292 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



salts have been found to be specially useful in the case of 

 sewages containing an excessive amount of grease, e.g., at 

 Wakefield, where much wool-scouring refuse enters the 

 sewers. 



All processes of chemical precipitation, while they are 

 capable of yielding effluents containing less suspended matter 

 than either of the processes considered in the foregoing para- 

 graphs, result in the production of considerable quantities of 

 sludge, which needs special care in its disposal, as its con- 

 stituents are still capable of undergoing ofiensive decomposi- 

 tion, differing thus from the residuum left after well-conducted 

 anaerobic or aerobic treatment. 



The choice of one or the other of the methods of tank 

 treatment (a), (b), (c) or (d) depends on local conditions. In 

 the case of small communities, where constant attention 

 cannot be given, and also where the fall is Umited, anaerobic 

 tanks find useful appUcation. In certain cases also, notably 

 at Birmingham and to some extent at Manchester, anaerobic 

 treatment has been found useful, in the first case in order to 

 produce an inofiensive sludge, and in the second case to 

 neutrahse to some extent the effect of antiseptic trade effluents 

 present in the sewage, before the latter is finally treated on 

 filter beds. In both these cases, however, the presence of 

 considerable quantities of iron salts in the sewage diminishes 

 the chance of nuisance, owing to the combination of any 

 sulphuretted hydrogen produced with the dissolved iron, to 

 form black ferrous sulphide. It must be emphasised that 

 anaerobic treatment, carried out in ill-designed tanks and 

 with imperfect supervision, may be, and often has been, a 

 serious source of nuisance, and, for this reason, prehminary 

 aerobic treatment has often much to recommend it. The 

 slate process of Dibdin requires, however, an amount of fall 

 depending on the depth of the bed, in addition to that required 

 for the subsequent filtration processes. If this is available, the 

 process can often find useful appHcation, it being understood 



