Biodegradation C-N-P Removal

Question 1

Why do we need to model biodegradation

Answer 1

• Substrate concentration in effluent (has to be below BOD or COD norm)
• Biomass concentration in waste stream to have an idea of the waste sludge that will have to be treated

Question 2

Carbon removal parameters

Answer 2

• Hydraulic Retention Time (HRT)
• Sludge Retention Time (SRT)
• Sludge Loading Rate (F/M)
• Volumetric Loading Rate

Question 3

Plug Flow

Answer 3

• Advantage with settling and creating a microbial community
• C-removal through tapered aeration before the clarifier (lower demand for oxygen)
• Toxic shock loads can kill microbial community so stepwise loading (loading over several areas) to dilute toxicity

Question 4

Conventional CSTR configuration

Answer 4

• biodegradation tank then sedimentation tank
• complete mixing occurs due to the design of tanks and the way oxygen is supplied

Question 5

Aeration in C-removal

Answer 5

• aeration can supply oxygen in two ways:
• air diffusion
  > air diffusion where oxygen transfer takes place at the interface of air bubbles-water
  > compressed air is introduced at the bottom of the aeration tank
  > can occur from a submerged aerator which is simple and cost effective but depends on the depth of the basin or a membrane aerator where a blower introduces air at a certain pressure and pushes it through tubular or planar membranes
• surface aeration
  > oxygen transferred during the residence of the water drops in the air phase and the formation of thin air bubbles by water jet created by mechanical device into the liquid
• aeration device selected based on kind of wastewater, activated sludge process, SS characteristics, construction of aeration tank and climate

Question 6

Contact stabilisation in C-removal

Answer 6

• during first wastewater contact with activated sludge a rapid adsorption of suspended solids, colloidal matter and other components of high molecular weight occurs
• during the stabilisation the adsorbed material is biodegraded (reactivated)

Question 7

Oxidation ditch in C-removal

Answer 7

• based on principle of extended aeration and sludge starvation
  > low loading
  > long residence time
  > high O2 supply

Question 8

Deep Shaft in C-removal

Answer 8

• deep but limited in diameter so takes up less space
• high hydrostatic pressure means higher solubility of gas so transport of oxygen is more sufficient
• however leads to low sludge production so organisms need to survive under stress

Question 9

Sequence Batch Reactor

Answer 9

• all steps done in one reactor
• Fill reactor, react with circulation and aeration, let it settle, extract water (and biomass from bottom) and repeat
• Pro: flexible as different wastewaters have different compositions, reactor time etc
• Con: discontinuous so need holding tank or systems in parallel (since WW effluent is continuous)

Question 10

Nitrogen Removal

Answer 10

• NH3 = toxic at high concentrations, NO3- leads to eutrophication
  > NO3 comes from households, industry, run-off or leaching from agricultural soils
• Majority of N components have to be removed, want to form N2
• NH4+ -> NO2- -> NO3- -> N2
  > First 3 = nitrification carried out by nitrifiers
  > Last step = denitrification

Question 11

Nitrification

Answer 11

• NH4+ -> NO2- -> NO3-
• needs oxygen, pH drop (create protons), long SRT (doesn’t generate a lot of energy, so growth is slow)
  > Conditions: pH = 7.5, DO > 0.5 mg/L, T of 30 °C

Question 12

Denitrification

Answer 12

• N2O -> N2
• N2O = greenhouse gas, need to ensure we go all the way to N2
• Oxygen is preferred over nitrate as e- acceptor so need to make sure no oxygen is present (anoxic)
• Carbon source (e- donor) + nitrate + H+ -> N2 + CO2 + H2O
  > In theory, need 3x more carbon than nitrate but for design use ratio of 10 to ensure total nitrate dissimilation
• Conditions: pH of 6-8, 5-60 °C, limiting DO, availability of nitrate/nitrite
  > Lower pH = incomplete reduction = undesirable nitrite accumulates

Question 13

N-Removal Configuration: Post denitrification

Answer 13

• bCOD into aeration tank to form nitrate then into anoxic tank with methanol
• All carbon broken down in first tank so need to add carbon source ($)

Question 14

N-Removal Configuration: Pre denitrification

Answer 14

• First step is denitrification where majority of carbon is used up (recycle nitrate as it is needed here)
• Nitrate formed in second tank

Question 15

N-Removal Configuration: Time Dependent Systems

Answer 15

• sequence batch reactor
• one reactor (filling -> reaction -> sedimentation), aeration switched on and off, nitrate transformed to N2 in the same tank, accurate monitoring of nitrate and O2 needed

Question 16

P removal - Physio chemical

Answer 16

• Physio-chemical removal: precipitation with Fe3+, Fe2+, Al3+… - deposits at bottom of tank then removed
  > BOD also precipitates, good but also disadvantage if you want to use carbon
  > Expensive
  > What to do with precipitate?

Question 17

P Removal - Biological

Answer 17

• luxury uptake (every cell needs P but if we just rely on that = too much growth BUT some organisms are able to take up more P than they need)
• Anaerobic phase
  > need phosphate accumulating organisms (PAOs)
  > Microorganisms transform bCOD to fatty acids which is then converted to cell mass by breaking down poly-P and glycogen reserves, phosphorous is released in this step
• Aerobic Phase
  > PAO are full of stored carbon material and in aerated environment they breakdown this material = energy release = growth and energy stored in the form of poly-P
  > Stored P exceeds amount released in 1st phase
• Conditions needed for biological 2 step removal:
  > Sequence of anaerobic and aerated reactors, sufficient readily biodegradable COD, no nitrate present in anaerobic reactor (no fermentation means no VFA and even if fermentation occurs, denitrifiers consume VFA instead of poly-P organisms), no anaerobic phases during further treatment (will re-release P)