Lecture 9 Activated Sludge process Flashcards
What 3 mechanisms does the activated sludge process use for the removal of organic matter
- Adsorption and also agglomeration onto microbial flocs
- Assimilation which is the conversion to new microbial cell material
- Mineralization which is complete oxidation
Discuss conditions which favour assimilation
These remove substrate by assimilating it to biomass. This results in a higher proportion of the operating cost required for sludge separation and disposal
Discuss conditions which favour mineralization
The volume of biomass is reduced under endogenous respiratory conditions. This results in lower sludge handling costs but higher aeration costs
What are 6 general design considerations
- Reactor selection
- Loading Criteria
- Sludge production (and therefore removal)
- Aeration requirements (Supply and mass transfer)
- Nutrient requirements (supplementation)
- Discharge requirements (treated water quality)
6 considerations for reactor selection
Reactor kinetics Oxygen - transfer requirements Type of wastewater Local environment Mode of secondary settling Costs (Construction, operation, maintanence)
What is the Loading criteria and name the considerations (2)
Food to micro-organisim ratio
Mean cell residence time
5 considerations for sludge production
High sludge concentrations promote high treatment rates but can lead to:
High pumping costs
High oxygen demands
High disposal costs (30% operating)
Dumping at sea or to landfill has been phased out
Dumping on land has problems - heavy metals may accumulate or too much material to dispose of
6 Aeration requirements
Oxygen demand
Proportional to the amount of biomass and rate of growth
can therefore be determined from the BOD of the waste, and the amount of excess sludge produced
Oxygen transfer must be able to
Satisfy BOD of the waste
Satisfy endogenous respiration of the biomass
provide adequate mixing
maintain a minimum level of dissolved oxygen; typpicall 1-2 mg/L
4 Assumptions when designing the activated sludge process
V is the effective reactor working volume, where all the microbiology and biochemistry will take place
There is no separation effect across gravity settling stage with respect to substrate or DO, so therefore S = Se = Sr = Sw and O = Oe = Or = Ow
There is no effective biomass in the feed stream therefore Xf = 0
There is no DO in the Feed stream therefore Of = 0
Using notes go through mass balances on Biomass, substrate and O2
Notes
Using notes go through the dynamic model and the steady state model
notes
Draw the activated sludge general diagram
Slide 9
What are the advantages of plug flow (2)
- no short circuiting within the aeration tank, although baffles can increase the HRT even more
- plug flow systems produce sludges with good settleability
What are the disadvantages of plug flow (3)
- DO deficiency occuring at the inlet where oxygen demand is greatest
- oxygen is usually in excess at the outlet where oxygen demand is least
- toxic loads are not diluted they pass through as a discrete plug which results in effects on performances
Advantages of complete mixed reactors (3)
- minimising the effects of toxic loads
- MLSS concentrations are higher permitting higher BOD loadings
- nitrification is possible
