Mathematical modeling of dissolved oxygen in fish ponds

A mathematical model was developed to predict the effects of wind speed, light, pH, Temperature, dissolved carbon dioxide and chemical oxygen demand (COD) on Dissolved Oxygen (DO) in fish ponds. The effects of organic feeds, aeration and fish activity were added to the model developed by Kayombo et al. for Waste Stabilization Ponds (Ecological Modelling 127(2000): 21 - 31) to reflect the situation in fish ponds. Model calibration and validation was done by use of average DO, pH, temperature, COD, CO2 and algae biomass data measured from RETCO fish ponds in Dar es Salaam; and light intensity data were adopted from Kayombo et al. (2000). Model results showed a linear relationship between simulated DO and measured DO in fish pond (r2 = 0. 87) for model calibration and (r2 = 0.88) for model validation. Simulation results also showed a general decrease of DO with time in 13 days by 28 and 38% for first and second batch, respectively. Thus, the model developed in this study could be used to predict the DO dynamics in fish ponds. Based on the model results, successful cultivation of healthy fish may require that retention time for water in the fish pond be 10 days.Key words: Calibration, light, pH, substrate, temperature, validation

Construction and Demolition Waste Characteristics in Tanzania

The construction industry generates a lot of construction and demolition (C&D) waste which puts some challenges to its management. For example, currently, in many towns in Tanzania, there are no landfill sites for solid waste disposal; and as a consequence open air dumping sites are used. Dumping C&D waste puts pressure for acquisition of large portions of land in order to accommodate the disposal of the growing waste generated from construction and demolition sites. Others include imposed economic burdens, social discomfort as well as sources of environmental pollution like air and water pollutants. Due to population growth and land limitation for waste disposal, the current practices will put extra pressure on C&D waste management in future as well. This paper aims to investigate the quantity and quality of C&D waste in Tanzania and the possibilities for reusing and recycling this waste in the production of building materials. The use of C&D waste for building material production can be a best option not only for waste management but also for providing alternative building material for present and future generations. Materials used in this study were cementitious rubble recovered from eight building construction and demolition sites in Dar es Salaam. Two samples from natural sources were used for comparison purposes. Secondary data from Dar es Salaam City Council was used to estimate the amount of C&D waste generated in Tanzania annually. Furthermore, the recovered C&D waste samples were crushed to get recycled aggregates that were used in laboratory analysis. The results showed that the C&D waste generation in Tanzania increased from 3.03 million tonnes to 7.9 million tonnes in the period ranging from 1994 to 2010 years. Furthermore, the results showed that the recycled aggregates were weaker than natural aggregates, however, mineralogically were not significantly different from natural aggregates. Thus, their chemical composition similarities suggest that recycled C&D waste is suitable for production of building materials in Tanzania. The recycling of C&D waste into building material will contribute to sustainable social, economic, and environmental improvements