Methods and system for treating blackwater are disclosed. A pyrolysis chamber and a condensing heat exchanger are provided. A blackwater slurry, consisting of blackwater solids, water, and volatiles, is passed into the pyrolysis chamber. The blackwater slurry is heated to produce steam, to separate at least a portion of the volatiles, and pyrolyze the blackwater solids, the separated volatiles passing out of the pyrolysis chamber. The steam and separated volatiles are cooled in a condensing heat exchanger, condensing at least a portion of the steam and at least a portion of the separated volatiles as a contaminated water stream. A water-clarifying chemical is added into the contaminated water stream that reacts with the volatiles, resulting in a treated water stream.