A wastewater treatment facility with a rotating biological contactor (“RBC”) system. The RBC sits in a tank that receives wastewater to be purified. The tank supports a rotatable shaft. One or more stages of rotating disk assembles progressively purify incoming wastewater. Each stage is oriented transversely to the shaft and has a plurality of disk assemblies. Each disk assembly has a number of disks. Each of the disks comprises multiple concentric arcuate segments. The segments are provided with two generally radially extending edges, two circumferential edges that extend between the two radially extending legs, and a front face and a back face. Posts extend from the front and back faces of the disks for stirring wastewater and for offering a site for adherence of biomass suspended in the wastewater. Connecting members interconnect adjacent disks and optionally define a flow channel therebetween. First and second engagement features also extend from one of the front and back surfaces of a disk that enable adjacent segments of a disk to be joined.