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Sewage treatment using anaerobic digestion in a Biotank
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Anaerobic Digestion

Aerobic Digestion
Anaerobic Digestion
Solids in Wastewater
Sewage bacteria
Domestic compounds
Health Hazards

 

anaerobic [ār “bik]
requiring little or no oxygen to live
occurring with little or no oxygen
relating to or caused by anaerobic bacteria






Anaerobic digestion is a complex biochemical reaction carried out in a number of steps by several types of micro-organisms that require little or no oxygen to live. During the process, a gas principally composed of methane (CH4) and carbon dioxide (CO2),...

...otherwise known as biogas, is produced. The amount of gas produced varies with the amount of organic waste fed to the digester and temperature influences the rate of decomposition (and gas production).

Figure II-1: Anaerobic digestion pathway






Anaerobic digestion occurs in four distinct steps, illustrated in Figure II-1:
bulletHydrolysis: complex organic matter is decomposed into simple soluble organic molecules using water to split the chemical bonds between the substances.
bulletFermentation or acidogenesis: the chemical decomposition of carbohydrates by enzymes, bacteria, yeasts, or molds in the absence of oxygen.

bulletAcetogenesis: the fermentation products are converted into acetate, hydrogen and carbon dioxide by so-called acetogenic bacteria.
bulletMethanogenesis: methane (CH4) is formed from acetate and hydrogen/carbon dioxide by methanogenic bacteria...

...The acetogenic bacteria grow in close association with the methanogenic bacteria (Figure II-2) during the fourth stage of the process. The reason for this is that the conversion of the fermentation products by the acetogens is thermodynamically only possible if the hydrogen concentration is kept sufficiently low. This requires a close symbiotic relationship between both classes of bacteria.

Figure II-2: Methanosarcina barkeri
The anaerobic process only takes place under strict anaerobic conditions (i.e. absence of oxygen and very low redox potential). It requires specific adapted biosolids and particular process conditions, which differ considerably from those needed for aerobic treatment.




The advantages of anaerobic wastewater treatment


After completion of the anaerobic digestion steps, the pollutants in the wastewater are transformed into methane (CH4), carbon dioxide (CO2) and a small amount of biosolids. Since the solubility of methane in water is very low, it escapes as methane gas.

As a result, a significant part of the energy originating from the pollutants leaves the system as biogas, leaving only...

... a fraction of the initial energy for assimilation by the biomass.
Hence the amount of energy generated, by this pathway, which can be used by the biomass is only a small fraction of the total energy content of the incoming pollutants.

Biomass growth is therefore much lower compared to the one prevailing in the aerobic processes.

Figure II-3: Simplified comparison aerobic vs anaerobic


For that reason, anaerobic treatment produces between five and ten times less biosolids (sludge) than aerobic processes (Figure II-3).

In addition, anaerobic biosolids have the advantage of being much more compact than aerobic biosolids. The dry solids content of anaerobic biosolids range from 2% (for a digestor) to more than 8% for an upflow anaerobic biosolids blanket process.

Furthermore, anaerobic biosolids have much better dewatering characteristics compared to aerobic biosolids,...

... As a result, the volume of dewatered biosolids coming from an anaerobic treatment is 7 to 12 times lower compared to aerobic processes.

Another advantage is that anaerobic digestion does not require energy-consuming aeration equipment.  the production of biogas during the process even results in a positive energy balance.

 




...and its drawbacks


However, anaerobic digestion has its drawbacks. The main one is that it requires more stringent process control and only reduces the organic pollution by 85 to 90%, which means a second step is usually needed to guarantee high effluent quality. This is usually an aerobic stage for polishing before discharge.

As anaerobic biosolids production is rather low, the nutrient removal (nitrogen and phosphorus) is equally low as well.

This is one more reason for applying an aerobic second stage, which removes the residual nutrients which would otherwise cause eutrophication of the river in which the treated effluent is discharged.


 

 
  All enquiries should be made to:

Chris White
Biotank Ltd.
Birch Farm
Southery Road
Feltwell
Norfolk
IP26 4EN.  UK

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