Sludge Settling Problems Flashcards
1
Q
Well settling sludge
A
- if settling velocity > overflow velocity
- formation/presence of firm, compact, heavy flocs
- filamentous backbone theory
> backbone of floc = filamentous bacteria (macrostructure) of which
> a variety of floc forming bacteria (microstructrue) are attached as microcolonies - glued together by extracellular Polymeric Substances (EPS) which are produced bybacteria resulting from cell decay macromolecules
2
Q
What are the consequences of sludge settling problems
A
Bad separation of sludge and purified water:
- sludge escaping with the effluent
- a shortage of sludge in the biology (decrease of biodegradation capacity)
- not enough dewatering of sludge (higher sludge treatment costs)
3
Q
Types of sludge settling problems
A
- Dispersed growth
- Pinpoint flocs
- Rising sludge
- Vicious bulking and foaming
- Filamentous bulking
4
Q
Dispersed growth
A
- = no cell aggregation
- caused by low EPS production due to a high concentration of biodegradable substrates in the influent
- and toxic components in influent
- occurrence in conventional WWTPs is rare (only at very low sludge retention times)
5
Q
Pinpoint flocs
A
- = if the amount of filaments is too low (very weak flocs that break up easily)
- causes absence of filaments and high sludge residence times
-occurrence in conventional WWTPs is rare
6
Q
Rising Sludge
A
- = well settling sludge can start to rise due to the production of nitrogen gas in the settler if anoxic conditions is present
- caused by high sludge concentration (rapid oxygen consumption)
- if the gas has disappeared, sludge will settle again
7
Q
Vicious bulking and foaming
A
- =production of foam and slimy sludge
- causes include
> Zooglea is known to produce high amounts of biopolymers which make the sludge flocs slimy and which can withold a lot of water
> Nocardia, Rodococcus, Microthrix parvicella produce tensio-active biopolymers = biosurfunctants and will induce foam production. A stable foam layer will develop which will attract and retain biomass
-occur very frequently
8
Q
Filamentous Bulking
A
- = when the filamentous organisms overgorw the floc forming one
- consequences include
> open floc structures (too many filaments in the floc center and water retention)
> filaments can protrude from the floc and interact with other filaments to form a network
9
Q
Which bacteria cause filamentous bulking
A
- Aerobic bulking inducers
> strict aerobic bacteria who easily degrade readily biodegradable matter
> high affinity for oxygen - Foam forming actinomycetes
> aerobic bacteria of which growth is favoured by specific influent components - Low F/M types
> sludge loading is low - presence of long fatty acids
- Sulfide oxidising organisms
> presence of sulfides
10
Q
Causes of filamentous bulking
A
- low oxygen concentration
- shortage of nutrients
- low sludge loading
- high sludge retention times (N-removal)
- presence of sulfides
- too low a pH
11
Q
Filamentous bulking: Kinetic selection theory
A
- microbial growth in WWTP = Monond kinetics
- difference in maximum specific growth rate constants (u#max) and affinity constants (Ks) between floc forming and filamentous bacteria
- floc forming organisms have high u#max values = fast growth when a lot of substrate is available
- floc forming organisms have high Ks values = low affinity for the substrate
- low sludge loading, low nutrient concentration and low oxygen concentration can be explained directly
12
Q
Filamentous bulking: Intermittently aerated systems (N-removal)
A
- due to the presence of nitrite, or more specifically the (intracellular) presence of the intermediate NO:
> if the denitrification during the not-aerated phase was not fully completed
> NO can even be produced during aerobic denitrification starting from nitrite - NO inhibits the oxygen uptake of aerobic sludge
- some of the filamentous organisms reduce nitrate only to nitrite and not further: they do not accumulate NO (-> no inhibition)
13
Q
Filamentous bulking: presence of reduced S-components
A
- sulfides are toxic -> inhibition of floc formers
- in the presence of oxygen or nitrate, some organisms can use sulfides as e-donor
- presence of sulfides
> if the incoming water has been through an anaerobic phase in sewer
> formed by sulfate reducing organism (the center of large flocs = anaerobic)
14
Q
Filamentous bulking: too low a pH
A
- in acid wastewater streams
- growth of fungi
- hyphen will have the same negative effects as the filamentous organisms
- the problem is not common
15
Q
Monitoring of filamentous bulking
A
- Microscopy
> Eikelbooms classification based on cell diameter, presence of S granules, gram coloring and coloring of polyphosphates - Sludge Volume Index
> the volume occupied by 1g of sludge (MLSS) after 30 minutes of settling in a 1L cone
> for SVI > 200, diluted (DSVI)
> DSVI = (sludge volume30*2^n)/MLSS with n the number of two-fold dilutions - Molecular-genetic techniques
> Fluorescent in Situ Hybridisation
> based on specific DNA probes that recognise specific nucleotide sequences in the bacteria cell
> probe is labeled with a fluorescent dye
> advantage is the high specificity without isolation
> disadvantage is the high cost and the need for specific probes
