A design approach for facultative and maturation wastewater stabilization ponds
The aims of the research project were to observe the operation of stabilization ponds with a view to investigating the value of various existing approaches used in the design of both facultative and maturation wastewater stabilization ponds and, if possible, to produce a realistic and reliable method for pond design. This latter aim was to be carried out either through the modification of existing equations or by the development of a new approach. In addition it was also intended to investigate the phenomena of dissolved oxygen and thermal stratification within stabilization ponds. The required data for this work was obtained as a result of detailed observation of the four-pond wastewater treatment installation in the Cayman Islands, British West Indies. Details of flows, flow strengths, removal of BOD and faecal coliform organisms were collected and numerous profiles of the pond contents were carried out to investigate dissolved oxygen and thermal stratification. Information concerning temperatures, solar radiation, hours of sunshine, relative humidity, rainfall, evaporation and wind speed and direction were also recorded over the three year investigational period. The most common design methods, the first-order complete-mix approach of Marais and Shaw and the modified empirical approach of McGarry and Pescod as modified by Mara and Arthur, were studied in detail using the results collected. These two design approaches were those used for the initial design of the Cayman ponds. Suggestions were then made for modifications to the first-order, complete-mix equation with regard to the values of the reaction coefficients employed for the removal of organic matter and of faecal coliform bacteria. Modifications of the equations to obtain K (the first-order reaction coefficient for either BOD or faecal coliform removal) were suggested. Following that a new empirical design approach was developed using the data collected from the operational ponds together with the climatological information. The models were developed in three steps. Firstly, matrices of correlation coefficients were produced to discover the existance of any strong correlations between any of the dependent and independent variables considered. Secondly, the Stepwise Backward Selection procedure for multiple regression (SBS) was applied to select the most relevant preliminary variables for multiple regression models. Then finially, the relevant variables were grouped in ordinary least-squares multiple regression models using a Hierarquical Multiple Regression procedure (HMR) to construct the models. As a result of this work groups of semi-empirical and empirical models were produced_ Two semi-empirical design models were developed to express K as a function of loading and climatic factors. These two equations were for the removal of BOO and faecal coliforms in facultative ponds. Two other groups of empirical design equations with a total of fourteen possible design models relating pond efficiency to factors such as loading, retention time and climatic conditions were also produced for the design of facultative and maturation ponds in the Caribbean region treating weak and saline wastewater. Also carried out were extensive field investigations into the temporal and spatial distributions of dissolved oxygen and water temperature in the ponds and as to how these varied with climatic conditions.