Waste BATs for Water Flashcards
What is clarification?
The removal of solids from the effluent
What steps are involved in clarification in wastewater treatment works?
- 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.
What is sludge?
Sludge are the solids contained in effluent from digesters. It is made of suspended microorganisms (biomass that have fed and grown on the BOD).
Give some techniques of solid removal in wastewater treatment works.
- Screening which is used for larger solids
- Filtration for example in deep bed sands
- Sedimentation
How is sedimentation achieved in settler tanks?
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.
What is the settling velocity of particles dependent on?
Density and size
For biomass what is more critical for the settling velocity: density or size?
Size because the density of the biomass is similar to that of water.
What is the problem of particles with a size of 1 micrometre or less?
It takes them too long to settle. This is especially significant when considering the bacterial sludge from a digester.
What is one possible solution (albeit unrealistic) for the long settling times for bacterial sludge?
Centrifugation. However, this technique would be very expensive to install on a large scale.
What is a far more realistic solution to solve the long settling times for bacterial sludge?
Flocculation. This is the addition of flocculating agents so that the smaller particles agglomerate into larger flocs.
Describe the sludge which is produced by flocculation.
It is still very watery which can be concentrated by filtration - press belt or deep bed sand filters which retain solids.
How do flocculating agents work?
They add counterions to be able to neutralise the surface charges of particles.
What are some traditional examples of flocculating agents.
Al2 (SO4)3 and FeCl3
What is the relationship between the charge of the ion and their effectiveness as flocculating agents?
The more charged, the more effective as a flocculating agent. One positive charge makes the flocculation process 10 times more effective.
In which two streams can the flocculating agent end up?
- The treated water
- The sludge
How do we remove flocculating Al 3 from water if necessary?
It is precipitated in alkaline conditions from the addition of Ca(OH)2
How do we remove flocculating agent that ends up in sludge?
We minimise the amount of coagulant that we are using in the first place. This avoids the high content of heavy metals in sludges.
What is the alternative to using heavy metals as flocculating agents?
Use polymers
What will happen with the light and heavy hydrocarbons in an oil slick?
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.
What is the first priority in removing hydrocarbons from water?
Prioritise installing a physical barrier as this has less chemical and energy inputs and there will be a comparatively high recovery.
What is the problem with liquid hydrocarbons in water?
They disrupt cell membranes and kill organisms in waste water treatment.
What are the two phases in which the liquid hydrocarbons can become distributed?
- Some in their own phase which is dispersed droplets.
- Some dissolved in water
How do we remove dispersed oil i.e. the hydrocarbons that are dissolved within their own phase?
Flotation and assisted centrifuge settlers
How do we remove dissolved oil?
- Stripping but this is heat and energy intensive
- Adsorption on activated carbon and the activated carbon is then regenerated using steam stripping.
