It is, therefore significant to consist of 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 facilely dewatered and dried.
Anaerobic digestion is the most tasteless 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 output of a beneficial by-product, a combustible gas, which can be used as a source of energy.
The advantages of this process:
The organic article of the sludges is significantly reduced by conversion into gaseous end-products; the offensive 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 allowance in the ability of pathogenic bacteria; there is a marked chemical convert after digestion. The liquid fraction (supernatant) contains increased levels of ammonia as a succeed 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 compound 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 introductory 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 ordinarily monitored and if healthful action is not taken in time.
Anaerobic digestion is a multi-stage biological waste medicine 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 offensive odour is removed. The process can, however be described adequately and simply as occurring in two stages, consuming two dissimilar 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 succeed 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 while digestion is removed continuously. In high-rate digestion, stabilised sludge is displaced from the digester while feeding. In low-rate digestion, sludge 'supernatant' is ordinarily removed as the feed sludge is added, stabilised sludge is removed at less frequent intervals.
It is significant that the organic acids formed in the first stage of the waste medicine process are converted to methane at the same rate at which they are formed. If not, they obtain and ultimately lower the pH, foremost to inhibition of the second stage of the digestion process and digester failure. temperature must be maintained within definite ranges - heating increases the action 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 adequate buffer capacity (alkalinity) introduced from the breakdown of organic matter.
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