Saturday, 19 March 2022 11:24

Long-term effects of mineral precipitation on process performance, granules’ morphology and microbial community in anammox granular sludge


When treating wastewaters prone to inert precipitation with granular sludge systems, mineral formation needs to be properly addressed and controlled to ensure system’s long-term stability. In this work, an extensive study on mineral precipitation on the surface of anammox granular sludge is presented. A 7-L gas-lift reactor was inoculated with one-year stored biomass. A fast start-up procedure lead to volumetric load of up to 0,48 gN-NO2-/l/d, with a maximum specific nitrogen load of 0,28 gN-NO2-/gVSS/d and stable nitrite removal higher than 95%. Severe mineral precipitation was observed on the granules’ surface, after a few months of hard-water feeding and resulted in a dramatic deterioration of reactor performance. Substrate diffusion limitation from the bulk to the inner layers, insufficient mixing due to higher granule density and shear stress increase due to hard-particle collision were the main mechanisms that lead to progressive process disruption. Macroscopic evidences, SEM-EDX and ad hoc chemical analyses revealed that the inert formation was a calcite-like mineral. The criterion of gravimetric selection was applied to discard the highly dense granules affected by severe precipitation, which can be adopted for inert accumulation control in real-scale installations through waste sludge withdrawal from the bottom of up-flow reactors. The present study provides valuable knowledge for addressing operational problems due to intense inert formation in real-scale granular systems and for understanding the mechanisms behind related systems’ failures.

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