Field evaluation of membrane distillation followed by humidification/dehumidification crystallizer for inland desalination of saline groundwater

Abstract

Membrane distillation (MD) and humidification/dehumidification (HDH) are emerging desalination technologies suitable for inland or “zero liquid discharge” (ZLD) applications. A1m3/d pilot MD system desalinated saline groundwaters with up to 6.3% total dissolved solids (TDS) and produced a brine at 10.2% TDS. This brine served as the feed for a pilot HDH crystallizer.The MD unit operated at 40% recovery producing distillate with <20mg/L TDS at a stable flux of 5L/m2-h. Pretreatment, either by adding antiscalant or concentrated acid, was critical for stable MD operation. HDH was an effective crystallizer capable of generating solids from the MD brine while producing additional distillate with TDS<100mg/L. Total energy input for MD and HDH were 260kWh/m3 and 220kWh/m3 respectively.Because cooling water is typically not available at inland applications, removal of latent energy is the key challenge faced by inland thermal desalination processes. This paper also describes the distinctly different processes used by MD & HDH for heat removal and presents an enthalpy balance for a 500m3/d MD desalination plant.Pilot results showed MD & HDH to be technically feasible for inland desalination but both processes need to increase energy efficiency to improve process economics.