History of and Current Trends in Wastewater Treatment With input by Lee Walker History of Wastewater...
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Transcript of History of and Current Trends in Wastewater Treatment With input by Lee Walker History of Wastewater...
History of and Current Trends inWastewater Treatment
With input by Lee Walker
History of Wastewater Treatment
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 into rivers
History of Wastewater Treatment
1500 BC: Isle of Crete
•advanced plumbing and drainage systems
•open sewers built of stone
•royal household had flushing toilet
• last group to use flushing toilets until 1596
History of Wastewater Treatment
History of Wastewater Treatment
300 BC
•Greece: most developed waste management of any civilization prior to the nineteenth century.
•Banning the dumping of waste into the streets.
• For 800 y Greek government removed waste at the expense of landowners.
•Greeks and Romans discovered the link water quality public health.
•Underground sewer network in Rome Tiber river
History of Wastewater Treatment
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 management became a priority
History of Wastewater Treatment
16th Century
•No change in the understanding and disposal of human wastes.
•Some idea of the capacity of polluted rivers to clean themselves (microbes were not understood yet)
•Successful for smaller communities.
•London collected sewage but dumped into Thames
•Cheap method dead river.
•With population increases water bodies could no longer treat the high wastewater flows.
•What was limiting ? Oxygen Anaerobic rivers
•Alternative treatment became necessary.
History of Wastewater Treatment
1860 Septic tank •Perceived link between solids and health•Treat sewage from an entire community •Remove 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 filtersto treat septic tank effluent(Better oxygen supply, little bacterial biomass present)
Lagoons
History of Wastewater Treatment
•Pathogens Epidemics
•Solid Organics Building up in environment Long term pollution (river sediments)Oxygen depletion in rivers Death of higher life
•Dissolved organics Oxygen depletion Death of higher life
•Nutrients (N and P) Algal blooms Buildup of solid organics Decay Oxygen depletion Death of higher life
•Odor, colour,…
Effects of Waste Water Disposal
Pathogens Bioessays
Solid Organics Filter or centrifuge sample. Dry residue Total suspended solids TSS. Ash the TTS Loss is solid organics = volatile suspended solids = VSS
Dissolved organics COD : Chemical Oxygen Demand (mg/L of O2) = The amount of oxygen required to oxidize soluble organics by an acidic dichromate solution.
BOD : (Biological Oxygen Demand) (mg/L of O2) = The amount of oxygen required for microbial removal of soluble organics over a 5 day period.
•Nutrients (N and P) : Present as ammonia and phosphate algae blooms algae decay sec. pollut.
Waste Water Analysis
Wastewater Required composition 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
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 Fish
Not 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
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
X
S
D
Ste
ady
Sta
te C
once
ntra
tion
Dotted line no feedback:•Washout occuring early
•4-fold Feedback approximately:•4*X 4*R 1/4* S•allows 1/4 reactor size to do same work
•Feedback essential for pollutant removal. Can be used 100-fold 100-fold smaller treatment plant
•Note: same assumed feed concentration (SR)
R
Dcrit
SR
Theoretical Effect of biomass feedback
Biomass Retention in WWTP
How to Retain Biomass ?
Filters don’t 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 can’t happen during aeration and mixing
Use external settlers = Clarifiers
Intermittent stopping of aeration and mixing =
Sequencing batch reactor
Biomass Retention in WWTP
Biomass Retention in WWTP
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) will favour suspended or filamentous bacteria growth no settling breakdown of plant performance.
Single cells or filaments have • a higher surface area and allow facilitated diffusion.• a lower apparent Ks value for substrate.
Running treatment plant like a chemostat would result in continuous substrate (BOD) limitation no flocs no settling low biomass breakdown
Biomass Retention in WWTP
Growth of filamentous bacteria favoured by low substrate (BOD) concentrations;detrimental to gavity settling
floc
Biomass Retention in WWTP
Use of Clarifier for Biomass Retention via external biomass feedback
• Centrifuging of recycle liquid
• Membrane filtration of recycle liquid
• Flocculation• Gravity settling of
flocculated biomass
Recycle(Feedback)
Inflow
Outflow
Clarifier
Biomass Retention in WWTP
Problems with floc formation
To encourage floc formation: need to expose biomass to high feed levels (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 plugIntermixing with the previous and following plug is minimised
Biomass Retention in WWTP
Return Activated SludgeAir Line
Influent
Effluent
Waste Sludge
Clarifier
Plug flow waste water treatment allowing high 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 system before being removed.The liquid retention time (hydraulic retention time = HRT) is a few hours while the SRT is about 15 -40 days
Biomass Retention in WWTP
Activated sludge reactors
ThickenerBiomass Sedimentation
Elledge WWTP
Influent
Effluent
Waste SludgeCycle
Fill
Aeration
Settle
Decant
Use of Sequencing Batch Reactor (SBR) for a) Biomass Retention via internal biomass feedbackb) floc formation by oxposing biomass to a sudden high inflow of biomass
Biomass Retention in WWTP
Use of Bioselector to allow contact with bacteria and high BOD
(not examinable)
Hybrid between plug flow reactor and SBRIncoming wastewater is mixed with return activated sludgein an SBR. System used at Woodman Point Treatment plant
Biomass Retention in WWTP
SBR treatment plant in Western Australia
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 decant•Not a continuous process - batch
Biomass Retention in WWTP