Deals with the biological removal of nitrogen and phosphorus, Nitrification-denitrification removal of nitrogen, and Phosphate accumulating organisms and poly-hydroxibutirate in the phosphorus removal.

1. Biological Treatment for Nitrogen
and Phosphorus Removal

2. Organic-N
Ammonical-N
Nitrite-N
Nitrate-N
Organic-N
Nitrogen gas
Organic-N
(net growth)
decomposition
and hydrolysis
oxygen
oxygen
assimilation
lysis & auto-oxidation
denitrification
Organic carbon
nitrification
nitrification
Scheme of Biological Nitrogen Removal

3. Biological Nitrification
• Wastewaters have organic-N and ammonical-N
– Ammonical-N is DO depleting and toxicity causing
– Nitrogen removal is needed for controlling eutrophication
and in the water disposed through recharging ground water
– Nitrate nitrogen limits in drinking water (45 mg/l as nitrate
and 10 mg/l as nitrite bitrogen)
• Nitrification is a 2-step process (NH3-N to NO2-N to NO3-N)
and usually achieved along with BOD removal in the same
biological treatment unit
• When concentration of potentially toxic and inhibitory
substances is high, two-sludge systems, each with an
aeration tank and a clarifier, in series are used
– First system is for BOD removal (operated at shorter SRT)
– Nitrification occurs in the 2nd
system – raw influent is partially
bypassed into the 2nd
system to facilitate sufficient
flocculation & clarification – have much longer HRT & SRT
• In attached growth reactors nitrification is accomplished
after BOD removal (sometimes in separate reactor)

4. nitrificationBOD removal
• Aerobic autotrophic bacteria are responsible for
nitrification
– Nitroso bacteria (Nitrosomonas, Nitrosococcus, Nitrosospira,
Nitrosolobus, Nitrosorobrio, etc.) are responsible for step-1
– Nitro bacteria (Nitrobacter, Nitrococcus, Nitrosdpira,
Nitrospina, Nitroeystis, etc.) are responsible for step-2
Biological Nitrification

5. • Oxygen requirement for complete oxidation of ammonia is
4.57 g/g of N (3.43 for nitrite production and 1.14 g for
nitrate production)
• Use of ammonia as nutrient by the autotrophs reduce the
oxygen requirement to 4.25 g/g
• Requires alkalinity of 7.14 g as CaCO3 per g of nitrogen
(7.07g if use of ammonia as nutrient is considered)
• Nitrification is inhibited by
– Low DO levels (<0.5 mg/l is inhibitory - rate increases with
DO upto 3 to 4 mg/l)
– pH below 6 is inhibitory and 7.5 to 8 is optimal
– Nitrifiers are sensitive to a multitude of organic toxicants
(solvents, amines, proteins, tannins, phenols, alkohols,
cyanates, ethers, carbamates, benzene, etc.)
– Metals are inhibitory (complete inhibition at 0.25 mg/l for
nickel and chromium, and 0.1 mg/l for copper)
– Unionized ammonia can also be inhibitory
Biological Nitrification

6. Biological Denitrification
• Denitrification: reduction of nitrate by heterotrophic
bacteria to nitrous oxide and nitrogen gas
– Wide range of bacteria (but not algae and fungi) are capable
• Nitrate removal occurs in assimilating or dissimilating
modes
– Assimilative mode involves reduction to ammonia for use in
cell synthesis (occurs if ammonical-N is not available)
– Dissimilative mode is biological denitrification and is coupled
with respiratory electron transport chain
• Oxygen equivalence of using nitrate or nitrite in place of
oxygen is 2.86 g/g for nitrate and 1.71 g/g for nitrite
• Alkalinity is produced in the denitrification process (3.57 g
of alkalinity (as CaCO3) per gram of nitrate reduced
• Denitrification is inhibited at higher DO (>0.2 mg/l for
pseudomonas & >0.13 mg/l for highly dispersed growth)

7. • Two basic schemes of ASP are used for denitrification
– Known as substrate denitrification or preanoxic denitrification
• anoxic tank followed by an aeration tank
• organic matter of the influent acts as electron donor
– Known as postanoxic denitrification (occurs after nitrification)
• Endogenous decay of microbial mass supplies electron donor
• Often exogenous carbon source (methanol or acetate) is used
• Rate of reduction is relatively slower
• Either a suspended growth or an attached growth system can
be used for the denitrification
• Granular medium filter can be used both for the denitrification
and for the filtration removal of suspended solids
• Nitrification & denitrification can also occur simultaneously
– Nitrification on the floc surface (if DO in the bulk liquid is high
enough) and denitrification in the floc interior (if DO in the
interior is low enough)
– Depending on the mixing conditions, nitrification and
denitrification can occur in the same tank
• Nitrogen level controls substrate utilization kinetics only
when it is <0.1 mg/l.(BOD demand is 4 g/g NO reduced)
Biological Denitrification

8. Aerobic
section
Anoxic
section
Aerobic
section
Anoxic
section
Biological Denitrification
substrate denitrification or preanoxic denitrification
postanoxic denitrification

9. Biological Phosphorus Removal
Involves use of Phosphorus Accumulating Organisms (PAO)
in an anaerobic – aerobic system
• Phosphorus is incorporated into sludge (as polyphosphate) in
volutin granules and removed through sludge wastage
Under anaerobic conditions
• Proliferation of PAOs occur
• PAO assimilate fermentation products (specially acetate) into
storage products (polyhydroxybutyrate-PHB) and concomitantly
release stored polyphosphate as orthro phosphate
• Acetate is essential for forming PHB and for providing
competitive advantage to the PAOs
• Presence of nitrate can be inhibitory (acetate can be depleted ,
through denitrification!, and become not available to PAOs)

10. Under aerobic conditions
• Stored products (PHB) are oxidized to release energy and in
return phosphate get stored within the cell as polyphosphate
• Molar ratios for Mg, K and Ca of 0.71, 0.5 and 0.25 to
phosphorus respectively are believed to facilitate the
polyphosphate storage
• pH and DO should be >6.5 and >1.0 mg/L respectively
Reactor for phosphorus removal is comprised of an anaerobic tank
with HRT 0.5 to 1 hour and placed ahead of an aeration tank
• Return activated sludge and influent are brought in contact in
the anaerobic tank
• Requires >2.5 days SRT in anaerobic and aerobic systems
Typical microbial biomass has 1.5 to 2% phosphorus - in PAOs
phosphorus content can be as high as 20-30%
Stoichiometrically about 10 grams of bCOD is needed for the
removal of one gram of phosphate from wastewater
PAOs form very dense, good settling flocs in the activated sludge
Biological Phosphorus Removal

11. Anaerobic
system
Aenobic
System
Anaerobic – aerobic system
for biological phosphorus removal