The Basics Of The Anaerobic Digestion Process
By Michael Russell
It is, therefore necessary to contain and treat these wastes so that the treated waste sludge is stable; the offensive odour is removed; the quantity of pathogenic bacteria is reduced; the mass and volume of sludge is reduced and the sludge can be readily dewatered and dried.
Anaerobic digestion is the most common method in use today for treating waste water sludges. Its attractiveness comes from it being a relatively stable process if properly controlled, with low operating costs and the production of a useful by-product, a combustible gas, which can be used as a source of energy.
The advantages of this process:
The organic content of the sludges is significantly reduced by conversion into gaseous end-products; the obnoxious odour of the sludge is removed and the final digested sludge has a characteristic 'tarry' odour; fats and greases are broken down by the process; there is a significant reduction in the quality of pathogenic bacteria; there is a marked chemical change after digestion. The liquid fraction (supernatant) contains increased levels of ammonia as a result of the breakdown of organic nitrogen (proteins). This makes the digested sludge liquor potentially suitable for agricultural use; the biogas that is formed is a mixture of carbon dioxide (CO2) and methane (CH4) that can be used for digester heating or to generate power.
The disadvantages of this process:
A relatively high initial capital cost is involved, which tends to limit the process to medium to large size waste water works. The slow rate of bacterial growth requires long periods of time for start-up and limits the flexibility of the process to adjust to changing feed loads, temperatures and other environmental conditions. The process is prone to upsets if not regularly monitored and if corrective action is not taken in time.
Anaerobic digestion is a multi-stage biological waste treatment process whereby bacteria, in the absence of oxygen, decompose organic matter to carbon dioxide, methane and water. In this way, the waste sludge is stabilised and the obnoxious odour is removed. The process can, however be described adequately and simply as occurring in two stages, involving two different types of bacteria. The first stage, the organic material present in the feed sludge is converted into organic acids (also called volatile fatty acids) by acid forming bacteria. In the second stage, these organic acids serve as the substrate (food) for the strictly anaerobic methane-producing bacteria, which converts the acids into methane and carbon dioxide. The end result of the process is a well-established sludge in which 40 to 60% of the volatile solids are destroyed. Finally, a combustible gas consisting of 60 to 75% methane and the remainder largely being carbon dioxide.
The digestion process is continuous. Fresh feed sludge must be added continuously or at frequent intervals. The gas formed during digestion is removed continuously. In high-rate digestion, stabilised sludge is displaced from the digester during feeding. In low-rate digestion, sludge 'supernatant' is normally removed as the feed sludge is added, stabilised sludge is removed at less frequent intervals.
It is essential that the organic acids formed in the first stage of the waste treatment process are converted to methane at the same rate at which they are formed. If not, they accumulate and ultimately lower the pH, leading to inhibition of the second stage of the digestion process and digester failure. Temperature must be maintained within certain ranges - heating increases the activity of the anaerobic bacteria reducing the required digestion time. A pH of 7,0 to 7,5 is recommended to encourage the methane-producing stage. A correctly operating digester will have sufficient buffer capacity (alkalinity) introduced from the breakdown of organic matter.
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