The byproduct are usually comprised of gas, water, oil sludge/residue and catalysts.
The gas will be purified before emission or to be recycled as heating fuel;
The water will be filtrated and purified as cooling water;
The oil sludge could be used for burning or to be raw material for producing asphalt or road constructions;
The catalysts can be reactivated by microwave or other technologies for reuse. Or to be handed over to the professional recycler for disposal.