Influence of Leachate Recirculation on Landfill Degradation and Biogas Production

Population growth combined with the rising standard of living of people around the world is the reason for the ever-increasing production of waste which management is costing states a lot of money for its disposal. Among available waste treatment techniques, landfill is one of the most promoted waste management techniques with the emergence of the bioreactor concept. However, the control of biodegradation parameters in order to accelerate waste stabilization is an important issue. For environmental and economic reasons, the technique of leachate recirculation by injection into the waste is increasingly used to improve the degradation of landfilled waste. The injection of leachate is possible using vertical boreholes, horizontal pipes, infiltration ponds or a combination of these. Indeed, moisture is the main factor in waste degradation and biogas production. The migration of leachate to the bottom of the landfill creates low moisture in the upper areas of the landfill reducing the growth of microbial populations. This results in low or no biogas production. The main objective of the present work is to develop a numerical model of leachate recirculation by injection into the waste to rewet the waste and restart biological activity. The analysis of the results shows that the diffusion of the wet front increases with time and depth. The lateral widening of the wet front is slow in relation to the progression of the wet front towards the bottom of the waste cell. This indicates the predominance of gravity effects over diffusion phenomena. The results reveal that the distributed re-injection is the best mode of leachate recirculation because the moisture distribution on the whole waste mass is totally satisfactory and the biogas generation is more important. Leachate recirculation campaigns should be done periodically to rewet the waste, boost microbial activity and hope for a quicker stabilization of the landfill.

Numerical Study of Mixed Convection in a Photovoltaic Trombe Wall Channel Coupled with a Solar Chimney Integrated into a Building for Habitats Passive Cooling

The solar chimney can generate airflow through the living space of the building to provide cooling. Hence, solar energy represents the best renewable, environmentally friendly source of energy that can be used for heating and cooling of houses. The present paper reports the numerical study of the performance of the mixed convection in the associated hybrid Photovoltaic/Thermal chimneys integrated into building for natural habitat ventilation. The front side glass plate of the chimneys is heated by a non-uniform daily solar radiation flux. Air is considered to be the cooling fluid. The stream fucntion-vorticity formulation with a finite difference numerical discretization solution scheme has been adopted. The system of algebraic governing equations is solved by Thomas algorithm method. The aim of the present paper is to study and to predict the dynamic fields and particularly of the mass flow rate of the air thermosiphon drawing in the associated hybrid Photovoltaic-Thermal chimneys integrated into a building for passive cooling in the habitats. The effects of the governing parameters, namely Reynolds number (30 ≤ Re ≤ 200), Rayleigh number (103 ≤ Ra≤ 105), the integrated chimney width on the fluid flow and the heat transfer characteristics, are studied in detail. The local Nusselt number, streamlines, isotherms, PV cells electrical efficiency and the outlet velocity at the top of the channels are the results represented versus the above controlling parameters.

Numerical Analysis of the Transient Process of Flow and Heat Transfer at High Rayleigh Number in a Partially Open Habitat Heated from Above

In this work, we studied the thermoconvective instabilities in a pentagonal cavity containing a Newtonian fluid. The cavity provided with a side opening is uniformly heated from above by a constant heat flux. The natural ventilation phenomenon in the classic habitat of the hot climate is thus numerically analyzed with unsteady natural convection equations formulated with vorticity and stream-function variables. The finite volume predictions of two-dimensional laminar natural convection at high Rayleigh number are presented. Results show that the incoming fresh air and the hot air discharge begin with the late start of the convection. The phenomenon intensifies with time and the birth of instabilities improves the homogenisation of temperatures which imply the elimination of very cold and very hot areas. However, the competition between the incoming fresh air and the hot air expansion leads to a perpetual displacement of the thermal front. The cross-sections at the opening of the incoming fresh and outgoing hot air are time-varying and the penetration depth of the fresh air is highlighted by the large convective cells originated from the aperture. The non monotonic variation of the Nusselt number reflects not only the multicell nature of the flow but also expresses the heat lost by the active walls due to the fresh air.