Augmenting water supply by combined desalination/water recycling methods: an economic assessment

Dry coastal communities increasingly need to consider non-traditional methods of augmenting their water supply. While seawater reverse osmosis (SWRO) is a popular option, existing municipal wastewater flows allow for other options. The current study presents a preliminary economic comparison of three alternatives for increasing the water supply by 50% for a hypothetical baseline coastal scenario: increasing desalination (Scenario A), direct potable water reuse (DPWR) (Scenario B), and a novel retrofitted configuration of a hybrid Forward Osmosis – Reverse Osmosis (FO-RO) plant (Scenario C). The latter used the dilution of the seawater feed to increase the recovery and overall output water of the original RO step. To account for the time value of money, levelised cost (LC) was used as the primary economic metric. The hybrid FO-RO configuration had a comparable LC to DPWR (0.59 vs 0.61 $ m^-3) and was 12% cheaper than desalination (0.67 $ m^-3). Furthermore, hybrid FO-RO was 7% more energy efficient than conventional desalination due to reduced intake and pretreatment flows. Sensitivity analyses demonstrated that incremental reductions in LC were possible for increased FO membrane flux, including in pressure assisted osmosis (PAO) scenarios with applied pressure ranging from 2 – 6 bar. The viability of DPWR and hybrid FO-RO was dependent on the extent of existing wastewater treatment in the baseline scenario. Desalination was comparable or superior in baseline scenarios featuring treated wastewater of less than secondary quality. These findings validate the examination of hybrid FO-RO configurations that deviate from the energy-reduction paradigms typically studied