PhD dissertation “Optimization of osmotic pressure usage for safe water production in the context of combining water reuse and desalination”, by Rajashree Yalamanchili

Growing water scarcity continues to push regions around the world to lookout for alternative water resources such as seawater desalination and wastewater reuse. While desalination by reverse osmosis (RO) provides a reliable supply, it remains constrained by high specific energy consumption and the environmental impacts of brine disposal. At the same time, discharge of treated wastewater is a missed opportunity for water reuse and resource recovery. In this context, a forward osmosis-reverse osmosis (FO–RO) hybrid system that simultaneously dilutes seawater and pre‑concentrates wastewater, is envisioned to lower the energy intensity of desalination while enhancing water and resource recovery from wastewater.

This thesis demonstrated, through modelling and design approaches, that by optimizing osmotic dilution processes, it is possible to achieve wastewater recovery as high as 90% in FO while reducing RO desalination energy below 1 kWh/m³. Furthermore, processing diluted seawater in an existing desalination plant showed feasibility to retrofit to FO-RO hybrid and also enhance permeate production as high as 67%. However, when comparing FO-RO hybrid with independent schemes energetically, standalone water reuse remained attractive. Beyond these valuable findings, the work also showed that integrating FO as a concentration step hinges on understanding how hydraulic conditions, flow orientation, module arrangement, and intrinsic FO mass‑transfer limitations shape the achievable recovery and proximity to osmotic equilibrium. Insights from combined pilot‑scale experiments and modelling revealed that a tree‑shaped, multi‑module configuration operated in counter‑current flow promotes more balanced hydraulic distribution and maintains the osmotic driving force more effectively than a simple series layout.

Having established the potential of FO-RO hybrid systems, Rajashree Yalamanchili’s thesis then turned to broader questions: Is the water produced suitable for safe reuse? And, what is the resource‑recovery potential when the FO‑pre‑concentrated wastewater is integrated with downstream biological processes such as anaerobic digestion (AD) and microalgae? Employing literature-based modelling, the hybrid system showed potential to reach RO recovery as high as 80-82% under varying operating conditions. Moreover, the RO permeate satisfied relevant agricultural guidelines, although it would require more advanced disinfection steps to meet drinking-water standards and reduce ammonium and pathogen log‑removal values. With FO pre‑concentration, the impacts of elevated salinity and ammonium on downstream biological processes (AD and microalgae systems) need further experimental validation, as both parameters may influence process stability and productivity. Even when the system is powered using energy generated from AD, standalone water reuse remains a low‑energy option, although methane losses in the absence of pre‑concentration could limit the net energy gains.

Finally, the researcher focused on the removal of contaminants of emerging concern. In a FO-RO-AD hybrid system, FO achieves high initial rejection, but persistent and recalcitrant compounds accumulate in the anaerobic digestion effluent due to limited biodegradation. On the seawater side, osmotic dilution combined with RO’s high rejection ensures consistently high‑quality permeate, with residual contaminants confined to the RO brine. Operational variations mainly affect contaminants of emerging concern levels in the anaerobic digestion effluent, while permeate quality remains stable. Overall, compared to standalone seawater RO and AD–RO trains, the FO–RO–AD hybrid delivers the lowest concentrations of contaminants of emerging concern in permeate and brine, while concentrating the highest load in the AD effluent, which requires appropriate management.

Altogether, the thesis gathers new scientific-technical knowledge on integrating water reuse and desalination through FO-RO hybrid systems for safe water reuse. It also identifies relevant operational parameters and processes that should be studied in future research works. Directed by Dr Gaëtan Blandin and Dr Ignasi Rodriguez-Roda, the work aligns with ongoing research on membrane technologies for sustainable water treatment at LEQUIA research group of the University of Girona.   

Main publications:

(1) Rajashree Yalamanchili et al, Can a forward osmosis-reverse osmosis hybrid system achieve 90 % wastewater recovery and desalination energy below 1 kWh/m³? A design and simulation study, Desalination, 585, 2024, 117767, https://doi.org/10.1016/j.desal.2024.117767.

(2) Rajashree Yalamanchili et al, Single-pass forward osmosis for efficient feed concentration: Optimizing multiple modules arrangement and flow distribution, Desalination, 615, 2025, 119224, https://doi.org/10.1016/j.desal.2025.119224