Chemical Precipitation

Chemical precipitation for the purposes of purifying sewage was used in Britain following the Public Health Act of 1875 which was aimed at protecting rivers which had become grossly polluted by the combination of water-carriage technology and discharge into the nearest watercourse. The Act insisted that sewage be treated before discharge. Sewage farming had been the preferred method but land was often scarce or unsuitable in British inland towns and cities. Chemical precipitation before land treatment reduced the amount of land required.

The first chemical precipitant patented was lime. Between 1856 and 1876 it is estimated that over 400 patents were granted for chemical precipitants. Little was understood about the science behind precipitants and a writer at the time observed:

Inventors seem mainly to have looked out for articles which were cheap, or entirely worthless, and heaped them together without any definite notion of the part which they were separately and collectively to play. This alone can count for the recommendation of such bodies as coal-ashes, soot, salt, gypsum, etc., which in almost every case would do more harm than good. Very often we see, especially in the older specifications, materials given as alternatives whose action, if any, must be evidently quite dissimilar the one to the other.

Often the precipitants were unwanted by-products of industrial processes used with some other material.

Many limited liability companies were formed to exploit the situation and make profits from patented precipitation processes. They promoted their processes using test results from experiments, often undertaken by their own employees, and literature giving a misleading interpretation of the results. By 1884 most had gone into liquidation and their treatment works had become the property of the local authorities.

It was generally recognised by opponents and proponents alike that chemical precipitation did not purify the sewage but merely clarified it and that the chemical precipitation had to be used in conjunction with some sort of filtering process.

Chemical precipitation was experimented with for a very short time at North Sydney because ocean disposal was cosidered to be too expensive yet the disposal of raw sewage into the Harbour was no longer acceptable. It was proposed that the sewage would be screened before having lime and sulphate of iron mixed with it and then filtered through 6 feet of sand before being discharged into Middle Harbour. The place for the treatment was later named Folly Point. The sludge would be made into sludge cake using filter presses and then burnt in furnaces since "it was deemed inadvisable to rely solely on any demand for the product as a means of disposal" and because burning was the most "efficacious" method of disposal.

The sewage treatment works were duly handed over to the Water Board on their completion in 1899. But in their annual report the following year the Board claimed that there were not enough tanks "to meet the requirements of the rapid expansion of the sewerage system" and that additional works had been authorised. The year after that the precipitation process was abandoned because the process was too expensive and the sand filtering area "had every appearance of becoming sour and sewage sick" and required regular harrowing to keep it aerated. In a later report, the Board also admitted that there had been a number of complaints of nuisances.

At first it had been hoped in Britain that the expense of treating sewage in this way could be recouped from turning the precipitated sludge into a valuable fertiliser. A British Local Government survey in 1894 of 234 towns that had or were still using chemical treatment found that none had made a profit from manufacture of fertiliser although 174 were still using chemicals. When it was realised that fertiliser manufacture was not profitable the disposal of the precipitated sludge became the biggest problem facing those using chemical treatment.

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