Recycling water from spent dialysate by osmotic dilution: Impact of urea rejection of forward osmosis membrane on hemodialysis duration

Highlights

• Osmotic dilution was used for recycling spent dialysate of hemodialysis.

• A model was developed to evaluate the effect of several factors in the osmotic dilution on the hemodialysis duration.

• A relatively low urea rejection of an FO membrane has a minor impact on the subsequent hemodialysis duration.

• The reasons for the counter-intuitive result in recycling the spent dialysate by FO were revealed.

Abstract

Hemodialysis consumes significant amounts of pure water and produces large amounts of spent dialysate. Recovering water from spent dialysate is a new attempt in medical industries to save water and costs. The recent proposal of using forward osmosis (FO) to concentrate the spent dialysate showed economic viability. However, there is still one key concern of potentially extended hemodialysis duration because commercial FO membranes have limited urea rejection, which would reduce the urea concentration gradient during hemodialysis. In this paper, a model was developed to quantify the effect of incomplete urea rejection in FO on subsequent hemodialysis duration. The impacts of average urea rejection, water recovery, and the number of hemodialysis sessions were systematically analyzed. Counter-intuitively, the results indicated that relatively low urea rejection of FO membranes actually has little impact on hemodialysis duration. Low urea concentration in the spent dialysate, partial urea rejection by the FO membrane, and further dilution of pre-diluted dialysis concentrate, largely reduces this impact. The result further supports the viability of FO in spent dialysate reuse. Although FO membrane with 100% rejection is expected, the present work indicates that commercial FO membranes with a low rejection of neutral matters are of practical utility in certain applications.

Introduction

Hemodialysis requires large volumes of purified water produced by the water purification system (primarily consists of a pretreatment unit and a reverse osmosis unit) to prepare dialysate, rinse and reprocess dialysis machines and membranes [1,2]. A typical facility-based patient uses about 80 tons of tap water to produce sufficient high-grade water per year for hemodialysis [2]. In 2010, the number of patients needing renal replacement therapy was estimated to range between 4.902 and 9.701 million worldwide, and is expected to increase dramatically in the coming decades [3]. Recycling water from spent dialysate with a low-cost method is of great significance for saving water resources and costs, as well as reducing the potential pollution risk of spent dialysate discharge, especially in the water shortage area.

A major challenge to recycle spent dialysate is the removal of urea, the main nitrogenous waste from metabolism [4]. Hydrolysis by enzymes to ammonia and subsequent adsorption by zirconium hydrophosphate is the most common technique to remove urea from spent dialysate [5]. The high cost and the frequent sorbent cartridge replacement are limiting for the wide application of the technique [6]. Electrochemical methods have been reported for efficient urea degradation [4,7], but the generation of toxic chlorine species, metal leaching from electrodes, and development of acidosis represent severe disadvantages of such an approach [7].

Forward osmosis (FO) is an emerging membrane technology utilizing natural osmotic process for water desalination [[8], [9], [10], [11], [12], [13]]. Without the energy-intensive draw solution recovery, osmotic dilution is a truly low-energy FO process with lower energy consumption than other desalination technologies [[14], [15], [16], [17], [18], [19], [20], [21], [22]]. In our recent work, osmotic dilution was reported to be promising to recover water from the spent dialysate [18]. Dialysis concentrate was used as the draw solution to extract water spontaneously from the spent dialysate (Fig. 1). Complete rejection of creatinine and uric acid by the FO membrane, as well as low membrane fouling and scaling were observed. Centralized treatment of spent dialysate from a dialysis center by FO would save substantial water and cost in case of complete urea rejection. Nonetheless, low urea rejection by the FO membrane raised great concerns since the small molecular weight (60 Da) and electroneutrality of urea presumably lead to a diluted dialysis concentrate containing urea. In hemodialysis, the urea concentration gradient between the blood and dialysate side represents the driving force for urea transport across the hemodialysis membrane [23]. As a consequence, the reduced urea concentration gradient leads to prolonged hemodialysis duration, which further results in increased dialysate consumption and costs.

Unfortunately, FO membranes reject urea only partially. To clarify the validity of FO technology in the application, we developed a mathematical model that correlates FO with hemodialysis to quantitatively determine the effect of partial urea rejection in osmotic dilution on the subsequent hemodialysis duration. The effect of three factors, i.e. average urea rejection in FO, water recovery, and the number of hemodialysis sessions, on the hemodialysis prolongation rate was systematically analyzed. This work provides a theoretical basis for the potential application of FO in water recovery from the spent dialysate.