8a-advanced waste water treatment
TRANSCRIPT
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Advanced Wastewater Treatment
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Removal Capacity of Conventional Treatment Processes
BOD & SS = 85 – 95 % RemovalCOD = 65% Removal
Remaining organic matter needs to be removed further
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Advanced Wastewater Treatment
Remove constituents not adequately removed by secondary treatmentNitrogen, phosphorus, other soluble organic and inorganic compoundsAccelerate plant growth, toxic, concentrate in food chain, increase chlorine demand, taste and odour problem
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Limit for Sewage Treatment Plants (> 10,000 pe) in the EU Countries
Effluent concentration Minimum elimination rate
Total nitrogen
Total phosphorus
15 mg/l for plants < 100,000 pe10 mg/l for plants >100,000 pe
2 mg/l for plants < 100,000 pe1 mg/l for plants >100,000 pe
70 – 80%
80%
Nitrogen level in treated effluent in EU Countries
Total N NH4-N Source
German - Ruhr new old
8.9*14
2.36.3
Bode and Klopp (2001)
Italy 2** 6+ Carucci et al (1999)
Denmark 8 2 Rindel (2002)
Netherland 9 - Van der Graaf (2001)
note: * inorganic N ** as NH4+ + NO3-N nitrate nitrogen
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Common Processes
Biological nutrient removalChemical coagulation and precipitation of phosphorusAmmonia strippingBreakpoint chlorinationFiltrationCarbon adsorptionIon exchangeMembrane separationOzonation
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Biological Nutrient Removal BNR
Integrated with conventional biological process used for BOD and TSS removalAnaerobic, anoxic and aerobic zones for phosphorus release and uptake and nitrification and denitrificationEffluent quality; BOD5/TSS/TP/TN = 5/5/1/5 mg/l
Package constructed wetlands = 1 mg/l P
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Enhanced Nitrogen Removal
High sludge ageLong detention timeBOD/TKN = not less than 3 for biological nitrification
Nitrified Effluent
Nitrifiers are slow growth microorganisms as compared with heterotropic bacteria (in carbonaceous oxidation)
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Nitrogen Removal - Nitrification
Organic Matter + O2 NH3 + Microorganisms + CO2
NH3 + 2O2
NO3- + H2O + Microorganisms
Organic Matter + NO3-
N2 + Microorganisms
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Bacteria
Nitrosomonas (the rate limiting bacteria) converts ammonia to nitrite and Nitrobacter converts nitrite to nitrate
NH4+ + 2O2 NO3
- + 2H+ + H2O
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Alkalinity
Nitrification consumes alkalinityNH3 + 2O2
NO3- + H+ + H2O + Microorganisms
Need buffering capacity; maintain pH at 6.5 – 8.0Abrupt drop in rate of nitrification beyond pH 6.5
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Denitrification
Anaerobic & Anoxic condition required 3 – 4 g methanol per gram of nitrate to be
removed Denitrifiers are sensitive to temperature variation pH 6-8 2 – 3 h retention time Minimum 1 – 2.5 d sludge age to produce
flocculating sludge
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How much methanol is consumed by oxidation of 30 mg/l NO3- ?
Methanol demand for oxidation of nitrate =Half reaction for methanol
- [ 1/6 CO2 + H+ + e- = 1/6CH3OH + 1/6H2O ]
Half reaction for nitrate
+ [ 1/5 NO3- + 6/5 H+ + e- = 1/10 N2 (g) + 3/5 H2O]
+
1/6CH3OH + 1/5 NO3- + 1/5 H+ 1/10N2 + 1/6 CO2 + 13/30 H2O
Molecular Wt NO3 = 48Molecular Wt CH3OH = 32
Amount of methanol consumed, M = 5.33/9.6 x 30 mg/l = 16.7 mg/l
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Nitrification & Denitrification
Carbonaceous Oxidation and Nitrification
(in the same tank)
Carbonaceous Oxidation
Nitrification
DenitrificationDenitrification
Add methanol, ethanol, acetate, molasses
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Biodenipho process
Process works using Anoxic-oxic cycle
Denitrification(Nitrate decrease)
Nitrification(Nitrate increase)
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Lynetten, Copenhagen 750,000 pe - Biodenipho
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Lynetten STP is Biodenipho (Nitrification-denitrification) that produce Total Nitrogen in effluent below 8 mg/l
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Capacity Lynetton STP - Biodenipho 750,000 PEFlowrate max Dry weather
m3/hrm3/hr
23,0007,083
Flowrate storm m3/hr 41,500
Waste Characteristic Raw effluent treated
BOD 150 15
(all in mg/l) COD 400 75
TSS 200
NH4-N 40 8 (TN)
TP 9 1.5
Retention time (hr) in cubic metres:
Primary settling tanks 2.7 19,200
BioP tanks 3.4 24,000
Aeration tanks 20.8 147,000
Final settling tanks 8.3 59,100
Thickeners 1.4 10,000
Digesters 22 days based on SRT 18,000
Biogas production 12,863 m3/d 0.7 m3/m3/day
Sludge incineration 7 tonnes/hr
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Case study:
http://www.princeton-indiana.com/wastewater/index.html
BNR at Princeton, Indiana USA
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Phosphorus Removal
Biological uptake by sludge Chemical precipitation = lime and iron &
aluminium salts Luxury uptake = Sludge being treated
anaerobically before returning the phosphorus deficient sludge to the reaction tank. The sludge would then assimilate greater quantity of phosphate than usual.
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Enhanced Phosphorus Uptake
CAS = 1 – 2 % PEnhanced system = 3% PDetention time = 0.5 – 2 h
Anaerobic Aerobic
Produce short chain fatty acid & P released
Acid utilised & P taken up
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Layout
Anoxic Aerobic
AerobicAnoxicAnaerobic
Nitrified recycle
Anoxic recycle
Nitrified recycle
Anaerobic > Anoxic > Aerobic system ensures denitrification & P removal
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Membrane Separation
What is a membraneWhat drives the separationPore sizes, RO, UF, MF
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Schematic Diagram of Membrane Separation
MEMBRANE MEMBRANE
MEMBRANE
Water
Salt Macromolecules
Selected Macromolecules
ULTRAFILTRATION
MICROFILTRATION
REVERSEOSMOSIS
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Membrane Filtration Spectrum
0.001 0.01 0.1 1 10 100 1000m (log scale)
Reverse Osmosis
Microfiltration
Particle FiltrationUltrafiltration
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Different Blocking Mechanisms
Complete blocking Intermediate blocking
Cake filtration Standard blocking
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Crossflow and Dead-end Filtration
MEMBRANE
Feed Reject
FiltrateFilter Cake
Flux
Time
MEMBRANE
Feed
FiltrateFilter Cake
Flux
Time
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Solute Rejection & Breakthrough
Functional groups present in the membraneNature of membrane surfaceSize of solute moleculeDissociation of solute molecule (pH dependent)Operation time
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Ideal Membrane
High flux rateTolerant to chlorineResistant to biological attackResistant to foulingMechanically strongInexpensiveChemically stableExcellent filtrate quality
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Ozonation
Ozone is the triatomic form of oxygen (composed of three oxygen atoms), O3. Under normal conditions ozone is unstable and quickly decomposed to the more stable gaseous oxygen, O2. Because ozone is unstable and cannot be stored successfully, it must be generated at the point of application.
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Ozonation
Ozone can be generated by passing oxygen, or air containing oxygen, through an area having an electrical discharge or spark. (A clean smell in the air after a thunder and lightning storm was most likely caused by ozone formed by lightning bolts passing through the atmosphere).
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Ozonation
Typical ozonators have two large area metal electrodes separated by a dielectric and an air gap. An alternating electric current is applied to the electrodes creating an electrical discharge. At the same time air or oxygen is passed through the air gap. As the air or oxygen flows through the air gap, and the electrical discharge, a portion of the oxygen is converted to ozone. The dielectric is necessary to spread the electric discharge over the entire electrode area and avoid producing an intensive single arc.
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Ozonation
The concentration of the ozone leaving the ozonator is approximately 1 to 2% by weightAs with chlorination, the effectiveness of disinfection using ozone is depended on the concentration of the disinfectant, thorough mixing and contact time. To satisfy the mixing and contact time requirements, three general types of contactors are usually used: (1) packed bed, (2) sparged column, and (3) sparged column with mixing.
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Advantages of Ozonation 1. eliminates odours 2. reduces oxygen demanding matter, turbidity and
surfactants 3. removes most colours, phenolics and cyanides 4. increases dissolved oxygen 5. production of no significant toxic side products6. increases suspended solids reduction
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Disadvantages of Ozonation
1. high capital cost 2. high electric consumption3. highly corrosive, especially with steel or iron
and even oxidizes Neoprene
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Disinfection Using Ozone
Ozone is thirteen times more soluble in water than oxygen. When first introduced into wastewater, very little disinfection occurs. The ozone is rapidly consumed, satisfying the ozone demand of inorganic salts and organic matter dissolved in the wastewater. The disinfecting properties of the ozone come into play only after the ozone demand is satisfied. When the demand is satisfied, research studies indicate, ozone brings about disinfection 3100 times faster than chlorine.
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Disinfection Using Ozone
It has also been found that disinfection occurs within contact times of 3 to 8 seconds. Typical ozone dosages needed to reach the disinfection stage vary with the quality of the effluent. Dosages between 5 to 15 mg/L are commonly cited for disinfection of secondary wastewater effluents. Ozone also exhibits excellent virocidal properties at these dosages but with longer contact time of about 5 minutes needed. It has also been found that any residual ozone in the effluent of the contactor disappears in a matter of seconds outside the contactor.
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Other Uses of Ozone in Wastewater Treatment
Ozone has the ability to remove solids from wastewater by oxidation and physical floatation. A foam develops when wastewater is ozonated. It has been found that this foam traps a significant amount of solids and nutrient material such as phosphates and nitrates.
pH has been found to increase very slightly because of ozonation. This is probably the affect of carbon dioxide being driven out of the solution by the gas feed in the ozone contactor.
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Other Uses of Ozone in Wastewater Treatment
Colour and turbidity are reduced by addition of ozone. This is brought about by chemical oxidation of the substance causing the colour or turbidity.
Some minor nitrification occurs, but not at levels high enough to consider ozonation as an effective nitrification process.
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Safety of Ozone
The Maximum Allowable Concentration (MAC) of ozone in air, as established by the American Council of Governmental Industrial Hygienists is 0.1 ppm by volume for continuous human exposure. The threshold odor of ozone is 0.01 ppm. This means a person working near an ozone-handling area should be able to detect the presence of ozone at levels far below the MAC.
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Other Uses of Ozone in Wastewater Treatment
The odor of ozone has been described as similar to that of cloves, new mown hay, nitric acid, etc., depending on the concentration. Concentrations greater than 1 ppm are extremely pungent and are considered unsafe for prolonged human exposure, and therefore should be avoided.