ENVIRONMENTAL ENGINEERING LECTURE 3: WATER TREATMENT MISS NOR AIDA YUSOFF

Transcription

1 ENVIRONMENTAL ENGINEERING LECTURE 3: WATER TREATMENT MISS NOR AIDA YUSOFF

2 LEARNING OUTCOMES Define the concept and process of water treatment. Describe the concept of coagulation, flocculation, and sedimentation applied in water treatment. Propose a simple water treatment system.

3 WATER POLLUTION Water pollution is the contamination of water bodies such as lakes, rivers, oceans, and groundwater by human activities. All water pollution affects organisms and plants that live in these water bodies and in almost all cases the effect is damaging either to individual species and populations but also to the natural biological communities. It occurs when pollutants are discharged directly or indirectly into water bodies without adequate treatment to remove harmful constituents.

4 Water Pollution The specific contaminants leading to pollution in water include a wide spectrum of chemicals, pathogens, and physical or sensory changes such as elevated temperature and discoloration. While many of the chemicals and substances that are regulated may be naturally occurring (calcium, sodium, iron, manganese, etc.) The concentration is often the key in determining what is a natural component of water, and what is a contaminant. Many of the chemical substances are toxic. Oxygen-depleting substances may be natural materials, such as plant matter (e.g. Leaves and grass) as well as man-made chemicals.

5 WATER TREATMENT / PURIFICATION The treatment of water intended for human consumption is a very old practice. Example: boiling and filtering of drinking water. Water treatment describes those processes used to make water more acceptable for a desired end-use. Water purification is the removal of contaminants from untreated water to produce drinking water that is pure enough for its intended use, most commonly human consumption. Substances that are removed during the process of drinking water treatment include bacteria, algae, viruses, fungi, minerals such as iron, manganese and sulphur, and man-made chemical pollutants including fertilizers.

6 WATER TREATMENT / PURIFICATION The processes involved in treating water for drinking purpose may be solids separation using physical such as settling and filtration, chemical such as disinfection and coagulation. World Health Organization (WHO) guidelines are generally followed throughout the world for drinking water quality requirements. In addition of the WHO guidelines, each country or territory or water supply body can have their own guidelines in order for consumers to have access to safe drinking water.

7 PROCESSES FOR DRINKING WATER Basically, the combination of following processes is used for municipal drinking water treatment worldwide, however selection of the treatment based on the source of the water. Pre-chlorination - for algae control and arresting any biological growth Aeration - along with pre-chlorination for removal of dissolved iron and manganese Coagulation - for flocculation Coagulant aids also known as polyelectrolytes - to improve coagulation and for thicker floc formation

8 Sedimentation - for solids separation, that is, removal of suspended solids trapped in the floc Filtration - for removal of carried over floc Disinfection - for killing bacteria There is no unique solution (selection of processes) for any type of water. Also, it is difficult to standardize the solution in the form of processes for water from different sources. Treatability studies for each source of water in different seasons need to be carried out to arrive at most appropriate processes.

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12 Water Treatment plant

13 Water Treatment plant

14 Flow Diagram of conventional surface water treatment plant ( filtration plant )

15 1. Coagulation Surface waters must be treated to remove turbidity, colour and bacteria. The objective of coagulation is to turn the small particles of colour, turbidity and bacteria into the larger floc through precipitation. Definition: a method to alter the colloids so that they will be able to approach and adhere to each other to form larger floc particles. A coagulant is a substance (chemical) that is added to water to accomplish coagulation.

16 COAGULANT PROPERTIES: i. Trivalent cation: colloids most commonly negative charge, hence cation is required to neutralize the charge. ii. Nontoxic iii. Insoluble in the natural ph range: the coagulant must be precipitate out of solution so that high concentrations of the ion are not left in the water Two most commonly coagulant are Al 3+ and Fe 3+ Aluminum can be purchased as dry or liquid alum Al2(SO4)3.14H2O. Iron as sulfate salt or chloride salt Check for the reaction occurs when the Al 3+ or Fe 3+ is added to a water.

17 When aluminum is added to a water containing alkalinity, the following reaction occurs: Such that each mole of alum added uses six moles of alkalinity and produces six moles of carbon dioxide The above reaction shifts the carbonate equilibrium and decreases the ph However, as long as sufficient alkalinity is present and CO 2 (g) is allowed to evolve, the ph is not drastically reduced and is generally not an operational problem.

18 When sufficient alkalinity is not present to neutralize the sulfuric acid production, the ph may be greatly reduced: Lime or sodium carbonate may be added to neutralize the acid Two important factors in coagulant addition are ph and dose The optimum dose and ph must be determined from laboratory tests

19 One of the most common methods to evaluate coagulation efficiency is to conduct jar tests. 1. Six beakers are filled with water and each is mixed and flocculated uniformly by a gang stirrer. 2. A test is often conducted by first dosing each jar with the same alum dose and varying the ph in each jar. 3. The test can be repeated in a second set of jars by holding the ph constant and varying the coagulant dose. 4. The water is mixed rapidly for 1 min to ensure complete dispersion of the chemicals and the mixed slowly for 15 to 20 min to aid in the formation of floc. 5. The water is allowed to settle for 30 min. 6. The water is tested using turbidity test for the settled water.

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21 2. Softening Hardness: define as the sum of all polyvalent cation and the common units of expression is mg/l as CaCO3 The term hardness is used to characterize a water that does not lather well. Causes a scum in a bath tub Leaves hard, white, scale on coffee pots, tea kettle and hot water heater. The failure to lather well and formation of scum on the bath tubs is the result of the reaction of calcium and magnesium with the soap. A common water treatment goal is to provide water with a hardness in the range of 75 to 120 mg/l as CaCO3. Chemical use are lime and soda

22 Removing hardness in water by addition of calcium hydroxide and sodium bicarbonate, Na 2 CO 3 Objective= precipitate calcium as CaCO 3 and magnesium as Mg(OH) 2. Ca 2+ + CO 2-3 CaCO 3 (1) Mg OH - Mg (OH) 2 (2) Precipitation of CaCO 3 require ph 10.3 and Mg (OH) 2 require ph 11.

23 Natural Process by which water is made hard

24 Summary of softening reactions

25 3. Flocculation The most important factor affecting particle-removal efficiency Objective: is to bring the particles into contact so that they will collide, stick together and grow to a size that will readily settle. Enough mixing is important throughout the process.

26 INTRODUCTION TO MIXING & FLOCCULATION Mixing (rapid mixing): - Process whereby chemicals are quickly and uniformly dispersed in the water. Precipitates are formed from the chemical reactions that take place in rapid mixing during coagulation and softening - Coagulation: form Al(OH) 3 or Fe(OH) 3 - Softening : form CaCO 3 & Mg(OH) 2

27 FLOCCULATION : - Contacting process of the precipitates formed from coagulation and softening with one and another - So that they can agglomerate and form flocs (larger particles) - Accomplished by slow and gentle mixing.

28 Flocculation is accomplished with: a)axial-flow IMPELLER

29 b) PADDLE FLOCCULATOR

30 c) BAFFLED CHAMBER

31 POWER REQUIREMENTS Power given to the liquid in a baffled tank by an impeller for FULLY TURBULENT FLOW: Note: N p (Impeller constant) of a specific impeller can be obtained from the manufacturer.

32 For radial-flow impeller: N p = 5.7 For axial-flow impeller: N p = 0.31

33 For paddle mixer:

34 For pneumatic mixing:

35 For static-mixing devices: Note: At normal temperature: of water = 9.81 kn/m 3

36 UPFLOW SOLIDS-CONTACT BASIN FOR MIXING, FLOCCULATION AND CLARIFICATION

37 UPFLOW SOLIDS CONTACT TANK. EFFLUENT WEIR ON RIGHT.

38 4. Sedimentation Particle that will settle within a reasonable period of time can be removed in a sedimentation basin (clarifier). The basin can be divided into 4 zones: inlet, settling, outlet and sludge storage

39 5. Filtration The water leaving the sedimentation tank still contains floc particles. The settled water turbidity is generally in the range from 1 to 10 NTU (typical is 2 NTU). In order to reduce the turbidity to less than 0.3 NTU, a filtration process is normally used. Definition: is a process for separating suspended or colloidal impurities from water by passage through a porous medium. Medium usually used are sand or coal.

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41 Granular media filtration systems

42 6. Disinfection Used in water treatment to reduce pathogens (disease-producing microorganisms) to an acceptable level. Not the same as sterilization, which implies the destruction of all living things. (No need for drinking water) It should have a fast rate of kill and should be persistent enough to prevent regrowth of organisms

43 Chlorine may be applied to water in gaseous form (Cl2) or as an ionized product of solids (Ca(OCl) 2 or NaOCl. The reactions in water are as follows: Cl 2 + H 2 O H + + HOCl Ca(OCl) 2 Ca OCL - NaOCL Na + OCl - HOCl H + + OCL - (hypochlorous acid) and (hypochlorite ion) The sum of HOCl and OCL -1 is called the free chlorine residual and is the primary disinfectant employed.

44 At low concentrations, chlorine probably kill microorganisms by penetrating the cell and reacting with the enzymes and protoplasms. At higher concentrations, oxidation of the cell wall will destroy the organism Factors affecting the process are: Form of chlorine ph Concentration Contact time Type of organism Temperature

45 Giardia lamblia Cryptosporodium

46 Chlorine disinfectant tablet Chlorine disinfectant unit

47 HYDROFLUOROSILICIC ACID DRUM AND PUMP METERING SYSTEM FOR FEEDING FLUORIDE INTO WATER.