Wastewater Treatment

Bülent Mertoğlu

Wastewater Contaminants

Suspended solids

Priority pollutants: metals (Cd, Hg), benzene compounds, and

chlorinated compounds

Microorganisms: pathogenic and nonpathogenic

Organics: refractory and biodegradable

Nutrients:

– Phosphorus, Nitrogen (ammonia, nitrites, nitrates)

Typical Composition of Untreated Wastewater

Constituent Weak (mg/l) Strong (mg/l)

Alkalinity 50 200

BOD5 100 300

COD 250 1000

Suspended Solids 100 350

TDS 200 1000

Total Nitrogen 20 80

Total Organic Carbon 75 300

Total Phosphorous 5 20

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The amount of organic carbon present determines:

the amount of O2 needed for biological treatment

the size of waste treatment facility needed

the efficiency of the treatment process

biochemical oxygen demand

total organic carbon

chemical oxygen demand

Objective 1 - Reduce organic content (reduction of BOD)

There are three methods to determine carbon present:

Biochemical Oxygen Demand (BOD)

Definition: The amount of dissolved oxygen utilized by microbes for

the biochemical oxidation of organic (carbonaceous BOD) and

inorganic (autotrophic or nitrogenous BOD)

The BOD test was developed in 1930’s. This is a five day test that

measures the amount of O2 consumed in a wastewater sample by a

mixed population of heterotrophic bacteria in the dark at 20oC

BOD of wastewater is typically 110-440 mg/L and must be reduced

to 20 mg/L for discharge

BOD = Di – Df

P

where:

Di = initial dissolved O2 concentration

Df = final or 5-day dissolved O2 concentration

P = volumetric fraction of wastewater

Example: 5 ml wastewater is added to a 300 ml BOD flask

P = 5 = 0.0167 Di = 8 mg/L Df = 2 mg/L300

BOD = 8 – 2 = 359 mg/L0.0167

Oxidation is usually 60-70% complete after 5 days

Total Organic Carbon (TOC)

TOC is measured using a TOC analyzer. The sample is combusted

and organic carbon quantified using infrared detection.

Chemical Oxygen Demand (COD)

COD is measured following digestion at high temperature with

strong oxidant such as chromic acid, or sulfuric acid/potassium

dichromate. The chromate ion reacts with the COD producing a

color that is measured.

If COD >> than BOD what does this mean?

Total Kjeldahl Nitrogen

TKNOrg-N (Amino Acids, Urea)

NH4-N

N2: Nitrogen gas (79 % of air)NH3: ammoniaNH4- : ammonium ionNO2- : nitrite ionNO3- : nitrate ionOrg N: organic nitrogenTKN: Total Kjeldahl Nitrogen(% 40 org N -- %60 NH4H )

Nitrogen Removal:

Activated Sludge Process

Introduction

Biological wastewater treatment systems, microorganisms are the main players.

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Applications

1. Carbonaceous removal - aerobic

- anaerobic

2. Nitrogen removal - nitrification

- denitrification

3. Phosphate removal - anaerobic/aerobic

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Microbial Group Electron Donor Electron Acceptor Carbon Source

Aerobic Heterotrophs

Org. Carbon O2 Org. Carbon

NitrifiersNH4

+

NO2-

O2

O2

CO2

CO2

Denitrifiers Org. Carbon NO3- Org. Carbon

MethanogensAcetate

H2

AcetateCO2

AcetateCO2

Activated Sludge Principles

• The microorganisms are “activated” for rapid uptake of new

substrate, thus the term activated sludge

• Bacteria are encouraged to grow by providing

− Oxygen

− Food (Organic compounds - BOD)

− Nutrients

− Correct temperature

− Time

• As bacteria consume BOD, they grow and multiply

• The basis of the activated sludge process is the growth of floc

forming bacteria on wastewater organics

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Bacterial Growth Curve

• Lag-phase During this phase bacteria become acclimated to their

new surroundings. They are digesting food, developing enzymes

and other things required for growth.

• Accelerated Growth-phase The bacteria are growing as fast as

they can, since there is an excess of food. The cells are mostly

dispersed, not sticking together.

• Declining Growth-phase Reproduction slows down because there

is not an excess of food. A lot of food has been eaten and there are

now a large number of bacteria to compete for remaining food, so

the bacteria do not have enough remaining food to keep the growth

rate at a maximum.

Bacterial Growth Curve

• Stationary-phase The number of bacteria is the highest possible,

but not much food is left, so the bacteria cannot increase in number.

There is some reproduction, but some cells are also dying, so the

numbers of bacteria remain relatively constant. The bacteria have

now lost their flagella and have a sticky substance covering the

outside of the cell, allowing them to agglomerate into floc. In fact,

the floc gets big enough that if aeration and mixing were stopped,

the floc could settle to the bottom.

• Death-phase The death rate increases with very little if any growth

occurring. Therefore, the total number of living bacteria keeps

reducing. The bacteria are just trying to keep alive.

Activated Sludge

Activated sludge process consists of an aerated tank and a settler.

Monod Equation

Oxidation Ditch

Treatment Plants in Istanbul

Paşaköy Wastewater TreatmentPlant

Anoxic Tank

Oxic Tank

Aeration Tank

Overflow

Mechanical Aerators

Fixed aerator Floating aerator

Turbine aerator

Diffusers

Air Pipe Header Diffusers

Recycle Pipe

Aeration Tank