This study is designed to assess the safety, efficacy and dialytic capabilities of hemodiafiltration with on-line prepared substitution fluid using the AK200 ULTRA in comparison to conventional hemodialysis. This study is motivated by the need for a commercially available product capable of administering hemofiltration and hemodiafiltration in a clinical setting. The ultimate goal of this study is to improve morbidity and mortality in the ESRD patient population in the United States.