Solid build-up and consolidation within biofiltration of wastewater: A new multicretria analysis to early detect irreversible clogging
Accumulation et consolidation de matières solides au sein des biofiltres : comme détecter précocement la mise en place d'un colmatage irréversible
Résumé
Submerged biofiltration, using a granular medium colonised by biofilm, is indeed suitable for the treatment of
urban wastewater from large cities. Unfortunately, while periodic backwashing usually controls fouling, some
plants require extensive maintenance (i.e., irreversible clogging requiring declogging with soda cleaning) due to
head loss development and porosity reduction that conventional backwashing cannot overcome. A new multicriteria
analysis method has been proposed and tested on a full-scale plant where irreversible clogging
occured twice a year. This work first cross-analyses the on-line indirect measurements recorded over 2.2 years to
explain the occurrence of irreversible clogging. We have successfully shown that excessive volumetric loading
rates (in suspended solids or chemical oxygen demand) have provoked irreversible clogging. Secondly, this work
proposes scenarios of formation of cohesive matter, resistant to backwashing through, a succession of three
states, one for the accumulation of solids within the void volume (storage due to the partial efficiency of periodic
backwashing) and two states for the transformation of the accumulated solid matter with time, starting with the
initiation of consolidation (reversible) and ending with its consolidation (irreversible). Finally, this work proposes
a new method based on the analysis of the magnitude of the increase of the initial relative head loss with
time (ΔPi / ΔPref) to gradualy describe the clogging of a biofilter: <1.3 (no solids accumulation) for unclogged
biofilters, 1.3–1.6 (solids accumulation), 1.6–2 (initiation of consolidation) and > 2 (consolidation of solids). It
provides recommendations on how to process the data in a hierarchical manner to detect the transition towards
irreversible clogging at an early stage, and suggests corrective actions to carry out to the operator.
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