A nucleus replacement mimics a native annulus in shape and function for use in partial disc arthroplasty. The opening of the inlet throttle valve or the exhaust throttle valve during the closing process from the normal opening to the recovery opening and during the returning process from the recovery opening to the normal opening is changed at a gradually decreasing or gradually increasing rate of change of the opening. The biaser provides a positive downforce urging the forward end of the nozzle assembly toward the surface being cleaned. The frustro-conical surface may be an external surface or an internal surface, depending upon the desired configuration of the connector. The invention also provides a method of filtering raw wastewater through a bed of fermenting organic solids suspended by gas evolved in a fermentation zone. The customized patient-specific acetabular reaming guide includes a longitudinal passageway for an acetabular reamer and a plurality of arms with attached feet.