Water acquisition and treatment

Question 1

Industrial wastewater can be led into the sewage system, when?

Answer 1

It meets the environmental permit conditions, legislation and water supply requirements of the industrial plant.

Question 2

Wastewater should not what?

Answer 2

Hinder function of sewer system, wastewater treatment plant or its unit operations. Wastewater may not pose threat to the environment.

Question 3

Two types of wastewater?

Answer 3

Domestic- and industrial wastewater.

Question 4

Municipal wastewater treatment system consist of five steps:

Answer 4

Preliminary treatment
Primary treatment
Secondary treatment
Advanced treatment
Final treatment
Solids processing

Question 5

Why does industrial wastewater must be pre-treated prior to being discharged into municipal sewer system?

Answer 5

Removes materials that will not be treated by municipal system, remove materials that inhibit the biological processes in secondary treatment.

For example: silver ions are toxic to bacteria which might affect biological process. Thus silver ions are removed at pre-treatment before biological process so that bacterial performance do not get affected.

Question 6

Preliminary treatment steps?

Answer 6

Upon arrival via the sewer system, the wastewater is sent through a bar screen, which removes large solid objects such as sticks and rags.

Leaving the bar screen, the wastewater flow is slowed down entering the grit tank, to allow sand, gravel and other heavy material that were small enough not to be caught by the bar screen to settle to the bottom.

Question 7

Primary treatment steps?

Answer 7

It allows for the physical separation of solids and greases from the wastewater.

The screened wastewater flows into a primary settling tank where it is held for several hours allowing solid particles to settle to the bottom of the tank and oils and greases to float to the top.

Question 8

Secondary treatment steps?

Answer 8

Biological treatment process that removes dissolved organic material from wastewater. The partially treated wastewater from the settling tank flows by gravity into an aeration tank.

Liquid mixture (solids with micro-organisms and water) is sent to the final clarifier.

In clarifier, solids settle out to the bottom where some of the material is sent to the solids handling process and some is recycled back to replenish the population of micro-organisms in the aeration tank to treat incoming wastewater.

Question 9

Final treatment steps?

Answer 9

Treated water is disinfected and then it is send out for wastewater reuse activities or for discharging in river/streams.
Mostly chlorination and/or ultra violet irradiation is used for disinfection purposes.

Question 10

How are solids being processed?

Answer 10

The primary solids from the primary settling tank and the secondary solids from the clarifier are sent to a digester.
Micro-organisms use the organic material present in the solids as a food source and convert it to by-products such as methane gas and water.

Digestion results in a 90% reduction in pathogens and the production of a wet soil-like material called “biosolids” that contain 95-97% water.

Question 11

What is used to squeeze water from the biosolids?

Answer 11

In order to remove some of this water, mechanical equipment such as filter presses or centrifuges are used to squeeze water from the biosolids to reduce the volume prior to being sent to landfill, incinerated or beneficially used as a fertilizer or soil amendment.

Question 12

Wastewater treatment steps? (Again)

Answer 12

Preliminary Treatment (screening)
Primary Treatment (primary settling)
Secondary Treatment (e.g. activated sludge)
Advanced Treatment (e.g. P removal)
Final Treatment (disinfection)
Solids Processing (sludge treatment)

Question 13

What is the purpose of equalization basins?

Answer 13

Improves effectiveness of primary & secondary treatment. It is not a treatment process.

Question 14

Settling / sedimentation in primary treatment?

Answer 14

Solid liquid separation process in which a suspension is separated into two phases:
Clarified supernatant leaving the top of the sedimentation tank (overflow).
Concentrated sludge leaving the bottom of the sedimentation tank (underflow).

Question 15

What is the purpose of settling in primary treatment?

Answer 15

To remove coarse dispersed phase.
To remove coagulated and flocculated impurities.
To remove precipitated impurities after chemical treatment.
To settle the sludge (biomass) after activated sludge process / tricking filters.

Question 16

What is sedimentation?

Answer 16

also known as settling, may be defined as the removal of solid particles from a suspension by settling under gravity.

Question 17

What is clarification?

Answer 17

usually refers specifically to the function of a sedimentation tank in removing suspended matter from the water to give a clarified effluent. In a broader sense, clarification could include flotation and filtration.

Question 18

What is thickening?

Answer 18

in sedimentation tanks is the process whereby the settled impurities are concentrated and compacted on the floor of the tank and in the sludge-collecting hoppers.

Question 19

What it the basic approach in secondary treatment?

Answer 19

To use aerobic biological degradation.

‘‘organic carbon + O2 -> CO2 + new cells’’

Question 20

How is aerobic biological degradation achieved?

Answer 20

Created by a very rich environment for growth of a diverse microbial community.

High density of microorganisms (keep organisms in system)
Good contact between organisms and wastes (provide mixing)
Provide high levels of oxygen (aeration)
Favorable temperature, pH, nutrients (design and operation)
No toxic chemicals present (control industrial inputs).

Question 21

Dispersed growth vs fixed growth method in secondary treatment?

Answer 21

Dispersed Growth (suspended organisms):
Activated sludge
Oxidation ditches/ponds
Aerated lagoons, stabilization ponds.

Fixed Growth (attached organisms):
Trickling filters
Rotating Biological Contactors.

Question 22

What is activated sludge?

Answer 22

Process in which a mixture of wastewater and microorganisms is agitated and aerated.

Leads to oxidation of dissolved organics.

After oxidation, separate sludge (mostly microbial cells, water, and other contaminants) from wastewater.