ch5 wastewatertreatment ii

7/29/2019 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



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Coarse bubble diffusers



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Surface aerators



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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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ch5 wastewatertreatment ii

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