In the contest of the study of Natural-Technological hazard (also named Na-Tech), some works investigating the effects of the volcanic ash fallout on chemical plants were published. The literature highlighted that volcanic ash emissions can endanger human health by means the inhalation of solid particles or can cause damage on critical structures, infrastructures, transport systems, chemical plants and lifelines due to the ash accumulation. In some cases, the impact on chemical plants and lifeline networks could also be significant for the environment. The effects of volcanic ash accumulation on primary wastewater treatment equipment have been recently analysed with the aim to determine the conditions leading to malfunctions of fine screens. The study highlighted that the critical ash thicknesses, causing damages, depend on the specific surface area and the voidage of the emitted material. To complete the previous investigation, in this paper the effects of these parameters are dealt and, then, the critical thresholds of volcanic ash deposit causing equipment malfunctions are updated. The area surrounding Mt. Etna is investigated.