boues finales

CONT: [...] l'importance de ne pas sous-estimer le problème préoccupant de l'élimination des boues finales.

CONT: ... the importance of not underestimating the serious problem of disposal of the final sludge.
OBS: The term ""sludge"" is considered to be an uncountable noun. However, it is often found in the plural form (sludges), particularly when speaking of different types of sludge.