Lecture 8: Wastewater treatment 4

Question 1

What is meant by nutrient removal in activated sludge

Answer 1

Can be operated to remove nitrogen and phosphorus

Important because:
1. P and N may cause eutrophication of surface waters
2. ammonia is toxic to fish
3. high levels of nitrate getting into drinking water may cause methemoglobinemia (blue baby syndrome). In GI tract nitrate-reducing bacteria convert NO3- to NO2-. NO2- binds to Hb, converting Fe2+ to Fe3+ reducing O2 carrying capacity

WW treatment effluents into sensitive regions may have limits of 1ml/L total P and 10mg/L total N

Question 2

What is the source of N in WWTPs

Answer 2

organic nitrogen is converted to ammonium (in sewage pipes going to WWTPs and during primary treatment)

Important reactions

H2NCONH2 + H2O (urease ->) 2NH4+ + CO2
Amino acids (deaminase ->) NH4+

Question 3

How is nitrogen chemically removed

Answer 3

1. Liming and air stripping
   liming raises pH to ~10 - 11
   NH4+ + OH- -> NH4OH -> NH3 + H2O
   Air stripping removes NH3 from solution
2. Breakpoint chlorination
   ammonium is oxidised by N2
   3Cl2 + 2NH4+ + O2 -> N2 + 6HCl + 2H2O

Question 4

How is nitrogen removed

Answer 4

1. microbial assimilation of N
   microbes req N for own growh
   No3-, NO2-, NH3 can be used
   (only removes a small amount of N)
2. Dissimilative N removal
   occurs in 2 major stages - nitrification and denitrification

Question 5

What is nitrification and denitrification

Answer 5

Nitrification - aerobic, and mostly autotrophic
NH3 -> NO2- -> NO3-
reaction 1: ammonia oxidation. Reaction 2: nitrite oxidation

Denitrification - dissimilatory nitrate reduction to gaseous nitrogenous compounds mostly N2, NO. Anaerobic by chemoheterotrophs. Requires reducing conditions and carbon substrate

NO3- + NOD -> N2 + CO2 + H2O + OH- + biomass

Question 6

What bacteria are ammonia oxidising

Answer 6

Major genera: Nitrosomonas (Beta proteobacteria), Nitrosococcus (gamma proteobacteria), Nitrosospira (beta proteobateria)

Nitrosomonas: dT 8h - several days, optimal temp and pH 25 -35C & 7.5 - 9

Reaction:
NH3 + O2 + 2H+ (ammonia monooxygenase ->) NH2OH + H2O (hydroxylamine oxidoreductase ->) NO2- + 5H+

Question 7

What bacteria are nitrite oxidising

Answer 7

Nitrobacter (alpha proteobacteria)
Nitrococcus (gamma proteobacteria)
Nitrospira
Nitrospina (sigma proteobacteria)

Mostly chemoautotrophs utilising energy forom nitrite oxidatin to fix CO2

Question 8

How is phosphorus removed

Answer 8

In WW, P usually 10 - 20 mg/L in form of:
orthophosphate (PO4^3-) (60-80%)
Organic compound P, e.g., nucleic acids, polyphosphate

P in WW effluent may cause eutrophication

EU Urban WW Treatment Directive states max P and N levels

Methods
1. Phosphorus assimilation by microbes (only 10-20% of P)
2. Chemical precipitation (with Al3+, Fe3+, Fe2+, or Ca2+)
3. Physical methods (eg membrane filtration)
4. Combined biological/chemical process
5. Enhance biological phosphorus removal (EBPR)

Question 9

What is Enhanced biological phosphorus removal (EBPR)

Answer 9

Involves biological accumulation of P as polyphosphate by polyphosphate accumulating organisms (PAOs)

carried out by activated sludge bacteria that accumulate P in excess of theri metabolic needs

phosphate accumulated intracellularly as phosphate granules

Question 10

What are the pros and cons of enhance biological phosphorus removal (EBPR) over chemical removal

Answer 10

+lower running costs
+ better environmentally

-systems intermittently fail
- set-up costs high
- need back up (ie chemicals)
- details of microbiology not weel understood

Question 11

What is A. phosphatis-like PAOs

Answer 11

Acetate and propionate-fed bioreactors select for Candidatus A phosphatis

Metagenomic sequencing of EBPR sludges has let to complete reconstration of A phosphatis genome