Report of the Medical Officer of Health on the public health and sanitary circumstances of Johannesburg.

  • Johannesburg (South Africa)
Date:
[1933]
Catalogue details

Licence: Public Domain Mark

Credit: Report of the Medical Officer of Health on the public health and sanitary circumstances of Johannesburg. Source: Wellcome Collection.

    41/52 (page 35)
    Other means of deodorisation have been investigated, and since scrubbing by activated carbon seems the most satisfactory method, supplies of activated carbon have been ordered overseas for large scale scrubbers. Very interesting problems have arisen out of these deodorisation studies, but details will be reserved for a future special report. Primary Filter Beds. Alteration of the fan spray jets of the distributors at Bruma Works to solid swing-arm jets resulted in great diminution in the odour of filter beds. At certain times when small spiders are least active, the psychoda filter fly is very prevalent. Weekly doses of bleaching powder are added to the tank effluent to keep down the numbers of this fly. In June, 1933, an achorutes species was observed on thn surface of one of the land effluent samples from the northern side of Klipspruit Farm. A search by the manager of Klipspruit Farm revealed ample supplies of achorutes at Klipspruit and at the same time the smell observer at Bruma, in his volunteer capacity of biologist, found a smaller and apparently different species on the Portuguese Gardens adjoining Bruma Works. Achorutes from both these sources have been used to seed the percolating beds at Bruma and Cydna- Works. Curves showing the flows, the crude sewage and final effluent figures for each purification plant have been prepared by the Superintendent of Sewage Works; these, and tables of average figures, are included herewith. Digester Overflow Liquids. The overflow liquid from the Dorr digesters at Antea and Bruma Works has still continued to present a serious problem. The discharge of the digester overflow into the raw sewage at both these works caused the whole of the sewage during sedimentation to become septic, with noticeable development of hydrogen sulphide which, when the tank effluent was distributed over the percolating beds, gave rise to aerial nuisance, and neces¬ sitated heavy dosage with chlorine. At Antea Works, ash drainage beds have been constructed to filter the digester overflow, which contains some 20—50 per cent, by volume of solids settleable in one hour. The filtrate is run into the tank effluent and then to the primary beds, whilst the retained solids are removed from the beds when sufficiently dry. This can only be regarded as a temporary expedient, but it has relieved the situation at Antea. At Bruma Works a small upward-flow conical settlement tank has been installed and it has been found that with continuous sludge removal such a tank is satisfactory for removing the solids from the digester overflow liquid. The sludge is handled on ash drainage beds, whilst the settled liquid is temporarily being given filtration through ashes and irrigation over land. The overflow from the Pruss digester at Cydna Works has by careful opera¬ tion of the central settlement compartment and lessened stirring in the digester been kept fairly harmless, and has been returned to the screened sewage through¬ out the year. Use of Chlorine. The use of chlorine for the deodorisation of tank effluent prior to distribution on the percolating beds was studied at the Bruma and Cydna Works. In the case of Cydna tank effluent, it was found that 30-40 parts of chlorine per million were required to produce any noticeable reduction in odour. In the case of Bruma tank effluent, it was not possible, with the means available, to give a large enough dose for deodorisation. Small doses, about 10 parts per million, actually caused development of a more unpleasant odour; this intensified odour required 15 to 30 minutes to diminish its former intensity. As no period of stay of 15 to 30 minutes between settlement tanks and percolating filter was possible, the use of chlorine was discontinued. Chlorine of lime and lime are being used in the Bezuidenhout Valley sewrer. This retards development of free hydrogen sulphide, and being added about 1J 'miles above the sewage works, gives an efficient clotting action, especially with the soaps which are a predominant feature of Bruma sewage.