,
21 Maintenance and operation of stabilisation ponds, 632,
22 Management of the sludge from stabilisation ponds, 644,
PART FOUR ANAEROBIC REACTORS,
23 Introduction to anaerobic treatment, 659,
24 Principles of anaerobic digestion, 663,
25 Biomass in anaerobic systems, 697,
26 Anaerobic treatment systems, 709,
27 Design of anaerobic reactors, 728,
28 Operational control of anaerobic reactors, 774,
29 Post-treatment of effluents from anaerobic reactors, 805,