Petroleum refinery waste stream sludges are recycled by segregating the sludges according to their oil content. Sludges of high oil content are developed and then injected into a delayed coking unit during the coking phase so that they are converted to coke and liquid coking products. High water content sludges are used to quench the coke during the quench phase of the coking cycle, with minimal increases in coke volatile matter. The process increases the capacity of the delayed coking unit to process and recycle refinery waste sludges and produce a coke of lower volatile content.