Forward osmosis (FO) is considered as an energy-efficient process for numerous applications. Although its performance is determined by the spatially varied operation factors and the length of the channel, most of the reported simulation studies rely on length-averaged lumped models. Here, we introduce a one-D model based on heat and mass transfer and transport behavior for both bulk draw and feed channel flows. We find prediction results to be in good agreement with two different experimental results at inlet feed temperatures below 25 °C. However, the difference of water flux (Jw) and reverse salt flux (RSF) between measured and predicted data increases when both feed and draw temperatures also increase. Our theoretical simulation study first reveals that the feed temperature near the membrane active layer surface is the main factor for improving water and salt permeabilities. We find that, with a channel width of 0.3 m and a channel length of 2.5 m, Jw and RSF calculated using the length-averaged based lumped model are overestimated by 13.01% and 13.12%, respectively, compared to those obtained using our new spatial variation model. Our study demonstrates that the length-averaged based lumped model is not an appropriate simulation model to predict the performance of large-scale FO modules at lower inlet velocities.

The major challenge of membrane distillation (MD) is the low gain output ratio (GOR) resulting from the high thermal energy requirement and the low water production. Here, the use of hot streams such as geothermal groundwater or produced water that are suitable to be treated by MD without the need for thermal energy input is discussed. Hot groundwater from the Northwest of Himalaya, India, is used as a case study. In this paper, we propose a novel multi stage air gap MD reversal design coupled with natural/forced cooling system. A 1-D rigorous simulation model is developed to estimate the performance of the proposed system. Water production capacity and GOR of various scenarios were estimated and reported. Based on our results, the increase in temperature difference between feed and brine discharge cooled down temperature can have a positive effect to improve both water production capacity and GOR. The increase of inlet feed flow rate increases the water production but not the GOR. The influence of increasing module width on water production and GOR is negligibly small. Results showed that the maximum GOR was about 3.89 without natural (passive) cooling system. Meanwhile, when natural (90%)/forced (10%) combined cooling system was applied, a GOR as high as 24.4 could be achieved.