Download - Lectut CE 202 PDF Tertiary Treatment
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WASTEWATER TREATMENT – TERTIARY TREATMENT
PROCESS
SUDIPTA SARKAR
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Typical Process flow Diagram –
Different Treatment Blocks
Bar Screens Grit RemovalPrimary Clarifier
O2
Aeration
tank
Secondary
ClarifierNutrient
Removal
D
I
S
P
O
S
A
L
Dewatered
Sludge to
landfill
AnaerobicDigester
Gravity Sludgethickener
Filter Press
Screenings Grit
PRELIMINARY PRIMARY SECONDARY TERTIARY
Advanced
Treatments
SLUDGE PROCESSING
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Treated Wastewater Effluent Still Contains…
• A portion of initial organic load (residual BOD)
– Carbon matter, depletes O2, causes biomat growth
• TSS (total suspended solids)
– Depletes O2• NH3 (ammonia)
– Toxic to fish, depletes O2, a nutrient that promotes biological growth
• NO3- (nitrate)
– Toxic to babies, drinking water regulated, a nutrient
• Total P (total phosphorus)
– A nutrient
• Pathogens (bacteria/viruses)
– Disease causing
There is a need of tertiary treatment to tackle the contaminants still remaining
in the treated effluent
• To better protect public health and environment from creating a potentially
hazardous condition such as eutrophication
• To provide additional treatment when soils or receiving waters cannot
naturally degrade the small amount of contaminant released.
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Problems associated with nutrients presence in wastewater are
• accelerating the eutrophication
• stimulating the growth of algae & rooted aquatic plants
• aesthetic problems & nuisance
• depleting D.O. concentration in receiving waters
• Toxicity towards aquatic life
• increasing chlorine demand
• presenting a public health hazard
• affecting the suitability of wastewater for reuse
Nutrients Removal
Basic nutrients present in the domestic wastewater are
– Nitrogen (ammonia, nitrite, nitrate)
– Phosphorus (soluble and insoluble. Orthophosphate, polyphosphate. Organic P)
– Sulfate
– Other compounds of nitrogen & phosphorus
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Control and Removal of Nitrogen (Biologically):
• Removal of Nitrogen by Nitrification/Denitrification Processes:
– It is a two step processes
Conversion of Ammonia to Nitrite (Nitrosomonas)
NH4+ + 1.5 O2 Bacteria (Nitrosomonous) NO2
- + 2 H+ + H2O
Conversion of Nitrite to Nitrate (Nitrobacter)
NO2- + 0.5 O2 (Nitrobactor) NO3
-Nitrification
Process
Denitrifying bacteria obtain energy from the conversion of NO3- to N2 gas,
but require a carbon source
NO3- + CH3OH + H2CO3 C5H7O2N + N2 + H2O + HCO3
-
(organic matter) (cell mass)
Denitrification
Process
There is another reaction, mediated by a special group of bacteria, known as ANAMMOX
(ANaerobic AMMonium OXidation) bacteria, recently discovered in the year 2000, which
directly produces nitrogen from nitrite as per the following reaction:
NH4+ + NO2- N2 + 2H2O
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SuspendedGrowth SeparateStage Nitrification
Single State Nitrification
Nitrification Processes
The following factors affect nitrification:
a) Conc. of NH4+ and NO2
-, b) BOD/TKN ratio (BOD should be gone/removed) ;
c) Dissolved oxygen conc. (need oxygen); d) Temperature; e) pH (7.5 to 8.6)
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• Need low (no) oxygen (< 1 mg/L)
• Need carbon source (Methanol or alternative)
• Neutral pH (pH 7)
• Concentration of nitrate
Denitrification
Separate-stage denitrification process using a separate carbon source
Air
Return SludgeReturn Sludge
Source of Organic
carbon (methanol or
alternative
NitrificationTank
Nitrification
Clarifier
Denitrification
Clarifier
Denitrification
tank
Effluent
Waste Sludge Waste Sludge
Anoxic Zone
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Combined Nitrification-Denitrification Process – Oxidation Ditch
The influent stream is discharged into the upstream limit of the anoxic zone , organicloading of the wastewater serves for the denitrification purpose. The wastewater at
the end of the anoxic zone has the organic load as well as ammonia source, it is put
into extended aerobic zone where nitrification occurs. Because there is no polishing
stage provided at the end of aerobic region for denitrification, the nitrogen removal
efficiency is low, about 50-60%. Effluent is taken just before the anoxic zone.
Controlling the Dissolved oxygen level is crucial to the efficiency of the system.
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Four-stage Plug Flow Bardenpho Process
Separate reaction zones are used for carbon oxidation nitrification and anoxic denitrification. The influent
initially enters into anoxic denitrification zone where nitrified mixed liquor is recycled from subsequent
combined carbon oxidation nitrification compartment. The carbon in the wastewater is used to denitrify
the recycled nitrate. Organic loading is high, the process of denitrification proceeds rapidly. Ammonia
passes on unchanged to the next compartment.
In the 2nd compartment, ammonia of the wastewater gets nitrified, because of extended aeration.
In the 3rd compartment, denitrification need is rather low; it is carried out anoxically by the utilization of
endogenous carbon source still remaining.
In the 4th compartment, aeration is done to strip the nitrogen gas out of the system, so that the final
sludge is not fluffy and settles down well in the clarifier.
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Enhanced Biological Phosphorus Removal
Phosphorous appears in the wastewater as orthophosphate (PO43-), Polyphosphate (P2O7)
and organically bound phosphorus. Polyphosphate and org-P account up to 70% of total P
in the wastewater. Microbes need P for cell synthesis and energy transport. 10-30 percentof P is thus removed in the secondary treatment process where new cells are synthesized
and wasted. In order to attain even lower P concentration at the outlet, additional uptake
of P by the microbes is required.
When some aerobic microbes such as Acinetobactor is subjected to anaerobic situation,phosphorus is released from the cells. If the microbes are then subjected to aerobic zone,
they tend to uptake more phosphorus than needed because they want to store more
phosphorus to counteract the future stressed situation.
Thus, microbes when subjected to alternate anaerobic and aerobic situations, tend to
extract more P from the wastewater.
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PAO- Phosphorus
accumulating
organism
A/O Process for P removal
Net Result:
Phosphorus
accumulation by the
microorganism. And,removal as
Phosphate-rich
sludge
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Inorganic Phosphorus Removal
• Chemical Precipitation
– Calcium (lime) addition at high pH (>10)
– Alum precipitation
– Precipitation with ferric hydroxide
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Combined Nitrogen and Phosphorus Removal
A2/O Process
Anaerobic Anoxic Aerobic
Air
Wastewater