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Advantages and Disadvantages of Anaerobic Treatment

The principal advantages and disadvantages of anaerobic treatment when compared with the aerobic equivalent processes are listed, and then discussed below.

Advantages of Anaerobic Treatment

 

  • Less energy required
  • Less biological sludge produced
  • Lower nutrient demand
  • Methane production: Providing potential energy source with possible revenue both from sale of the energy, and benefit from government tax, and (Kyoto agreement) CDM etc. payments arising from renewable fuels/non-fossil fuel incentives
  • Methane production: Anaerobic digestion contributes to reducing greenhouse gases by reducing demand for fossil fuels
  • Smaller reactor volume required
  • Biomass acclimatisation allows most organic compounds to be transformed
  • Rapid response to substrate addition after long periods without feeding
  • End product can be potentially saleable products biogas, soil conditioner and a liquid fertiliser.
  • Process more effectively provides sanitisation/removal of diseases.

Disadvantages of Anaerobic Treatment

  • Longer start-up time to develop necessary biomass inventory
  • May require alkalinity and/or specific ion addition
  • May require further treatment with an aerobic treatment process to meet discharge requirements
  • Biological nitrogen and phosphorus removal is not possible
  • Much more sensitive to the adverse effect of lower temperatures on reaction rates
  • May need heating (often by utilisation of process gas) to achieve adequate reaction rates
  • May be more less stable after ‘toxic shock’(eg after upsets due to toxic substances in the feed)
  • Increased potential for production of odours and corrosive gases.
  • Hazards arise from explosion. (In the EU, such additional Health & Safety Regulations as the ATEX Directive, and possibly also Gas Institute Regulations will require various compliance measures to be applied for AD.)
  • Anaerobic treatment is not effective for treatment of methanogenic landfill leachate, it may (rarely) be efficacious for the early stage leachate production period while the waste is still acetogenic.

 

Discussion of the advantages of Anaerobic Treatment Processes

Of the advantages cited in the bullet points above, energy considerations, lower biomass yield, fewer nutrients required, and higher volumetric loadings are usually considered the most important, and are examined further in the following discussion.

Energy Considerations

Anaerobic processes may be net energy producers instead of energy users, as is the case for aerobic processes.

Table 1: Comparison of energy balance for aerobic and anaerobic processes for the treatment of a Wastewater with a flow rate of 100m3/day; wastewater strength 10kg/m3; and at 20oC.

 

Value, kJ/d

Energy

Anaerobic

Aerobic

Aerationa,b

 

-1.9 x 106

Methane producedc,d

12.5 x 106

 

Increase wastewater temperature to 30C

-4.2 x 106

 

Net energy, kJ/d

8.3 x 106

-1.9 x 106


      a - Oxygen required = 0.8 kg/kg COD removed.

      b - Aeration efficiency = 1.52 kg O2/kWh and 3600 kj = 1 kWh.

      c - Methane production = 0.35 m3/kg COD removed.

      d - Energy content of methane = 35,846 kj/m3 (at 0C and 1 atm).

For the conditions given, the aerobic process requires 1.9 x 106 kJ/d. On the other hand, the anaerobic process produces a total of 12.5 x 106 kJ/d. Of the total energy produced anaerobically, about 4.2 x 106 kJ/d is required to raise the temperature of the wastewater from 20 to 30C, the low end of the mesophilic temperature range, a more desirable temperature for anaerobic treatment. Thus, the potential net energy production that can be achieved with anaerobic treatment is on the order of 8.3 X 106 kJ/d.

This is a very significant benefit, especially when the pressing need for the reduction in fossil fuel use due to the severe consequences of current predictions of impending climate change. A summary of the 3rd UNFCCC Climate Change report here reinforces this point.

The wastewater strength is important for comparing energy balances for aerobic and anaerobic processes, where the wastewater temperature must be increased.

With the same assumptions used to generate the energy balance presented in Table 10-2, both the aerobic and anaerobic processes would require the same amount of energy input if the wastewater biodegradable COD concentration is 1270 mg/L. At lower COD concentrations, the aerobic process requires less energy. However, heat recovery from the anaerobic effluent stream can modify these values. Further, the lower biomass yield discussed below is still a major advantage offered by anaerobic treatment.

Lower Biomass Yield. Because the energetics of anaerobic processes result in lower biomass production by a factor of about 6 to 8 times, sludge processing and disposal costs are reduced greatly.

The major environmental and economic issues associated with the reuse and disposal of biomass produced from aerobic processes are discussed in Metcalf & Eddy, McGraw Hill ‘Wastewater Engineering, Treatment & Use”.

The fact that less sludge is produced in anaerobic treatment is a significant advantage over aerobic treatment.

Fewer Nutrients Required

Many industrial wastewaters lack sufficient nutrients to support aerobic growth. The cost for nutrient addition is much less for anaerobic processes because less biomass is produced.

Higher Volumetric Loadings

Anaerobic processes generally have higher volumetric organic loads than aerobic processes, so smaller reactor volumes and less space may be required for treatment. Organic loading rates of 3.2 to 32 kg C0D/m3/d may be achieved which compares with 0.5 to 3.2 kg C0D/m3/d for aerobic processes.

Disadvantages of Anaerobic Treatment Processes

Potential disadvantages also exist for anaerobic processes as shown by Table 1.

Operational Considerations

Operational considerations, the need for alkalinity addition, and the need for further treatment are highlighted further in the following discussion.

The major concerns with anaerobic process are their longer start-up time (months for anaerobic versus days for most aerobic processes).

  • AD  process sensitivity to possible toxic compounds, operational stability, the potential for odour
  • The production, and corrosiveness of the digester gas. However, with proper wastewater characterisation and process design these problems can be avoided and/or managed.
  • Need for Alkalinity Addition. The most significant negative factor that can affect the economics of anaerobic versus aerobic treatment is the possible need to add alkalinity. Alkalinity concentrations of 2000 to 3000 mg/l as CaCO3 may be needed in anaerobic processes to maintain an acceptable pH with the high gas phase COD concentration. If this amount of alkalinity is not available in the influent wastewater or cannot be produced by the degradation of proteins and amino acid, a significant cost may be incurred to purchase alkalinity, which can affect negatively the overall economics of the process.
  • Need for Further Treatment. Anaerobic processes can also be followed by aerobic processes for effluent polishing to utilise the benefits of both processes. Series reactors of anaerobic-aerobic processes have been shown feasible for treating municipal wastewater in warmer climates resulting in lower energy requirements and less sludge production (Goncalves and Avanjo, J999; Garuti et al., 1992)
  • Higher capital and operating costs
  • Visual impact of large digester units.

Conclusion

In general, for municipal wastewaters with lower concentrations of biodegradable COD, lower temperatures, higher effluent quality needs, and nutrient removal requirements, aerobic processes are favoured at present.

For industrial wastewaters with much higher biodegradable COD concentrations and elevated temperatures, anaerobic processes may be more economical.

See also “the Environmental Benefits of Anaerobic Digestion” here.

check out the Energy from Waste swicki at eurekster.com 

 

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