First-Order Removal Rates for Organic Matter in Full-Scale Waste Stabilization Pond Systems in Northeastern Brazil

Abstract

This paper investigated the organic matter removal in waste stabilization ponds (WSP) based on ten full-scale wastewater treatment plants (WWTP) located in Rio Grande do Norte, Northeast Brazil. Although many systems are operating at high organic loading rates, in some cases, over 1000 kg BOD5 ha−1 d−1, the ponds promoted satisfactory removals of biochemical oxygen demand (BOD5) and chemical oxygen demand (COD). The first-order removal rates (k) for BOD5 and COD were obtained by assuming the ideal hydraulic patterns of completely mixed and plug-flow models. The k values proved to be a function of physical and operational parameters that affect fluid movement in a pond. The increase in the organic loading rate caused an increase in first-order removal rates. Moreover, increasing the ratio between hydraulic retention time (HRT) and depth (H) led to a reduction in k values. This information can be adapted into predictive models to be applied in WSP designs with similar characteristics. In comparison with the empirical equations reported in the literature, these models lead to removal rates more appropriate to the local reality.