Developments to the first-order, complete-mix design approach for stabilisation ponds

The results of an extended investigation into the operation of a stabilization pond system in the Cayman Isles, incorporating an appreciably saline wastewater, were used to discover any correlations existing between a number of wastewater, pond and environmental factors and four different reaction coefficients employable in the first-order, complete-mix equation for the design of ponds. The four reaction coefficients included are derived using the BOD5 results for unfiltered inlet and outlet samples (ufk), another derived using the unfiltered inflow samples but filtered outflow samples (uffk), a third calculated employing filtered inflow and outflow samples (fk) and the fourth was derived from the inflow and outflow counts of faecal coliforms (kb). In the first stage of statistical analysis, no reliable simple correlations were discovered between any of the k values and any one of the individual pond, wastewater or environmental factors employed. However, two further stages involving Stepwise Backward Selection and Hierarchical Multiple Regression produced reliable models for ufk, uffk, fk and kb) for the ponds in the system in which log kb was dependent on the rate of inflow and the log of the faecal coliforms concentration and ufk, uffk and fk were influenced by solar radiation, rainfall, faecal coliform concentrations, BOD loading and pond depth. Only two design equations could finally be selected. These were for uffk and kb for facultative ponds