In my experience working at a coal plant the water from the water side of the boiler never contacts any coal.  Otherwise you have huge problems.  The water is treated with oxygen scavengers corrosion inhibitors and and ph boosting chemicals.  Usually the boiler blow down water comes out with a yellow tint very low conductivity and has about a 11 ph however most blow down water is ph neutralized and cooled down before discharge.  The other wastewater streams are usually concentrate from RO systems which is concentrated city water or well water.  There is also blow down from cooling towers which depending on how they are chemically treated with antiscalants biocides bleach and sulfuric acid but with the huge amount of water blow down this is very diluted.