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

  • Johannesburg (South Africa)
Date:
[1936]
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.

    36/48 (page 34)
    The experimental digester at Klipspruit has been fed -with crude sewage sludge only during the year, the dose being 5 per cent, of digester contents daily. The colder weather of May and June greatly impeded digestion, the pH value dropped, and the odour was worse, indicating that digestion was insufficient. The digester is to be operated with a proportion of night-soil added to the raw sludge. For the purpose of obtaining data, for the reconstruction of the treatment works erected nearly 30 years ago, the construction of a pilot plant has been authorised by the Council. The work is well advanced, and should be ready for operation at an early date. The plant consists of a screen, hopper-bottom vertical flow sedimentation tank circular in plan, measuring weirs, automatic dosing syphon, and a 40ft. diameter percolating filter (bacteria bed) approximately 6 feet deep. Adjacent to this plant there has been erected a Truss type totally enclosed filter, for which remarkable results are claimed by the suppliers. The diameter of the covered filter is 40 feet, its depth exceeds 12 feet, the tank effluent is pumped to the distributor, and air is introduced into the compartment by means of a fan. By operating the two types of percolating filters side by side, using the same tank effluent, under the same conditions, the comparative merits can be accurately ascertained. The information obtained will be invaluable when consideration is given to the design of the units of the proposed new works at Klipspruit. Anteci Works. During the year construction of the extensions mentioned in the last annual report and two new 100ft. diameter circular bacteria beds have been completed. A forced ventilating system and ozone-producing apparatus for deodorising the foul gases exhausted by this system from the sedimentation tank house have been installed. Construction of an open, earth-sided concrete slab lined, secondary digester has been started. The two original 100ft. diameter bacteria beds have taken the flow, on an average 160,000 gallons per day, giving a dosage of approximately 50 gallons per cubic yard per day, and the effluent has been satisfactory, especially considering the strength of the sewage. Bruma Sewage Works. The operation curves for the activated sludge plants at these works are appended, and examination of these curves will reveal points of interest that would require pages of description. The activated sludge plants at these works were installed to produce a partially purified and inodorous liquid to be further treated at high rates of application on the existing rectangular percolating beds. Elimination of odour from the percolating beds having become of supreme importance, the operation of the activated sludge plants has had to be modified to achieve this object. It has not been found possible to operate a straightforward partial treatment process under Johannesburg conditions without giving rise to serious aerial nuisance. Freedom from objectionable smell has, up to the present, only been obtained by the use of highly nitrifying sludge, and this, of course, involves the production of a highly oxidised effluent. It has been necessary, therefore, to adopt a dual treatment process in order to obtain a partially treated effluent without smell. The main plant was operated to give complete treatment to some 650,000 gallons of settled sewage per day. Twenty-five per cent, of the aeration capacity has been used for reconditioning of sludge. A completely stable and highly nitrified effluent has been produced. The surplus sludge from the main plant, which also contains much of the humus from the percolating beds, is gravitated to the small diffused air unit of 95,000 gallons capacity, where partial treatment is given to a further quantity of settled sewagp. The whole of the sludge from this overloading plant is then pumped back to waste into the raw sewage channel, prior to the Dorr Settlement Tanks. The flow of settled sewage to the overloading plant has to be varied according to the condition of the activated sludge coming from the large plant, and it is of interest to note that observation of the behaviour of this overloading plant gives a most reliable indication of the state of activity of the sludge on the main plant, for falling off in quality of the effluent in the overloading plant usually slightly precedes noticeable deterioration of sludge in the large plant.