tiếng anh cấp thoát nước nxb khoa học kỹ thuật 2006 đỗ văn thái 262 trang

The challenge of water treatment plant design 22 Bài 2.Thách thức của việc th k ế nhà máy nước Bài 3.H oà trộn,keo tụ và tạo bông Bài 4.. WASTE WATER TREATMENT PHẦN II.XỬ LÝ NƯỚC THẢI Bà

TH Ư VIỆN ĐH NHA TRANG

NHA XỤ.

Đ ỗ VĂN THÁI (Đại học Xây Dựng)

CẤP THOÁT NƯỚC

NHÀ XUẤT BẢN KHOA HỌC VÀ KỸ THUẬT

HÀ NỘI - 2006

TRƯỜNG ĐẠI HỌC XÂY DựNG

40 NĂM THÀNH LẬP (1966 - 2006)50 NĂM ĐÀO TẠO (1956 - 2006)

Bài 1.Kỹ thuật môi trường / vệ sinh

Unit 2 The challenge of water treatment plant design 22

Bài 2.Thách thức của việc th k ế nhà máy nước

Bài 3.H oà trộn,keo tụ và tạo bông

Bài 4 Công nghệ lắng

Bài 5.Công nghệ lắng nhanh

Bài6 Lọc tốc độ cao bằng vật dạng hạt

Bài 7 Vật liệu lọc

Bài8 Rửa vật liệu lọc

3

Bài 9 Vận hành và diều khiển

Unit 10 Oxidation and disinfection

Bài 10 Ôxy hóa và khử trùng

119

PART II WASTE WATER TREATMENT

PHẦN II.XỬ LÝ NƯỚC THẢI

Bài 1.Kỹ thuật xử lý nước thải

Unit 2 Wastewater treatment objectives and regulations 147

Bài 2 Những mục tiêu và quy tắc xử lý nước thải

Unit 3 Classification of wastewater treatment method 160

Bài 3.Phân loại các phương pháp xử lý nước thải

Bài 4 ứng dụng các phương pháp xử lý

Bài 5.Những vấn dề cấp bách

Bài 6 Thải bùn và sử dụng lại

Bài 7 Vai trò của người kỹ sư

Bài 8 Những yếu tố cần xem xét về môi trường

4

Unit 9 Implementation of wastewater

Bài9 Thực hiện các chương trình quản nước thải

Unit 10 Startup and operation of treatment plants 226

Bài 10 Khởi dộng và vận hành các nhà máy xử lý

APPENDIX

Abbreviations commonly used in the water industry 235

Những chữ viết tắt thường dùng trong ngành nước

5

LỜI NÓI ĐẦU

Nước là tài nguyên quý giá nhất mà thiên nhiên ban tặng cho con người Bản thân con người cũng coi "nước là quà tặng quý nhất mà con người có thể tặng nhau" - "water is the best gift man can give to man”

Cũng như những quốc gia khác Việt Nam luôn nỗ lực đầu tư trí tuộ và vật chất để bảo vệ tài nguyên nước, nhằm bảo vệ môi trường nói chung và môi trường nước nói riêng Xuất phát từ quan điểm đó hàng loạt các dự án nước đã và đang được thực hiện trên toàn quốc như dự án cấp nước Hà Nội - Phần Lan, Hải Phòng - Phần Lan, Chương trình nước sạch nông thôn do UNICEF tài trợ và các dự án cấp, thoát nước của các thành phố và thị xã có vốn vay của ADB hay WB

Các dự án trên đều có sự tham gia của các chuyên gia nước ngoài ở hầu hết các lĩnh vực lừ tư vấn, giám sát dự án, thiết kế, giám sát thi công Để hoàn thành tốt được nhiệm vụ trong các dự án nước, các cán bộ chuyên ngành cần phải trang bị một trình độ tiếng Anh chuyên ngành nhất định Cuốn sách ra đời nhằm giúp các cán bộ ngành nước giảm bớt khó khăn trong công việc của mình

Sách bao gồm ba phần : Phần I trinh bày các bài về công nghệ; kỹ thuật xử lý nước cấp; Phần II có các bài về xử lý nước thải và phần Phụ lục trình bày các ký hiệu viết tắt và lời giải thích thuộc ehưyôn ngành

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Tác giả hy vọng cuốn sách không những giúp bạn đọc nâng cao trình độ tiếng Anh mà còn đưa tới bạn đọc những thông tin chuyên môn cập nhật khác.

Cuối cùng tác giả xin cám ơn ban lãnh đạo Trường Đại học Xây dựng Hà Nội và Nhà xuất bản Khoa học và Kỹ thuật đã nhiệt tình giúp đỡ để cuốn sách được xuất bản kịp thời phục vụ bạn đọc

Tác giả

8

PHAN I

XÛ LŸ CAP NUÔC

PARTI

W ATER S U P P L Y TREATM EN T

Unit 1ENVIRONMENTAL / SANITARY ENGINEERING

Environmental / Sanitary engineering is concerned with providing clean, safe water supply systems for towns, cities, and rural areas It is also concerned with disposing of excess water and waste materials by means of sewer systems Many aspects of environmental / sanitary engineering are directly related to hydraulic engineering; indeed, some of the projects are parts of water supply systems The Hoover Dam, for example, supplies water to the city of Los Angeles, with which it is connected by a series of canals, tunnels, and aqueducts across the deserts and mountains of the southwestern United States

A great deal of archeological evidence has revealed the importance of water supply systems in the ancient world Probably the most impressive systems were built by the Romans, whose aqueducts still stand in modem Italy, Spain, France, and Turkey Rome itself had a water supply estimated at 50 million gallons a day, or about 50 gallons a day for each resident of the city The water was delivered to fountains, where people collected it in pots and then carried it to their homes; only a few buildings and residences had connections to the main pipelines Rome also had a sewer, the Cloaca Maxima, part of which is still used today Like other sewer systems of ancient times, it was intended to carry of the water from storms or the waste water from the public baths There

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were penalties for disposing of solid wastes in it.

After the fall of the Roman Empire, water supply and sewer systems received relatively little attention until modem limes In the Middle Ages in Europe, water came from streams and wells,

while wastes were disposed of in cesspools or even returned to the

same streams from which the water was taken After the connection between water supply and certain diseases such as typhoid was established in the nineteenth century, cities and towns all over the W'orld built safe water supply systems The experience of three Aunerican cities illustrates some of the engineering problems - and economic and legal problems as well - in constructing large water supply systems

Chicago grew from a tiny trading post on Lake Michigan to a great metropolis in only a few decades Providing water for the fast­growing population was a problem from the city's earliest days The primary source was the lake that bordered the city, but the water was frequently polluted by discharge of water from the Chicago River, which acted as a sort of open sewer during the city's early years After storms or floods, the pollution was especially severe, and not even moving the pumps for the water farther from the shore w as an adequate solution It was finally decided to reverse the flow olf the Chicago River so that it discharged not into the lake but into thie Illinois River, a tributary of the Mississippi After this had been done by means of the Chicago Sanitary and Ship Canal, however, there were cries of outrage, followed by lawsuits, from people in towns and cities downstream from Chicago because their water supply sources were being polluted by wastes from Chicago

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Finally, the city was forced to build several large sewage treatment plants to remove harmful substances from waste water before it was released into the Chicago River.

New York City has had to construct a system that brings water to the city from considerable distances As the city has grown, it has been necessary to range farther and farther away to find new sources At present, much of New York's water comes from the watershed of the Delaware River, northwest of the city, across the Hudson River and beyond the Catskill Mountains It is delivered to the city’s system of holding reservoirs through the longest tunnel in the world, the Delaware Aqueduct, which extends for the most part through bedrock for a distance of 169 kilometers At some points the aqueduct reaches a depth of 750 meters The New York system delivers an average supply of approximately 1.3 billion gallons a day - considerably more than 100 gallons a day for every person in the city

Los Angeles is located in an area of low rainfall When the population of the city began to grow rapidly, water was brought in from the Colorado River, more than 350 kilometers away, across deserts and mountains The Colorado River Aqueduct, supplying water to Los Angeles and other southern California communities, is 1,081 kilometers long An even more ambitious project now under construction will divert water from rainy northern California to the drier central and southern portions of the state The Oroville Dam, a great earth-fill embankment dam, is part of the overall California State Water Project When the project is completed, it will deliver water for both irrigation systems and town and city systems from

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San Francisco and Sacramento in the central part of the state to I os Angeles and areas even farther south The main aqueduct and its principal branches will be several hundred kilometers long, some of it in concrete lined canals and some of it in pipes and tunnels High - capacity pumping stations will raise the water over intervening mountain ranges.

In addition to transporting water over long distances, modem water supply systems also use several techniques for purification

One of them is filtration The water is passed through a filter that

consists of a bed of sand or gravel, which removes a large proportion of the solids that might otherwise contaminate the

supply Another process is aeration Sprays of water are shot into

the air, where sunlight and oxygen help to kill bacteria and also remove gases with an unpleasant odor or taste; or air is bubbled into or through the water A third method involves treatment with chemicals, usually chlorine, to kill harmful bacteria The process is

known as chlorination.

Part of providing a safe water supply is disposing of liquid and solid wastes This problem has become acute in recent years noit only because of world-wide population growth, but also because of the vast amount of waste created by industrial processes and by the great mountains of trash that are the by-product of increased consumption

A large number of modem drainage systems use the same sewers to dispose of domestic wastes and runoff water from storms Many of these systems were designed to empty into streams or other bodies of water where nature itself purified the water over a

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period of time Now, however, the amount of waste has become so great that many streams and lakes and even the seas have become polluted More and more treatment plants are being built to purify water before it is released back into the environment Therefore, the modem trend is to build separate drainage systems for storm runoff and for domestic wastes so that the treatment plants do not have to

process the runoff water, which is relatively unpolluted.

There are a number of different methods by which solid wastes can be removed or rendered harmless Several of them are ordinarily used in combination in treatment plants One of the processes is filtration Another is , in which wastesare allowed to settle until they become solid or semisolid and can be removed There are also techniques in which water can be treated by biological means, by using some kinds of bacteria to kill other kinds, or by chemical means, as in chlorination One of the

most successful methods is called the activated-sludge process It

involves using compressed air to increase and control the rate of biological reactions that purify the wastes In effect, tratment plants speed up natural purification processes so that the water that is finally released from them is essentially harmless Present-day concern over environmental pollution has increased the demand that waste water should be treated to the fullest degree possible before it is returned to the environment

Vast amounts of trash have also posed problems in disposal Much of it has been used as landfill by dumping in swampy areas

or in shallow water so that the area can be made useful A great

deal of it has also been burned in incinerators, huge furnaces that

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reduce the wastes to ash Incinerators, however, are out of fashion today because they release harmful fumes into the air Many of them are being redesigned to control these emissions more effectively; at the same time other solutions are being sought.

One modem method of disposing of trash and domestic wastes is recycling,which simply means using the waste material

again The wastes from treatment plants, for example, can be used as fertilizer It can also be used as fuel In fact, some treatment plants fill their own energy needs by burning their waste products to provide steam for generating electricity Similarly, some kinds of trash can be collected separately - glass, newspapers, and aluminum cans, for example All of these materials can be processed for reuse In some cases, trash has also been compacted to serve as fuel

The concern for a cleaner environment together with need to conserve and reuse our resources has created a challenge for which sanitary engineers, working with environmentalists, wil be called upon to find new solutions over the next few years

VOCABULARY

/ vệ sinh

cạnh của kỹ thuật mồi trường / vệ sinh

15

♦ aqueduct kênh / cầu dẫn nước

khảo cổ

«

nước (sạch) cho dân số tăng trưởng nhanh

nhiễm

1 6

nghiêm trọng

♦ adequate solution giải pháp thích hợp

♦ trib u tary sông nhánh

♦ Chicago Sanitary and Ship Canal Công ty vệ sinh và giao

thông đường sông Chicago

♦ sewage treatment plant 1) nhà máy xử lý chất thải; 2)

công trình của hệ thống xử lý chất thải

Angeles (nằm) ở vùng ít mưa

water was brought in from the Colorado river khi dân số

thành phố bắt đầu tăng nhanh, nước được cấp về từ sông Colorado

2 TACTN-A

17

♦ ambitious project dự án nhiều tham vọng

2) dự án nước quốc gia

dẫn chính và các nhánh chính của nó

tủa bằng cách phun tơi nước vào không khí hay phun không khí vào trong nước, sự sục khí

bằng clo

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2.TACTN-B

♦ industrial processes 1) các quy trình cổng nghiệp; 2) các

quá trình sản xuất công nghiệp

other bodies of water where nature itself purified the water over a period of time những hộ thống này được thiết

kế để thoát hết (nước thải) vào các dòng suối hay các chỗ chứa nước khác, ở đó tự nhicn sẽ tự làm sạch nước sau một thời gian

water which is relatively unpolluted các nhà máy xử lý

không cần phải xử lý nước mưa vì nó tương đối không bị ô nhiễm

♦ filter 1) lọc; 2) cái lọc, dụng cụ lọc

lắng trong công nghệ xử lý nước; 3) sự hình thành trầm tích

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♦ settle lắng xuống, lắng cặn

♦ chemical (thuộc) hóa học

vi sinh để diệt khuẩn có hại

nước cần được xử lý tới mức độ đầy đủ nhất có thể

♦ pose 1) đưa ra, đặt ra (vấn đề, câu hỏi )', 2) đặt, sắp đặt

♦ landfill 1) san lấp; 2) sự san lấp

20

♦ fuel nhiôn liệu

trường

21

Unit 2THE CHALLENGE OF WATER TREATMENT PLANT OESIGN

When water treatment engineering first evolved in the early part of the twentieth century, its main goal was to ensure that infectious organisms in drinking water supplies were removed or inactivated Chlorination and filtration practices were applied with tremendous success to the point that major death-causing waterborne disease outbreaks in the United States were virtually eliminated by the 1930s

As a result, for engineers trained in the 1960s, 1970s, and 1980s, both education and industry belief was that all concerns of microbiological contamination in surface waters could be eliminated by providing filtration (with suitable pretreatment) to produce water of sufficient clarity (turbidity less than 1.0 or 0.5 ntu), and then chlorinating Groundwater was thought to be already filtered, requiring only chlorination to maintain a distribution system residual Any additional treatment was generally considered necessary only to address non-health-related parameters, such as excessive hardness or water discoloration caused by iron and manganese

The principal challenge to water treatment engineers in the 1960s and 1970s was engineering cost-effectiveness; how to accomplish these simple treatment goals at the lowest total cost to the water utility Thus, in these decades many new techniques and

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processes were developed to clarify surface water economically These developments included improvements to sedimentation basin designs; high-rate clarification processes such as tube settlers, plate settlers, and dissolved air flotation, high-rate filtration processes; and proprietary pre-engincercd or package equipment integrating flocculation, settling, and filtration processes.

In the 1970s and 1980s a new drinking water concern arose: the potential long-term health risks posed by trace amounts of organic compounds present in drinking water A wave of regulations ensued with new maximum contaminant Içvels (MCLs) established for total trihalomethanes (TTHMs), pesticides, and volatile organic chemicals (VOCs) This trend continues today In response to this concern and resulting treatment needs, water treatment engineers have successfully devised new methods of water treatment to remove organic compounds These methods, such as air stripping, activated carbon adsorption, and enhanced coagulation, have been the primary focus of water treatment engineering over the last 15 years

contamination is reemerging as the primary focus of water treatment engineers The main driving forces behind this development have been:

♦ The promulgation of the Surface Water Treatment Rule and Total Coliform Rule by the U S Environmental Protection Agency (USEPA) and the monitoring and enforcement actions that have oceured since 1989, when they went into effect

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♦ Recent documented cases of contamination of drinking water supplies by waterborne diseases, mainly giardiasis and cryptosporidiosis, caused by cysts rather than bacteria.

New approaches and processes are evolving to address the:se concerns These approaches include renewed emphasis on source water protection, optimising plant performance, and recycle stream management, plus consideration of new technologies, especially membrane treatment and ozonation

TODAY'S CHALLENGES

Engineers who design water treatment systems today face many challenges The most important of these are described as follows

Integrated Treatment Systems

Traditional treatment engineering has focused on tlhe treatment plant as the sole vehicle for controlling drinking water quality The engineer's role was to characterize the quality of tlhe source water to enter the plant and devise treatment facilities to produce water meeting drinking water standards The point of measurement for drinking water standards was the finished water exiting the plant

Today's engineer must view the water treatment plant as ontly a major component in a multistep treatment process This process includes consideration of the path that the water travels upstream of the plant in the watershed and the elements of the water transmission and distribution system downstream of the plant

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Changing water quality must be managed in each of these steps, and new regulations require that drinking water standards be met at the customer's tap.

Regulatory Uncertainties

The definition of "safe" drinking water, which remained relatively fixed in the 1950s, 1960s, and 1970s, now seems to be constantly changing or under review as the water utility industry grapples to understand the potential health effects of trace amounts of an increasing variety of chemical compounds and infectious organisms Today's treatment system engineer, in addition to addressing current drinking water standards, must anticipate potential future requirements A water system designed today must be designed with sufficient flexibility to be modified to meet these potential requirements

Regulatory uncertainties extend to other environmental concerns important to water treatment plant design, including waste management practices and chemical storage and feed operations

New Technologies

The state of the art of water treatment plant design is continually changing as new technologies emerge, offering new unit processes for water treatment or making currently used processes more efficient or economical In addition, advances in computer technology and building materials are rapidly changing and improving the support systems associated with water treatment plants

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Multidiscipline Teams

A water treatment plant enginering design team traditionally consisted simply of a small group of civil engineers This single­discipline team performed the majority of design work for vitually all plant components Support disciplines of architects and structural electrical, and mechanical engineers were used to execute the basic decisions madè by the design team

requirements dictates that a far more multidisciplined approach be used Typically, a small group of civil engineers remains as the "project" engineers, but this group uses the expertise and resources of many different specialists to execute the design In addition to traditional design support disciplines, these may include :

♦ Process engineers♦ Plant operations specialists♦ Instrumentation and control engineers♦ Health and safety specialists

♦ Environmental scientists♦ Specialists in environmental permitting and public

participationMajor design decisions today are no longer made unilaterally by the project team Instead, a consensus is reached after participation by members of the design team and by individuals outside the team, including owners, operators, regulatory agencies, and the general public

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Project Delivery

The traditional procedure for construction of a new water treatment plant was for engineering design and specification to be prepared by an engineering firm or the owner's in-house staff Bids were then taken and the contract awarded to the lowest responsible bidder The design team then usually monitored the construction to see that the design intent was carried out, and after construction was completed, the facilities were operated by the owner

Today, a number of changes and variations to this traditional approach are being considered or implemented Two of the principal alternatives are:

♦ Design-build approaches, in which one entity is responsible for both design and construction

♦ Privatization approaches, in which the facility is owned by a private entity providing treatment service for the water utility

In addition, a global marketplace for water treatment engineering is evolving Ideas and practices are being exchanged among countries all over the world In North America, there is increasing consideration of European treatment practices, technologies, and firms

VOCABULARY

lây nhiỗm

sinh từ nước gây sự chốt chóc lớn

27

♦ outbreak bùng nổ

được nghĩ rằng đã qua lọc

♦ address to 1) toàn tâm toàn ý với; 2) chú ý triệt để tới,

nhắm tới

necessary only to address to non - health - related parameters bất kỳ sự xử lý thêm nào chủ yếu chỉ được coi

là tới những thông số không liôn quan gì tới sức khoẻ

trong nước nhanh

28

♦ dissolve air flotation sự tách khí không hoà tan

kế công nghệ trước, có bản quyền riêng

filtration processes thiết bị trọn gói tích hợp các công nghệ

tạo bông, lắng và lọc

of organic compounds present in drinking water những

nguy hiểm tiềm tàng lâu dài đối với sức khoẻ được thể hiện bởi dư lượng các hợp chất hữu cơ trong nước uống

♦ ensue 1) xảy ra sau đó; 2) (+ from; on) sinh ra từ, nảy sinh

từ

cực đại

29

♦ Surface Water Treatment Rule (S w T R) Quy định xử

lý nước mặt

nước)

plant performance tối ưu hóa hoạt động của nhà máy

hoàn

diện; 2) hệ thống xử lý (nước) thống nhất

xuất nước đáp ứng các tiêu chuẩn nước uống

2) công nghộ xử lý nhiều bước

30

♦ upstream thượng lưu (dòng

♦ watershed 1) đường phân nước; 2) lưu vực sông

M ater travels upstream of the plant in the watershed and the elements of the water transmission and distribution system downstream of the plant quy trình công nghệ này

bao gồm việc xem xct đường nước chảy thượng lưu trong phạm vi phân nước và các thành phần truyền tải nước và hệ thống phân phối nước từ nhà máy

uống hiện hành

tương lai

qui định; những điều hay thay đổi trong quy định

♦ state of the art sự tiên tiến; sự tối tân; the State of the art of water treatment plant design mức độ tiên tiến trong

thiết kế nhà máy nước

trợ

31

♦ majority số đông; đa số

điều khiển

participation chuyên gia cho phép về môi trường và sự

tham gia của công chúng

sự đấu thầu đã được thực hiộn

♦ bid 1 ) bỏ thầu; 2) trả giá

32

Unit 3

MIXING, COAGULATION AND FLOCCULATION

Coagulation and flocculaton may he broadly described as chemical and physical processes that mix coagulating chemicals and flocculation aids with water The overall purpose is to form particles large enough to be removed by the subsequent settling or filtration processes Particles in source water that can be removed by coagulation, flocculation, sedimentation, and filtration include colloids, suspended material, bacteria, and other organisms The size of these particles may vary by several orders of magnitude Some dissolved material can also be removed through the formation of particles in the coagulation and flocculation processes

Terms used in this unit are defined as follows:

♦ Coagulation: the process in which chemicals are added to

water, causing a reduction of the forces tending to keep particles apart Particles in source water are in a stable condition The purpose of coagulation is to destabilize particles and enable them to become attached to other particles so that they may be removed in subsequent processes Particulates in source waters that contribute to color and turbidity are mainly clays, silts, viruses, bacteria, fulvic and humic acids, minerals (including asbestos, silicates, silica, and radioactive particles), and organic particulates At pH levels above 4.0, particles or molecules are generally negatively charged The coagulation process physically occurs in a rapid mixing process

3.TACTN-A

33

♦ Mixing', commonly referred to as flash mixing, rapid

mixing, or initial mixing The purpose of rapid mixing is to provide a uniform dispersion of coagulant chemical throughout the influent water

♦ Enhanced c o a g u l a t i o n : a phrase used by the U S.

Environmental Protection Agency (USEPA) in the Disinfectants and Disinfection By-Products Rule The Rule requires that the coagulation process of some water supplies be operated to remove a specified percentage of organic material from the source water, as measured by total organic carbon (TOC) Enhanced coagulation (removal of TOC) can be achieved in most cases by either increasing coagulant chemical dosage or adjusting the pH during the coagulation reaction

♦ Coagulant chemicals', inorganic or organic chemicals that,

when added to water at an optimum dosage, cause particle destabilization Most coagulants are cationic when dissolved in water and include chemicals such as alum, ferric salts, lime, and cationic organic polymers

♦ Flocculation: the agglomeration of small particles and

colloids to form settleable or filterable particles (floes) Flocculation begins immediately after destabilization in the zone of decaying mixing energy following rapid mixing, or as a result of the turbulence of transporting flow In some instances, this incidental flocculation may be an adequate flocculation process A separate flocculatioa process is most often included in the treatment train to enhance contact of destabilized particles and to build floe particles of optimum size, density, and strength

34

3.TACTN-B

♦ Flocculation aids: chemicals used to assist in forming

larger, denser particles that can he more easily removed by sedimentation or filtration Cationic, anionic, or nonionic polymers are most often used in dosages of less than 1.0 mg/L

♦ Direct filtration: a treatment train that includes coagulation,

flocculation, and filtration, hut excludes a separate sedimentation process With direct filtration, all suspended solids are removed by filtration In the process sometimes called in-line filtration, flocculation occurs in the conduit between the rapid mixing stage and the filter, in the volume above the filter media, and within the filter media

♦ Solids contact clarifiers: proprietary devices that combine

rapid mixing, flocculation, and sedimentation in one unit These units provide separate coagulation and flocculation zones and are designed to cause contact between newly formed floe and settled solids

THE COAGULATION PROCESS

Coagulation reactions occur rapidly, probably taking less than one second Principal mechanisms that contribute to the removal of particulates when coagulating chemicals such as alum or ferric chloride arc mixed with water include chemical precipitaion, reduction of electrostatic forces that tend to keep particles apart, physical collisions between particles, and particle bridging

Several factors affect the type and amount of coagulating chemicals required, including the nature of suspended solids and

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th e c h e m ic a l c h a r a c te ris tic s o f th e in flu e n t w a te r.

Coagulant Chemicals

The most commonly used coagulants are:♦ Alum (aluminum sulfate), A12(S 04)3.14H20 is the most common coagulant in the United States and is often used in conjunction with cationic polymers

♦ Polyaluminum chloride, Al(OH)x(Cl)y, is efficient in some waters requiring less pH adjustment and producing less sludge

♦ Ferric chloride, FeClj, may be more effective than alum in some applications

♦ Ferric sulfate, Fe2(S 0 4)3, is effective in some waters and more economical in some locations

♦ Cationic polymers can be used alone as the primary coagulant or in conjunction with aluminum or iron coagulants

Although alum is by far the most widely used coagulant chemical, ferric chloride of ferric sulfate form a better-settling floe in some waters and may be more consistently effective in removing natural organic matter

Flocculation Aids

Floe formed in many waters with alum is light and fragile and somewhat difficult to settle Polymers and other additives can often help form a floe that is more efficiently removed by settling and filtration Typical additives used for floculation aids are:

♦ High-molecular-weight anionic or nonionic polymers

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♦ Activated silica

These chemicals are normally added after applying

coagulants, from 5 to 600 seconds after mixing If the water to be

treated with a flocculent aid is already in the flocculation stage, the chemical should be added so that it can be spread across the flocculation basin

Chemical Selection

The selection of coagulant chemicals and flocculation aids for use in a particular plant is generally based on economic considerations along with reliability, safety, and chemical storage considerations The best method of determining treatability, the most effective coagulants, and the required dosages is to conduct bench scale and, in some cases, pilot tests Jar tests can be used to determine treatability and estimate chemical dosages If possible, testing should cover all critical seasonal conditions

When designing for coagulant application, as much flexibility as possible should be allowed to accommodate changing conditions Several points of addition for coagulant chemicals, particularly polymers, should be provided in the rapid mixing and flocculation processes The order of chemical addition is also important in almost all waters

Sludge quantity and disposal are important considerations in selecting the coagulant to be used Metal ion coagulants produce considerably larger volumes of sludge than polymers The ability to predict the exact reaction and quantity of sludge that will be

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produces solely by the reaction formulas is limited For this reason, predictions of treatability, chemical dosages, and sludge quantities must generally be determined by laboratory and pilot plant tests.

The coagulation process may, in some cases, be improved by

requirements to the point where low residual solids (or filtration efficiency) make direct filtration feasible Oxidation with air and chemical oxidants such as chlorine and potassium permanganate may also aid coagulation by oxidizing iron and manganese, which can aid floe formation

VOCABULARY

and turbidity are mainly clays, silts, viruses, bacteria

những hạt nhỏ li ti trong các nước nguồn góp phần tạo nên mầu và độ đục chủ yếu là các sét, bùn, vừut, vi khuẩn

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♦ humic acid axit humic

generally negatively charged ở độ pH cao hem 4,0 các hạt

hoặc phân tử chủ yếu tích điện âm

dòng (nước) vận chuyển

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♦ a treatment train that includes coagulation, flocculation and filtration but excludes a separate sedimentation process công đoạn xử lý bao gồm giai đoạn kco tụ, tạo bông

và lọc nhưng khổng bao gồm quá trình lắng riêng biệt

♦ in-line filtration sự lọc liên tiếp; lọc nối tiếp

c

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