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Typical composition of untreated domestic wastewater
Total Solids ( 400 –1200 mg/L)
often a function of parasitic clearwater infiltration into the sewers
Inorganic (ash)
50%
Dissolved inorg.
35%
Non-settleable
inorganic
40%
Organic (volatile)
50%
Suspended (>1 µm) 45%
(15% inorg) (30% org)
Dissolved
organic
20%
Settleable (“>10µm”) Non-settleable
25% (10% inorg ;
organic
(15% organic)
35%
Trickling Filter
Activated sludge process
external medium
bacteria
hydrolysis through
exoenzymes
waste
constitutive
organic matter
Submerged aeration pump
Surface aerators
Submerged porous diffuser
Decoupling of the agitation / mixing function from the aeration provision
Activated sludge flocs
200 um
Activated sludge model
The modeling approach presented here is the
classical one, but variants exist.
Lawrence, A.W. and McCarty, P.L. (1970) Unified basis for biological treatment design and operation. J. Sanit. Engin. ASCE 96, 757–778.
We start from the simple case of a chemostat
which is a continuous stirred-tank reactor (CSTR).
For the moment we do not consider reinjecting the
newly produced biomass, back into the bioreactor,
after liquid-solid separation.
In that case the biomass budget simply reads:
Biomass budget
It can be shown that, in the case with recycling, exactly the
same rule applies, but with substitution of the hydraulic
residence time, by the cell residence time.
The importance of this formula is that it determines the
extent of newly produced sludge. One option is thus to play
on the negative term (endogenous decay) to try to
minimize the amount of biosolids wastes to be disposed
(see Table 15.4)
Based now on the
substrate budget, we find
if
if
It shows that the only two (professional) ways to operate
an activated sludge plant are to base on maintaining,
either a constant MLSS, or a constant sludge age. This is
the so-called Walker rule.
Food-to-Microbe ratio
kg BOD5/kg MLSS.d
Sludge Volume Index, SVI
(volume of sludge after 30 min. settling, ml) x 1000
SVI =
mg/L suspended solids
A mixed liquor has 4000 mg/L suspended solids. After 30
minutes of settling in a 1 L cylinder, the sludge occupied
400 ml.
SVI = (400 x 1000)/ 4000 = 100
Good settling if SVI < 100, if SVI > 200 …. problems
Bulking sludge, excessive development of filamentous bacteria,
poor settleability of the flocs.
If residence time is excessive (> 2hr) the risk in
secondary clarifier is that denitrification takes
place, generating N2 microbubbles which can
float sludge flocs. What we obviously want to
avoid is sludge loss into the receiving water
body we actually wanted to protect in the first
place.
On site, however, this problem should be
differentiated from the bulking one (due to
proliferation of filamentous bacteria).
To reach good nitrification rates, it is necessary to work at a low F/M ratio. The biomass
concentration thus needs to be higher than in the conventional process. These requirements
have consequences regarding the subsequent sludge manipulation options and disposal routes.
submerged membrane modules (Kubota ®)
0.4 µm porosity
allows improved separation of newly produced biosolids from final
treated effluent (even if activated sludge flocs have a poor settling
behaviour, chronic or episodic)
Recent evolution of biological wastewater treatment processes
- 1925
- 2010 Anammox (ANaerobic AMMonium OXidation)