Waste BATs for Water

Question 1

What is clarification?

Answer 1

The removal of solids from the effluent

Question 2

What steps are involved in clarification in wastewater treatment works?

Answer 2

• Primary treatment of the effluent including primary settlement
• Effluent from digesters or precipitation stages

It is any step that involves the removal of solids in the treatment of the wastewater.

Question 3

What is sludge?

Answer 3

Sludge are the solids contained in effluent from digesters. It is made of suspended microorganisms (biomass that have fed and grown on the BOD).

Question 4

Give some techniques of solid removal in wastewater treatment works.

Answer 4

• Screening which is used for larger solids
• Filtration for example in deep bed sands
• Sedimentation

Question 5

How is sedimentation achieved in settler tanks?

Answer 5

The solids settle to the bottom and then drain out of a hole in the centre from the inclined slopes. Clarified water remains on the top surface. A weir then collects the clarified water in spillover.

Question 6

What is the settling velocity of particles dependent on?

Answer 6

Density and size

Question 7

For biomass what is more critical for the settling velocity: density or size?

Answer 7

Size because the density of the biomass is similar to that of water.

Question 8

What is the problem of particles with a size of 1 micrometre or less?

Answer 8

It takes them too long to settle. This is especially significant when considering the bacterial sludge from a digester.

Question 9

What is one possible solution (albeit unrealistic) for the long settling times for bacterial sludge?

Answer 9

Centrifugation. However, this technique would be very expensive to install on a large scale.

Question 10

What is a far more realistic solution to solve the long settling times for bacterial sludge?

Answer 10

Flocculation. This is the addition of flocculating agents so that the smaller particles agglomerate into larger flocs.

Question 11

Describe the sludge which is produced by flocculation.

Answer 11

It is still very watery which can be concentrated by filtration - press belt or deep bed sand filters which retain solids.

Question 12

How do flocculating agents work?

Answer 12

They add counterions to be able to neutralise the surface charges of particles.

Question 13

What are some traditional examples of flocculating agents.

Answer 13

Al2 (SO4)3 and FeCl3

Question 14

What is the relationship between the charge of the ion and their effectiveness as flocculating agents?

Answer 14

The more charged, the more effective as a flocculating agent. One positive charge makes the flocculation process 10 times more effective.

Question 15

In which two streams can the flocculating agent end up?

Answer 15

• The treated water

- The sludge

Question 16

How do we remove flocculating Al 3 from water if necessary?

Answer 16

It is precipitated in alkaline conditions from the addition of Ca(OH)2

Question 17

How do we remove flocculating agent that ends up in sludge?

Answer 17

We minimise the amount of coagulant that we are using in the first place. This avoids the high content of heavy metals in sludges.

Question 18

What is the alternative to using heavy metals as flocculating agents?

Answer 18

Use polymers

Question 19

What will happen with the light and heavy hydrocarbons in an oil slick?

Answer 19

Light hydrocarbons will evaporate. Heavier hydrocarbons are broken down by microbes. But the microbes need the right temperatures and nutrients such as N, P etc.

Question 20

What is the first priority in removing hydrocarbons from water?

Answer 20

Prioritise installing a physical barrier as this has less chemical and energy inputs and there will be a comparatively high recovery.

Question 21

What is the problem with liquid hydrocarbons in water?

Answer 21

They disrupt cell membranes and kill organisms in waste water treatment.

Question 22

What are the two phases in which the liquid hydrocarbons can become distributed?

Answer 22

• Some in their own phase which is dispersed droplets.

- Some dissolved in water

Question 23

How do we remove dispersed oil i.e. the hydrocarbons that are dissolved within their own phase?

Answer 23

Flotation and assisted centrifuge settlers

Question 24

How do we remove dissolved oil?

Answer 24

• Stripping but this is heat and energy intensive

- Adsorption on activated carbon and the activated carbon is then regenerated using steam stripping.

Question 25

Other than reducing solid content and BOD what are the other benefits of solid removals?

Answer 25

The removal of heavy metals and persistent organic pesticides which tend to get incorporated into solid particulates and oily droplets.

Question 26

What other considerations must be made for oily droplets and solid particulates?

Answer 26

• pH
• Heavy Metals
• Temperature

Question 27

How much of a problem is pH for oily droplets and solid particulates?

Answer 27

Typically not much of a problem. Use the cheapest available materials to neutralise which would typically be calcium carbonate for acid wastes.

Question 28

How do heavy metals end up in oily droplets and solid particulates?

Answer 28

Catalysts, erosion of internal surfaces, combustion

Question 29

How do we separate heavy metals?

Answer 29

They can be precipitated as some salt with a low solubility product. However precipitated crystals are small and difficult to filter out.

Alternatively we could use combined techniques so increase the concentration by reverse osmosis and then precipitate.

Question 30

How are reed beds a composite treatment method?

Answer 30

Roots of the plant acts as a habitat and they are good for dealing with BOD and metals and contaminants. These accumulate in the root system and thus must be periodically dug out and dried.

Question 31

What is digestion?

Answer 31

Biological degradation of BOD-producing microbial biomass. This is known as sludge.
i.e. digestion is the degradation of sludge

Question 32

What are the two forms of digestion?

Answer 32

• Aerobic digestion

- Anaerobic digestion

Question 33

Describe aerobic digestion

Answer 33

Naturally occurring bacteria will use the soluble body as a carbon and energy source. Oxygen supply and mass transfer are crucial aspects in the design.

Question 34

Describe anaerobic digestion.

Answer 34

Specialised microbes convert both solid BOD and soluble BOD into useful CH4 gas. Oxygen must be excluded.