In some embodiments, this invention pertains to isotopically enriched N-substituted piperazine acetic acids. The sensor spacer is spaced apart from the sensor pad, and includes a sensor electrode disposed on a portion of the sensor spacer which faces the sensor pad. A continuous flow of wastewater is supplied to the rotatable device, and intermittent pulses of wastewater are supplied to the stationary nozzles. The support pillars, the support plate, and the flexible pads form a hollow area, and each support pillar has a hollow hole.