Modeling Wind Effect on Waste Stabilization Pond Performance

Wind has an important effect on the behavior of ponds as it induces vertical mixing of the pond contents. Good mixing ensures a more uniform distribution and higher dispersion number within the pond and hence a better degree of waste stabilization. This study developed a mathematical model for the prediction of Coliform bacteria ( ce /co) on waste stabilization pond performance. The model was developed based on two-dimensional steady dispersed flow model and Hulbult’s (1944) boundary conditions. The solution of the equation was restricted to method of separation of variable and fourier series expansion. The model on wind effect was carried out with different wind speed 2.27m/s, 1.88m/s and 1.64m/s respectively directed at different tanks at inlet, outlet and the side of the tanks labeled, B, C and D, while tank A was under control condition. The model was verified using the laboratory scale model ponds (LSWSP) experimental results. The solution of the model was obtained by writing Fortran computer programme for the computation of coliform bacteria ratio ( ce /co). The coefficient of correlation between the measured and the predicted values ranges between 0.8710 to 0.9980 indicating that the prediction are very good. Keyword: Dispersion number, waste stabilization pond, coliform bacteria, fourier series, separation of variable. Introduction The primary purpose of wastewater treatment is the reduction of pathogenic contamination, coliform bacteria, suspended solids, oxygen demand and nutrient enrichment. Those treating raw wastewater are referred to as facultative ponds, lagoons or oxidation ponds. Their purpose is to further reduce suspended solids, BOD, faecal micro-organisms and ammonia in the plant effluent. Wastewater stabilization ponds (WSPs) are a cheap and effective way to treat wastewater in situations where the cost of land is not a factor. Not only has it been found to be one thousand times better in destroying pathogenic bacteria and intestinal parasites than the conventional treatment plants (Mara and others, 1983). It is also more economical (Arthur, 1983). It is simple to construct, operate and maintain and it does not require any input of external energy. Although a waste stabilization pond system usually requires large land area because of its long detention time which is attributable to its complete dependence on natural treatment process, it will still be very suitable in several African countries and communities where land acquisition is not a problem. Besides, its efficiency depends on the availability of sunlight and high ambient temperature which are the prevailing climatic conditions in most of these communities. Many characteristics make waste stabilization pond substantially distinguished from other wastewater treatment methods. This includes design construction and operation simplicity, cost effectiveness, low maintenance requirements, easily adoptive for upgrading and high efficiency. Conventional treatment of liquid wastes involves mechanical treatment systems, and are the norm in developed countries. However, they are not the best option for less developed countries. Indeed, conventional treatment schemes were developed due to climatic and area constraints. These constraints are often not the case in developing countries. Moreover, the use of energy intensive mechanisms is not desirable in less developed countries, where energy supply is not reliable. Further, conventional treatment facilities require regular high-skilled maintenance, a thing that is either too expensive or impossible to find in developing countries. Wind has an important effect on the behaviour of facultative ponds as it induces vertical mixing of the pond liquid. © Centre for Promoting Ideas, USA www.ijastnet .com 156 Good mixing ensures a more uniform distribution of Biochemical Oxygen Demand (BOD), dissolved oxygen (DO), Coliform bacteria and algae and, hence, a better degree of waste stabilization. In the absence of windinduced mixing, the algal population tends to stratify in a narrow band some 20cm thick during day light hours (Mara and others, 2001). This concentrated band of algae moves up and down through the top 50cm of the pond in response to changes in incident light intensity and causes large fluctuations in effluent quality (especially BOD and suspended solids). Hence wind action promotes mixing and reaeration within the pond system and operation. Wind sweeping over water surfaces creates a zone of circulation within the top surface of the pond called the epiliomnion. The wind-created velocity is used up in transporting the water through the length of the fetch and back to its point of origin in the return flow (Wright, 1972; and Gallagher and others, 1973). There has been growing activity in the study of wind generated circulation in lakes and Lagoons. Representative studies in this direction are those of (Gallagher and others, 1973 and Gedney and others, 1972) respectively. Other investigators have studies circulation patterns in lakes and Lagoons. The effects of wind on velocity distribution and wave generation have been studied by (Wu, 1973). In many cases of practical importance, the need to know the velocity distribution in a lake or Lagoon stems from the need to know the distribution of transportable substances, such as the BOD and DO. In these cases, wind action has more than a single role to play, not only does it determine the main features of the velocity distribution but also it establishes the magnitude of the turbulent diffusion and surface reaeration. The longitudinal dispersion coefficient in a pond or wide channel can be calculated under various combinations of stream flow and wind conditions. The effect of wind, which produces drift currents in the stream, on dispersion has hitherto been neglected. The mechanism of longitudinal dispersion in turbulent shear flow was discussed first by (Elder, 1959). Restricting his study to a long straight circular pipe, Taylor not only proposed a scheme of calculation, but also verified his analytic results experimentally. Other extensive experiments have also been conducted in laboratory Flumes by (Elder, 1959, Fischer, 1966 and 1967) respectively and in natural streams by (Thackstone and Krenkel, 1967). Also (Wu, 1969) expressed the combined velocity distribution in a channel as: