Dispersion studies in anaerobic ponds: implications for design and operation

The design procedure for anaerobic ponds is based on calculations of the influent volumetric organic load as a function of temperature and wastewater strength. Hydraulically either a plug flow or a completely mixed model is assumed. However, factors such as sludge sedimentation patterns, biogas bubbling and sludge accumulation may influence the hydrodynamic behaviour of the pond and hence its removal efficiency. The results obtained show that sludge accumulation and incorrect positioning of inlets and outlets influence the hydraulic performance of the ponds. The dispersion number values obtained from experimental data (δ> 0.01) show the predominance of flow mixing patterns within the ponds. Nevertheless, short-circuiting as well as dead zones coexist. Pond desludging as well as improvement of the inlet-outlet arrangements increased the removal efficiency of both BOD and TSS. It would seem that desludging of the anaerobic pond has a higher impact on the effluent quality compared with improvements to the inlets and outlets. Furthermore,the removal efficiency of suspended and soluble organic matter might be improved by enhancing mixing within the pond volume. Finally, apart from conventional dispersion studies, other tools like computational fluid dynamics are essential in order to understand the complex hydrodynamic phenomena taking place in anaerobic ponds.