Integrating project scheduling and material ordering adds more realism to project scheduling and considers additional trade-offs that could lead to reductions in the overall project cost. In this paper we review the evolution of the integrated problem, and investigate the impact of treating the activity duration as a decision variable on the activities schedule and materials plan. The effects of introducing rewards (penalties) for early (late) completion as well as materials quantity discounts on the project schedule and cost are also considered. These additions provide scheduling flexibility that might lead to further reduction in the project's total cost or makespan. Considering the various project costs, we found that there exists an optimal schedule that either starts as early as possible (at time zero) or completes as late as possible. We also show that if the project starts at time zero, then its duration cannot be longer than that for the case where the schedule ends as late as possible. It is also shown that the material ordering policy does not follow the Wagner and Whitin model when activity duration is variable or in the presence of quantity discounts. These results have led to modeling and solving the problem in a more efficient manner. Extensive computational work shows the validity of the model and the solution approach.
Analysis of the chemical components of Municipal solid waste (MSW) indicated that the presence of high concentrations of toxic heavy metals. The aim of this study was to select the best leaching condition, in order to generate a compost product, which adheres to the environmental standards for safe use.To determine the role of organic acids produced in the bioleaching process, chemical leaching experiments were compared using inorganic (H 2 SO 4 ) or organic acids (oxalic, and gluconic acids). These results revealed that addition of H 2 SO 4 correlated with a lower retained yield when comparing with addition of the other acids. Bioleaching was then studied by culturing fungi on the MSW. Addition of Aspergillus niger correlated with an efficient removal of heavy metal elements. The effect of pH was also determined and assays at pH 5.5 using a clay medium (32.4%) or at pH 2.5 using a sorghum medium (28%) were characterised by a lower retained yield. In the above conditions, we looked at elemental ions which have been deemed beneficial to the environment (P and K) and those which are toxic (Cd, Cu,Cr, Ni, Pb and Zn), these elements were measured in the solid fractions.
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