Dissolved Oxygen (DO):

Transcription

1 Section VIII Water Treatment- Introduction Dissolved Oxygen (DO): - The source of D.O in water is photosynthesis and aeration - It is one of important parameters to measure the water quality. - It gives pleasure taste to water - As the temp D.O - If the D.O concentration decreases to less than 4mg/l all fish die - If the D.O concentration is less than 2 mg/l all organism dies and the water is called septic water - Best D.O concentration is between 8-10 ppm. Optimum is 9ppm. - The maximum naturally accrued is 14 mg/l.

2 Section VIII Water Treatment- Introduction Biochemical Oxygen demand (BOD) : - is the quantity of oxygen that is used by microorganism to stabilize the wastewater, Usually measured after 5 days. - a BOD test can be used to measure waste loadings to treatment plants, plant efficiency and the effects of a discharge on a receiving stream, and to control the plant process. - It is indicator for the required aeration amount. - The main equation describes the process is: DO + organic matter CO 2 + biological growth - Drinking water usually has a BOD of less than 1 mg/l - Ordinary domestic sewage may have a BOD of 200 mg/l. -Any effluent to be discharged into natural bodies of water should have BOD less than 30 mg/l. Test Summary: 1- The sample is filled in an airtight bottle and incubated at 20 o C for 5 days. 2- The dissolved oxygen (DO) content of the sample is determined before and after five days of incubation at 20 C 3- and the BOD is calculated from the difference between initial and final DO. The initial DO is determined shortly after the dilution is made; all oxygen uptake occurring after this measurement is included in the BOD measurement

3 Section VIII Water Treatment- Introduction Calculations: BOD 5 mg/l = (Initial DO - DO 5 ) x Dilution Factor Bottle Volume (300 ml) Dilution Factor = sample Volume BOD At any time: BOD t = L(1-10 -kt ). Or BOD t = L(1-exp -kt ). Where: BOD t : BOD at any time L: ultimate bio-oxygen demand k: oxygen decay constant, [day -1 ] t: time, [days] However: The BOD reaction rate constant (K) is dependent on the following: 1. The nature of the waste: - degradability of the organic matter. For example, Simple sugar and starches are rapidly degraded 2. The ability of the organisms in the system to utilize the waste 3. The temperature - the water temperature may vary from place to place for the same river; hence, the BOD rate constant is adjusted to the temperature of receiving water using following relationship: KT = K20 θ (T-20) Where T = temperature of interest, o C KT = BOD rate constant at the temperature of interest, day -1 K20 = BOD rate constant determined at 20 o C, day -1 θ = temperature coefficient. This has a value of in general and for higher temperature greater than 20 o C

4 Section VIII Water Treatment- Introduction BOD incubator Example: Determine ultimate BOD for a wastewater having 5 day BOD at 20 o C as 160 mg/l. Assume reaction rate constant as 0.23 per day (base exp). Solution BOD5 = Lo ( 1 exp -k.t ) 160 = Lo (1 exp -5 x 0.23 ) Therefore, Lo = mg/l Example: A BOD test is done by pipiting 5 ml of waste water into 300 ml testing bottle. If the initial DO was 8.4 mg/l and the DO after 5-days of incubation at 20 o C was 3.7mg/l, calculate the BOD and estimate the 20-days BOD value assuming the reaction decay constant k = 0.1 day -1. (USE base 10) Solution: Dilution factor = 300/5 = 60 BOD 5 = ( )*60 = 282 mg/l.* b- to determine the BOD after 20 days: from (*) calculate L

5 Section VIII Water Treatment- Introduction 282 = L ( ) L = 412 mg/l Thus BOD 20 = 412 ( ) BOD 20 = mg/l Try the solution using Base (Exp ) but in this case the K = 0.23 day -1 Example: The wastewater is being discharged into a river that has a temperature of 15 o C. The BOD rate constant determined in the laboratory for this mixed water is 0.12 per day. What fraction of maximum oxygen consumption will occur in first four days? (Base Exp) Solution Determine the BOD rate constant at the river water temperature: K15 = K20 (1.056) (T-20) = 0.12 (1.056) (15-20) = per day... note 1: per day mean day note 2: as the temp decrease the reaction rate (K) decreases Using this value of K to find the fraction of maximum oxygen consumption in four days: BOD4 = Lo (1 e x4 ) Therefore, BOD4 / Lo = 0.305

6 Section VIII Water Treatment- Introduction Chemical oxygen demand (COD): -Measure the amount of organic compounds in water that can be oxidized by strong oxidant like mixture of sulfuric and chromic acids. -Most applications of COD determine the amount of organic pollutants found in surface water. - It indicates of the strength (degree of pollution) of industrial WW that are not biodegradable - the test is faster than BOD test. PH - Very important parameter that affects treatment processes, especially coagulation, anddisinfection - any unusual change may reflect a major event H 2 O H + + OH -1, A- k w = [H + ] [OH -1 ];;;; k w = B- PH = -log[h + ] [H + ] = 10 -PH

7 Section VIII Water Treatment- Introduction Conventional water treatment: is a combination of coagulation, sedimentation, filtration and disinfection process. 1- Primary treatment involves : pumping, screening and grit removal - Screening: aims to remove large objects, such as stones or sticks, that could plug lines or block tank inlets. - grit chamber- slows down the flow to allow grit to fall out - sedimentation tank (settling tank or clarifier)- settleable solids settle out and are pumped away, while oils float to the top and are skimmed off 2- Secondary treatment : can remove up to 90% of the organic matter in wastewater by using biological treatment processes. It performed by attached or suspended growth processes. A- Activated Sludge: is the most common option with aeration and agitation, then allows solids to settle out. - Bacteria-containing activated sludge is continually recirculated back to the aeration basin to increase the rate of organic decomposition. B- Trickling Filter: is bed of coarse media (often stones or plastic) 3-10 ft. deep. - Wastewater is sprayed into the air (aeration), then allowed to trickle through the media. - Microorganisms, attached to and growing on the media, break down organic material in thewastewater. - Trickling filters drain at the bottom C- Lagoons: These are slow, cheap, and relatively inefficient, but can be used for various types of waste water. - They rely on the interaction of sunlight, algae, microorganisms, and oxygen (sometimes aerated).

8 Section VIII Water Treatment- Introduction 3- Tertiary treatment as ultra filtration Aerobic conversion of organic matter: the general simplified equation is C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + energy In reality there are more complex equations. Un-aerobic conversion

9 Section VIII Water Treatment- Introduction C 6 H 12 O 6 3CH 4 + 3CO 2 + energy + bio mass It is two step reaction (i) Acidogenic phase where the organic matter converted to organic acids (ii) Methanogenic phase where organic acids are concerted into methane and carbon dioxide. The typical function of each water treatment units can be summarized as the following: Treatment unit Screening Sedimentation Coagulation Filtration Softening Chemical treatment Aeration Disinfection Function (removal) Floating matter Suspended matter Small suspended matter, part of colloid and bacteria Remaining colloidal, dissolved matter and bacteria Hardness Iron, manganese, heavy metals..etc Color, Odor and taste Pathogenic bacteria, organic matter The type of treatment required for various water sources are as the following: Water source Ground and spring water fairly free from contamination Ground water with chemical, minerals and gasses Lake, surface water reservoirs with low amount of pollution Other surface water such as river, canals and impounded reservoirs with considerable amount of pollution Recommended treatment No treatment or chlorination Aeration, coagulation (if necessary) filtration and disinfection Disinfection Complete treatment

10 Aeration: - It removes odor, color and taste due to reducing the concentration of volatile gasses like H 2 S (hydrogen sulfide), and algae and other related organisms. - It oxidizes iron and manganese, increases [DO], removes CO 2, CH 4 and other flammable gasses, reduces corrosion. - Its work principle is based on the fact that the atmospheric oxygen will replace the volatile gasses in water, while it will escape into atmosphere. - The replacement will continue till reaching the equilibrium depending on the partial pressure of each specific gasses. Type of aeration: 1- Gravity aerators: In this type water is allowed to fall by gravity, such that large area of water will exposed to atmosphere. 2- Fountain (Spray) aerators A special nozzle is used to produce a fine spray. Each nozzle is 2.5-4cm diameter. Discharging about L/h Nozzle spacing should be such that each meter cube of water (m 3 ) has aerator area of m 2 for one hour. 3- Injection or Diffused aerator : - It consists of tank with perforated pipes, tube or diffuser plates, fixed at the bottom to release fine air bubbles from compressor unit. - The tank depth is 3-4m and the tank width is 1.5 times of tank depth. - If the depth is more the diffusers must be placed at 3-4m below the water surface

11 - Aeration time is min and litter of air is required per litter of water. - Increase the diffuser depth increases the aeration rate and efficiency - Decrease the orifice size increases the aeration rate and efficiency 4- Mechanical aerator: Mixing paddle as in flocculation are used.

12 The puddle may be submerged or at surface.

13 Settling (Sedimentation) it is a type of solid liquid separation process in which a suspension is separated into two phases: a- Clarified supernatant leaving the top of the sedimentation tank (overflow). b- Concentrated sludge leaving the bottom of the sedimentation tank (underflow). Purpose of Settling To remove coarse dispersed phase. To remove coagulated and flocculated impurities. To remove precipitated impurities after chemical treatment. To settle the sludge (biomass) after activated sludge process / tricking filters. Principle of Settling Suspended solids present in water having specific gravity greater than that of water (1) tend to settle down by gravity as soon as the turbulence is retarded by offering storage. Basin in which the flow is retarded is called settling tank. Theoretical average time for which the water is detained in the settling tank is called the detention period. Sedimentation tanks are circular or rectangular. Sedimentation types: itexist, based on characteristics of particlesa. 1- Discrete or type 1 settlingparticleswhose size, shape, and specific gravitydo not change over time(has a density (ρ) = kg/m 3 ). 2- Flocculating particles or type 2 settling; particles that change size, shape and perhaps specific gravity over time(has a density (ρ) = kg/m 3 ). 3- Hindered settling or type 3 settling: blanket sedimentation occrues at Lime softening sedimentation and Sludge thickeners in water treatment 4- Compressed settling or type 4 settlingoccruessludge thickeners in water treatment

14 Applications in Wastewater Treatment: 1. grit removal 2. suspended solids removal in primary clarifier 3. biological floc removal in activated sludge Place of Sedimentation in various WW plant: Or

15 For treating hard water to removes flocculated solids. The sedimentation tank comes after the flocculation tank.

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19 Sedimentation Theory: If a particle is suspended in water, it initially has 2 forces acting upon it. The forces of gravity F g = mg... but m = ρ. V p Thus F g = g* ρ p * V p The buoyant force quantified by Archimedes F b = Thus F b = g* ρ w * V p Once motion has been initiated, a third force is created due to viscous friction that is drag force F d F d = C D *A p * ρ w * v s 2 2

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21 CD = 24/Re For laminar Re < 1 CD = 24/Re + 3/ Re For transitional 10 4 > Re > 1 CD = 0.4 ForturbulentRe > 10 4

22 Sedimentation process: It is a physical treatment that allows for particles having specific gravity higher than water to settle under its own weight. Factor affecting the sedimentation: 1- Water prosperities: b- Sed. Decreases when water viscosity increase c- Sed. Decreases when water density increase d- Sed. Increase when water temp. Increase. 2- Suspended solids concentration, size and shape 3- Detention time: sed. Efficiency increase by increasing the water retention at the tank, but after time the precipitation is decreased sharply thus the time should be determined correctly. (2-4hr) 4- Flow velocity: decrease the flow velocity increase the sed. The max. Allowable velocity in the tank is 0.3 m/sec 5- Tank shape: circular tanks are more efficient for sedimentation Ideal sedimentation tank: 1- The flow is laminar 2- There are no dead zones 3- The horizontal velocity is constant 4- Good arrangement of inlet and outlet weirs.

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