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CVC109 Water > Wastewater Treatment > Flashcards

Wastewater Treatment Flashcards

1
Q

What is the difference between water treatment and wastewater treatment

A

The process of extracting water from a natural source and treating it so that it is of potable quality

Wastewater treatment is water collect from human consumption which has been polluted and is treated prior to discharge

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2
Q

The suitability of wastewater treatment depends on

A
  • Income levels
  • Community size
  • Population density
  • Water usage
  • Resources and skills available
  • Institutional Capacity
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3
Q

Different types of wastewater

A

-Black Water- from toilets

-Grey Water- from baths, washbasins, showers and kitchens

-Storm water- from runoff

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4
Q

What is the difference between separate and combined sewer systems

A

In a combined system stormwater and sewage there is one pipe that contains wastewater and surface runoff.

Whereas in a separate there is two separate pipes- however there is nearly always some leakage

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5
Q

Advantages of a separate sewage system

A

-No combined sewer overflows (CSOs) following heavy rain- potentially less pollution of watercourses

-Smaller wastewater treatment requirement. Each type can follow own optimum line

-Wastewater sewer can be smaller, and therefore greater velocities can be maintained even at low flows

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6
Q

Disadvantages of separate sewer systems

A

-Extra cost of 2 pipes

-Additional space occupied in narrow streets

-No flushing of deposited wastewater solids by stormwater

-No treatment of stormwater occurs

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7
Q

Advantages of a combined sewer system

A

-Lower pipe construction costs

-Economical in space

-House drainage simpler and cheaper

-Deposited wastewater solids flushed out in times of storm

-Some treatment of stormwater

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8
Q

Disadvantages of a combined sewer system

A
  • CSOs necessary to keep main sewers and treatment works to a feasible size. May cause serious pollution of watercourses
  • Larger treatment works inlets necessary
  • Higher pumping cost
  • Line is a compromise and may necessitate long branch connections
  • Slow, shallow flow in large sewers in dry weather may cause deposition of solids
  • Grit removal necessary
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9
Q

Make-up of domestic wastewater

A

99.9% water and 0.1% solids, the aim of wastewater treatment is to remove this small portion

70% of the solids are organic (fats, proteins, carbohydrates) 30% are inorganic (grit, salts, metals)

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10
Q

What does sewage contain

A
  • Thermotolerant coliform bacteria
  • Pathogens
  • Aerobic and anaerobic bacteria
    o Not all bacteria are harmful
    o Treatment reduces the number of harmful bacteria
    o Wastewater treatment encourages useful bacteria to breakdown organic matter into simple chemicals and produce new bacterial cells
  • Organic matter
  • Inorganic matter
  • Water
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11
Q

Reasons for wastewater treatment

A

-Reduce the spread of diseases by pathogens present in wastewater

-Reduce environmental damage caused by polluting matter

-Reduce concentrations of nitrates, phosphates and endocrine disruptors

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12
Q

Objectives of wastewater treatment

A
  • Encourage the growth of saprophytic bacteria- this is bacteria that feed on dead and decaying organic matter
  • To accelerate natural treatment processes
  • Reduce Environmental Pollution
  • Provide treatment within a confined area
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13
Q

2 parameters for assessing wastewater

A

BOD
-Measure of the concentration of organic pollutants in the water
-Amount of dissolved oxygen that is used by aerobic microorganisms when decomposing organic matter
-High BOD= more organic matter, and less oxygen for other animals

Suspended Solids
-Measure of the concentration of suspended material present, affecting the clarity of wastewater

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14
Q

What does BOD, COD and OD stand for

and compare their magnitude

A

Biochemical Oxygen Demand

Chemical Oxygen Demand

Oxygen Demand

OD>COD>BOD

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15
Q

Conditions for discharge into sewers

A

-pH range of 6-10, low pH release toxic gas

-Hydrogen cyanide gas can be formed from methane and ammonia

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16
Q

Conditions for wastewater to be released into watercourses

A

-Maximum compliance limits for BOD and COD

-Annual mean compliance limits for phosphorous and nitrogen

17
Q

How does nature deal with pollution

A

-Dilution- Mass Balance. This can be used to work out how much you can safely release without impacting the downstream conditions outside of the allowable tolerances

-Degradation and Recovery -as rivers continue to flow new oxygenated water continues to flow. But will eventually reach a point if there is too much polluted material then the septic zone will extend further

18
Q

What takes longer to breakdown groundwater or water pollutants

A

Groundwater, because there is less oxygen present in the soil

19
Q

Aerobic vs Anaerobic decomposition of organic matter equations

A

Aerobic:
Organic Wastes +Oxygen=Oxidised Wastes and New Bacteria

Anaerobic:
Organic Wastes=Methane + Carbon Dioxide + New Bacteria

20
Q

Aerobic vs Anaerobic decomposition of organic matter process

A

Aerobic:
-Processes are parallel
-Suit domestic waterways
-Produces good quality effluent
-Oxygen supply needed
-There is little smell
-Treatment is complete

Anaerobic:
-Processes are sequential
-Used where sewage is very strong, and oxygen can’t be supplied sufficiently quickly
-Suitable for problem wastes
-Additional sludge production is relatively low
-Unpleasant smells are produced
-Treatment is only partial

21
Q

Treatment stages for domestic wastewater

A
  1. Screening
    2.Grit Removal
  2. Primary Sedimentation
  3. Biological Treatment
  4. Secondary Sedimentation
22
Q

Describe methods of screening and their objective

A

Removal of large solid particles. Important to prevent future stages being damaged by large particles

-Set of vertical parallel bars in the flow that catch large particles. Simple and effective as no moving parts

-Perforate metal plates

By product is compressed screenings and grit which are generally sent to landfill

23
Q

Objective and methods of primary sedimentation

A

Remove light, organic particles, fats and greases.

-Rectangular, horizontal flow tank

-Circular radial flow tank

Both work in a similar way. The flow is reduced to allows fats, oils and greases to float to the surface and are collected by a scraper bar. While organic solids are heavier and fall to the bottom of the tank. This sludge is then removed from the outlet at the bottom

24
Q

Objectives and methods of biological treatment

A

Encourage biological decomposition of remaining organic matter into stable end-products, or to produce additional microbiological organisms

25
Q

Objectives and methods of secondary sedimentation

A

Remove any waste products following biological treatment

26
Q

2 types of secondary treatment

A
  1. Fixed Film- where bacteria are attached to fixed surfaces

2.Suspended Growth treatment- bacteria remain in suspension in the wastewater

27
Q

Examples of Fixed Film

A

AEROBIC CONDITIONS
Percolating Filters- Circular tanks filled with gravel
Bacteria grow on the surface of the gravel
Sprinkler system distributes wastewater

Rotating Biological Contactors
-series of circular disks rotated through wastewater
-Microorganisms grow on the surface of the disks and degrade the wastewater pollutants

28
Q

Examples of Suspended growth

A

Activated Sludge
Introducing air bubbles to feed bacteria, allowing them to consumer organic material

Oxidation Ditches

29
Q

Disposal of wastes removed from wastewater

A

Screening
-Often compressed to remove water, then disposed to landfill
-Sometimes dried and then burnt

Grit
-Either buried locally or disposed to landfill

Sludge
-Sometimes dried, often digested anaerobically. Then often spread on agricultural land
-Some sludges are incinerated or buried in landfill

30
Q

Wastewater treatment in developing countries

A

Stabilising Ponds

31
Q

Advantages of Waste Stabilising Ponds

A

-Low Cost
-Easy to maintain
-Excellent removal of faecal bacteria
-Well suited to warm climates

32
Q

Disadvantages of waste stabilising ponds

A

-Require large areas of land
-Insects may breed
-Unpleasant odours may be released

33
Q

Three different types of waste stabilisation ponds and order they typically occur in

A

Anaerobic
Facultative
Maturation

A-F-M-M-M

34
Q

Characteristics of Anaerobic ponds

A

-Only required where there is large volumes of strong waste

-Deep (2-5m) to allow for anaerobic conditions

-Solids sink to the bottom where they are digested anaerobically

-Retention time 1-5 days

-Desludging every 3-5 years due to high amounts of solids

-Sometimes a crust forms on the top- this helps prevent oxygen

35
Q

Characteristics of Facultative Pond

A

-Usually largest in the system

-1-2m deep, upper levels aerobic and lower regions anaerobic

-Retention time 20-40 days

-Solids sink to the bottom where them are digested anaerobically

36
Q

Characteristics of maturation pond

A

-Principally aerobic

-Shallow (1-1.5m) to allow sunshine

-Main function is the destruction of pathogens

-Retention time 3-6 days each (normally a series of about 3)

CVC109 Water (13 decks)

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