Anaerobic Treatment

Question 1

What is Anaerobic Water Treatment

Answer 1

• in anaerobic conditions no DO or nitrate available as electron acceptor so only other organic matter can serve as electron acceptor
• anaerobic water treatment produces biogas which is a fuel in a four step process
• needs a higher temperature otherwise biogas would just be used to heat up waste stream
• anaerobic water treatment produces 5kg of sludge/ 100kg of COD with 75% of COD becoming biogas which can be used for energy. However, 10-20kg of COD is still in the effluent which may need treatment

Question 2

Describe the four step process of anaerobic treatment

Answer 2

1) Hydrolysis
- bigger molecules (polymers) cut into smaller pieces (monomers)
- performed by common bacteria and are fast growers
2) Acidogenesis
- smaller molecules transformed into simple acids called Volatile Fatty Acids
- performed by common bacteria and are fast growers
3) Acetogeneicis
- Previous simple components are transformed into acetate or hydrogen and carbon dioxide
- bacteria are slow/very slow growers
4) Methanogenesis
- Formation of methane by combining hydrogen and carbon dioxide or reducing acetate to methane
– bacteria are slow/very slow growers and strictly anaeroibic

Question 3

Types of Anaerobic Reactors

Answer 3

• Lagoons
• CSTR digesters
• Fixed film reactors
• Fluidised bed reactors
• Granular reactors (via auto granulation)
• Anaerobic Membrane Bioreactors

Question 4

Lagoons

Answer 4

• Simple
• Pond where wastewater resides for very long time (HRT = 3-100 days)
• High residence time requires large volume and a lot of space
• System cannot be overloaded and this is reflected by a volumetric loading rate which is the reactor loading rate of 0.5 to 1.5kg COD/m3/d
• Lagoons often not covered so release methane to environment (BAD!) and have rather low effluent quality

Question 5

CSTR digesters

Answer 5

• continuously stirred tank reactor as biodegradation reactor (not aerated) and a sedimentation tank to separate sludge and cleaned water
• Efficient sludge settling and separation from water, inducing an efficient sludge recycle is more important in these digesters compared to aerobic counterpart as methanogens are slow growers
• HRT approx 20 days and VLR 2-5 kg COD/m3/d
• especially used when there are a lot of suspended solids and/or fatty components in the influent

Question 6

Fixed Film Reactors

Answer 6

• some parts of anaerobic digestion microbial community are such slow growers that alternative systems were required to keep biomass within the system by using biofilm formation on support material
• support material needed to have high specific surface
• fixed bed biofilm reactor is when tank is completely filled with the support or packing material and the material is not moving, but wastewater can flow through
• Lot of biofilm can grow as larger surface however, biofilm growth can clog pores of the packing material which can impede proper flow through of the water so not all biofilm remains in contact with water, decreasing its effectiveness
• VLR < 10 kg COD/m3/d

Question 7

Fluidised bed reactor

Answer 7

• some parts of anaerobic digestion microbial community are such slow growers that alternative systems were required to keep biomass within the system by using biofilm formation on support material
• support material needed to have high specific surface
• support/packing material takes up only part of reactor volume and material floats around due to water flowing in and out
• enables good contact with wastewater and biofilm but less surface area for contact
• less prone to clogging but there is a risk of loss of carrier and can be difficult to operate
• VLR > 30 kg COD/m3/d

Question 8

Auto-Granulation and Granular Reactors

Answer 8

• apart from biofilm on the support material, also aggregates of microorganisms spontaneously developing
• aggregates were big and dense so could settle even within the biodegradation tank
• due to upstream flow of water, less flocculated and dense material is washed on and a selection of settling granules stay in the reactor
• this process occurs in upstream anaerobic sludge blanket (UASB)
  > influent comes in the bottom of the tank and flow up towards effluent
  > granules entrained with liquid move upwards but they fall due to mass and size and settle again, and are predominantly present as a sludge blanket at the bottom of the tank
  > biogas bubbles are generated and captured on top such that biogas can be collected and used
• variant of this is Extended Granular Sludge Bed (EGSB)
  > smaller diameter and taller so has higher upflow velocity so granules are entrained a bit more
  > this enable optimal contacts between granules and water but they still settle
• granules are 1-3mm in diameter and have high sedimentation velocities
• UASB copes with 5 - 15 kg COD/m3/d and EGSB was 10-30 kg COD/m3/d
• Have HRTs less than 1 to 2 days (better than biofilm)
• major drawback is that granulation process is hampered when influent contains a lot of suspended solids and/or FOG so CSTR digesters still used

Question 9

Anaerobic Membrane Bioreactors (AnMBR)

Answer 9

• created for waste that is high in FOG and TSS so can’t be treated by granular reactors
• 100% certainty that biomass remains in system and high COD conversion efficiency
• very low surface requirements
• VLR = 10 kg COD/m3/d

Question 10

When do you use anaerobic treatment and with what reactor type

Answer 10

• Depends on TSS content and COD concentration in influent
• systems up to 500 mg/L COD (should be ~1000)in the influent does not produce much biogas and if the influent does not have a higher temperature by itself it is not economically viable to treat in anaerobically
• granular reactors preferred when TSS is limited and COD is high but not too high
• more TSS, higher influence conc leads to CSTRs becoming more prominent
• not all compounds can biodegrade anaerobically so lab testing required for new waste streams
• some compounds are toxic to the anaerobic organisms (inhibiting system) so also require lab testing
• ions (saline or salty water) and ammonium at high concs can be inhibiting
• temperatures higher than 37 degrees kills microorganisms rapidly whilst 25 degrees reduced production to 50%
• pH should be neutral can can be controlled easily
• best for industrial wastewater

Question 11

Advantages of Anaerobic Water Treatment

Answer 11

• can generate energy while removing pollutants and produce less excess biomass (however higher amount of COD)
• 4 to 5 times less volume in reactor is needed (even in combined)
• surface requirement is less and can work with deeper reactors as oxygen distribution not needed
• no aeration costs and limited mixing costs as biomass helps in mixing liquid in the reactor so sludge handling costs are lower and produced biogas has an economical value
• Start up time can be very long for anaerobic reactors but when there are more systems running, the extra sludge can help in starting new systems