High strength wastes generally require anaerobic treatment prior to aerobic treatment. A low rate anaerobic digestion process provides several advantages:
o Efficient and reliable treatment
o Robust, with superior ability to cope with peak loads and spikes for limited period of time.
o Low manpower requirements and low operating and maintenance costs due to it being a simple-to-operate system with low energy consumption.
o Satisfactory performance over a wide range of temperature conditions.
o Eliminates the need for primary treatment of the liquid waste stream (such as the equalization tank and primary clarifier); raw solids after screening are added directly to reactor where they are digested, producing biogas and minimizing waste sludge handling.
o Reduced sludge handling, as this system produces very little waste sludge on its own due to the prolonged solids retention time.
o Able to store sludge within the system such that wasting can be controlled. Sludge is typically wasted at a solids concentration of 3 or 4 percent; this sludge is very stable, does not have an offensive odor, and has good fertilizer and soil amendment value.
The Process:
Anaerobic treatment comprises of four steps, namely:
1. Hydrolysis;
2. Acidification
3. Fermentation; and
4. Methanogenesis.
When activated sludge is kept in an anaerobic environment, specialised bacteria will develop that use the excess sludge as a source of organic matter for fermentative metabolic processes. The end products of the fermentation are mainly methane and carbon dioxide. The overall conversion process of complex organic matter into methane and carbon dioxide can be divided into above four steps.
In an anaerobic digester, the four processes occur simultaneously. In the hydrolysis process, macro molecules like proteins, poly saccharides and fats that compose the cellular mass of the excess sludge are converted into molecules with a smaller atomic mass that are soluble in water: peptides, saccharides and fatty acids. The hydrolysis or solubilisation process is carried out by exo-enzymes excreted by fermentative bacteria. Hydrolysis is a relatively slow process and generally it limits the rate of the overall anaerobic digestion process.
The second step of the anaerobic digestion process is acidogenesis or acidification, a process that results inthe conversion of the hydrolysed products into simple molecules with a low molecular weight, like volatile fatty acids (e.g. acetic-, propionic- and butyric acid), alcohols, aldehydes and gases like CO2, H2 and NH3.
Acidification is effected by a very diverse group of bacteria, the majority of which are strictly anaerobic, i.e. the presence of oxidants like oxygen or nitrate is toxic. The presence of these bacteria is important to remove all oxygen that might be introduced into the system, for instance together with the excess sludge. The acidogenic bacteria are able to metabolise organic material down to a very low pH of around 4.
In the third step, acetogenesis, the products of the acidification are converted into acetic acids, hydrogen, and carbon dioxide by acetogenic bacteria. The first three steps of anaerobic digestion are often grouped together as acid fermentation. It is important to note that in the acid fermentation, no organic material is removed from the liquid phase: it is transformed into a form suitable as substrate for the subsequent process of methanogenesis.
In the final step of the anaerobic digestion process, the products of the acid fermentation (mainly acetic acid) are converted into CO2 and CH4. In order to maintain an anaerobic sludge with a high metabolic activity, it is necessary to apply favourable environmental conditions. Among these factors the most important ones are temperature, pH, the absence of toxic materials and the availability of nutrients.
The anaerobic digester is equipped with internal baffles and internal mixers to prevent short circuiting and uniform solids contact. The wastewater enters the anaerobic digester via a lateral pipe network located below the sludge bed. A recycle pump re-circulates anaerobic sludge with the incoming effluent through the digester to promote acclimation. As the effluent rises upward through the sludge blanket, organics and solids are degraded, generating biogas. The anaerobic digester is equipped with a geomembrane liner cover for collection of biogas. Biogas rises to the surface and accumulates at the tank perimeter. An external blower operates at slightly negative pressure to draw gas from the tank headspace and divert the same to a future biogas generation system. The external blower system can be replaced by a negative draft fan that creates the negative pressure, as well as a ducting system along the perimeter can be used to capture the biogas.
Wastewater after anaerobic digestion is generally sent to downstream aerobic treatment.