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.
Other than reducing solid content and BOD what are the other benefits of solid removals?
The removal of heavy metals and persistent organic pesticides which tend to get incorporated into solid particulates and oily droplets.
What other considerations must be made for oily droplets and solid particulates?
- pH
- Heavy Metals
- Temperature
How much of a problem is pH for oily droplets and solid particulates?
Typically not much of a problem. Use the cheapest available materials to neutralise which would typically be calcium carbonate for acid wastes.
How do heavy metals end up in oily droplets and solid particulates?
Catalysts, erosion of internal surfaces, combustion
How do we separate heavy metals?
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.
How are reed beds a composite treatment method?
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.
What is digestion?
Biological degradation of BOD-producing microbial biomass. This is known as sludge.
i.e. digestion is the degradation of sludge
What are the two forms of digestion?
- Aerobic digestion
- Anaerobic digestion
Describe aerobic digestion
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.
Describe anaerobic digestion.
Specialised microbes convert both solid BOD and soluble BOD into useful CH4 gas. Oxygen must be excluded.
