Sludge treatment plant at sheffield.

One of the world's largest sewage sludge dewatering plants employing filter presses was put into operation at the city of Sheffield. The new installation of thirty-six presses produces 30 tons per hour of sludge cake containing 55% to 65% moisture. The plant supersedes 20 acres of open-air drying beds and 40 acres of sludge lagoons which occupy the only available land adjacent to the sewage works and which have become inadequate to cope with the 350 000 tons of liquid sludge produced annually.

The new pressing plant occupies about 2 acres. It has cost £ 620 000 and its completion marks the end of the first phase of reconstruction programme which also includes new screening and grit separation plant and new primary settlement tanks to replace tanks now forty to eighty years old. The plant in question serves a population of 500 000 and the dry weather flow of 26 m.g.d. includes about 12 m.g.d. industrial wastes, much of it from the steel Industry.

The liquid sludge dewatered in this installation is the combined total production of primary and activated sludge, without prior digestion.

The design of the filter press installation at Sheffield was likely to reflect present conditions. The shortage of available land, the fact that the industrial wastes contain much iron, indicated a mechanized sludge dewatering system and a filter press

plant was selected. The plant at present is handling about 17000 tons of dry sludge solids per year, producing 55 000 tons of filter cake/year. This output is being produced by day work only.

The filter press house is set midway between settling tanks and bio-aeration treatment channels. The sludge is withdrawn from the primary settling and activated sludge tanks and pumped into three covered sludge storage tanks which have a total capacity of 900 000 gallons.

There are three conditioned sludge storage tanks each of 63 000 gallon capacity sited along the front of the filter press house. They are 30 ft diameter tanks by 11 ft side wall depth. Sludge is pumped to the tanks through a mixer consisting of an l8 in. pipe with 8 in. diameter propeller rotating at 960 r.p.m. Here milk of lime(10% lime slurry) is injected into the sludge flow, mixed and delivered into the storage tanks.

The lime itself is brought in road tankers and blown into one three tanks containing effluent water (20 000 gallon capacity each) and then dosed into the mixer.

Fully automated effluent treatment plant.

Many branches of industry are known to produce toxic effluents. The technical process is such that when the liquors have come to the end of their useful life, they must be disposed of and this can create problems if not subjected to treatment before discharge.

Doncaster Ltd of Leeds was faced with such a problem and it was a requirement of the local authority that no toxic metal or acids should be allowed to flow to its treatment works. The total plant was designed, it having certain novel design concept. In common with all effluent treatment plants it was necessary to define the problem which existed and to anticipate future problems by predicting the development of electro-process technology; to establish all definite, probable and possible outlet points for effluent from the process, define both the quantities which would be discharged from each individual outlet and the chemical character of that effluent. Supervisory staff bad to control general effluent production.

With the problem defined it was then possible to approach the treatment procedure. The constituents of this particular effluent were hexavalent chromium in the form of chromic acid together with hydrofluoric, nitric, phosphoric, sulphuric and hydrochloric acids present in either highly concentrated form a very dilute form as general swill waters. While acids can be neutralised by alkali addition, chromium in the hexavalent form does not react. It was necessary to reduce the chromium to the trivalent state by the use of sulphur dioxide before precipitation as the hydroxide.