Tannery effluent treatment success

5 February 2001

Seminars and workshops are often mere talking shops, ending with a few pious recommendations and little action thereafter! But the Unido seminar held late 2000 was different. Designed to demonstrate the successes of Unido sponsored (Netherlands financed) effluent treatment and cleaner technologies project applications in Zimbabwe, the seminar achieved its objectives. Few of the 57 participants, drawn from all over the Eastern and Southern Africa region, could have returned home unimpressed by the scale of success and the relative ease with which similar advances could be achieved in their home countries given Unido sponsorship, donor aid and most important of all, motivation at managerial level. Unido tannery and pollution control expert, Hans van Os gave an overview of the project's scope and intentions to contain tannery pollution in Zimbabwe. This involved an initial survey of needs with three tanneries assisted in the upgrading of their effluent treatment systems. In addition, a cleaner technology demonstration unit has been established in Bulawayo and a research study into solid waste treatment started at the Bata plant in Gweru, including a pilot sludge treatment plant. Municipalities were also helped under the project. Project applications elsewhere in Africa, notably Kenya and Ethiopia, were of a very similar nature. Assistance provided has invariably involved technical advice and monitoring of implementation by Unido engaged consultants. In the case of Zimbabwe this was by tannery effluent specialist, Dr Guiseppe Clonfero and Johan Barnard, formerly a senior staff member at Liri Technologies, and now an independent consultant specialist with EnViroTan (Pty) Ltd, South Africa. Barnard supervised and implemented the research applications at Bata. The project, van Os said, had financed effluent treatment equipment needs on a revolving fund basis. The assisted tanneries had invested considerably in the installation of equipment. Training workshops were held to advance environmental awareness and the ability to deal with pollution factors. 'Significant improvements have been achieved at all plants concerned both in Zimbabwe and elsewhere', van Os said. Highlights of the seminar were: * Dr Clonfero on 'Sludge dewatering by drying beds' and 'Hair saving technology' * Johan Barnard dealing with 'Lagooning systems for tannery effluent treatment' and the 'Solid waste research project' at the Bata plant, Gweru * Carlington Maravanyika, Supierior Group of Companies, concerning high-exhaustion chrome tannage procedures instituted at the group's Imponente Tannery, Harare * Yassin Awale, Unido coordinator of projects on cleaner tanning technologies, broad brushing the several approaches open to tanneries pursuing the goal of 'cleaner tanning technology' Bata's Manuel Makiwa said when the solid sludge research project began, sludge production stood at 12 tons daily (20% water) and cost US$10,000 a year for removal (four trailer loads/day). He added that a larger experimental digestor of five m³ capacity, to investigate feeding procedures, gas handling and usage, would be commissioned shortly. For 2001/2002, a 300m³ sludge digestor was planned and would cater for the entire effluent plant sludge production. This could reduce the volume of sludge to 1.2 tons/day or two trailer loads/week, with savings of $9,000 on transport costs alone plus gas recovery of 40,000 litres (coal equivalent of 40 tons) valued at $4,000. Both Barnard and Makiwa freely admitted to problems during the research project. These involved blockages in feed pipes, difficulties in measuring gas production during small scale trial runs and lowered biological reaction rates during winter months. Tannery director at Imponente Tanning, Harare, Carlos Touguinha, told the seminar that limited space at the plant had precluded the use of drying beds. The company had, instead, installed a belt filter press on Unido advice. The results were positive: dry matter percentage had increased from 3.5% to 25% and 7 m3 of recovered water is recycled. The savings on solid waste transport and disposal costs are impressive, about Z$1 million (US$ 18,000) annually. With purchase and installation costs of near Z$2 million (US$36,000), the pay back time for this environmental investment will be a mere two years. Combined with other process adjustments, the tannery is saving round Z$6 million a year on operating expenses, the biggest saving being from reduced chrome tanning salts purchases. Carlington Maravanyika, also of Imponente, explained further. He said the company had achieved a balance between chrome offer and chrome uptake. He quoted dry weight averages of 3.4% chrome content in wet-blue against 0.25% Cr2O3 g/l in the used float. Results like this were relatively simple to obtain, given final pH values of between 3.9-4.2 and final temperatures of 45-50°C. However, this called for close supervision of operational procedures. 'Conventional tannages undoubtedly use more chrome than high exhaustion systems', Maravanyika said. The final pH and temperature, when high, ensured high chrome fixation. The results from Imponente had led not only to significant cost reductions during processing but also to reductions in discharged heavy metal pollutants.

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