08 Overview of Waste Water Treatment 2008

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History of and Current Trends inWastewater Treatment

With input by Lee Walker

History of Wastewater Treatment


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Before 10,000 BC nomadic tribes allowed the soil to treat it

After establishing townships approach continued throw wastes into the streets street levels rose raise the doors to their houses

Egypt 2100 BC only for elite: waste was removed and dumped intorivers



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1500 BC: Isle of Creteadvanced plumbing and drainage systems

open sewers built of stone

royal household had flushing toilet

last group to use flushing toilets until 1596



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300 BC

Greece: most developed waste management of anycivilization prior to the nineteenth century.

Banning the dumping of waste into the streets. For 800 y Greek government removed waste at theexpense of landowners.

Greeks and Romans discovered the link water quality

public health.

Underground sewer network in Rome Tiber river



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Dark Middle Ages:

Fall of the Roman Empire knowledge lost for 1000 y.

Old practice of simply throwing their waste into the streets.

No separation drinking water and human wastes.

Wastes transferred from waste pits into drinking wells

Epidemics raged in the cities dysentery, typhus (which comes from bad sanitation)typhoid fever (from human feces and urine) major plagues of the 12th century waste managementbecame a priority



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16th Century

No change in the understanding and disposal of humanwastes.

Some idea of the capacity of polluted rivers to cleanthemselves (microbes were not understood yet)

Successful for smaller communities.

London collected sewage but dumped into Thames

Cheap methoddead river.

With population increases water bodies could no longertreat the high wastewater flows.

What was limiting ? Oxygen Anaerobic rivers

Alternative treatment became necessary.



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1860 Septic tankPerceived link between solids and healthTreat sewage from an entire communityRemove solids, untreated liquid discharged to river

1868 Sand bed filter

to filter septic tank effluent before discharge to river(No oxygen supply)

1893 Rock Trickling filters

to treat septic tank effluent(Better oxygen supply, little bacterial biomasspresent)

Lagoons



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Pathogens Epidemics

Solid Organics Building up in environment Long term pollution (river sediments)Oxygendepletion in rivers Death of higher life

Dissolved organics Oxygen depletion Deathof higher life

Nutrients (N and P) Algal blooms Buildup ofsolid organics Decay Oxygen depletion Death of higher life

Odor, colour,

Effects of Waste Water Disposal


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Pathogens Bioessays

Solid Organics Filter or centrifuge sample. Dry residue Totalsuspended solids TSS. Ash the TTS Loss is solid organics = volatilesuspended solids = VSS

Dissolved organics

COD : Chemical Oxygen Demand (mg/L of O2) = Theamount of oxygen required to oxidize soluble organics byan acidic dichromate solution.

BOD : (Biological Oxygen Demand) (mg/L of O2) = Theamount of oxygen required for microbial removal ofsoluble organics over a 5 day period.

Nutrients (N and P) : Present as ammonia and

phosphate algae blooms algae decay sec. pollut.

Waste Water Analysis


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Wastewater Requiredcomposition Levels

BOD (mg/L) 200 45

TSS (mg/L) 200 45

NH3 Nitrogen (mg/L) 30 1

Phosphorus (mg/L) 10 No Limit

Fecal Coliforms (/100 mL) 107 < 14 (CFUs)

Example WW composition


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Large shallow ponds, 1.2 to 2.4 meters in depth.

Not mixed or aerated Mostly anaerobic.

Long treatment times, odor emission.

Algae growth Secondary pollution

Can work as Integrated System for agricultural areas

Nutrients Algae Zooplankton FishNot suitable for highly populated areas

Average treatment time = Hydraulic Retention time = HRT

= 20 to 200 days Huge reactor volume

(for Perth about 500 to 1000 Subiaco Stadiums).

Why not Lagoon Treatment


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Why long treatment times?

Lagoon = chemostat with low productivity. Why?

Efficiency limited by biomass levels and by oxygen.(Efficiency ~ Productivity (R) of chemostat is proportional to the amount of

biomass (X) present)

Design a waste water treatment plant with high X.

Purpose of plant:

Remove organics (COD, BOD)

Remove nutrients (N and P)

Allow re-use of water in the future.

Biomass must be retained longer than the water

Why not Lagoon Treatment


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X

S

D

SteadyS

tateConcen

tration

Dotted line no feedback:Washout occuring early

4-fold Feedbackapproximately:4*X 4*R 1/4* Sallows 1/4 reactor size todo same work

Feedback essential forpollutant removal. Can beused 100-fold 100-foldsmaller treatment plant

Note: same assumedfeed concentration (SR)

R

Dcrit

SR

Theoretical Effect of biomass feedback

Biomass Retention in WWTP


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How to Retain Biomass ?

Filters dont work.Gravity separation needed.

Settling velocity of small particles is proportional to their size (Stokes

law).

Floc formation is essential to allow gravity separation (Settling

velocity must be > 1m/h)

Settling cant happen during aeration and mixing

Use external settlers = Clarifiers

Intermittent stopping of aeration and mixing = Sequencing batch

reactor (SBR)




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Problems with floc formation

Pros and Cons of Floc formation for bacteria?

+ Shelter from predators (Protozoa)- Diffusion problems of BOD and O2

Continuous presence of low levels of BOD (feed) destroy flocs, why?

It favours suspended or filamentous bacteria growth (higher surface area)

Higher surface area more effective uptake of low substrateconcentrations (lower apparent kS value for substrate)

Running treatment plant simply like a chemostat

would result in continuous substrate (BOD) limitation no flocs no settling low biomass breakdown

In addition to batch, fed-batch, chemostat a different process is used Plugflow reactor with biomass feedback o

Sequencing Batch Reactor (SBR)Biomass Retention in WWTP

G h f


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Growth offilamentousbacteria favouredby low substrate

(BOD)concentrations;detrimental to gavitysettling

floc



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Principle of Biomass Retentionvia external biomass feedback

Centrifuging ofrecycle liquid

Membrane

filtration of recycleliquid

Flocculation

Gravity settling of

flocculatedbiomassRecycle(Feedback)

Inflow

Outflow

Clarifier



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Plug Flow Systems to avoid FeedLimitation

To encourage floc formation: need to expose biomass to high feedlevels (BOD) by:

a) Plug flow system and clarifyerb) SBRc) Using of a bioselector (not examinable)

Plug Flow system :The feed and biomass is mixed at entry and moves through the

process as plug

Intermixing with the previous and following plug is minimised



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Return Activated SludgeAir Line

Influent

Effluent

Waste Sludge

Clarifier

Plug flow waste water treatment allowinghigh BOD levels at the beginning

BOD Gradient

A fraction of the sludge is wasted and provides a Solids Retention Time

(SRT). SRT is the average length of time the sludge is in the systembefore being removed.The liquid retention time (hydraulic retention time = HRT) is a few hourswhile the SRT is about 15 -40 days



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Activated sludge reactors

Thickener

Biomass Sedimentation

Elledge WWTP


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Influent

Effluent

Waste Sludge Cycle

Fill

Aeration

Settle

Decant

Use of Sequencing Batch Reactor (SBR) fora) Biomass Retention via internal biomassfeedback

b) floc formation by oxposing biomass to asudden high inflow of biomass



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Use of Bioselector to allow contact withbacteria and high BOD

(not examinable)

Hybrid between plug flow reactor and SBRIncoming wastewater is mixed with return activated sludge

in an SBR. System used at Woodman Point Treatment plant



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SBR treatment plant in Western Australia


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Comparison between Plug flow and SBR

Traditional plug flow wastewater treatment

liquid pumped from one compartment to another phases were separated in time and space

Sequencing batch reactor

all phases occur in the one reactor phases separated only by time no need for additional clarifyer Phases of operation

fill, aerate, settle and decantNot a continuous process batch

In both cases, bacteria undergo changes of feedsaturation and feed limitation


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08 Overview of Waste Water Treatment 2008