Predicting long-term solid accumulation in waste stabilisation lagoons through a combined CFD-process model approach

Abstract

Sludge accumulation in anaerobic lagoons is one of the major issues determining long-term operating costs. However, very little mechanistic analysis has been done on long-term sludge behaviour. A coupled hydrodynamic-biochemical model was developed using computational fluid dynamics (CFD), and results from this applied to a compartmental based model (CBM) for long-term simulation. The CFD model incorporates a mixture method for the spatial-and temporal evolution of fluid and solids with a non-Newtonian rheology. CFD was used to evaluate short term hydrodynamics, and a common CBM used to understand the fluid movement and sludge behaviour of full-scale anaerobic lagoons (with varying depths, sidewall slopes, and loading rates), operating in commercial piggeries located in Southern Queensland and Southern New South Wales, Australia. The results found that the lagoons had varying hydrodynamics, and sludge accumulates rapidly in sloped sidewall lagoons, forming a variable depth bed which occupied a substantial fraction of the lagoons. Shallow lagoons were dominated by significant surface recirculation dynamics, and were susceptible to solids accumulation, while deep lagoons allowed the formation of a well developed settled fraction. Predicted lagoon lifetimes varied substantially, but predicted long-term accumulation rates were approximately double that observed, due to long-term degradation of slowly degradable material.