1. Wastewater Treatment Activated sludge
Professor Nick Gray
Centre for the Environment
Trinity College
University of Dublin
© Tigroney Press

2. Wastewater Treatment Activated sludge
Learning objectives:
To explain the operational basis of this unit process
To understand the importance of f/m to operation
Fully comprehend the concept of floc formation
To understand ecology of the process
To be able to conduct basic assessment procedures
To be able to identify the main design types.

3. Wastewater Treatment: activated sludge
Process overview
The most widely used process worldwide is activated sludge
Comprised of five main components:
Reactor
Mixed liquor (MLSS)
Aeration/mixing
Settlement tank
WAS/RAS streams

4. Wastewater Treatment: activated sludge

5. Wastewater Treatment: activated sludge

6. Wastewater Treatment: activated sludge

7. Wastewater Treatment: activated sludge
The operation management of activated sludge and level of treatment achieved is based on the microbial growth curve

8. Wastewater Treatment: activated sludge

9. Wastewater Treatment: activated sludge
Process control:
Biomass control
MLSS or MLVSS
SRT or Sludge age
Plant Loading
Volumetric
Organic
Sludge
Sludge settleability
SVI
SSVI
Sludge assessment
Microbial activity (SOUR)
Morphological
microbiological

10. Mixed Liquor Suspended Solids (MLSS)
Wastewater Treatment: activated sludge
Mixed Liquor Suspended Solids (MLSS)
Crude measure of biomass (mg L-1 or g m3)
Used to calculate other operating parameters
Used daily to ensure optimum biomass in reactor and controls sludge wastage
Concentration in aeration tank controlled by sludge wastage
Operational concentration depends on operational mode
Low rate systems: 1,500-3,500 mg L-1
High rate systems: up to 8,000 mg L-1
Higher the concentration the greater the efficiency of substrate removal
Optimum MLSS determined by
Availability of O2 (aeration efficiency)
Ability of sedimentation tank to separate and recycle biomass
Does not distinguish between active and non active fraction of sludge activity
MLVSS – volatile biomass (500oC)

11. Wastewater Treatment: activated sludge
Sludge age
Controls the activity of the sludge and also degree of secondary colonization
Sludge age = Total amount of biomass (MLSS) in the system
Rate of loss of Biomass from system
Sludge age :
Controlled by altering sludge wastage rate
<0.5 d (Low) biomass has high growth rate + high volume of waste sludge produced
>0.5d (High) biomass has low growth rate + low volume of waste sludge produced
Conventional systems 3-4d for good removal of substrate, low biomass production and good settling properties

12. Sludge Loading rate – food:micro-organism ratio
Wastewater Treatment: activated sludge
Sludge Loading rate – food:micro-organism ratio
Where Q is the influent flow rate (m3d-1) , BOD the biochemical oxygen demand of the influent (g m-3), V the volume of the aeration tank (m3) and X the mixed liquor suspended solids (MLSS) concentration (g m-3).
The lower the f/m ratio the lower the rate of metabolism and the greater the rate of BOD removal and the better sludge settleability.

13. Wastewater Treatment: activated sludge
Relationship between f/m and final effluent quality

14. Range of process operating parameters
Wastewater Treatment: activated sludge
Range of process operating parameters

15. Wastewater Treatment: activated sludge
Sludge Settleability
Sludge Volume Index (SVI)
One litre graduated cylinder filled with mixed liquor.
Settlement for 30 minutes
Volume of settled sludge (V) measured in mL
What the values indicate:
>120 ml g-1 = poor settlement (bulking)
<80 ml g-1 = Good settlement
<50 ml g-1 = Excellent settlement

16. Wastewater Treatment: activated sludge

17. Wastewater Treatment: activated sludge
Stirred Specific Volume Index (SSVI)

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Floc is treatment unit
Bacteria are held together by ECPs
Adsorption
Release of ECEs by non-viable bacteria
Absorption
Secondary colonization (metazoa) as floc ages
Flagellate, ciliate protozoa
Rotifers, nematodes
Active bacteria on outer surface , non-viable cells in centre which retain active enzyme systems

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21. Wastewater Treatment: activated sludge
Floc structure and health is vital not only for good substrate removal but also for good separation of biomass from the treated effluent
Most operational problems manifest themselves as changes in biomass structure
These are then related to basic factors such as
Wastewater characteristics
Sufficient oxygen
Sufficient food
Trace elements
Temperature
Toxicity

22. Wastewater Treatment: activated sludge
Concept of floc structure is vital in optimizing performance
Presence of filaments is also crucial

23. Wastewater Treatment: activated sludge

24. Wastewater Treatment: activated sludge
25 species frequently recorded
Commonest: Types 0041, 021N, Nocardiforms, M. parvicella and H. hydrossis
Associated with floc structure:
Bridging : Types 021N , 0961, S. natans, H. hydrossis
Open floc: Types 1701, 0675, N. limicola, M. parvicella

25. Dominant filament type can indicate conditions causing bulking
Wastewater Treatment: activated sludge
Dominant filament type can indicate conditions causing bulking
Low DO S. natans, H. hydrossis, Type 1701, M. parvicella
Low f/m Types 021N , 0041, 0675, 0092, 0961, 0803,0961, H hydrossis , M. Parvicella, Nocardia spp.
Septic/ sulphide Thiothrix spp., Beggiatoa, Type 021N
Low pH Fungi
Nutrient deficiency Thiothrix spp., S. natans, Type 021N (also possibly H. hydrossis, Types 0041,0675)

26. Wastewater Treatment: activated sludge
Filament density
Direct correlation between TEFL and poor settleability
Physical length of filaments assessed, total extended filament length (TEFL) um per g MLSS or um per L MLSS
Subjective index
Good correlation to TEFL based on series of images and rated 1-6 or 1-10

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Secondary colonization is normally a function of f/m ratio and sludge age

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Nitrification
Nitrification also takes place in aerobic reactors
Require higher oxygen concentration
Very susceptible to inhibition
Slow growing so can be overgrown by heterotrophs
Nitrosomonas
Nitrobacter
Denitrifcation
Takes place in anoxic reactors with <0.1 mg/l oxygen
Fast and robust reaction involving wide range of facultative bacteria
Like all bacterial reactions - temperature dependent

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Underlying principle of reactor design

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Oxidation ditches

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33. Wastewater Treatment: activated sludge Sequencing Batch Reactors
(SBRs)

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Conclusions
Activated sludge is a microbial system using a wide range of bacteria
Treatment is by adsorption followed by absorption leading to respiration or synthesis
Mesofauna, predominately protozoa, colonize the biomass and improve clarity by removing bacteria (including pathogens) as well as fine particulate matter
The rate of reaction and ecological balance of the system is controlled by the f/m ratio Activated sludge is an aerobic process
By incorporating anoxic and anaerobic phases within reactors denitrification and enhanced phosphorus uptake can be achieved