Some low-yield sulfite pulping operations ferment spent sulfite liquor (SSL) to remove biochemical oxygen demand associated with dissolved sugars while at the same time generating ethanol as a salable product. Simultaneous saccharification and fermentation (SSF) of primary clarifier sludge in a medium of SSL was proposed as a means of reducing the amount of sludge to be disposed of while at the same time increasing ethanol productivity. In this article, the option of fortifying existing SSL fermenting precesses with the sugars produced via in situ enzymatic hydrolysis of sulfite primary clarifier sludge (PCS) has been explored. In 100% SSL PCS hydrolysis rates as high as 3.4 g/(L . h) were observed at an initial enzyme loading of 10 filter paper. units (FPU)/g PCS. To reduce the deleterious effects of glucose inhibition, single-stage SSF was carried out using cellulase enzymes and Saccharomyces cerevisiae. The production rate of ethanol in SSL was increased by as much as 25% through the SSF process.