AIR POLLUITON P2

1. WHAT KIND OF SEWER SYSTEMS YOU KNOW?

• Combined system of sewers: both storm water runoff and sewage in the

same pipe

• Separate system of sewers: transports sewage alone and storm water runoff

alone (or directly to surface water)

On the other hand, waste water can be conveyed by gravity, can be pumped

or conveyed by vacuum.

2. WHAT KIND OF POLLUTANTS ARE IN COMMUNAL WASTE WATER?

From physical point of view:

Solids:

• solid of high density (e.g. stones)

• swimming solid (e.g. wood)

• suspended particle (grit, sand, small organic particles, drops of

oil etc.)

Soluble materials

Emulsions

Gases

From chemical point of view:

Organic

Inorganic

3. WRITE THE DEFINITION OF BOD5 AND COD. WHAT ARE THEY USED

FOR?

BOD = Biochemical Oxygen Demand

The BOD is a measure of the rate at which micro-organisms use dissolved

oxygen in the bacterial breakdown of organic matter under aerobic

conditions.

The BOD5 test indicates the organic strength of a waste water and is

determined by measuring of BOD of a sample at 20°C for five days in the

dark.

COD = Chemical Oxygen Demand

COD is a measure of the amount of oxygen consumed from a chemical

oxidising agent under controlled conditions. The COD is generally greater

than the BOD as the chemical oxidising agent will often oxidise more

compounds than is possible under biological conditions.

4. MAKE A SKETCH OF THE FLOW DIAGRAM OF A COMMUNAL WASTE

WATER TREATMENT PLANT. MAKE A SHORT DESCRIPTION OF THE

EQUIPMENT OPERATION PRINCIPLES.

5. MAKE A SHORT DESCRIPTION OF PHYSICAL TREATMENT. WHAT

PROCESSES CAN BE USED IN THIS PART OF WASTE WATER

TREATMENT?

The screening can be:

• Coarse screening, for spacing of over 40 mm

• Medium screening, for spacing of 10 to 40 mm

• Fine screening, for spacing under 10 mm

6. HOW CAN BE CALCULATED THE SETTLING VELOCITY OF A SOLID

PARTICLE IN WATER?

Considering:

Re < 1

Free settling

Ball shape for the particle

𝒗𝒔 = 𝒅𝒔

𝟐( 𝝆𝒔 − 𝝆𝒇) 𝒈

𝟏𝟖𝝁𝒇

Where:

𝑣𝑠= settling velocity from Stokes law [m/s]

𝑑𝑠= diameter of the solid (particle considered) [m]

𝜌𝑠= density of the solid [kg/m³]

𝜌𝑓= density of the fluid [kg/m³]

𝑔 = gravity acceleration [m/s²]

𝜇𝑓= dynamic viscosity of the fluid [kg/(m·s)]

7. HOW CAN YOU CALCULATE THE MINIMUM LENGTH OF A HORIZONTAL

FLOW RECTANGULAR SETTLEMENT TANK, IF THE SETTLING

VELOCITY, THE INLET VOLUME FLOW RATE, THE WIDTH AND THE

DEPTH OF THE BASIN ARE GIVEN?

Considering a laminar flow:

𝑣𝑤𝑎𝑡𝑒𝑟= water velocity [m/s];

𝑣𝑠= settling velocity [m/s];

𝑣𝑤𝑎𝑡𝑒𝑟 = �̇�

𝐴𝑤 =

�̇�

𝐵. 𝐷

Settling time and detention time:

𝑡𝑠 = 𝐷

𝑣𝑠 ; 𝑡𝑑 =

𝐿

𝑣𝑤𝑎𝑡𝑒𝑟

But 𝑡𝑑 > 𝑡𝑠, therefore:

𝐿𝐵𝐷

�̇�>

𝐷

𝑣𝑠

𝐿 > �̇�

𝑣𝑠 . 𝐵

8. WHAT KINDS OF BIOLOGICAL TREATMENT YOU KNOW?

On the basis of oxygen consumption:

Aerobic

Anaerobic

On the basis of microorganisms’ growth:

Biofilm (attached growth, fixed film) processes:

Percolating filter (Trickling filter)

Rotating biological contactor

Activated sludge (suspended growth) treatment

9. WHAT KIND OF AERATION POSSIBILITIES CAN BE USED IN ACTIVATED

SLUDGE TREATMENT (SKETCH + DESCRIPTION)?

1) Mechanical surface aerator with draft tub: mix of water in

which there is air dissolved into the waste water.

2) Submerged air diffusion:

3) Combined surface mixing and submerged diffuse aeration:

4) Diffused air pipes

5) Bubble diffusers

6) Jet aeration equipment

10. WHY HAS TO BE RETURNED A PART OF SLUDGE FROM SECONDARY

SETTLEMENT TANK TO THE ACTIVATED SLUDGE TREATMENT PLANT?

The recirculation of part of the sludge from the secondary settlement tank

ensures that the same amount of microorganism will remains in the treatment

system.

11. MAKE A SKETCH AND A SHORT DESCRIPTION OF A TRICKLING FILTER.

12. MAKE A SKETCH AND A SHORT DESCRIPTION OF A ROTATING

BIOLOGICAL CONTACTOR.

Biofilm process, since the rotating leaves are covered with biofilm at which

the waste water pass through.

13. WHAT PROCEDURES CAN BE USED IN TERTIARY TREATMENT? GIVE A

ONE SENTENCE COMMENT TO EACH OF THEM.

Processes in tertiary treatment can be:

Disinfection: treatment of the effluent for the destruction of all

pathogens.

Chemical treatment (precipitation, neutralization etc.): used for acidic

and caustic waste water, if contaminants can be well precipitated; to

improve settling properties, to remove heavy metals and phosphorus

removal.

Adsorption: adhesion of atoms, ions, molecules of gas, liquid, or

dissolved solids to a solid surface. This process creates a film of the

adsorbate (the molecules or atoms being accumulated) on the surface

of the adsorbent.

Membrane technology: semi-permeable layer with two main features

of selectivity and permeability. The inlet flow is separated in permeate

(cleared water) and retentate (concentrated contamination).

Distillation:

Evaporation: thermal process, widely used for concentrating

solutions, suspensions, and emulsions. Concentration is accomplished

by boiling out one part of the solvent, normally water, from the liquid

phase.

14. WHAT CATEGORIES OF DISINFECTION YOU KNOW? WRITE THEIR

ADVANTAGES AND DISADVANTAGES.

Disinfection Techniques of Wastewater

Chemical by Chlorine, Ozone, Hydrogen peroxide.

Advantages are elimination of odors; oxidation of residual organic

compounds; ozone can be generated from air, making its supply

dependent only upon a source of power.

Disadvantages are high cost of production and potential to cause

localized air pollution.

Physical.

Irradiation.

15. WHAT IS PRECIPITATION? WHAT IS IT USED FOR? WHERE CAN IT BE

USED IN WASTE WATER TREATMENT?

Chemical precipitation is a method of wastewater treatment. Metal salts are

generally used for the precipitation of phosphate. Wastewater treatment

chemicals are added to form particles which settle and remove contaminants.

Before primary settlement, biological treatment and secondary settlement.

16. DESCRIBE THE OPERATION OF A FIXED BED ADSORPTION COLUMN.

(SKETCH, BREAKTHROUGH CURVE)

A part of the bed is saturated at inlet point of water and reaches adsorption

equilibrum. This part of the bed is not able to further adhesion so moves

towards the outlet zone of the water. Zones in the bed are:

saturated zone

adsorption or operation zone

free zone

Water flow to be cleared into the adsorber has to be turned off at

breakthrough point or exhaustion point, and let into another adsorber

column. Saturated adsorber has to be regenerated (desorbed.)

17. WHAT TYPES OF SLUDGE WATER YOU KNOW? WHAT ARE THE

REMOVAL PROCESSES OF THEM?

Inter-particles water (~70%) - Thickening

Bonded water “colloidal water “ or “capillary water” (~22%) - First

conditioning or stabilization, then: Dewatering

Inside water (~8%) - drying

18. SHOW THE FLOW DIAGRAM OF SLUDGE TREATMENT. WHAT TYPES OF

SLUDGE WATER ARE REMOVED IN THE PARTS OF IT?

19. SHOW THE VOLUME OF SLUDGE VS. SLUDGE CONCENTRATION

DIAGRAM.

20. WHAT IS CONDITIONING USED FOR? DESCRIBE THE POSSIBLE

CONDITIONING METHODS.

The purpose of sludge conditioning is to provide a rigid sludge structure

of porosity and pore size sufficient to allow drainage and dewatering.

Preparation process before dewatering.

Sludge conditioning is a process whereby sludge solids are treated with

chemicals or various other means to prepare the sludge for dewatering

processes.

Physical

Chemical

Biochemical (aerobic, anaerobic)

21. WHAT IS DIGESTION USED FOR? (HEATING, MIXING, SHAPE, COVER,

BOTTOM)

The digestion process is a Anaerobic Biochemical Conditioning. The

products are:

Digested sludge for agriculture

Biogas

The digestion can occurs in different temperatures according with the

microorganisms into the sludge:

Cold digestion T < 15 °C (50-180 days)

Heated digestion

Mesophilic digestion T= 32- 38 °C (15-25 days)

Thermophilic digestion T= 55- 58 °C (5-12 days)

The reactor can be:

Cylindrical

Conical bottom

Conical cover

Egg-shaped

22. DESCRIBE THE OPERATION OF A DECANTER CENTRIFUGE (SKETCH).

Sludge dewatering (after conditioning)

1 – Feeding sludge pipe

2 – Dry sludge

3 – Clarified liquid

4 – Screw conveyor

5 – Rotating bowl

7 – Adjustable tresholds

8 – Casing

23. MAKE A SHORT DESCRIPTION OF BIOGAS. (PRODUCTION, COMPOUNDS,

UTILIZATION)

Biogas can be defined as a mixture of methan (60-65% CH4) and other

gases, as carbon dioxide (30-35%), what can be produced by digestion of

organic mater of the sludge, for example. The biogas can be used in the heat

energy recovery by its direct combustion or it can be used in electric energy

(gas engine, furnace).