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