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SECONDARY TREATMENT Main aim is to remove BOD (organic matter) to avoid oxygen depletion in the recipient Microbial action Aerobic/anaerobic microorganisms.

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Presentation on theme: "SECONDARY TREATMENT Main aim is to remove BOD (organic matter) to avoid oxygen depletion in the recipient Microbial action Aerobic/anaerobic microorganisms."— Presentation transcript:

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)

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

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


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

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)

Hydrocarbons Organic solvents Chlorinated hydrocarbons Aromatic compounds

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)

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

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.

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

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

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

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

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

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

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


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

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

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

1-litre glass cylinders with Kemira's flocculator device to compare the efficiency of different coagulants to determine optimal dosage


dissolved CODCr : mg/l 50-85% of the total organic matter

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