1. SECONDARY TREATMENT
Main aim is to remove BOD (organic matter) to avoid oxygen depletion in the recipient
Microbial action
Aerobic/anaerobic microorganisms that decompose organic material
Aerobic degradation is much faster and easier to control
Activated sludge treatment – bacteria suspended in the wastewater (most common type of biological WWT)
Sludge contains bacteria
Activated because they are hungry (spend some time without food (easily biodegradable organics) in the settling tank)

2. ACTIVATED SLUDGE TREATMENT
Organic material + O2 + nutrients + microorganisms  new cells + CO2 + H2O
Same process occurs in nature
Protection of water quality
Controlled process
Intensified process
Bacteria are reused – recycling from secondary clarifier (recycling sludge or returned activated sludge)
Microbial growth (continuous food supply) – bacteria have to be removed  waste activated sludge (excess sludge)

3. AERATION TANK Oxygen has to be provided – aeration tank (reactor)
Wastewater = liquid containing food (organic pollution)
Biomass (bacteria – concentrated by recycling)
Combination of the liquid and microorganisms undergooing aeration = mixed liquor
The suspended solid = mixed liquor suspended solid (MLSS)
Biomass is mostly organic material – it can be measured as VSS (volatile suspended solid) - MLVSS

4. ACTIVATED SLUDGE PLANT
recycling sludge
Q3, S3, X3
Q1, S1, X1
V2, X2, S2
Q4, S4, X4 O2
Q5, S5, X5
excess sludge
inlet
effluent
aeration tank
sedimentation basin
Q: wastewater volume (m3/d)
S: BOD5 concentration = soluble substrate (mg/L  S1= mg/L)
X: concentration of biomass (sludge) (mg/L, g/L  X2=3-6 g/L)
V: volume (m3)

5. Q3, S3, X3
Q1, S1, X1
V2, X2, S2
Q4, S4, X4
Q5, S5, X5
Mass balance: Inflow rate = outflow rate
Q1 = Q = Q3+Q5  Q3 = Q1-Q5 (Q3 = Q-Qw)
In the aeration tank and in outflow streams the same dissolved organic matter (substrate) concentration
S2 = S3 = S4 = S5 = S (there is a profile in AS tank)
In waste streams the same biomass concentration
X4 = X5 = XR

6. Q, S0, X0 (Q-Qw), S, Xe V, X, S Qw, S, XR Qr, S, XR
V: reactor (aeration tank) volume, m3
Q: influent flow rate (m3/d)
X0: concentration of biomass in the effluent (g VSS/m3)
Qw: waste sludge flow rate(m3/d)
Xe: concentration of biomass in effluent (g VSS/m3)
XR: concentration of biomass in return line from clarifier (g VSS/m3)

7. Treatment efficiency (in terms of soluble BOD): Recycle rate:
(Q-Qw), S, Xe
Q, S0, X0
V, X, S
Qr, S, XR
Qw, S, XR
Treatment efficiency (in terms of soluble BOD):
E = (S0-S)/S0
Recycle rate:
ratio of return sludge volume to raw wastewater volume
R = Qr/Q

8. Volumetric organic loading rate (volumetric load):
(Q-Qw), S, Xe
Q, S0, X0
V, X, S
Qr, S, XR
Qw, S, XR
Volumetric organic loading rate (volumetric load):
Organic matter BOD (or COD) applied to the aeration tank volume per day
BV= Q×S0/V = kg BOD5/m3d

9. Sludge load = F/M (food to microorganisms) ratio:
(Q-Qw), S, Xe
Q, S0, X0
V, X, S
Qr, S, XR
Qw, S, XR
Sludge load = F/M (food to microorganisms) ratio:
Organic matter load applied to unit mass of sludge (biomass)
BX = Q × S0 / (V × X) kg BOD5/kg VSS/d
kg BOD5/kg VSS/d  high load
kg BOD5/kg VSS/d  normal load
kg BOD5/kg VSS/d  low load

10. Sludge production (we grow bacteria: product-inflow):
(Q-Qw), S, Xe
Q, S0, X0
V, X, S
Qr, S, XR
Qw, S, XR
Sludge production (we grow bacteria: product-inflow):
FSP = Xe×(Q-Qw)+ XR×QW – (X0×Q0)
can be expressed by yield (= g biomass produced/g substrate utilized)
Excess sludge production:
QW×XR 0

11. Sludge age (solid retention time):
(Q-Qw), S, Xe
Q, S0, X0
V, X, S
Qr, S, XR
Qw, S, XR
Sludge age (solid retention time):
average time during which the sludge has remained in the system
SRT = X = (V × X)/ (Xe×(Q-Qw) + XR×QW) [d]
kg sludge present in the aeration tank
sludge leaving the system kg/d
SRT < 2 days  high load
SRT = 2-6 days  normal load
SRT > 6 days  low load

12. SLUDGE VOLUME INDEX

13. SLUDGE VOLUME INDEX
SVI = (settled volume of sludge, mL/L)(1000 mg/g)/(suspended solids, mg/L) = mL/g

14. SLUDGE VOLUME INDEX
Mixed liquors with a 3000 mg/L TSS concentration that settles to a volume of 300 mL in 30 minutes in a 1-L cylinder would have an SVI of 100 mg/L – good settling characteristics
SVI>150 filamentous growth

15. TERTIARY TREATMENT (PHYSICO-CHEMICAL TREATMENT)
pH control
Nitrogen removal (ammonia stripping)
Stripping (volatile organic compounds)
Filtration
Adsorption
Chemical phosphorus removal

16. pH CONTROL
Treatment processes (biological, chemical) have optimal pH range
Communal wastewaters: pH between
If not  pH adjustments (biological treatment)
+ Assuring the stability of pH (buffering capacity) – NaHCO3

17. pH CONTROL CaO, Ca(OH)2 = lime NaOH H2SO4 HCl CO2
Cheap, but with CO2  CaCO3 (not suitable for pH control)
Only rough control
NaOH
Expensive
No effect of CO2
Fine control
H2SO4
Easy to calculate and apply accurately
Concentrated sulphuric acid is dangerous
HCl
Good control
Volatile – corrosion
CO2
Very rough control (weak acid)

18. STRIPPING
Mass transfer of a gas from the liquid phase to the gas phase
The liquid is stripped with another gas (usually air)
Removal of ammonia, odorous gases, volatile organic compounds

19. STRIPPING Nitrogen (ammonia) removal
at high loaded biological WWTP or at low temperature there is no significant nitrification
ammonia stripping is the most suitable
ratio of ammonia (gas) - ammonium depends on pH
NH4+  NH3 + H+
between pH mostly NH3

20. STEPS OF NH3 STRIPPING pH control ( increase it to 10.5-11.0)
– usually by lime
air stripping (intensive aeration of water)
sedimentation (calcium-carbonate + other solids)

21. STRIPPING OF VOLATILE COMPOUNDS
Hydrocarbons
Organic solvents
Chlorinated hydrocarbons
Aromatic compounds

22. STRIPPING OF HYDROGEN SULPHIDE
The problem of hydrogen sulphide – odour, corrosion
Stripping and oxidation
2H2S + O2  2S + 2H2O
Other possibilities
pH change (liming)
Oxidation of H2S (nitrate feeding)
Conversion to non-soluble form (precipitation with iron(III)-hydroxide)

23. SULPHIDE REMOVAL DUE TO PRECIPITATION WITH IRON-SALTS
Fe(III) ion reacts easily with sulphide ions - weakly water soluble compounds are formed
dissolved sulphides can be precipitated rapidly in the presence of ferric salts ( mmol/L)
<5 mg/L sulphide in raw sewage - less than 0.1 mg/L after treatment
precipitation is not significant in case of aluminium salt

24. ADSORPTION
Adsorption is the physical and/or chemical process in which a compound is accumulated at an interface between phases (solid-liquid interface)
Adsorbate: the substance being removed from the liquid phase to the interface
Adsorbent: the solid phase on which the accumulation occurs

25. ADSORPTION
Ion exchange adsorption = ions of a given species are displaced from an insoluble exchange material by ions of a different species in solution - softening
Ion change on natural matters (zeolite) – softening, ammonia removal
Ion change on ion exchange resin (synthetic aluminosilicates, phenolic polymers)
Adsorption on activated carbon
PAC
GAC adsorbers

26. ADSORPTION ON ACTIVATED CARBON
Activated carbon is able to remove dissolved organic substances from water directly. The problem is that activated carbon is not specific for pollutants, so during the activated carbon adsorption a lot of natural, non-pollutant type organic matters will be removed as well. The activated carbon adsorption is almost the only process for removal of organic micropollutants from water.
The activated carbon is applied in two forms: powdered activated carbon = PAC (one time use) and granular activated carbon = GAC (usable till saturation of adsorbent). The granular activated carbon has higher specific organic matter removal capacity than powdered activated carbon. Although the powdered activated carbon is cheaper than granular, for long time, permanent applying the granular activated carbon is more economical.

27. ACTIVATED CARBON GAC PAC
granulated – filled into a tower
powdered – mixed into the water
chance of contact is high  more efficient adsorption  widespread
GAC
Water flows through the column filled with granulated activated carbon
chance of contact is lower (in the water it is not possible to achieve as high activated carbon concentration as in GAC) - cheaper
PAC
Activated carbon powder is mixed into the water

28. CHEMICAL WASTEWATER TREATMENT
Definition of chemical wastewater treatment:
WIDER SENSE:
treatment of wastewaters with chemical methods
chemical coagulation
chemical precipitation (removal of P and heavy metals)
chemical disinfection
advanced oxidation processes
ion exchange
chemical neutralization
MORE SPECIFIC:
addition of Fe-, Al-, Ca-, Mg-salts with the aim of phosphorus or organic removal

29. CHEMICAL PHOSPHORUS REMOVAL
Phosphorus removal (chemical precipitation)
Al3+ + PO43-  AlPO4
= converting of dissolved P compounds to a low solubility metal
phosphate (through use of a metal salt)
Precipitants:
Aluminium salts
Iron salts
Lime

30. CHEMICAL PRECIPITATION OF PHOSPHORUS
Precipitation chemicals precipitate the dissolved inorganic phosphates as insoluble compounds (to be more exact: compounds with small solubility)
At the same time metal-hydroxides are formed
 jelly-like flocs which bind the precipitated metal phosphates and any other suspended substances in the water (coagulation-flocculation)
This also removes organically combined P, as the amount of suspended matter is greatly reduced by chemical precipitation

31. CHEMICAL PHOSPHORUS REMOVAL
Phosphorus removal (chemical precipitation)
Al3+ + PO43-  AlPO4
Removal of organic matter (coagulation-flocculation)
Al3+  aluminium-hydroxide
Good coagulant: contacts suspended matters (mainly organics) of wastewater rapidly and strongly
Organics are originally mainly in colloidal form – do not settle well – settling characteristics can be improved due to coagulation-flocculation

32. Coagulation:
destabilization of the colloidal particles
Flocculation:
increase the size of flocs

33. CHEMICAL TREATMENT as the only treatment process
primary (direct) precipitation
or in combination with biological treatment processes
pre-precipitation
simultaneous precipitation
post-precipitation
significant part of the organic pollutants is connected to suspended solids
 increasing of their removal efficiency in the primary settling tank results low organic pollutant load in the activated sludge processes

34. CHEMICAL PHOSPHORUS REMOVAL
Addition of calcium
Usually in the form of lime (Ca(OH)2)
Reacts with the natural bicarbonate alkalinity to precipitate CaCO3
As pH increases beyond 10, excess Ca ions react with the phosphate to precipitate hydroxylapatite
10 Ca PO OH-  Ca10(PO4)6(OH)2
pH has to be adjusted back before biological treatment
No simultaneous P removal can be applied

35. CHEMICAL PHOSPHORUS REMOVAL
Addition of aluminium or iron
Al3+ + HnPO43-n  AlPO4 + nH+
Fe3+ + HnPO43-n  FePO4 + nH+
1 mole aluminium or iron ion will precipitate 1 mole of phosphate
Many competing reactions (the above ratio never occurs)
We can not estimate the required dosage based on stoichiometry
Dosages established based on bench-scale tests
Solubility of AlPO4 is the smallest around pH = 6
Solubility of FePO4 is the smallest around pH = 5

36. PRECIPITANT ADDITION

37. activated sludge basin
PRE-PRECIPITATION
screen
grit chamber
sedimentation
activated sludge basin
flocculator
metal salt
min
BOD removal  90%
TP removal > 90%

38. PRE-PRECIPITATION
Direct precipitation followed by a biological treatment stage
Introduced to biological treatment plants to reduce the loading to the biological stage
Reduction in energy consumption and in hydraulic retention time

39. SIMULTANEOUS PRECIPITATION
screen
grit chamber
sedimentation
activated sludge basin
metal salt
min
BOD removal: 90%
TP removal: %

40. SIMULTANEOUS PRECIPITATION
Phosphorus is chemically precipitated at the same time as biological treatment in an activated sludge process
The biological stage also serves as a flocculation tank, with both the biological and chemical sludge being separated in a subsequent stage
Results 1 mg/L TP

41. activated sludge basin
POST-PRECIPITATION
screen
grit chamber
sedimentation
activated sludge basin
metal salt
20 min
10 min
coagulation tank and flocculator
BOD removal  90%
TP removal > 95%

42. POST-PRECIPITATION
Phosphorus is separated from biologically treated water in a separate post-treatment stage
TP below 0.5 mg/L

43. COAGULATION-FLOCCULATION LABORATORY JAR TESTS
1-litre glass cylinders with Kemira's flocculator device
to compare the efficiency of different coagulants
to determine optimal dosage

44. TP REMOVAL

45. RESIDUAL CODCR CONCENTRATIONS VS. COAGULANT DOSAGE
dissolved CODCr :
mg/l
50-85% of the total organic matter