ABSTRACT
This study investigates the performance of laboratory pond system containing immobilized cells of Saccharomyces cerevisiae in the treatment of raw tannery wastewater as compared to a pond system without cells. The performance of the pond system was evaluated by the measurements of BOD, COD, sulphate, TS, chloride, pH, chromium and cadmium. Also, the hydraulic characteristics of the pond configuration such as the average retention time and flow rate were estimated.
The results obtained indicated that the system containing the immobilized cells yielded better results in the reduction of some of the parameters when operated under the same hydraulic and organic loadings. For example, the overall BOD, COD and chloride removal were 73.5%, 62.96% and 69.56% respectively. Also, chromium and cadmium respectively attained a reduction of an average of 96.21% and 53.77%.
Using statistical tools in further analyzing the results showed that the immobilized cells of s. cerevisiae are solely responsible for the reduction of the parameters.
CHAPTER ONE
1.0 INTRODUCTION
Almost all industries discharge waste waters at some stage of their manufacturing processes. The effects of different types of wastes when discharged for example to a stream are very complex. Leather industries are also one of the chief polluting industries that are responsible for the discharge of harmful and inorganic substances to the environment. The presence of tannery waste in water usually inhibits the development of such water for drinking and industrial use. Examples of such pollutants are suspended matter, which may kill fish and render water unsuitable for use as a domestic supply.
Tanneries have for many years, been associated with bad smells and the dumping of untreated waste effluent in rivers. In Nigeria, tanneries dispose their waste indiscriminately into sewer systems or nearby streams (Aliyu 1997).
There are difficulties in the treatment of tannery wastewaters due to the complexity of .the waste. Constituents that can cause difficulty at treatment works are suspended solids including hair and fleshing, lime (giving too high a pH value for effective treatment), sulphide and compounds of chromium (Ernest, 1980).
Due to the high cost of treatment of their wastes, most of the local tanners in Nigeria may not and are unable to adopt any form of treatment of their waste (Aliyu, 1997). However, some form of preliminary treatment is possible in the case of mechanized tanners who produce larger volumes of waste. Tanneries are heavy consumers of water (Ernest, 1980) and thus, large volume of waste waters are released daily. Most of the tanneries in Nigeria simply discharge their effluent without prior treatment into sewers or streams due to the high cost of treatment (Aliyu, 1997).
In view of the high cost of treating tannery effluent, this study therefore, focuses on the use of biotechnology which provides and environmentally accepted technique in treating tannery effluents.
1.2 TANNERY PROCESSES
Several processes are involved in the production of leather from raw hides and skins. Generally, a tannery is divided into three sections: the lime yard or beam house, the tan yard, and the finishing yard (Sarkar, 1981). The type of process is of more importance with respect to pollution control than the type of hide or skin processed (Tannery wastes, 1972).
1.2.1 THE LIME YARD
Hides and skins normally come into tanneries in wet-salted or dry-salted forms which preserve them from putrefaction and deterioration. They are
soaked in water in pits for 16 to 24 hours. When well soaked, the skins are then treated in a suspension of hydrated lime and sodium sulphide to swell the skin and to facilitate the removal of hair from the skin. They are un-haired, fleshed, washed and de-limed with ammonium sulphate or chloride (Sastry and Prasad, 1977). All the hairs, fleshing, waste liquors and washings make up the lime yard wastewater (Aliyu, 1997).
1.2.2 THE TAN YARD
Tanning is the process by which hide and skin molecules are structurally affected by substances known as tannins, such that the end product is a tender, non-putrefying material called leather. Tannins are applied to skins in water solutions. Two types of wastewater are normally produced in the tan yard: chrome tan wastewater is acidic, bluish green in colour and
contains Cr3+ ions between 1,000 and 2,200 mg/l. This form of chromium,
unlike Cr6+, is non-toxic (Sastry, 1977). The vegetable tan liquor is acidic, brown and contains organic tannins.
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