i
A GUIDE
TO
WASTEWATER
TREATMENT
ii CONTENTS
SECTION PAGE
1 INTRODUCTION ... 0
1.1 Wastewater sources, composition and flow rate estimation ... 1
1.1.1 Domestic wastewater ... 1
1.1.2 Industrial Wastewater ... 2
1.2 Water Quality Standard-Measures of Water Quality- When is water contaminated ... 3
1.2.1 Dissolved oxygen ... 3
1.2.2 Biochemical oxygen demand ... 4
1.2.3 Solids ... 5
FIGURE1 CLASSIFICATION OF TOTAL SOLIDS (BASED ON FILTRATION) (VESILIND & ROOKE, 2003) ... 5
1.2.4 Nitrogen ... 5
1.2.5 Phosphorous ... 6
1.2.6 Bacteriological measurements ... 6
1.3 Wastewater characteristics ... 7
1.3.1 Physical characteristics of wastewater ... 8
1.3.2 Chemical wastewater characteristics ... 9
1.3.3 Biological Characteristics of Wastewaters ... 11
1.4 Effects of Untreated liquid effluents ... 12
1.4.1 Health effects ... 12
1.4.2 Increase in the B.O.D. & C.O.D. content of water bodies ... 13
1.4.3 Increase in nutrient content ... 13
1.4.4 Increase of soil deposition ... 14
1.4.5 Effects of odours ... 14
1.4.6 Effects of Increased Temperatures ... 14
1.5 Wastewater collection systems ... 15
1.5.1 Sanitary sewer systems ... 16
1.5.2 Storm sewer systems ... 16
1.5.3 Combined sewer systems ... 17
1.5.4 Collection System Components... 17
FIGURE2 (A) JUNCTION BOXES (B) INTERCEPTOR PIPES (DRINAN & WHITING, 2001) ... 18
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2 WASTE WATER TREATMENT METHODS ... 18
FIGURE3 TYPICAL STAGES IN THE CONVENTIONAL TREATMENT OF SEWAGE 19 FIGURE4 ... 20
constituent ... 20
Unit operation or process ... 20
Suspended Solids ... 20
3 INDUSTRIAL WASTE WATER TREATMENT METHODS ... 22
3.1 Physical/chemical treatment methods ... 23
3.1.1 Screening ... 23
FIGURE5 INCLINED BAR SCREEN ... 24
FIGURE6 CURVED BAR SCREEN ... 25
FIGURE7 RADIAL BAR SCREEN ... 25
FIGURE8 STEP TYPE SCREEN ... 25
FIGURE9 BRUSH TYPE SCREEN ... 26
3.1.2 Sedimentation... 26
FIGURE10 CIRCULAR AND RECTANGULAR SETTLING TANKS ... 26
FIGURE11 PRIMARY CLARIFIER ELEVATION VIEW ... 27
FIGURE12 PRIMARY CLARIFIER PLAN VIEW ... 27
FIGURE13 SUCTION TUBE CLARIFIER ELEVATION ... 27
FIGURE14 PICKET FENCE SLUDGE THICKENER ... 28
3.1.3 Flotation and Skimming ... 28
FIGURE15 ... 29
3.2 Chemical treatment methods ... 29
3.2.1 Chlorination ... 30
FIGURE16 CHLORINATOR ... 30
3.2.2 Ozonation ... 31
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FIGURE18 OZONATOR ... 32
3.3 Biological treatment methods ... 32
3.3.1 Activated-sludge Process ... 34
FIGURE19 AN AERATION BASIN (PEPPER, GERBA, & RUSSEAU, 2006) ... 35
FIGURE20 DENITRIFICATION SYSTEMS: (A) SINGLE-SLUDGE SYSTEM. (B) MULTISLUDGE SYSTEM (PEPPER, GERBA, & RUSSEAU, 2006). ... 36
FIGURE21 DENITRIFICATION SYSTEM: BARDENPHO PROCESS (PEPPER, GERBA, & RUSSEAU, 2006). ... 37
3.3.2 Trickling Filters ... 38
FIGURE22 (A) A UNIT OF PLASTIC MATERIAL USED TO CREATE A BIOFILTER. THE DIAMETER OF EACH HOLE IS APPROXIMATELY 5 CM. (B) A TRICKLING BIOFILTER OR BIOTOWER. THIS IS COMPOSED OF MANY PLASTIC UNITS STACKED UPON EACH OTHER. DIMENSIONS OF THE BIOFILTER MAY BE 20 M DIAMETER BY 10–30 M DEPTH (PEPPER, GERBA, & RUSSEAU, 2006)... 39
3.3.3 Oxidation Ponds ... 40
FIGURE23 AN OXIDATION POND. TYPICALLY THESE ARE ONLY 1–2 METERS DEEP AND SMALL IN AREA. ... 40
3.3.4 Aerobic ponds ... 40
FIGURE24 AEROBIC WASTE POND PROFILE (PEPPER, GERBA, & RUSSEAU, 2006)... 41
3.3.5 Anaerobic ponds ... 41
FIGURE25 ANAEROBIC WASTE POND PROFILE (PEPPER, GERBA, & RUSSEAU, 2006)... 41
3.3.6 Facultative ponds ... 41
FIGURE26 MICROBIOLOGY OF FACULTATIVE POND (PEPPER, GERBA, & RUSSEAU, 2006)... 42
3.3.7 Aerated lagoons or ponds ... 42
4 INTRODUCTION ... 44
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FIGURE27 SCHEMATIC DIAGRAM OF DIFFERENT PROCESSING SECTORS
IN TEXTILE INDUSTRY (RAMESH BABU, 2007) ... 45
4.1.1 Cultivating and harvesting ... 46
4.1.2 Preparatory Processes ... 46
4.1.3 Spinning- Yarn manufacture ... 46
4.1.4 Weaving- Fabric manufacture ... 47
4.1.5 Finishing- Processing of Textiles ... 47
4.2 Textile Industry Chemicals ... 49
Hydrophobic/ Oleophobic Agents ... 52
1.5.5 ... 52
1.5.6 Antistatic Agents ... 52
1.1.1 ... 52
1.5.7 Oxidative compounds ... 52
4.3 The origin of textile effluents ... 53
4.3.1 Colour ... 53
4.3.2 Persistent Organics... 53
4.3.3 AOX and heavy metals ... 54
4.3.4 Toxicants ... 54
4.3.5 Surfactants ... 54
4.3.6 Temperature ... 55
4.4 Waste disposed from each section ... 57
4.5 Treatment Methods ... 57
4.5.1 Primary treatments ... 58
FIGURE28 MECHANICAL WASTEWATER SCREENING (HH AG, 2005) ... 58
4.5.2 Secondary treatments ... 60
FIGURE29 ... 61
FIGURE30 COMPACT CHEMICALLY ENHANCED-TRICKLING FILTER SYSTEM (AHMED, 2006) ... 63
FIGURE31 ACTIVATED SLUDGE (BABU B.V., 2008) ... 64
FIGURE32 ... 64
FIGURE33 ... 67
FIGURE34 SCHEMATIC DIAGRAM OF THE EXPERIMENTAL APPARATUS FOR PHOTOCATALYTIC REACTION ... 70
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FIGURE35 ADSORPTION COLUMN ... 71
FIGURE36 SCHEMATICS OF A THERMAL EVAPORATOR ... 71
4.6 Example 1 - WASTEWATER CHARACTERISTICS IN TEXTILE FINISHING MILLS ... 71
FIGURE37 SCHEMATIC DIAGRAM OF THE TEXTILE FINISHING MILL SHOWING DIFFERENT SECTIONS ... 72
4.7 Example 2- Textile Wastewater Treatment Plant ... 74
4.7.1 Plant operation ... 75
FIGURE38 AMARAVATHI COMMON EFFLUENT TREATMENT PLANT ... 78
5 INTRODUCTION ... 79
5.1 EFFLUENT SOURCE ... 80
FIGURE39 OIL STORAGE TANK ... 81
5.3 EFFLUENT PARAMETERS ... 81
5.4 EFFLUENT TREATMENT ... 83
5.4.1 PRE TREATMENT ... 83
5.4.2 PRIMARY TREATMENT ... 84
FIGURE40 PROCESS DIAGRAM OF TREATMENT METHODS SOURCE: STEFAN T. O, 2008 ... 84
5.4.3 SECONDARY TREATMENT... 84
5.4.4 TERTIARY TREATMENT ... 84
5.5 LEGISLATION ... 85
FIGURE41 FIG. 1 DISSOLVED AIR FLOTATION SYSTEM ... 87
FIGURE42 FIG. 2 HYDRO-CYCLONE SEPARATOR ... 88
FIGURE43 API OIL-WATER SEPARATOR ... 89
FIGURE44 A TYPICAL BIOLOGICAL TREATMENT PLANT ... 90
5.7 LIQUID EFFLUENT MONITORING ... 90
FIGURE45 WASTE WATER ANALYSIS LABORATORY ... 91
6 LEGISLATIONS ON TEXTILE INDUSTRY CASE STUDY: ... 95
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0 1 INTRODUCTION
Liquid effluents refer to water discharged from a community after it has been contaminated by various uses. It is often referred to as wastewater and it is combination of water removed from residences, institutions, industrial establishments, and surface and ground waters (Metacalf & Eddy Inc, 1991). It consists of 99.94 percent water by weight and the remaining 0.06 percent is suspended or dissolved material (Shun & Lee, 2000).
In the United States in the early 19th century, liquid effluents from residences, commercial premises and industries were generally discharged in to large bodies of water on to land directly without treatment. However, as the cities got larger, population increased and the demand for land became higher. Waste could no longer be dumped into the land untreated. A similar method was also being applied in the UK. Most settlements of old were located where there was easy access to water supply. However, as the clean water was used up, it was replaced by used dirty water. The polluted water was then sent back into the homes for use. Population explosion of urban areas produced massive outbreak of cholera and in 1848 and 14,000 people died of the disease. However, at this time there was no link between polluted water and disease. It was not until 1852 that a link was made between the polluted water and disease. Soon after this, laws were then enacted and certain actions were carried out. For instance in the London, efforts were made to clean up the river Thames which was regarded as biologically dead. Water from the Thames was first treated before being sent to homes for use. The water was also treated after use before being discharged back into the Thames. Today, the river Thames is considered as one of the cleanest rivers to run through a city (Read & Vickridge, 1997).
Today, in most modern cities, wastewater is treated before being discharged in to natural water bodies. In the US, 15,000 wastewater treatment plants treat approximately 150 billion liters of wastewater per day (Pepper, Gerba, & russeau, 2006). In this chapter, a brief introduction into wastewater, its sources, water quality standards, its effect and collection mechanism is described. Brief notes are also given to describe some references that can be consulted for more detailed information.
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Wastewater sources, composition and flow rate estimation
Wastewater is usually generated from residences, institutions and commercial houses, industries, farms, and run-offs from storms. Waste waters from residences, institutions, commercial houses, and farms are referred to as domestic wastewaters. However, recently farmers are required to set up on-site treatment systems for animal waste (Ministry for the Environment of Manatu Mo Te Taiao, 2009). Industrial wastewaters are sometimes treated separately but this depends on the type of industry and the size of the community. Most communities collect and treat both domestic and industrial wastewaters together in municipal wastewater treatment plants. Below is a brief introduction into some of the sources of wastewaters, their composition and flow rate.
1.1.1 Domestic wastewater
The components of domestic wastewater are; wastewater from homes, commercial places, water from rain runoff and infiltration wastewater (Kiely, 1997). Residential wastewater usually comprises of water from toilets, laundry, washing dishes etc and they are usually referred to as sewage. They can also be divided into two groups; Black water-which is basically water from toilets and Grey water which is water from every other source like kitchen sinks. Humans excrete 100–500 grams wet weight of faeces and 1–1.3 Liters of urine per person per day.
The composition and concentration of domestic water varies depending on the time of the day, the day of the week, the month of the year and other conditions (Metacalf & Eddy Inc, 1991). Table 1 gives a data of typical composition of domestic wastewater. The composition refers to the amount of physical, chemical, and biological pollutants present1.
To reduce the load of water in the treatment plant, some countries separate the pipe network for rain runoff from the main sewer water body. However, some countries do not have such and it will be too expensive to embark on the project of creating new sewer networks
1
For details on the composition of domestic wastewater further reading can be carried out in the book by (Metacalf & Eddy Inc, 1991)
2
TABLE 1 Typical composition of untreated domestic wastewater (Pepper, Gerba, & russeau, 2006).
CONTAMINANTS CONCENTRATION (mg/l)
LOW MODERATE HIGH
Solids, total 720 1200 350 720 1200 Dissolved, total 250 500 850 250 500 850 Volatile 105 200 325 105 200 325 Suspended solids 100 220 350 100 220 350 Volatile 80 164 275 80 164 275 Settleable solids 5 10 20 5 10 20 Biochemical oxygen demanda 110 220 400 110 220 400
Total organic carbon 80 160 290 80 160 290 Chemical oxygen demand 250 500 1000 250 500 1000 Nitrogen (total as N) 20 40 85 20 40 85 Organic 8 15 35 8 15 35 Free ammonia 12 25 50 12 25 50 Nitrites 0 0 0 0 0 0 Nitrates 0 0 0 0 0 0 Phosphorous (total as P) 4 8 15 4 8 15 Organic 1 3 5 1 3 5 Inorganic 3 5 10 3 5 10 5-day, 20°C (BOD, 20°C). 1.1.2 Industrial Wastewater
This is wastewater generated from industrial processes. The wastewater generated from industries varies in flow and composition depending on the type of industries. Metacalf &
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Eddy Inc. (1991) suggest that for industries with little or no wet processes, the estimated flow is about 1000 – 1500 gal/acre.d (9-14 m3/ha . d) for light industries and 1500-3000 gal/acre . d (14-28m3/ha . d) for medium industrial development. Generally, to determine the wastewater flow from an industry, a flow duration curve is created by taking measurements from the wastewater streams continuously using automatic continuous flow recorders. However, as this is expensive and time consuming, adequate measurements can also be obtained by using autosampling-autoanalytical equipments (Kiely, 1997).
As mentioned earlier, wastewater composition from industries vary and before treatment processes can be set up, waste flow diagrams or mass balance of waste flows and characteristics have to be carried out. Kiely, (1997) identifies five major steps required in the survey;
• Identifying the unique process from start to finish • Identifying the liquid waste streams
• Calculating flows of all wastewater streams
• Determining the pollutant load of all wastewater streams
• Analysing the pollutant load for the most suitable parameter to identify the waste stream
Water Quality Standard-Measures of Water Quality- When is water contaminated The quality of water is relative to its use. What may be considered as a pollutant for a particular water use may be of importance in another application. For example, organics in water help to support plant and animal life. However, organics in water will have an adverse effect if the water were to be used in a cooling tower (Vesilind & Rooke, 2003). The standard reference for water quality based on physical, chemical and biological characteristics is “Standard Methods for Examination of water and wastewater” The book is a compilation of test methods for measuring water quality. Some of the parameters measured are discussed below.
1.1.3 Dissolved oxygen
This is a very important parameter in the determination of water quality. Water devoid of oxygen will have odours and facilitate anaerobic conditions which will also result in odours
4
and loss of aquatic life. The oxygen content can be measured using an oxygen probe and meter (Vesilind & Rooke, 2003).
1.1.4 Biochemical oxygen demand
This is another very important parameter. It is a measures of both the rate at which oxygen is used up by microorganisms to break down organic matter and the amount of organic matter present in the water. In wastewater treatment, removal of BOD is essential as if left untreated and the rate of oxygen consumption is greater than re-oxygenation from the atmosphere unfavourable conditions will develop in the water body the wastewater is being discharged into.
The BOD in wastewater can be detected using the standard BOD test known as the 5-day BOD test which is run at 20°C for five days. The test is also carried out in the dark to prevent algae from producing oxygen. However, the test is not accurate as it depends on the use of oxygen by microorganisms. Other tests that have been employed are determination of chemical oxygen demand (COD test) and Total Oraganic Carbon (TOC test). The COD test makes use of strong oxidants to destroy the organic compounds present in the wastewater. It is based on the assumption that all the organics are destroyed. The organics present can then be estimated from stoichiometry. The TOC test measures the total carbon content of the wastewater. This is done by injecting the sample wastewater into a heating coil and measuring the amount of carbon dioxide gas produce and relating it stoichiometrically to the amount of carbon. Since the test does not measure the organic food material alone, the %5 day test is still used for the determination of BOD (Vesilind & Rooke, 2003).
The BOD content for most domestic wastewater discharge is approximately between 150 and 250 mg/L but that of industrial wastewater maybe as high as 30,000mg/L (Vesilind & Rooke, 2003). The BOD test carried out to;
• To determine the amount of oxygen that will be required for biological treatment of the organic matter present in a wastewater
• To determine the size of the waste treatment facility needed • To assess the efficiency of treatment processes, and
5
• To determine compliance with wastewater discharge permits.
1.1.5 Solids
In wastewater, anything other than water or gas is classified as solids. However, the basic definition for solids is anything that remains after evaporation at 103°C (Vesilind & Rooke, 2003). These solids are often referred to as Total solids. They can be classified into two groups based on filtration; suspendes solids and dissolved solids( as shown on figure 1.1). If left untreated, these solids can serve as serious pollutants leading to several effects which will be discussed later.
FIGURE1 CLASSIFICATION OF TOTAL SOLIDS (BASED ON FILTRATION) (VESILIND & ROOKE, 2003)
To determine the amount of total solids present, a known volume of wastewater is placed on an evaporating dish until all the water has evaporated. The total solids is expressed in milligrams per liter. As the name implies, dissolved solids are those components that dissolve in the water and will crystallize upon evaporation. Solids can also be classified in another way based on combustion into; volatile suspended solids and fixed suspended solids. Volatile suspended solids are generally organic in nature and a considered to combust at about 600°C.
1.1.6 Nitrogen
The presence of Nitrogen in wastewater being discharged untreated into a water body can cause euthrophication (presence of excess nutrients leading to the increase in microbial life). Nitrogen is an important element in biological reactions and is present in the organic form (i.e as amino acids and amines) and in ammonia form. It oxidises to nitrate reducing the oxygen levels in the stream.
Nitrogen presence can be detected analytically by calorimetric techniques. A known sample of wastewater can be reacted with Nessler reagent (a solution of potassium mercuric iodide). A yellow-brown colloid is formed and then it indicates the presence of Nitrogen. The precise amount can’t be determined from photometric analysis of the colloid.
Total
6 1.1.7 Phosphorous
Phosphorous is the limiting nutrient that prevents euthrophication and limits the rate of metabolic activity. If allowed to exceed natural limits can disrupt the ecological balance of the water body (Vesilind & Rooke, 2003).
1.1.8 Bacteriological measurements
Pathogens are organisms that cause illness and their determination is very important. However, the detection of pathogens is challenging for some reasons: Each pathogen has a specific detection procedure. Also, their concentration is so small as to make detection difficult. Yet, the presence of one or two of these organisms in water may be sufficient to cause infection (Vesilind & Rooke, 2003). In the United states pathogens of importance include Salmonella, Shigella, the hepatitis virus, Entamoeba histolytica, Giardia lambilia, Crptosporidium, and Escherichia coli H57 strain2. Some of these pathogens cause gastro intestinal disease and sometimes can lead to death. As mentioned earlier, it is impossible to measure all the pathogens carried by wastewater hus an indicator is used to define the bacteriological water quality. The most commonly used indicators are a group of microbes called coliforms (Vesilind & Rooke, 2003). Coliforms have five important attributes which is why they have become universal indicators;
• They are normal inhabitants of the digestive tracts of warm-blooded animals; • They exist in abundance and thus are not difficult to find
• They are easily detected
• They are generally harmless except in unusual circumstances • The can survive longer than most known pathogens
The amount of coliforms in water can be measured by passing a known amount of water through a sterile filter, then placing the filter in a Petri dish and soaking it with sterile agar solution that promotes the growth of coliforms alone. The number of dark blue-green dots formed after 24 or 48 hours indicates the coliform colonies present and it is expressed as coliforms/100mL. The removal of coliforms has become perfected by most wastewater treatment plant and the US EPA is tending towards the use of enterococci as an indication for
2
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contamination. Table 2 below shows some of the pathogenic organisms found in water and their typical concentration.
TABLE 2 Types and numbers of microorganisms typically found in untreated domestic wastewater (Pepper, Gerba, & russeau, 2006)3.
ORGANISM CONCENTRATION (per ml)
Total coliform 105–106 Fecal coliform 104–105 Fecal streptococci 103–104 Enterococci 102–103 Shigella Present Salmonella 100–102 Clostridium perfringens 101–103 Giardia cysts 10_1–102 Cryptosporidium cysts 10_1–101 Helminth ova 10_2–101 Enteric virus 101–102
The objective of wastewater treatment is to prevent the receiving water body from being contaminated by reducing;
• Biochemical oxygen demand (BOD) • Total suspended solids (TSS) • Nitrogen and Phosphorous • Faecal coliforms
However, other objectives may be set depending on the country and the legislation set up regarding the disposal of wastewater.
Wastewater characteristics
A combination of domestic and industrial wastewater is often referred to as municipal wastewater. Some countries have separate sewer networks for domestic and industrial effluents. However, in most countries the sewer systems are combined. In order to ensure that
3
(Pepper, Gerba, & russeau, 2006) this book contains detailed information on test that are used to detect amount of BOD, COD and TOD. It also contains detailed calculations and example. The book generally looks into all forms of environmental pollution with a chapter dedicated to water pollution.
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toxic substances are not released into the municipal wastewater system, industrial wastewaters have to be pre-treated to a certain standard depending on the country (Hammer, 1986).
In order to effectively collect, treat and dispose of wastewater an understanding of the basic characteristics of wastewater is essential. Wastewater is characterised in terms of;
• Physical • Chemical • Biological
However, some of these characteristics are interrelated. For example, temperature is a physical property but affects both the biological activity and the solubility of gases in wastewater (Metacalf & Eddy Inc, 1991). A summary of the characteristics is shown on Table 1.2. However some properties will be discussed briefly below.
1.1.9 Physical characteristics of wastewater 1.1.9.1 Solids in wastewater
Solids can exist in water either as suspended or dissolved solids as mentioned earlier. They are made up of organic or inorganic particles or immiscible liquids like oils and grease. The total solid content in wastewater is regarded as the residue upon evaporation at 103 to 105°C (Metacalf & Eddy Inc, 1991). They are often characterised by their size distribution, state and chemical characteristics. Solids are of importance in wastewater treatment as they serve as adsorption sites for micro organisms and chemicals and thus reduce the efficiency of treatment (Drinan & Whiting, 2001). Domestic wastewaters usually contain suspended solids that are organic in nature while industrial wastewaters contain a diverse variety of both organic and inorganic pollutants. Solids can be removed by primary sedimentation. However for particles of size 0.001 to 1 µm, secondary methods can be used to remove the solids (Metacalf & Eddy Inc, 1991). The TSS standards for primary and secondary effluents are usually set at 30 and 12 mg/L (Shun & Lee, 2000)4
1.1.9.2 Colour
The colour of waste water indicates how septic the waste is. At the initial stages, the wastewater is brownish or light grey in colour. As it flows further down the collection system, anaerobic reactions occur and it becomes dark grey or black in colour (Drinan & Whiting,
4
Shun & Lee, 2000 gives more detials on how to measure total suspended solids in wastewater and it also includes detailed calculations and examples.
9 2001).
1.1.9.3 Odour
Treatment basins, clarifiers, aeration basins, and contact tanks are some of sources where bad odour is generated in a wastewater treatment plant. Odours are generated from the anaerobic decomposition of organic compounds in wastewater. The units are normally covered to prevent the odours from escaping. However explosive gases may ensue and cause problems. Thus the units are vented to a scrubber to prevent that (Drinan & Whiting, 2001). Odours can be detected using olfactory systems. (Koe & Tan, 1998) in their work came up with a method to quantify wastewater odour strength using an olfactometer and a first order model5. Although there are four independent factors for the characterisation of odours: intensity, character, hedonics and detectability, the only factor commonly used in statutory development is detectability (Metacalf & Eddy Inc, 1991).
1.1.9.4 Temperature
The temperature of the wastewater is a very important parameter as it affects the rate of both the chemical and biological treatment. If temperatures are high, the solubility of the chemicals for treatment increases and microbial action is more effective. However if temperatures are low, microbial activity is slow and more chemicals will be required (Drinan & Whiting, 2001).
1.1.10 Chemical wastewater characteristics
The chemical characteristics of wastewater refers to the total dissolved solids (TSD) which comprises majorly of alkaline minerals, organics, PH, chlorides and nutrients. They are related to the solvent capabilities of the wastewater (Drinan & Whiting, 2001).
1.1.10.1 Total dissolved solids
These are the solid compounds that remain as residue after the wastewater has been filtered and has undergone evaporation. They can be removed from wastewater by filtration and evaporation, and also by electrodialysis, reverse osmosis, or ion-exchange (Drinan & 5 A method of quantifying the odor strength of wastewater samples has been investigated. Wastewater samples from two locations of a wastewater treatment plant were collected and subjected to air stripping. The off‐gas odor concentration was measured by a dynamic olfactometer at various time intervals. Applying a first order model to the decay of odorous substances in the wastewater under air stripping, the initial odor strength of the wastewater was determined. The model was found to be acceptable under five different air‐stripping rates studied. (Koe & Tan, 1998)
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Whiting, 2001). As discussed earlier they can be grouped into total suspended solids(TSS) and total dissolved solids(TDS). Each group can then be further divided into volatile and fixed fractions (Shun & Lee, 2000).
1.1.10.2 Metals
Metals such as cadmium, copper, lead, zinc,mercury, and others are of great concern in wastewater treatment because they are very toxic. And if discharged untreated can lead to severe complications and even death. Not only are they toxic, but the can greatly reduce the removal efficiency of some biological process (e.g. activated sludge process). The can be removed via chemical treatment and their presence in wastewater streams often increase the cost of the wastewater treatment plant. Their major source are from industrial wastes (Drinan & Whiting, 2001).
1.1.10.3 Organic Matter
Wastewater contains organic compounds which have their roots from both the plant and the animal kingdom. According to Metacalf & Eddy Inc, 1991, "In wastewater of medium strength, about 75% of the suspended solids and 40% of the filterable solids are organic in nature," Also organic compounds synthesised by man are found in wastewater. The compounds are usually made up of carbon, hydrogen, and oxygen. Compounds like sulphur, phosphorous,nitrogen and iron. The major organic compounds found in wastewaters are; proteins, carbohydrates, urea, fats and oils. The manmade compounds found are pesticides, surfactants and volatile organic compounds. As a result of industrialisation the amount of synthetic organic compounds in wastewaters are rapidly increasing. However, these compounds are not easily removed from wastewaters by biological treatment. Detailed brief description of some of these compounds can be found in (Metacalf & Eddy Inc, 1991) and (Drinan & Whiting, 2001)6.
1.1.10.4 pH
This is an indication of the hydrogen ion concentration present in the wastewater. The PH affects the chemical and biological processes in wastewater treatment. For instance, if the pH is high, the amount of chlorine required for the disinfection process will be greatly increased
6
(Metacalf & Eddy Inc, 1991) contains more details on types of organic compounds and the measurement of these organic constituents.
11 (Drinan & Whiting, 2001).
1.1.10.5 Nutrients
These are inorganic compounds that are essential to the growth and reproduction of plants and animals. The nutrients of greatest concern in wastewater treatment are nitrogen and phosphorous. Other nutrients include carbon, sulfur, calcium, iron, potassium, manganese, cobalt, and boron. The presence of nitrogen and phosphorous in surface waters is an indication of wastewater contamination. Their presence can lead to the growth of unwanted plants like algae and euthrophication. Typical ranges of nitrogen concentration in domestic raw wastewater are 25-85 mg/L for total nitrogen (the sum of ammonia, nitrate, nitrite, and organic nitrogen) and for phosphorus its 2 - 20 mg/L, which includes 1-5 mg/L of organic phosphorus and 1-15 mg/L of inorganic phosphorus (Shun & Lee, 2000).
1.1.11 Biological Characteristics of Wastewaters
Wastewater contains millions of microorganisms per milliliter. However many or these organisms are harmless. The water becomes contaminated with dangerous pathogens from waste discharged from people who are infected with them. Although micro organisms are used in various treatment processes, the final effluent discharge most not carry dangerous levels of pathogens. Basic orgfanisms of interest are bacteria, parasitic worms, protozoa, viruses and algae (Drinan & Whiting, 2001). Below is a brief description of these organisms.
1.1.11.1 Bacteria
Bacteria is common place in wastewater treatment procrsses. However the presence of some type of bacteria may cause gastrointestinal disorders.
1.1.11.2 Protozoa
These are single celled organisms that are widely distributed and highly adaptable. They are active participants of the activated sludge process. However they have to be revoved either by sedimation or filtration. Although most protozoans are harmless, two categories Entamoeba
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histolytica (amebiasis), and Giardia lamblia (giardiasis) are considered as very harmful. The levels of protozoa should be kept minimal in effluents.
1.1.11.3 Viruses
The presence of virus in wastewater is of great concer. This is because viruses are very small and cannot be easiliy removed by filtratio. Also, viruses remain inactive until they find a host and can reenter the water supply further downstream. Finally, testing for viruses is limited as there are limited methods available.
1.1.11.4 Algae
Algaes grow in fresh water, saltwater and ppolluted water. They are usually found at the surface as they require light for they metabolism. They are often used in waste water treatments like fluculative and aerobic ponds to generate oxygen. However, their growth is not easy to control, they encourage the formation of suspended solids and die of when the wheather is cold
1.1.11.5 Worms (Helminths)
These are organisms that metabolise organic compounds aerobically. They are indicators that a water body has been contaminated by wastewater. Parasitic worms like helminths are transmitted to humans via contact with untreated wastewater.
Effects of Untreated liquid effluents
The discharge of untreated effluents into local water may lead to several unwanted situations like the destruction of aquatic life, contaminations of drinking water which will lead to illness or even death, bad odours etc. Below some of the effects of untreated wastewater will be discussed.
1.1.12 Health effects
Of all the effects the health effects of untreated wastewaters are one of the most important ones. The first set of legislations on wastewater effluent discharge was focused towards their effect on human health. Wastewater contains millions of bacteria that originate from human
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faeces. However most are harmless. The organisms that may cause diseases are called pathogens. Contact with the contaminated water may lead to disease such as typhoid, cholera and gastrointestinal problems. The main class of viruses of concern are enteric viruses, which cause gastro-enteritis; for example, calcivirus (Norwalk virus), rotavirus, enterovirus (polio and meningitis) and hepatitis (Ministry for the Environment of Manatu Mo Te Taiao, 2009).
Asides from bacteria and viruses, other substances present in wastewater may lead to bad health or even death. For instance, compounds like mercury, volatile organic compounds, zinc, pesticides and other chemicals. Some of these compounds exist naturally but human activities have increased their concentration in natural water systems. They may not have immediate effects but may bio-accumulate in food and cause complications. According to the Ministry for the Environment of Manatu Mo Te Taiao, some ivestigations are being carried out to investigate the ability of some of these compounds to act as endocrine disruptor. Endocrine distruptors are chemicals that when absorbed into the body mimics or hinders the normal functions of hormones in the body.
1.1.13 Increase in the B.O.D. & C.O.D. content of water bodies
The discharge of untreated wastewater into springs, rivers and lakes will cause the BOD and COD to rise. This will reduce the amount of oxygen available to aerobic (oxygen demanding) aquatic animals like fish. Also this will encourage the growth of plants like algae and other anaerobic organisms. Eventually, this will render the water body septic and biologically dead (Weiner & Matthews, 2003)
1.1.14 Increase in nutrient content
Increased nutrient content (that is, organics from wastewater) will lead to algal bloom and eutrophication. Nitrogen in the form of nitrate (NO3) in surface waters indicates contamination with sewage and is an immediate health threat to both human and animal infants (Drinan & Whiting, 2001). Excessive nitrate concentrations in drinking water can cause death. The limiting factor for accelerated growth or some organisms is the absence of nutrients nitrogen and phosphorus in the water body. These compounds exist in water naturally but in limited quantities. An increased amount of these nutrients will cause the accelerated growth of some toxic organisms like algae which will slowly lead the water body to become septic.
14 1.1.15 Increase of soil deposition
Solids in water can have several harmful effects some of which are listed below • Solids can cause unsightly floating scum
• They can sink to the bottom of the stream or river and form potentially hazardous mud banks
• Most solids are organic in nature and upon decomposition create a demand for oxygen • Floating solids serve as sites where pathogenic organism can hide and pose a threat to
human health.
1.1.16 Effects of odours
Odours although cause no direct physical harm to humans have great psychological effects that may eventually lead to social and economic collapse in a community. Odours from wastewater treatment plant can cause; loss of appetite, water intake, impaired respiration, nausea, vomiting and mental perturbation. Offensive odours can also discourage capital investment and lower socio-economic status of the community if left untreated. In a paper prepared by Schiffman, et al., 2000, they proposed three paradigm which ambient odors may produce health symptoms in communities with odorous manures and biosolids7. Many communities have opposed the several wastewater treatment plant projects as a result of public perception of odours.
1.1.17 Effects of Increased Temperatures
The temperatures of wastewater is usually higher than the atmospheric temperature and the receiving water. High temperature decreases the solubility of oxygen. This combined with
7 Schiffman, et al., 2000, proposed three paradigm by which ambient odors may produce health symptoms in communities with odorous manures and biosolids. This site summarises the three paradigms
15
increased biochemical oxygen demand can greatly affect the oxygen content of the receiving water body. Eventually this will affect the aquatic life of the waterbody. Decreasing fish lifand supporting the growth of unwanted organisms.
Below shows a summary of some of the effects of pollutants contained in wastewater.
TABLE 3 Effects of pollutants in wastewater (Kiely, 1997)
Pollutants Effects Soluble organics Deplete dissolved oxygen
Suspended solids Deplete dissolved oxygen and
release undesirable gases Trace organics Affects taste odours and toxicity
Heavy metals Toxic to aquatic and human life
Colour and turbidity Affects aesthetics Nutrients (N and P) Cause eutrophication Refractory substances resistant to
biodegradation
Toxic to aquatic life
Oil and floating substances Unsightly Volatile substances e.g H2S and
VOC
Air pollution
http://www.woodlands-junior.kent.sch.uk/riverthames/pollution.htm dirty river thames
http://www.metrovancouver.org/services/wastewater/treatment/Pages/default.aspx
Wastewater collection systems
Wastewater collection systems are employed to transport wastewater form source to treatment plant before disposal (Read & Vickridge, 1997). Collection systems are made up of a series of network of pipes and pumping systems. In designing a collection system one must consider;
16
1. Health and Environmental aspect. That is the proposed collection system must not pose a risk to human health and to the environmental.
2. The area the sewer network is going to service. The treatment plant must be adequate in serving the allocated region.
3. The natural topography and drainage. Collection systems must be designed to take advantage of the natural systems and thus reduce cost of installing pumps.
There are three main types of sewer networks (Kiely, 1997); 1. Sanitary sewer systems
2. Storm sewers
3. Combined sewer systems
Below is a description of each of these systems.
1.1.18 Sanitary sewer systems
For these systems, wastewaters generated from both domestic and industrial sources are carried by separate systems of sewers to treatment plants while surface runoffs are carried of by another set of systems to natural watercourses. This type of system is mostly adopted in newer towns and cities (Read & Vickridge, 1997). Rain water washes contaminants from roofs, streets and other areas, however the contaminant load is considered insignificant compared to wastewater discharges from domestic and industrial sources (Hammer & Hammer, Water and Wastewater Technology, 2008). Sanitary contains majorly human waste as a result of the most important aspect of sanitary sewer design is the prevention of sewage overflow (as they contain pathogens dangerous to human health) (Drinan & Whiting, 2001).
1.1.19 Storm sewer systems
These systems handle wastewaters generated from run-offs as a result of rainfall or melting snow. They are becoming very important in developed and populated areas. This is because in such areas, the ground is paved and this prevents water from naturally percolating and
17
recharging ground water. Instead, heavy run-offs result which carry large amounts of contaminants (Drinan & Whiting, 2001).
Storm drains should be designed to handle contaminants like sand, silt and also sudden heavy flows. Since they do not contain sewage, they can be discharged directly into the natural environment although sometimes, primary treatment may be required (Hammer & Hammer, 2008).
1.1.20 Combined sewer systems
These are the oldest and most common type of collection system. For this type of system, both surface runoffs and municipal wastewater are transported by the same pipe networks to sewage treatment plants. This type of system is mostly found in older cities and towns (Read & Vickridge, 1997)8. The systems are designed to accommodate large flows, especially those resulting from heavy rain falls. However during storms, the system overflows and excess flow above the plant capacity is bypassed into natural water bodies. This may become a health hazard especially if water is used as supply for drinking water (Hammer & Hammer, 2008). Typical storm water contains a BOD of 30mg/l while overflow from a combined sewer contains contains 120mg/liter of BOD. Combined sewer overflows (CSO) are of great concern. However, it is very expensive to change the entire sewer network and other methods such as storage for later treatment are being explored (Shun & Lee, 2000).
1.1.21 Collection System Components
Most components of collection systems are built under streets easements, and right of way and they are designed to meet considerations of population size, estimated flowrates, minimum and maximum loads, velocity, slope depth, and need for additional system elements
8
This book contains detailed information on the history, construction, hydraulics and design of sewer systems. Focusing mainly on sewer rehabilitation, repair, and management.
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19
Preliminary: this includes simple processes such as screening (usually by bar screens) and grit removal. (through constant velocity channels) to remove the gross solid pollution.
Primary: usually plain sedimentation; simple settlement of the solid material in sewage can reduce the polluting load by significant amounts.
Secondary: for further treatment and removal of common pollutants, usually by a biological process.
Tertiary: usually for removal of specific pollutants e.g. nitrogen orphosphorous, or specific industrial pollutants
FIGURE3 TYPICAL STAGES IN THE CONVENTIONAL TREATMENT OF SEWAGE
20 FIGURE4
Shown in table1 are some constituents found in wastewater and conventional water treatment methods used to purify the water.
TABLE 4
CONSTITUENT UNIT OPERATION OR PROCESS
21 Grit removal Sedimentation High-rate clarification Flotation Chemical Precipitation Depth Filtration Surface Filtration
Biodegradable organics Aerobic suspended growth variation
Aerobic attached growth variation Aerobic suspended growth
variation
Aerobic attached growth variation Lagoon variation
Physical chemical systems Chemical oxidation Advanced oxidation Membrane filtration
Nitrogen Chemical oxidation
Suspended-growth nitrification and denitrification variations
Fixed-film nitrification and denitrification variations Air stripping
Ion exchange
Phosphorous Chemical treatment
Biological phosphorous removal
Nitrogen and phosphorous Biological nutrient removal variations
Pathogens Chlorine compounds
Chlorine dioxide Ozone
Ultraviolet radiation (UV) Colloidal and dissolved solids Membranes
22
Carbon adsorption Ion exchange Volatile organic compounds Air stripping
Carbon adsorption Advanced oxidation
Odours Chemical scrubbers
Carbon adsorption Bio filters
Compost filters
3 INDUSTRIAL WASTE WATER TREATMENT METHODS
The same way that you would know the steps of the process that you would be running in industry, a critical study should be carried out to familiarize yourself with the wastewater to find ways that the wastewater is generated in the plant.
Treatment methods can be divided into three general cases - Physical/Chemical treatment methods
- Thermal Treatment methods - Biological treatment methods
Physical Waste water treatment methods Biological Chemical Aerobic Septic tanks Lagoons Anaerobic Trickling Filtration Lagoons Chlorination Ozonation Neutralization Coagulation Screening Sedimentation
23 3.1 Physical/chemical treatment methods
After its biological treatment the waste water is almost clean and fresh again. However, the micro-organisms responsible for cleaning should now be kept in their individual basins and not run off with all the rest. Physical chemical treatment methods encompass a wide variety of technologies, including gravity separation, filtration, chemical precipitation, evaporation, oxidation, reduction, air stripping, carbon adsorption, ion exchange, adsorption on other media, electrolytic recovery and membrane separation.
Gravity separation is used to extract clean water when the waste is settled in the bottom of the tank. There are three types of separation methods which uses the same principal. Clarifiers, Oil water separators and catch basins and sumps.
3.1.1 Screening
Mechanical treatment is indispensable as the first process step of preliminary treatment for both municipal and industrial wastewater applications. It removes the bulk of the non biodegradable matter such as plastic, women materials, metallic items so that the subsequent treatment stages are protected against damage/pollution or to relieve them. There are many different types of screens in industry at present designed to suit different needs. Some examples as stated in EPCO, Australia are Inclined bar, curved bar, radial bar, step type, brush type, back-raked and static screens.
Equalization Degassification Flotation & skimming
24
In operation in all these types, the sewage flows through the screen which approaches it from the upstream side and after passing through exits from the downstream side. A mechanized comb system is attached between the two side chains and is driven through a head shaft and sprockets, to rake the screen periodically and the screenings collected are removed by a doctor blade at the top of the comb travel as stared in [Epco, Australia]. These screenings are dropped onto a skid plate which transports the screening down to a container.
25
FIGURE6 CURVED BAR SCREEN
FIGURE7 RADIAL BAR SCREEN
26
FIGURE9 BRUSH TYPE SCREEN 3.1.2 Sedimentation
Sedimentation, a fundamental and widely used unit operation in waste-water treatment, involves the gravitational settling of heavy particles suspended in a mixture, sedimentation tanks are also known as clarifiers in the wastewater treatment industry. This process is used for the removal of grit, particulate matter in the primary settling basin, biological floc in the activated sludge settling basin, and chemical flow when the chemical coagulation process is used.
FIGURE10 CIRCULAR AND RECTANGULAR SETTLING TANKS
Circular sedimentation tanks are preferred over rectangular tanks due to the ease of maintenance, faster sludge removal and higher removal efficiencies. There is a scraper mechanism adopted inside the tank which is used to collect the settled solids out of the tank with the use of a pump. As stated by [Hammer 2004, pg 370] the scraper mechanism takes different forms depending on which part of the treatment it is used for, i.e. primary, secondary or tertiary. As further stated in circular sedimentation tanks these sludge scrapers are attached to the rotating arm which scrapes the sludge towards the centre hopper where as in rectangular tanks the scrapers are carried along in the tank bottom which collects the sludge into a hopper which is situated at the influent end of the tank. There are 3 types of clarifiers which are named as primary, secondary and tertiary tanks.
Primary tanks
27
beginning of the primary process. The scraper mechanism as stated in [Epco] for primary tanks would also be fitted with scum removal equipment to remove the floating matter and in the primary stage as further stated approximately 65% of the organic solids and 35% pr the BOD in the sewage is removed.
FIGURE11 PRIMARY CLARIFIER ELEVATION VIEW
FIGURE12 PRIMARY CLARIFIER PLAN VIEW Secondary Tank
The water from the primary stage goes through some biological treatment and then enters the secondary sedimentation which separates the mixed liquids and suspended solids and humus sludge. The secondary clarifiers are fitted with scraper blades like in primary systems but could also adapt a suction tube system as shown in the picture below. These systems also are equipped with scum skimming systems.
FIGURE13 SUCTION TUBE CLARIFIER ELEVATION
28
These systems often adapt simple sweeper chains but could also be fitted with the same mechanisms as the primary and the secondary treatments. The final clarifiers are designed to use with biological aeration. Activated sludge is withdrawn through suction pipes located along the collector arm for rapid return to the aeration basin. Sludge thickeners and fermenters are also used to scrape heavier sludges as shown in the figure.
FIGURE14 PICKET FENCE SLUDGE THICKENER 3.1.3 Flotation and Skimming
Effluent Effluent
Settled solids discharge
29 FIGURE15
Dissolved air flotation is achieved by releasing fine air bubbles that attach to sludge particles and cause them to float. Small units tend to be rectangular for and fabricated using steel. Larger units are circular and manufactured in steel or concrete. Waste activated sludge enters the bottom of the flotation tank, where it is merged with recirculated flow that contains compressed air. A portion of the clarified effluent is pressurized in a separate retention tank under an air pressure of approximately 60 psi to force air into solution. On pressure release, the air dissolved in the recirculated flow forms fine bubbles to the suspended solids. The process underflow is returned to wastewater treatment, and the overflow, discharge by
3.2 Chemical treatment methods
This treatment method uses burning or exposure of wastewater to high temperatures to destroy the waste. Some waste that is burned could be used to recover energy in industrial furnace or cement kiln on site. Treatment facilities such as hazardous waste incinerators are another mean of for wastewater treatment but isn’t cost effective if used in small businesses as they are quite expensive, unless the facility generates a large amount of waste. Some industries tend to use off site facilities to treat wastewater but it is among the last choices to use such means. Wet air oxidation is another method used to treat waste water which is difficult to treat by other means. But this demands a huge amount of energy which in the long run is more cost effective if the wastewater is treated off site.
30 3.2.1 Chlorination
Chlorination is basically the process of adding liquid or gaseous chlorine in order to purify water. This process isn’t solely used for disinfection; it is also used for odor control and prevention of septicity, ammonia removal, destroying cyanides and phenols etc as stated by the water quality and health council. The liquid form of chlorine which is generally more expensive comes in the form of soluble salts (hypochlorites) while the gaseous form first needs to be dissolved in water before it is used in the waste water industry.
There are a few reactions that occur in chlorination when used in the waste water industry. When the chlorine is dissolved in water it firstly forms hypochlorous acid and hypochlorites.
Cl2 + H2O Æ HOCl + H + Cl HOCl Æ OCl + H
As chlorine is an active oxidizing agent when added into waste water even in small amounts it would react rapidly firstly with H2S, ferrous iron etc which are all compounds capable of reducing. After all inorganic reducing matter is converted chlorine subsequently reacts with the organic matter, ammonia or other nitrogeneous compounds to produce chloramines. The device used for the control of the chlorine added is called the chlorinator.
FIGURE16 CHLORINATOR
31 3.2.2 Ozonation
Before After
FIGURE17 OZONE WATER TREATED AREA
It is one of the modern methods used in wastewater treatment with a growing popularity. A device known as the ozone generator is used to break down pollutants in the wastewater. The ozone generator uses up the oxygen in the environment to produce ozone with the air of ultraviolet radiation which is discharged by an electric field. This ozone which is known to be highly reactive oxidise the bacteria, moulds and other pollutants in the wastewater.
As stated in the water pollution guide there are many advantages and disadvantages in using ozone in wastewater treatment.
• Advantages:
o Effective killing of bacteria.
o Ease of extracting irons and sulphur compounds as they are oxidised. o No nasty odours or residues hence precautions or measures for residue
treatment is not needed.
o As the oxygen to ozone conversion is a reversible reaction and the backward reaction is fast the ozone converts back to oxygen instantly leaving no traces of an oxygen use up.
32
o This method is unreliable as it needs electricity to run and would not run during an electric shortage and also costs money due to its power requirement. o The treatment cannot remove dissolved minerals and salts.
o Ozone treatment can sometimes produce by-products such as bromate that can harm human health if they are not controlled.
FIGURE18 OZONATOR
3.3 Biological treatment methods
Biological processes aid in the removal of non-settleable colloidal solids, inorganic compounds and some organic matter with the aid of micro organisms. Biological processes are often referred to as secondary wastewater treatment method as they aid in the removal of biodegradable organic matter that could not be removed during primary treatment (Kiely, 1997). In wastewater treatment, the main objectives is the reduction of organic contents and nutrients like nitrogen and phosphorus and also the removal of toxic organic compounds such that the discharge to a water body should lead to little or no removal of oxygen in it by bacterial action. During biological processes, organic pollutants are converted to less harmful compounds like water and cell tissues. These can then be removed by gravity settling.
The commonly used biological treatment processes: • Activated-sludge process
33 • Aerated lagoons
• Trickling filters
• Rotating biological contactors • Stabilization ponds
Fundamentals of Biological wastewater treatment process
Biological treatment processes all take place in a vessel called a reactor and designers are interested in the rate at which biodegradable compounds are removed from the inflow and also, the rate of growth of the biomass in the reactor. (Benefield & Randal, 1980). The biomass refers to the microbial body responsible for breaking down the pollutants. In designing a biological process, it is very important to understand the nature and biochemical activities carried out by the micro organism. Below, the types of micro organisms used and their nutritional requirements will be mentioned briefly.
Important microorganisms
The important microorganisms in biological treatments are; Bacteria, Fungi, Algae, Protozoa and Rotifiers.
Bacteria are single- celled organisms that reproduce mainly by binary fission although some species can produce asexually or by budding. They are made up of 80% water and 20% dry material. They also vary widely in size and their growth is greatly affected by the conditions of temperature and pH (Metacalf & Eddy Inc, 1991).
Fungi are multicellular organisms. Example of this is yeast. They can reproduce sexually or asexually. They have the ability to withstand lower pH and Nitrogen levels than bacteria and this makes them very important in wastewater treatment.
Algaes are unicellular or multicellular compounds. They are very important in wastewater treatment processes because of their ability to generate oxygen from photosynthesis. However, excess algae growth can lead to the biological death of a water body.
Protozoa and Rotifiers are single celled motile protists. Most protozoa are aerobic and are generally larger than bacteria and alsoeat bacteria as an energy source. Thus are used to polish effluents from biological waste treatments.
34
presence indicates a highly efficient biological process (Metacalf & Eddy Inc, 1991).
Nutritional requirements
For micro organisms grow, reproduce and function efficiently they must have; 1. A source of energy
2. Source of carbon for synthesis of new cellular material
3. Source of inorganic nutrients such as Nitrogen, phosphorous, sulphur, potassium calcium and magnesium (Metacalf & Eddy Inc, 1991).
The carbon and energy sources are considered as substrate
3.3.1 Activated-sludge Process
This is one of the most popular biological treatments adopted in most countries and it is also known as aeration-tank digestion. In this process, wastewater that has undergone primary treatment is pumped into a large tank and mixed with bacteria rich slurry known as activated sludge (Pepper, Gerba, & russeau, 2006). To encourage bacterial growth and decomposition of the organic materials present, air or oxygen is pumped into the tank. The mixture is then sent to a secondary settling tank where water is removed from the top and the bacteria rich sludge is removed from the bottom. About 20 percent of the sludge is recycled back into the primary aeration tank as inoculums while the remainder known as secondary sludge is removed (Kiely, 1997). Fig below shows an aeration basin.
The activated sludge culture is made up of bacteria, protozoa, rotifiers and fungi. The bacteria is mostly responsible for the break-down of organic material while the protozoa and rotifiers remove the bacteria.
35
FIGURE19 AN AERATION BASIN (PEPPER, GERBA, & RUSSEAU, 2006) The content of the aeration tank is referred to as the mixed-liquor suspended solids (MLSS) and the organic part of the MLSS is called the mixed-liquor volatile suspended solids (MLVSS), which consists of the non-microbial organic matter as well as dead and living microorganisms and cell debris (Kiely, 1997). The activated sludge process must be controlled to maintain a proper ratio of substrate (organic load) to microorganisms or food-to-microorganism ratio (F/M) (Pepper, Gerba, & russeau, 2006). This is expressed as BOD per kilogram per day. It is expressed as:
/ [1] where:
Q -flow rate of sewage in million gallons per day (MGD) BOD5 - 5-day biochemical oxygen demand
MLSS - mixed-liquor suspended solids (mg/L) V - volume of aeration tank (gallons)
It can thus be observed that the higher the wasting rate, the higher the food-micro organism ratio. A low F/M ratio indicates that the micro organisms are starved and will tend to have higher removal efficiencies. In conventional aeration tanks, the F/M ratio is 0.2–0.5 lb BOD5/day/lb MLSS, but it can be higher (up to 1.5) for activated sludge when high-purity oxygen is used (Hammer, 1986). The parameters that controll the operation of an activated sludge process are;
• organic loading rates • oxygen supply
• control and operation of the final settling tank
An important parameter to consider is the sludge settleability in the sludge tank. The biomass must settle well in order for it to be returned to the aeration tank. The best conditions for settling are achieved when carbon and energy sources are limited and the specific microbial growth rate is local. Conditions that hinder effective settleability are sudden changes in temperature, pH, absence of nutrients, and presence of toxic metals and organics. Another
36
important factor is the presence of filamentous bacteria. For effective settling, a residence time of three or four days is required (Metacalf & Eddy Inc, 1991).
Removal of Nitrogen and Phosphorous by activated sludge process
Activated sludge process can be modified such that they not only remove organic compounds but can also remove nutrients like nitrogen and phosphorous.
Nitrogen Removal
For nitrogen removal, the sludge is left to age for over four days to encourage nitrification of ammonia to nitrate by nitrifying bacteria. The nitrogen is then removed via denitrification process. Examples of avtivated sludge systems that have been modified for nitrogrn removal are:
• Single sludge system • Multisludge system and • Bardenpho process
Figure below shows a schematic diagram of these processes9
FIGURE20 DENITRIFICATION SYSTEMS: (A) SINGLE-SLUDGE SYSTEM. (B) MULTISLUDGE SYSTEM (PEPPER, GERBA, & RUSSEAU, 2006).
9
37 Phosphorous removal
Phosphorous can also be removed by modifying the activated sludge process. The process involves an uptake of the phosphorous during the aerobic stage and the release during the anaerobic stage. Two processes used are;
A/O (anaerobic/toxic) process: This process consist of and anaerobic zone upstream the conventional aeration tank. In the aerobic phase, soluble phosphorus is taken up by bacteria and is synthesised to polyphosphates and during the anaerobic stage, the phosphorus is released by the hydrolysis of the polyphosphates formed.
Bardenpho process: This process can also be used for the removal of nitrogen A schematic diagramof these process are shown on fig below
FIGURE21 DENITRIFICATION SYSTEM: BARDENPHO PROCESS (PEPPER, GERBA, & RUSSEAU, 2006).
Design of an activated sludge process
In designing an activated sludge process, the following conditions are considered; • Mixing regimes • Load criteria • Sludge viability • Oxygen requirement • Nutrient requirement • Temperature • Solid-liquid separation • Effluent quality
38
Another very important consideration is the choice of reactor as this will define the geometry of the reactor, the bath of effluent into the reactor and the mixing regime. There are two mixing regimes applicable in the activated sludge process
(1) Plug-flow: in this type of regime, the wastewater flows into the reactor (aeration tank) in an orderly fashion with no element of mixing. (2) Complete mixing flow: Here, the reactor is constantly stirred and kept
uniform. This type of mixing is often referred to as steady state.
However, complete mixing or plugflow is not achieved in the reactor but the design has to ensure that the conditions are almost met (Benefield & Randal, 1980)10.
3.3.2 Trickling Filters
This is one of the oldest biological treatment methods. However the main mechanism is not filtration as the name suggest. Treatment is achieved by diffusion and microbial assimilation. In the process, the effluent from the primary treatment is pumped through an overhead sprayer onto a bed of stones or plastic where bacteria and other organism reside. As the organic materials trickle past, the bacteria intercepts it and decomposes it aerobically. In older trickling filter designs, the beds were made of stones. But these had the disadvantage of limited depth of 3-10 ft, low void space, and requirement for structural design (Benefield & Randal, 1980). However, in modern trickling filter designs, the bed is made up of plastic units. Other materials that can be used are ceramic, hard coal. The most common type of plastic bed used is polyvinyl chloride (PVC) because of their light weight. Other advantages are the greater void space and larger specific area. The PVC are stacked in towers as shown on fig below.
10
Details on the design of activated sludge process and the associated kinetics involved can be found in this text.
39
(a) (b)
FIGURE22 (A) A UNIT OF PLASTIC MATERIAL USED TO CREATE A BIOFILTER. THE DIAMETER OF EACH HOLE IS APPROXIMATELY 5
CM. (B) A TRICKLING BIOFILTER OR BIOTOWER. THIS IS COMPOSED OF MANY PLASTIC UNITS STACKED UPON EACH OTHER. DIMENSIONS OF THE BIOFILTER MAY BE 20 M DIAMETER
BY 10–30 M DEPTH (PEPPER, GERBA, & RUSSEAU, 2006).
As the organic matter passes through the filter, it is converted to a microbial biomass that forms a bio film called zooleal on the filter surface. With time, the film thickens and the lower part has limited access to oxygen and as a result film sloughs off ( also called sloughing) and a new bio film is formed. Effluents from a trickling filter are sent to a clarifier for further removal of solid compounds. A typical trickling filter had a BOD removal efficiency of 85% (Pepper, Gerba, & russeau, 2006).
Two important properties of the filter media are; The specific area of the media
The percent void space
The greater the surface area, the greater the amount of biomass per unit volume. Also, the greater the void space, the higher the hydraulic loading can be without restricting oxygen
40 transfer (Benefield & Randal, 1980)11.
3.3.3 Oxidation Ponds
These are often referred to as sewage lagoons or stabilization ponds. They are the oldest of the wastewater treatment process requiring huge land space. Here, the wastewater is detained for a period of 1-4 weeks( sometimes longer) while microorganisms degrade the organic matter in them. A tyoical oxidation pond is shown below on fig
FIGURE23 AN OXIDATION POND. TYPICALLY THESE ARE ONLY 1–2 METERS DEEP AND SMALL IN AREA.
There are four categories of osidation ponds which are often used in series: aerobic ponds, anaerobic ponds, facultative ponds and aerated ponds.
3.3.4 Aerobic ponds
Here, the wastewater is detained for 3-5 days at a depth of about 1.5m to encourage the growth of algae which in turn promotes the generation of oxygen. A section of an aerated pond is shown on fig below.
11
41
FIGURE24 AEROBIC WASTE POND PROFILE (PEPPER, GERBA, & RUSSEAU, 2006)
3.3.5 Anaerobic ponds
These are about 1-10m deep and have a longer detention time of about 20 – 50 days. They are normally used to treat wastewater with high BOD content and do not requireany form of mechanical aeration. They also generate comparably small amount of sludge. Fig shows the profile of an anaerobic pond.
FIGURE25 ANAEROBIC WASTE POND PROFILE (PEPPER, GERBA, & RUSSEAU, 2006)
3.3.6 Facultative ponds
They are normally used for the treatment of domestic waste and they have a dentention time of 5-30 days. These type of ponds range in depth from 1- 1.25 m and are is made up of three sections: an upper aerated zone, a middle facultative zone, and a lower anaerobic zone as shown on fig . The make use of both aerobic and anaerobic treatment.
