General anaerobic treatment

General anaerobic treatment

Anaerobic treatment is a micro-biological process for the degradation of organic matter and is characterised by the production of biogas. The biogas mainly consists of methane (60 – 90 % CH4) and carbon dioxide (10 – 40 % CO2). Most of the anaerobic degraded organic matter is turned into biogas; a relatively small part is converted into new cell material.
The comparison of the anaerobic and the aerobic process is illustrated by the example of glucose conversion:
Anaerobic conversion:
C6H12O6 = 3 CH4 + 3 CO2 ( – 404 kJ)
Aerobic conversion:
C6H12O6 + 6 O2 = 6 CO2 + 6 H2O ( – 2844 kJ)
The anaerobic reaction of glucose releases 7 times less energy (free enthalpy) for the bacteria as compared to the aerobic reaction of glucose. Approx. 85 % of the energy will be available as methane and can be recovered as heat in a boiler or as heat and electricity in a cogenerator (CHP unit).
This is the main reason why the biomass growth, also called yield (surplus biomass production), in the anaerobic process is low. The biomass yield in an anaerobic installation is only 2 – 5 % of the converted organic matter quantity.
The active micro-organisms involved in the anaerobic conversion process, all belong to the group of anaerobic bacteria. This group has a great variety of bacteria that are able to, and in most cases only can, exist in an environment that excludes oxygen. The anaerobic degradation of organic material is a stepped process, each step consists of specific types of anaerobic bacteria.
All biologically degradable material is, through various intermediates, finally converted into biogas. Only during the last step, the methane production, the pollution (measured as chemical oxygen demand, COD) is removed from the wastewater. Large organic molecules like protein and starch, are converted by exo-enzymes into a form which makes them assimilative for acidifying bacteria. Subsequently they are converted into simple products like volatile fatty acids (VFA), CO2, H2, NH3 etc., products which in turn are used as substrate by methane producing bacteria. The organically bound carbon is released from the water as methane CH4 and carbon-dioxide CO2. In this context the methane bacteria play a key role in the total conversion process, they are responsible for the last step.
More than 70 % of the methane production originates from bacteria that use acetic acid, the remaining 30 % of the methane is produced by bacteria which are utilising hydrogen and carbon-dioxide.
For optimal anaerobic treatment, one should take the microbiological demands such as temperature, pH, nutrients, absence of toxic substances and other environmental factors into account. Methane producing bacteria are more sensitive to a change in one of these conditions, relative to the acidifying bacteria. A change in any of these conditions may result in an accumulation of (for example) VFA, which may cause a decrease in the pH and a reduction in the plant’s efficiency. It may even cause a complete shutdown of the digestion process.
The conversion rate depends on:
􀂃 The nature of the organic material (wastewater composition)
􀂃 Anaerobic biomass quantity and it’s adaptation and activity
􀂃 The intensity of the contact between organic material and biomass, mixing and contact time (design)
􀂃 Ambient factors such as temperature, pH and alkalinity
􀂃 Availability of macro- and micro-nutrients.
For this reason an adequate process control is required in order to maintain the optimal conditions. The most important parameters are:
􀂃 Temperature
􀂃 pH
􀂃 Nutrients