Involved in the biosynthesis of 2-keto-3-deoxy-D-glycero-D-galacto-nononic acid used in cell-wall polysaccharides (PubMed:18804026). Catalyzes the hydrolysis of 2-keto-3-deoxy-D-glycero-D-galacto-9-phosphonononic acid (KDN-9-P) to yield 2-keto-3-deoxy-D-glycero-D-galacto-nononic acid (KDN) (PubMed:18804026, PubMed:18986982, PubMed:23848398). Also able to hydrolyze N-acetylneuraminate-9-phosphate (Neu5NAc-9-P), 2-keto-3-deoxy-D-manno-octulosonate-8-phosphate (KDO-8-P), phosphoenolpyruvate (PEP), gluconate 6-phosphate, tyrosine phosphate ester and glucose-6-P as substrate (PubMed:18804026, PubMed:18986982, PubMed:23848398). 3-deoxy-D-glycero-beta-D-galacto-non-2-ulopyranosonate 9-phosphate + H2O = 3-deoxy-D-glycero-beta-D-galacto-non-2-ulopyranosonate + phosphate Binds 1 magnesium ion per subunit. kcat is 72 min(-1) for 2-keto-3-deoxy-D-glycero-D-galacto-9-phosphonononic acid (KDN-9-P) as substrate (PubMed:18804026). kcat is 44 min(-1) for N-acetylneuraminate-9-phosphate (Neu5NAc-9-P) as substrate (PubMed:18804026). kcat is 3.8 min(-1) for 2-keto-3-deoxy-D-manno-octulosonate-8-phosphate (KDO-8-P) as substrate (PubMed:18804026). kcat is 1.2 sec(-1) for 2-keto-3-deoxy-D-glycero-D-galacto-9-phosphonononic acid (KDN-9-P) as substrate (PubMed:18986982). Homotetramer. Belongs to the KdsC family.