Ch5 Wastewatertreatment II
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Transcript of Ch5 Wastewatertreatment II
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Wastewater treatment - II
Sudha Goel, Ph.D.
Assistant Professor (Environmental Engineering)
Department of Civil Engineering, IIT Kharagpur
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Suspended growth, mixed flow reactor or plug flow reactor for BOD removal
Activated sludge process (ASP)
Aeration tank to maximize DO levels in water and for biological growth Hydraulic detention time (HRT or ) = 3 to 8 hours Mean cell residence time (MCRT or c) = 4 to 30 d
Settling tank or clarifier for solids (biological particles) to settle Recycling of solids from clarifier to aeration tank to ensure a constant
bacterial population in aeration tank Recycle ratios from 0.25 to 5.0
Remaining solids are taken for sludge handling and disposal
Most popular for domestic wastewater treatment
CSTR type:
Advantages Dilution of wastewater; can withstand toxic shock, or shock loads
and flow fluctuations Disadvantages
Requires large land area High energy requirements for aeration
Wastewater treatment: ASP
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Activated sludge process
Brock 5ed
Aerobic oxidation of organicsEnd products: carbon dioxide, water and biomassAeration basin: air or oxygen is provided and biomass increasesClarifier: settling of biomass by natural flocculation
CSTR
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Aeration basin
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Aeration in ASP Oxygen utilization rate
Depends on nature of wastewater and reactor
Types of aeration Mechanical aeration in CSTR type ASP
Extended aeration: 10 mg/L-h
Conventional ASP: 30 mg/L-h
High rate ASP: 100 mg/L-h Diffused air in PFR -ASP
Types of air diffusers
Fine bubble diffusers: bubble size is 2 to 2.5 mm diameter
Coarse bubble diffusers: bubble size is
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Fine bubble diffusers
http://www.aqua-aerobic.com/library-aerationmixing.asp#
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Coarse bubble diffusers
http://www.aqua-aerobic.com/library-aerationmixing.asp#
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Coarse bubble diffusers
http://www.aqua-aerobic.com/library-aerationmixing.asp#
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Surface aerators
http://www.aqua-aerobic.com/library-aerationmixing.asp#
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Bhadreswar wastewater treatment plant:
aerated lagoon
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Bhadreswar wastewater treatment plant:
aerated lagoon
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Aerial View of Kruger Oxidation Ditches at Princeton WWTP
http://www.princeton-indiana.com/wastewater/pages/ditches/overview.html
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POP-I Submersible Mixer Direct Drive Horizontal Brush Aerator
http://www.princeton-indiana.com/wastewater/pages/ditches/overview.html
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Ponds and lagoons Used for small scale systems (or communities) especially suburban or
peri-urban type communities Longer HRT (no difference in HRT and MCRT) than conventional ASP,
therefore, large land area requirement Shallow structures with depths of 1 to 2.5 m All are modeled as completely mixed reactors without biomass recycle Ponds: no external mixing
Aerobic ponds: Shallow with DO present at all depths
Also called polishing or tertiary ponds Anaerobic ponds: deep ponds with no oxygen except in a thin layer at
the top Facultative ponds: both aerobic and anaerobic zones exist
Lagoons: various degrees of mixing Aerobic: completely mixed liquid and solids
Solids removal is required in a separate treatment unit Facultative: only liquid zone of lagoon is mixed; settled solids are
degraded anaerobically
Can be used in series to treat municipal wastewater
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Facultative ponds
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Design of ponds and lagoons
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Attached growth, plug flow reactor for BOD removal
Bed media: ordinary rocks or plastic; Bed diameter: as large as 60 m
Bed depths: with rocks it is restricted to 3 m, with plastic it can be much
higher (biological towers) Distributor arms sprinkle wastewater evenly at the top
Void spaces are large and allow air to circulate freely through the media
Biological growth includes bacteria, algae, fungi, protozoa, worms, insectlarvae
Sloughing of biomass when it gets too thick; f(hydraulic and organicloading rates)
Recycling of treated wastewater (effluent) enables higher efficiency,prevents slime from drying out
Advantages; Removal efficiency is higher than in ASP (suspended growth
processes) Less land area requirement than ASP
Problems: Washing out in monsoons and drying out in summer Extreme sensitivity to fluctuations in quantity and quality of influent
Cannot withstand toxic or shock loads
Wastewater treatment: trickling filters
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TricklingFilters
http://www.brentwood-ind.com/water/tricklingfilters.html
MPG, 2009
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Biotowers
http://www.ndsd.org/secondary.aspx
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Attached growth, mixed flow reactor for BOD removal
Series of closely spaced, circular disks mounted on a shaft
Diameter = 3.6 m, slime buildup on disk surface
Bottom 40% of disk area is submerged in wastewater
Wastewater treatment: rotatingbiological contactor (RBC)
Oxygen,organics (BOD) Boundary
layer
Bulk fluid
Flow dir
Concentrationin bulk fluid(Steady State)
BiofilmSubstratum
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Wastewater treatment: rotatingbiological contactor (RBC)
Wikipedia, 2008
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Rotating Biological Contactors
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Attached growth (AG) systems clarifier Design of secondary clarifier for AG systems is similar to
primary clarifiers Settling characteristics of bioparticles are similar to
flocculant particles; no coagulant needs to be added Approximates Type III settling (zone settling) since it
is a concentrated not dilute suspension Clarification not sludge thickening is the most important
objective A fraction of the sludge is sent back to the process while
the remaining is sent for sludge treatment Where solids conc is low, they are often sent to primary
clarifier for concentration of raw wastewater solids andremoval Cost-effective option; dont need to build another
clarifier since the settling characteristics are similar
Well settled sludge has 10 to 20% solids
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Wastewatertreatment plantflow charts fordifferentsecondarytreatment
processes
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Secondary clarification for suspendedgrowth processes
Objectives Produce effluent quality to meet discharge standards Concentrate biosolids to minimize quantity of sludge to be handled
At C1000 mg/L, it is a concentrated suspension
Settling as hindered and compression settling Results in zones with different types of settling as solids conc
increases from X in clarified zone to Xu in sludge underflow Hindered settling
Particles are in a fixed position relative to each other
Entire mass of particles settles as a whole Compression settling
As above, but due to higher conc., further settling takes placeonly by compression of the previously settled mass
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Secondary clarifiers: Zone settling
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Zone settling
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Solids fluxanalysis
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