Wastewater
-
Upload
mth-corporation -
Category
Engineering
-
view
583 -
download
7
description
Transcript of Wastewater
MID TERM TEST
1 st MARCH 2012
1
Wastewater Wastewater Treatment SystemTreatment System
Ms Noor Rosyidah Binti Sajuni
School of Engineering
3
Content
• Regulation and standard for wastewater
• Principle of wastewater treatment in industrial and domestic sewage,
• Primary treatment methods,
• Secondary treatment and
• Tertiary treatment methods
4
Sources of Wastewater
5
Wastewater Collection System
• Sewers are underground conduits for conveying wastewaters by gravity flow from urban areas to points of disposal.
• Storm waters enter a storm drainage system through inlets located in street gutters or depressed area that collect natural drainage.
• Sanitary sewer transport domestic and industrial wastewaters by gravity flow to treatment facilities.
• Sanitary sewers are placed at sufficient depth to prevent freezing and to receive wastewater from basement.
6
7
Principle of wastewater treatment in industrial and domestic sewage
• Municipal wastewater treatment systems are designed to serve the needs of towns and cities
• The steps in treatment:
– Collection
– Preliminary treatment
– Primary treatment
– Secondary treatment
– Advanced treatment
– Disposal of residue
8
Conventional Wastewater Treatment Process
Pretreatment involves:Screening Grit RemovalOil separationFlow equalization
Disinfection can use:Chlorine compoundsBromine ChlorideOzoneUV Radiation
Chemical Treatment is used in conjunction with the physical and chemical processes:Chemical precipitation Adsorption
Sludge Treatment and Disposal involves:grinding, degritting, blending, thickening, stabilization, conditioning, disinfection, dewatering, heat drying, thermal reduction, ultimate disposal
9
Considerations in Plant Design
• Effluent quality: normally govern by local government authority. Example:
– Environmental Quality (Sewage & Industrial Effluents) Regulation 1979
• Specify the requirement for new sources of discharges requiring written permission of the Director General of Environment.
• It regulates the acceptable conditions of discharge into inland waters by specifying parameter limit of effluent.
• There are 2 standard: Standard A & Standard B
• Standard A: discharge that into any inland water within catchment areas.
• Standard B: discharge into any other inland water.
10
11
• Design Loading.
– Wastewater quantity used for sizing basins and equipment varies with the nature of the equipment and whether the process is hydraulic or loading limited.
– The selection of the flow and load values must take into account hourly, daily, and seasonal variations.
– Flow and load values are typically expressed in terms of peak hour, max. day, min. day, max. average month; and annual average.
– Loading may be lbm of BOD, SS, concentration.
– Flow values are important during the sizing pumps, pipes, and hydraulically limited equipment.
– The loading value are important in sizing aeration and digestion equipment to meet peak demands.
12
Typical Design Criteria for Treatment Processes
Process Loading
Flow measurement Peak hrly flow
Bar Screen Peak hrly flow
Pump Peak hrly flow
Min. hrly flow
Grit chamber Min. hrly flow
Max. monthly flow
Primary settling Max. monthly flow
Biological treatment
Max. monthly BOD loading
Peak hrly BOD loading
Final settling Max. monthly flow
Disinfection Peak hrly flow
Thickening Max. daily sludge flow, max. solid loading
Digestion Max. monthly volatile solid load, max. monthly sludge flow
Dewatering Max. sludge flow, max. solid loading
13
On-site Disposal Systems
• About 85-90% of on-site wastewater disposal system are conventional septic systems.
• A conventional septic system consists of 3 parts: septic tank, a distribution box and absorption field.
14
• Septic tank: to remove large particles and grease, which would otherwise clog the tile field. Heavy solids settle to the bottom and undergo decompositions. Grease float on the surface and trapped.
– The size of the tank should able to accommodate a holding time for water in a septic tank of at least 24 hrs. For individual homes : 3m3 tank min., 4m3 for 3 bedrooms house; 5m3 for 4 bedrooms house, and 6m3 for 5 bedrooms house.
– Bacteria action in the tank helps to degrade the organic matter in the wastewater.
• Distribution box: to distribute the septic tank effluent throughout the absorption field.
• Absorption field: Consists of a series of trenches that contain perforated PVC pipes that are about 10cm in diameter.
– The pipes are placed over 15cm deep layer of drainrock and then buried with an additional layer of drainrock.
• Most septic system will fail eventually. The normal lifetime of an absorption field is 20-30 years.
15
• Water-tight sewer lines• Septic tank• Disposal system
– Subsurface drain field• Absorption trenches• Infiltration chambers
– ET/Absorption trenches– Aerobic system with sprinkler– Total retention lagoons– Pre-approved alternative disposal systems
• A wastewater system is made up of several connected components. Water-tight sewer lines convey wastewater from the house to the septic tank and from the tank to the disposal system.
• All systems will have a septic tank to digest organic solids. The effluent, or liquid waste discharged from the tank goes to some type of disposal system.
• The most common disposal system is a subsurface drain field. Traditionally this has been a perforated pipe buried in a gravel-filled trench. Recently polyethylene infiltration chambers have been used in place of the traditional absorption trench.
• In the case of sites where soils are not suitable for absorption trenches (usually because of poor infiltration rates, but also because of shallow water tables or rock outcrops) other types of disposal systems may be required.
16
17
Unit Operations of Pretreatment
• Pretreatment have little effect in reducing BOD5.• Physical unit operations are normally the 1st treatment methods to be used. • Screening is generally as the 1st unit operation in the wastewater treatment
plant.– A screen is a device with openings, generally uniform size, that is
used to retain solids found in the influent wastewater to the plant or combined wastewater collection systems.
– The principle role of screening is to remove coarse materials from the flow stream that could:i. Damage subsequent equipmentii. Reduce overall treatment process reliability and effectivenessiii. Contaminate waterways
– Sometimes, fine screens are used in place of or following coarse screens where greater removals of solid are required to:i. Protect process equipmentii. Eliminate material that may inhibit the beneficial reuse of
biosolids
18
Classification of Screens
Screening
Coarse Screen
Microscreen Fine screens
6-150 mm< 0.5 μm
< 6 mm
Hand cleaned
Mechanically cleaned
Static wedgewire
drum Step
Chain-driven
Reciprocating rake
CatenaryContinuous
belt
19
• Coarse Screen (Bar Racks)
– To remove large objects that would damage or foul pumps, valves, and other mechanical equipment.
– Hand-Cleaned Coarse Screens
• Used frequently ahead of pumps.
– Mechanically cleaned bar screens
20
21
Design Factor for Coarse Screens• Design consideration
– Location
– Approach velocity
– Clear opening between bars and mesh size
– Headloss through screens
– Screen handling, processing and disposal
– Control
22
Fine Screens
23
• Grit Chambers
– Grit chambers are basin to remove the inorganic particles to prevent damage to the pumps, and to prevent their accumulation in sludge digestors.
24
• Equalization
– Flow equalization is not a treatment process but a technique that can be used to improved the effectiveness of both secondary and advance wastewater treatment processes.
– Wastewater does not flow into a municipal wastewater treatment plant at a constant rate.
– The purpose of flow equalization is to dampen the variation so that the wastewater can be treated at a nearly constant flow rate.
– For existing plant, it can improve the performance.
– For new plant, it can reduce the size and cost of the treatment unit.
25
Primary Treatment• With the screening completed and the grit removed, the wastewater still
contains light organic suspended solids.
• Some of these can be removed from sewage by gravity in a sedimentation tank. The mass of settled solids is called raw sludge.
• Sludge can be removed from sedimentation tank by mechanical scrapers and pumps.
26
Sedimentation
• Sedimentation or clarification is the removal of particulate matter, chemical floc, and precipitates from suspension through gravity settling.
• The common criteria for sizing settling basins are detention time, overflow rate, weir loading, and with rectangular tanks, horizontal velocity.
– Detention time, expressed in hour as:
– Overflow rate (surface loading):
• There are 2 types of sedimentation tank: circular and rectangular tanks
hourtdaymflowdailyaverageQmvolumebaVwhereQ
Vt ),/(),(sin 33
areasurfaceAdaymmVwhereA
QV oo ,/ 23
27
Type of Sedimentation
• Type I: Discrete particle settling - Particles settle individually without interaction with neighbouring particles.
• Type II: Flocculent Particles – Flocculation causes the particles to increase in mass and settle at a faster rate.
• Type III: Hindered or Zone settling –The mass of particles tends to settle as a unit with individual particles remaining in fixed positions with respect to each other.
• Type IV: Compression – The concentration of particles is so high that sedimentation can only occur through compaction of the structure.
28
Comparison of Type I and II sedimentation
29
Coagulant
2422332342 3146)()(2614)( SOOHCOsOHAlHCOOHSOAl
Coagulation is the destabilization of colloids by addition of chemicals that neutralizes the charges of the colloids. The chemicals are known as coagulants, usually higher valence cationic salt (Al3+, Fe3+ etc.) Flocculation is the agglomeration of destabilized particles into a large size particles known as flocs which can be effectively removed by sedimentation or floatation. The addition of another reagent called flocculant may promote the formation of the floc.
OHSOHsOHAlOHSOAl 24232342 83)()(214)(
ClHsOHFeOHFeCl
OHClCOsOHFeHCOOHFeCl
3)()(3
733)()(37
323
223323
30
Biological Treatment
• Optimize primary treatment efficiency. BOD removed by primary treatment requires less energy than BOD removed by biological treatment, also surplus sludge should not accumulate in the primary clarifier to avoid dissolving and carry-over to the secondary process.
• Control amount of aeration to avoid excessive dissolved oxygen.
• Improve aeration system efficiency. For example, fine-bubble diffusers can increase oxygen transfer efficiency in comparison to coarse-bubble diffusers.
Types:
Aerobic ProcessesAnoxic ProcessesAnaerobic ProcessesCombined Aerobic-Anoxic-Anaerobic ProcessesPond Processes
Attached GrowthSuspended Growth Combined Systems
Aerobic MaturationFacultativeAnaerobic
31
• Air activated sludge is an aerobic process in which bacteria consume organic matter, nitrogen and oxygen from the wastewater and grow new bacteria. The bacteria are suspended in the aeration tank by the mixing action of the air blown into the wastewater. There are many derivations of the activated sludge process, several of which are described in this section.
• High purity oxygen activated sludge is an aerobic process very similar to air Activated sludge except that pure oxygen rather than air is injected into the wastewater.
• Aerated pond/lagoon is an aerobic process very similar to air activated sludge.
Mechanical aerators are generally used to either inject air into the wastewater or to cause violent agitation of the wastewater and air in order to achieve oxygen transfer to the wastewater. As in air activated sludge, the bacteria grow while suspended in the wastewater.
32
• Trickling filter is a fixed film aerobic process. A tank containing media with a high surface to volume ratio is constructed. Wastewater is discharged at the top of the tank and percolates (trickles) down the media. Bacteria grow on the media utilizing organic matter and nitrogen from the wastewater.
• Rotating biological contactor (RBC) is a fixed film aerobic process similar to the trickling filter process except that the media is supported horizontally across a tank of wastewater. The media upon which the bacteria grow is continuously rotated so that it is alternately in the wastewater and the air.
• Oxidation ditch is an aerobic process similar to the activated sludge process. Physically, however, an oxidation ditch is ring-shaped and is equipped with mechanical aeration devices.
33
Major Aerobic Biological Processes
Type of Growth
Common Name Use
Suspended Growth
Activated Sludge (AS) Carbonaceous BOD removal (nitrification)
Aerated Lagoons Carbonaceous BOD removal (nitrification)
Attached Growth
Trickling Filters Carbonaceous BOD removal. nitrification
Roughing Filters (trickling filters with high hydraulic loading rates)
Carbonaceous BOD removal
Rotating Biological Contactors
Carbonaceous BOD removal (nitrification)
Packed-bed reactors Carbonaceous BOD removal (nitrification)
Combined Suspended & Attached Growth
Activated Biofilter ProcessTrickling filter-solids contact processBiofilter-AS processSeries trickling filter-AS process
Carbonaceous BOD removal (nitrification)
34
Activated Sludge Process• The aeration tank contains a suspension of the wastewater and microorganisms, the
mixed liquor. The liquor is mixed by aeration devices (supplying also oxygen)• A portion of the biological sludge separated from the secondary effluent by
sedimentation is recycled to the aeration tank• Types of AS Systems: Conventional, Complete-Mix, Sequencing Batch Reactor,
Extended Aeration, Deep Tank, Deep Shaft
35
Advantages/Disadvantages
Advantages
• Flexible, can adapt to minor pH, organic and temperature changes
• Small area required
• Degree of nitrification is controllable
• Relatively minor odor problems
Disadvantages
• High operating costs (skilled labor, electricity, etc.)
• Generates solids requiring sludge disposal
• Some process alternatives are sensitive to shock loads and metallic or other poisons
• Requires continuous air supply
36
Trickling Filters
• The trickling filter or biofilter consists of a bed of permeable medium of either rock or plastic
• Microorganisms become attached to the media and form a biological layer or fixed film. Organic matter in the wastewater diffuses into the film, where it is metabolized. Periodically, portions of the film slough off the media
37
Advantages/Disadvantages
Advantages
• Good quality (80-90% BOD5 removal) for 2-stage efficiency could reach 95%
• Moderate operating costs (lower than activated sludge)
• Withstands shock loads better than other biological processes
Disadvantages
• High capital costs
• Clogging of distributors or beds
• Snail, mosquito and insect problems
38
Rotating Biological Contactors• It consists of a series of circular disks of polystyrene or polyvinyl
chloride that are submerged in wastewater and rotated slowly through it
• The disk rotation alternately contacts the biomass with the organic material and then with atmosphere for adsorption of oxygen
• Excess solids are removed by shearing forces created by the rotation mechanism
39
Advantages/Disadvantages
Advantages
• Short contact periods
• Handles a wide range of flows
• Easily separates biomass from waste stream
• Low operating costs
• Short retention time
• Low sludge production
• Excellent process control
Disadvantages
• Need for covering units installed in cold climate to protect against freezing
• Shaft bearings and mechanical drive units require frequent maintenance
40
Major Anaerobic Biological Processes
Type of Growth
Common Name Use
Suspended Growth
Anaerobic Contact Process Carbonaceous BOD removal
Upflow Anaerobic Sludge-Blanket (UASB)
Carbonaceous BOD removal
Attached Growth
Anaerobic Filter Process Carbonaceous BOD removal, waste stabilization (denitrification)
Expanded Bed Carbonaceous BOD removal, waste stabilization
41
Anaerobic Contact Process
• Untreated wastewater is mixed with recycled sludge solids and then digested in a sealed reactor
• The mixture is separated in a clarifier
• The supernatant is discharged as effluent, and settled sludge is recycled
Advantages/DisadvantagesAdvantages
• Methane recovery
• Small area required
• Volatile solids destruction
Disadvantages
• Heat required• Effluent in reduced chemical form
requires further treatment• Requires skilled operation• Sludge to be disposed off is
minimal
42
Upflow Anaerobic Sludge Blanket• Wastewater flows upward through a sludge blanket composed of biological
granules that decompose organic matter• Some of the generated gas attaches to granules that rise and strike degassing
baffles releasing the gas• Free gas is collected by special domes• The effluent passes into a settling chamber
43
Advantages/Disadvantages
Advantages
• Low energy demand
• Low land requirement
• Low sludge production
• Less expensive than other anaerobic processes
• High organic removal eficiency
Disadvantages
• Long start-up period• Requires sufficient amount of
granular seed sludge for faster start-up
• Significant wash out of sludge during initial phase of process
• Lower gas yield than other anaerobic processes
44
Major Anoxic and Combined Biological Processes
Type of Process
Type of Growth
Common Name Use
Anoxic Suspended Growth
Suspended Growth Denitrification
Denitrification
Attached Growth
Fixed-film Denitrification
Denitrification
Combined Aerobic, Anoxic, and anaerobic Processes
Suspended Growth
Single- or multi-stage processes, various proprietary processes
Carbonaceous BOD removal, nitrification, denitrification, phosphorus removal
Attached Growth
Single- or multi-stage processes
Carbonaceous BOD removal, nitrification, denitrification, phosphorus removal
45
Pond Treatment Processes
Common Name Comments Use
Aerobic Stabilization Ponds
Treatment with aerobic bacteria; oxygen is supplied by algal photosynthesis and natural surface re-aeration; depth of 0.15 to 1.5 m
Carbonaceous BOD removal
Maturation (tertiary) Ponds
Use aerobic treatment; applied loadings are low to preserve aerobic conditions
Secondary effluent polishing and seasonal nitrification
Facultative Ponds
Treatment with aerobic, anaerobic and facultative bacteria; the pond has 3 zones: a surface aerobic zone, a bottom anaerobic zone, and an intermediate zone partly aerobic-anaerobic
Carbonaceous BOD removal
Anaerobic Ponds Treatment with anaerobic bacteria; depths of up to 9.1 m to conserve anaerobic conditions
Carbonaceous BOD removal (waste stabilization)
46
Suspended Growth Separate Stage Nitrification
Single State Nitrification
47
Attached Growth
Attached Growth Nitrification following Act. Sludge
48
Biological Denitrification
• A modification of aerobic pathways (no oxygen)
– Same bacteria that consume carbon material aerobically
• Denitrifying bacteria obtain energy from the conversion of NO3- to N2 gas, but
require a carbon source
NO3- + CH3OH + H2CO3 C5H7O2N + N2 + H2O + HCO3
-
• Need low (no) oxygen (< 1 mg/L)
• Need carbon source (BOD in Wastewater)
• Neutral pH (pH 7)
• Conc of nitrate
49
Phosphorus Removal
• Chemical Precipitation
– Calcium (lime) addition at high pH (>10)
• Reacts with alkalinity
– Alum (Aluminum Sulfate) precipitation
– Iron precipitation
50
Sewage Treatment
Tertiary Treatment (Physicochemical Process)• Precipitation
• Filtration
• Chlorination
• Treated water is discharged to waterways
• Used for irrigation
• Recycled into drinking water
expensive process, sharply reduces inorganic nutrients (PO4, NO3)
51