Abstract: The feature of the present invention resides in that when a polyelectrolyte coagulant is added to sludge to dewater the sludge in a sewage treatment plant, the dosage level of the polyelectrolyte coagulant to be to the sludge is controlled most properly, whereby moisture content of dewatered cake can be minimized.

Abstract: A gallic acid alkyl ester or a gallic acid aryl ester is reacted in an inert solvent with 3 equivalents of naphthoquinone-(1,2)-diazido-(2)-sulfonyl chloride in the presence of an alkali to effect sulfonylation of 3 hydroxyl groups in the gallic acid moiety whereby a photodecomposable naphthoquinone-(1,2)-diazido-(2)-sulfonic acid ester is obtained. The new naphthoquinonediazido derivative thus obtained is mixed with an alkali-soluble phenol resin such as m-cresol novolac resin or phenol novolac resin to prepare a positive-type photoresist composition having high sensitivity and high resolving power as well as excellent dimensional accuracy and etching-resistance. In addition, this composition forms a good photosensitive film and can be a good ink receptor.