Co-Composting of Dewatered Sewage Sludge (Biosolids) and Sawdust for Agricultural use as an Organic Fertilizer: A Case Study of the KNUST Sewage Treatment Plant

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The objective of the study was to determine the best mix ratio of dewatered sewage sludge and sawdust in co-composting, to stabilizing the former and also managing the latter. Three different ratios (1:1, 1:1.5 and 1:2) of the dewatered sewage sludge to sawdust based on their carbon-nitrogen ratio and moisture content were composted for 120 days at the KNUST Sewage Treatment Plant. The compost heaps were turned over every three days for the first month and once a week afterwards to enhance aeration. Temperature was daily monitored at 20 cm and 30 cm depths of the heaps. The rate of decomposition was also assessed weekly by determining the rate of reduction of the compost heaps. Other parameters such as moisture content, dry solids, organic matter, ash, carbon, nitrogen, carbon-nitrogen ratio, phosphorus, potassium, pH, helminth eggs, total and faecal coliforms were determined monthly. The results showed a significant difference in the monthly levels of all the parameters listed above in each heap. There was no significant difference in the quality of the final compost produced from each heap, in terms of most of the listed parameters. Microbial parameters such as total and fecal Coliforms and helminth eggs decreased appreciably at the end of the composting process. The compost heaps with the ratios 1:1.5 and 1:2 reduced more than the heap with ratio 1:1 of sewage sludge and sawdust. Twenty eight cubic centimeters (28cm) of the different composts produced and dewatered dried sewage vi sludge were (set up as control) applied on different ten square meter beds to cultivate lettuce. Yield of lettuce, helminth eggs, total and faecal coliform organisms levels were determined upon harvesting. Yield was highest in lettuce cultivated with dewatered dried sewage sludge, followed by the compost with the formulation 1:1, 1:1.5 and lastly 1:2. Helminth eggs, total and faecal coliform levels on lettuce were higher than their levels in their respective compost and the dewatered dried sewage sludge. There was no significant difference in the quality of compost produced from sludge/sawdust of the various ratios (1:1, 1:1.5 or 1:2).
A thesis submitted to the Department of Theoretical and Applied Biology, Kwame Nkrumah University of Science and Technology, in partial fulfilment of the requirement for the award of the Degree of Master of Science on February, 2009.