Anaerobic digestion is among the oldest processes used for the stabilisation of Sewage sludge. Anaerobic digestion involves the decomposition of organic and, on a low level, inorganic matter (principally sulphate) in the absence of oxygen. The main products are CO₂ and CH₄ (methane). With this digester gas most of the energy needs for the plant operation can be met. Methane is highly relevant to global climate change, thus uncovered digesters without methane use (and burning to CO₂) should be avoided. According to the four-step model the following processes are involved:

1. Hydrolysis : particulate material is converted to soluble compounds that can then be hydrolysed further to simple monomers

2. Acidogenesis : in this step, also called fermentation, amino acids, sugars and higher (long chain) fatty acids are degraded further to volatile fatty acids, alcohols, acetic acid, hydrogen, carbon dioxide, ammonia and sulphide.

3. Acetogenesis : the volatile fatty acids and alcohols are degraded to acetic acid, hydrogen and carbon dioxide

4. Methanogenesis : this step is carried out by a group of bacteria called methanogens. Two groups are involved in methane production. One group split acetic acid into methane and carbon dioxide. The second group, termed hydrogen-utilising methanogens, use hydrogen as the electron donor and carbon dioxide as an electron acceptor to produce methane.

See also figure 2.

Figure 2: Four-step model of methane production

The microorganisms responsible for methane production are strict obligate anaerobes. They need darkness and heat speeds up their activity. Heated digesters are operated at 37°C (mesophilic) or 55°C (thermophilic).

As the four step model above shows, hydrogen is formed during the fermentative steps and consumed during Methanogenesis. If process upsets occur and the methanogenic organisms do not utilise the hydrogen produced fast enough, the Acetogenesis will be slowed with the accumulation of volatile fatty acids in the anaerobic digester and a possible reduction in pH. If an accumulation of volatile fatty acids is observed, the organic load has to be reduced. On the other hand the reaction

needs a certain H₂ - partial pressure to take place. Still the Methanogenesis is the limiting step.

The important difference to the aerobic metabolism, where carbon dioxide is produced, is that highly energetic source substances are split into carbon dioxide and methane with high energy content. This makes obvious that the energy gain from the anaerobic reactions is very low. In consequence the growth of anaerobic bacteria is comparatively slow.

The volume ratio between CH₄ and CO₂ in digester gas is about 0.65 : 0.35 and differs according to different substrate compositions (proteins, lipids, carbohydrates).

The quantity of methane gas can be calculated using the following equation:

V_CH4 = volume of methane produced at standard conditions (0°C, 1 atm) [m³/d]

Q = flow rate [m³/d]

S = COD in influent [mg/l]

S = COD in effluent [mg/l]

P_x = net mass of cell tissue produced per day [kg/d]

Y = yield coefficient [gVSS/gCOD] typical values: 0.05 to 0.1

k_d = endogenous coefficient [1/d] typical values: 0.02 to 0.04

The efficiency (ratio between S to S) can be estimated to 50 to 70 %.

Note: The methane volume is calculated at normal conditions. At higher temperatures the gas volume increases according to the gas law. To compute the volume of digester gas, the methane volume has to be divided by the methane content, e.g. 0.65.

As a result of the degradation of organic matter and the conservation of the water there is a remarkable attenuation of TS during digestion. The change in TS follows the following equation:

TS = total solids after digestion [kg]

TS = total solids before digestion [kg]

VSS = volatile solids after digestion [% TS]

VSS = volatile solids before digestion [% TS]

If the percentage of volatile solids (VSS) is reduced from 75% of TS to 50% of TS, the total solids will be halved. As a consequence the content of TS (%TS) will roughly halve as well. (Only roughly, because the sum of TS and water attenuates, too.) According to this fact it is advisable to keep the TS in the influent to a digester as high as possible. Therefore primary and secondary sludge have to be thickened in a static or mechanic thickener.

The design criteria for digesters are:

• retention time (20 to 25 days at 37°C)

• organic load (3 to 4 kg/(m³*d))

Digesters have to be mixed, usually by mechanical mixing, biogas injection or mechanical pumping. A group of digesters can be seen in figure 3.

Figure 3: Ankara Sludge Digester