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Marine Machinery 7 E Part 4

Tham khảo tài liệu 'marine machinery 7 e part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 110 Ship service systems bacteria is generated. These bacteria are said to be anaerobic whilst they are equally capable of breaking down sludge in so doing they generate gases such as hydrogen sulphide and methane. Continuing use of a biological sewage system after a failure of the air supply could result in propagation of anaerobic bacteria and processes. The gases produced by anaerobic activity are dangerous being flammable and toxic. Extended aeration plants used at sea are package plants consisting basically of three inter-connected tanks Figure 3.17 . The effluent may be comminuted i.e. passed through a device which consists of a rotating knife-edge drum which acts both as a filter and a cutter or simply passed through a bar screen from where it passes into the first chamber. Air is supplied to this chamber via a diffuser which breaks the air up into fine bubbles. The air is forced through the diffuser by a compressor. After a while a biological sludge is formed and this is dispersed throughout the tank by the agitation caused by the rising air bubbles. The liquid from the aeration tank passes to a settling tank where under quiescent conditions the activated sludge as it is known settles and leaves a clear effluent. The activated sludge cannot be allowed to remain in the settling tank since there is no oxygen supplied to this area and in a very short time the collected sludge would become anaerobic and give off offensive odours. The sludge is therefore continuously recycled to the aeration tank where it mixes with the incoming waste to assist in the treatment process. Over a period of time the quantity of sludge in an aeration tank increases due to the collection of inert residues resulting from the digestion process this Figure 3.17 Biological sewage treatment plant Hamworthy Ship service systems 111 build up in sludge is measured in ppm or mg litre the rate of increase being a function of the tank size. Most marine biological waste treatment plants are .