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The main aim of this research is to develop a mathematical model to predict the dissolved oxygen in recirculating aquaculture system. The oxygen consumption of the model through the fish respiration and nitrification and the oxygen addition of the model through oxygen generator and water pumping. The effect of different water temperatures (24, 26, 28, 30 and 32 °C) on the dissolved oxygen consumption through fish respiration, biofilter and nitrification and fish growth were studied. An experiment to measure oxygen consumed by fish respiration and biofilteration and fish growth with the growth period and to validate the model results was carried out. The oxygen consumption predicted by the model was in a good agreement with those measured by the system. The oxygen consumption by fish respiration ranged 12.04 to 47.53 g O2 m−3 h−1 experimentally, while it was from 12.01 to 46.06 g O2 m−3 h−1 theoretically. The predicted and measured oxygen consumption through biofilteration values ranged from 0.43 to 21.91 and 0.45 to 23.09 g O2 m−3 h−1, respectively. The individual fish weight from the system ranged from 3.00 to 209.52 g experimentally while it was from 3.00 to 226.25 g theoretically during the whole period.

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The shortages in conventional fuels along with the continuous rise in their prices have led to an increased emphasis on using solar energy as an alternative source of energy. Thereto, the objective of this study was to evaluate the thermal efficiency of three different types of greenhouse solar dryers and test their performance in Corchorus olitarius drying. Three different greenhouses namely Quonset, Gable-even span and Gothic arch were constructed and tested without load under different air flow rates (0.38, 0.57 and 0.855 kg s -1 ). The appropriate air flow rate which gives higher thermal efficiency was then applied to investigate the performance of the constructed dryers for drying the fresh Jew's mallow. The results indicated that the thermal efficiency of the dryers increased when the air flow rate decreased. The Quonset dryer reported a higher value of thermal efficiency of 58.48% under the lower air flow rate of 0.38 kg s -1 . Consequently, the Quonset dryer showed higher moisture removal of the Jew's mallow during drying. After 16 hours of drying, the moisture content of Jew's mallow decreased from initial moisture of 13.29 (kg water/kg dry matter) to less than 0.5 (kg water/kg dry matter). Meanwhile, the moisture content of the Jew's mallow samples in ambient air was still higher than 1.2 (kg water/kg dry matter) after the same drying time. These results emphasize the importance of using solar dryers, especially the Quonset greenhouse dryer, for the fast and safe drying process of agricultural products.

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