4) I saperi tradizionali della gondola veneziana
2.6. La Comunità patrimoniale nella Convenzione di Faro del 2005
Objective:
After reading this unit, you should able to Know what physicochemical treatment processes are.Know to what steps in physicochemical treatment process are.
Know the physicochemical treatment process keywords.
Reading 1.
Contaminated water contains particles of different sizes which can be classified as dissolved (< 0.08 μm), colloidal (0.08 - 1 µm), supracolloidal (> 100 - 100 mm) and settleable (> 100 µm). The type of treatment selected depends on the size of particles present in the wastewater. In practice, treatment efficiency also depends on particle size.
Solids of the size that are visible to the naked eye can be separated either by settling under the influence of gravity or by flotation, depending on the relative densities of solids and water. They may also be easily separated by filtration. However, very fine particles of a colloidal nature (called colloids, size < 1 µm) which have high stability are significant pollutants. The reason for this stability is that these particles have electrostatic surface charges of the same sign (usually negative). This means that repulsive forces are created between them, preventing their aggregation and subsequent settling. It has therefore proved impossible to separate them by settling or flotation. It is not possible to separate these solids by filtration because they pass through any filter. However, separation by physico-chemical treatments is possible.
Physico-chemical treatment of wastewater focuses primarily on the separation of colloidal particles. This is achieved through the addition of chemicals (called coagulants and flocculants). These change the physical state of the colloids allowing them to remain in an indefinitely stable form and therefore form into particles or flocs with settling properties.
TASK 1. COMPREHENSION QUESTIONS
Answer the questions below.1. According to the reading, how many types of particles can be classified? What are they?
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2. How can the coarse particle be separated from the wastewater?
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3. Why is colloid difficult to settle?
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4. Can we use membrane to remove colloid from water?
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5. To remove colloid, coagulants or flocculants are added to the solution, please tell the reasons
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Reading 2.
1.
ScreeningThe very first unit operation in waste water treatment is screening. Screening is an essential step in wastewater treatment for the removal of bigger suspended and floating solids which would otherwise damage equipment, interfere with the operation of treatment units or equipments. Screens are used ahead of pumping stations, meters and as a first step in all treatment works. A screen is a device with opening generally of uniform size for removing bigger suspended or floating matter in wastewater.
The screening element consists of parallel bars, rods gratings or wiremeshes or perforated plates and the openings may be of any shape although generally they are circular or rectangular, depending on the size of the opening screens classified as coarse, medium or fine.
2. Grit chamber
Grit chambers are designed to remove grit, consisting of sand, gravel, cinders or other heavy solid materials that have specific gravities much greater than those of the organic putrescible solids in the wastewater. Most of the substances of grit are abrasive in nature and will cause accelerated wear on pumps and sludge handling equipment with which it comes in contact. Grit deposits are not biodegradable and occupy large space in sludge digesters. It is therefore desirable to separate grit deposits from he organic suspended solids.
Grit chamber is assumed to be one in which particles settle as individual entities, and where there is no significant interaction with the neighboring particles, which is known as free settling or discrete settling or
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type I settling. Grit removal facilities basically consist of an enlarged channel area where reduced flow velocities allow grit to settle out. Mechanical scrapers remove the deposited grit.
For proper functioning of the grit chamber, the velocity through the grit chamber should not be allowed to change inspite of the change in flow. One of the most satisfactory type of automatic velocity control is achieved by providing a proportional Weir at the outlet. The shape of the opening between the plates of a proportional weir is made in such a way that the chamber depth will vary directly as the discharge, as a result of which the chamber velocity will remain constant for all flow conditions
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TASK 2: True or False
Decide whether the following statements are true (T) or false (F).
1. Grid chambers which remove, for example, sand, gravel and heavy metal are normally placed before the screen.
2. Materials removing by grid chamber are oftenly bigger than that removing by screen.
3. Screens are positioned behind pump to ensure that no coarse materials which affect the later process can pass.
4. Most of the substances of grit are chemically corrosive in nature and will damage pump.
5. We can use microbial to treat the substances removing by grit.
6. The chamber velocity can vary with the fluctuations of wastewater load.
Reading 3.
3. Flow equalizaion
Flow equalization is used to minimize te variability of water and wastewater flow rates and composition.
Each unit operation is a treatment train is designed for specific wastewater characteristics. Improved efficiency and control are possible when all unit operations are carried out at uniform flow conditions. If there exists a wide variation in flow composition over time, the treatment efficiency of the overall process performance may degrade severely. These variations in flow composition could be due to many reasons, including the cyclic nature of industrial processes, the sudden occurrence of storm water events, and seasonal variations. To dampen these variations, equalization basins are provided at the beginning of the treatment train. The influent water with varying flow composition enters this basin first before it is allowed to go through the rest of treatment process. Equalization tanks serve many purposes. Many processes use equalization basins to accumulate and consolidate smaller volumes of wastewater such that
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full scale batch reactors can be operated. Other processes incorporate equalization basins in continuous treatment systems to equalize the waste flow so that the effluent at the downstream end can be discharged at a uniform rate.
4. Neutralization
Neutralization is a common practice in wastewater treatment and waste stabilization. If a waste stream is found to be hazardous because of corrosivity, neutralization is the primary treatment used. Moreover, neutralization is used as a pretreatment system before a variety of biological, chemical and physical treatment processes. Since many chemical treatment processes, such as metal precipitation, coagulation, phosphorus precipitation, and water softening are pH dependent, the pH of these processes is adjusted to achieve maximum process efficiency. Furthermore, the pH of the effluent wastewater from different is industrial activities also requires adjustment prior to its discharge into receiving water bodies, the US EPA has set pH standards for different types of water; for example, the pH range required to protect marine aquatic life is 5-9.
Neutralization is the process of adjusting the pH of water through the addition of an acid or a base, depending on the target pH and process requirements. Some processes such as boiler operations and drinking water standards need neutral water at a p of 7. Water and wastewater is generally considered adequately neutralized if (1) its damage to metals, concrete, or other materials is minimal; (2) it has little effect on fish and aquatic life; (3) it has no effect on biological matter (i.e., biological treatment systems).
TASK 3. COMPREHENSION QUESTIONS
Answer the questions below.1. What is the role of flow equalization?
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2. Name some reasons causing the change in flow composition?
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3. Why equalization tank help full scale batch reactors can be operated?
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4. Which characteristics of watewater we should use neutralization?
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5. Which chemical treatment processes are effective with appropriate pH value?
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7. To not damage marine aquatic life, which range of pH value of the discharged wastewater?
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8. In which cases, water and wastewater is generally considered adequately neutralized?
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Reading 4.
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. Coagulation and FlocculationCoagulation and flocculation constitute the backbone processes in most water and advanced wastewater treatment plants. Their objective is to enhance the separation of particulate species in downstream processes such as sedimentation and filtration. Colloidal particles and other finely divided matter are brought together and agglomerated to form larger size particles that can subsequently be removed in a more efficient fashion. The traditional use of coagulation has been primarily for the removal of turbidity from potable water. However, more recently, coagulation has been shown to be an effective process for the removal of many other contaminants that can be adsorbed by colloids such as metals, toxic organic matter, viruses, and radionuclides. The researchers divided the coagulation process into three distinct and sequential steps:
1. coagulant formation 2. Particle destabilization 3. Inter-particle collisions
The first two steps are usually fast and take place in a rapid-mixing tank. The third step, inter-particle collisions, is a slower process that is achieved by fluid flow and slow mixing. This is the process that causes the agglomeration of particles and it takes place in the flocculation tank. Coagulation is usually achieved through the addition of inorganic coagulants such as aluminum or iron-based salts, and/or synthetic organic polymers commonly known as polyelectrolyte.
Properties of Colloidal Systems
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Colloids are very small particles that have extremely large surface area. Colloidal particles are larger than atoms and ions ut are small enough that they are usually not visible to the naked eye. They range in size from 0.001 to 10 micom resulting in a very small ratio of mass to surface area. The consequence of this smallness in size and mass and largeness in surface area is that the colloidal suspensions
1. Gravitational effects are negligible, and 2. Surface phenomenon predominates.
Because of their tremendous surface, colloidal particles have the tendency to adsorb various ions from the surrounding medium that impart to the colloids and electrostatic charge relative to the bulk of surrounding water. The developed electrostatic repulsive forces prevent the colloids from coming together and, consequently, contribute to their dispersion and stability.
Following the first step of coagulation, a second process called flocculation occurs. Flocculation, a gentle mixing stage, increases the particle size from submicroscopic microfloc to visible suspended particles. The microflocs are brought into contact with each other through the process of slow mixing. Collisions of the microfloc particles cause them to bond to produce larger, visible flocs called pinflocs. The floc size continues to build through additional collisions and interaction with inorganic polymers formed by the coagulant or with organic polymers added. Macroflocs are formed. High molecular weight polymers, called coagulant aids, may be added during this step to help bridge, bind, and strengthen the floc, add weight, and increase settling rate. Once the floc has reached it optimum size and strength, the water is ready for the sedimentation processes.
TASK 4. MULTIPLE CHOICE QUESTION
Answer the questions below.1. The coagulation process can be classified into…….steps.
a. 1 b. 2 c. 3 d. 4
2. Which step should be took place in slow mixing condition?
a. coagulant formation b.Inter-particle collisions c. Particle destabilization d. b&c
3. Colloids are very small particles that have
a. very large surface b. very large volume c. very large ratio volume/surface
d. very large ratio surface/volume
4. Colloidal suspensions are stable because
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a. of the electrostatic repulsive force between the particles.b. colloids are too small to collide with each other.
c. collision between the colloids are prevented by ions in the solution.
d. Adsorbed ions on the colloidal surface prevent colloids stick together.
5. Flocculation process occurs through sequential steps a. Microfloc - pinfloc macrofloc sediment.
b. Pinfloc microfloc macrofloc sediment.
c. Sediment microfloc pinfloc macrofloc.
d. Pinfloc mirofloc sediment macrofloc
6. Coagulant aids are added in flocculation process in order to:
a. Generate bridge between the microflocs or pinfloc.
b. Stick small flocs together.
c. Prevent macroflocs from breaking d. a, b and c are right.
Reading 5. Chemical Precipitation
Precipitation is a chemical unit process in which undesirable soluble metallic ions and certain anions are removed from water or wastewater by conversion to an insoluble form. It is a commonly used treatment technique for removal of heavy metals, phosphorus, and hardness. The procedure involves alteration of the ionic equilibrium to produce insoluble precipitates that can be easily removed by sedimentation. Chemical precipitation is always followed by a solids separation operation that may include coagulation and/or sedimentation, or filtration to remove the precipitates. The process can be preceded by chemical reduction in order to change the characteristics of the metal ions to a form that can be precipitated.
Precipitation processes should be distinguished from coagulation and flocculation. Coagulation is the removal of finely divided non-settleable solid particles, especially colloids, by aggregation into larger particles through the destabilization of the electric double layer (4). Flocculation is the formation of yet larger particles by the formation of bridges between coagulated particles through the adsorption of large polymer molecules and by other forces. Both coagulation and flocculation, which often occur together, result in particles that can be removed by sedimentation or filtration. Coagulation and flocculation occur subsequent to and concomitant with the precipitation processes as it is usually applied in waste treatment
Sedimentation
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Sedimentation is defined as a unit operation in which suspended particles are separated from a suspension by gravitational settling. The terms clarification and thickening of sludge apply to the same unit operation.
Coagulation, which is discussed before, involves the addition of chemicals to induce faster aggregation and settling of initially finely divided suspended and colloidal particles. The objective of sedimentation is to remove settleable particles from suspensions either with or without the addition of chemicals. When no chemicals are added to the process, it is called plain sedimentation. Plain sedimentation is usually employed in wastewater treatment, whereas in water- treatment plants, sedimentation, in most cases, is preceded by chemical coagulation. Sedimentation is also employed, to a limited scale, in separating particulates from air streams.
TASK 5. COMPREHENSION QUESTIONS
Answer the questions below.1. What is the principle of precipitation process?
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2. Chemical precipitation should be placed before coagulation and/or sedimentation, or filtration unit.
Why?
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3. Sometime, the metal ions need be change the oxidation state. What are the reasons?
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4. What is the driving force of sedimentation phenomenon?
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5. Whether particle size removing by coagulation or flocculation is larger that that by sedimentation?
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Reading 6. Dissolved Air Floatation Process
Dissolved Air Flotation (DAF) is the process of removing suspended solids, oils and other contaminants via the use of air bubble flotation. Air is dissolved into water, mixed with the wastestream and released from solution while in intimate contact with the contaminants. Air bubbles form, attach to the solids, increase their buoyancy and float the solids to the water's surface. A percentage of the clean effluent is recycled and super-saturated with air, mixed with the wastewater influent and injected into the DAF separation chamber.
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The dissolved air comes out of solution, producing millions of microscopic bubbles. These bubbles attach to the solids and float them to the surface where they are mechanically skimmed and removed from the tank. Dissolved air flotation systems are designed to remove fats, oils & grease (FOG), suspended solids, food/animal production/processing wastes, industrial wastes, hydrocarbon oils/emulsions and many other contaminants. Clarification rates as high as 97% or more can be achieved using our dissolved air flotation systems. Chemical pre-treatment can often help to improve the performance of contaminant removal.
TASK 6. COMPREHENSION QUESTIONS
Answer the questions below1. What does DAF stand for?
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2. In DAF technology, should the waste collector be placed at the bottom of the tank or at the surface water? Why?
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3. In your opinion, whether the big bubble gas size or small bubble size is good?
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TASK 7: How to find good keywords
Find the keywords in reading paragraphs 4 and 5 TASK 8: Summary
In about 5 sentences, summarize the main idea in paragraphs 4 and 5
TASK 10: Glossary
Search your knowledge, look up your dictionary, internet or ask your instructor to clarify the definition and Vietnamese meaning of the following terminologies.
No Terminology Definition Vietnamese
1 abrasive 2 accumulate
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3 agglomerate4 aggregation 5 aggregation 6 basin 7 coagulation 8 consolidate 9 contaminant 10 corrosivity 11 deposit 12 destabilization 13 equalization 14 filtration 15 flocculation 16 flocs 17 flotation 18 flotation 19 grit chamber 20 insoluble 21 ludge
22 Neutralization 23 pollutant 24 precipitation 25 pump 26 removal 27 repulsive 28 Screening 29 sedimentation 30 settle
31 supracolloidal