A multi-stage process for reducing the environmental contaminants in an ISO8217 Table 2 compliant Feedstock Heavy Marine Fuel Oil involving a core desulfurizing process and a Detrimental Solids Removal Unit as a pre-treating step or post-treating step to the core process. The product of the process is a Product Heavy Marine Fuel Oil compliant with ISO 8217 Table 2 for residual marine fuel including a maximum sulfur content (ISO 14596 or ISO 8754) less than 0.5 wt % and a Detrimental Solids content less than 60 mg/kg. A device for conducting the process and producing the product is disclosed.