As the precipitated sludge from chemical precipitation came to be considered to be an expensive nuisance rather than an asset, engineers searched for a means of treating the sewage which would not produce sludge: "It has been felt for some time that any means of treating sewage without the production of sludge, would be hailed by sanitary engineers as a great advance on present methods." Septic tank treatment was attractive because it held the promise of eliminating the sludge which was proving to be a nuisance with chemical precipitation. It was essentially a horizontal-flow primary sedimentation tank providing a very long retention period. Sewage entered and left the tank below the surface so that anaerobic microbes could operate. The sludge, which at first was not believed to accumulate, was not removed very often and never entirely removed so that there were always microbes present. Anaerobic tanks had been used as far back as 1860 but it was not until 1881 that it was found in France that organic solids liquified under such conditions and this was attributed to the anaerobic action taking place. By the end of the century septic tanks were being hailed as the answer to the sludge problem and an automatic process with no accompanying nuisance and no need for expensive chemicals. Although septic tanks were said to eliminate the sludge problem, at least one engineering writer has wondered in retrospect about the extent to which scientific judgement was influenced by wishful thinking. The precipitation tanks at North Sydney were all converted to open septic tanks in 1902 with the effluent from them still going onto the sand filter beds. The Board engineer claimed an excellent resulting effluent, no smells and a considerable cost saving. Septic tanks were also established elsewhere in Sydney. The Government analyst urged in that year's Water Board report that the success of the experiments with septic tanks and cultivation beds justified the whole of Sydney's sewage being treated in this way. Septic tanks were also given a vote of confidence by the President of the Royal Society of N.S.W., an engineer himself, in 1903 when he claimed that septic tanks had been recognised in England as being "an essential part of modern bacterial purification processes". Septic tanks replaced precipitation tanks in many parts of the world but it was soon realised that they were not the panacea that had been hoped for. The reduction in sludge volume was mainly caused by consolidation in the septic tank and loss of solids with the effluent. Not only that but septic tanks were found to be smelly and the effluent, which was more unpleasant than that from other tank processes, would often clog filters because of the high solids content. Also many tanks were built as septic tanks by people who did not understand the scientific principles involved, and their subsequent failure gave septic tanks a bad name. There were a number of complaints about the smells arising from the North Sydney tanks from nearby residents and from boating people. The local council had made representations to the Water Board in 1903 without success and the Mayor had declared conditions at Folly Point to be unsatisfactory. At a public hearing in 1905 witnesses described what they saw at Folly Point as "an abominable nuisance" and reported that many of the ladies on the wharf at the time were made sick by it. By 1912, the sand filters at Folly Point were overloaded and "sewage sick" and had to be relieved with the addition of artificial filters. The nuisance continued at Folly Point until it was decided to divert the sewage from there to the sea.
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