PURO: A unique RO-design for brackish groundwater treatment

Depletion of fresh groundwater sources as the result of overdraft, salinization and pollution becomes a major problem in parts of the world. Desalination of brackish groundwater by membrane technology, e.g. reverse osmosis (RO), seems to be a promising solution to water scarcity problems. However, energy consumption and concentrate disposal are considered as the main reasons for avoiding RO application. In order to overcome these drawbacks, the PURO concept, which consists of vertically-configured RO unit in an especially drilled well is designed, installed and is going to be tested. The installation operates without any chemical pretreatment and therefore, the concentrate can be injected into a deeper confined aquifer that contains water of similar concentration. To avoid chemical pretreatment, the system operates at lower recovery (50%) than conventional brackish groundwater reverse osmosis (BWRO). Higher energy consumption, as the results of lowering the recovery, is avoided by using natural hydrostatic pressure at the depth that RO is installed and by extracting the permeate water only. PURO consumes about 39% less energy when compared to a conventional BWRO installation of the same capacity. This article describes the PURO concept and discusses its advantages and disadvantages. It also provides a rough calculation of water cost for PURO and conventional BWRO with emphasizing on the energy costs. © 2015 Published by Elsevier B.V.

Brackish groundwater is becoming an important source of water for inland RO-installation due to the climate change as well as overdraft and contamination of fresh groundwater resources. When applying RO for treatment of brackish groundwater, the disposal of contaminated concentrate stream can be prevented by avoiding of chemicals pretreatment of feed water and direct application of feed water to the RO system. However, to prevent scaling of membrane modules, the operation recovery should be reduced. Therefore, more water should be extracted to have the same production capacity. As the consequence of more water extraction, more energy will be required. PURO is a solution to this higher energy consumption of RO systems operating at low recovery. PURO owes its lower energy consumption mainly to an efficient use of available hydrostatic pressure. Suitable hydrogeology conditions, dx.doi.org/10.1016/j.desal.2015.09.015 local regulations and local costs for primary study, drilling, material, etc. are important conditions for the application of projects such as PURO. Availability of an aquifer with large quantity of brackish water with a favorable ion concentration for proper feeding of the RO unit and a confined aquifer with high hydraulic conductivity and storativity are the main hydrogeology conditions required for the application of PURO. Since PURO operates inside the well, a good analysis of the location and feed water, a proper well construction and a good monitoring are essential for PURO. The extra equipment that is required for proper functioning of PURO results in extra investment cost of PURO. Furthermore, the maintenance of PURO is difficult and costly due to limited accessibility of PURO. Despite these higher investment and maintenance cost, the water cost by PURO is comparable with conventional BWRO. Although the production cost of PURO seems to be the same as conventional BWRO, it is a big step toward more energy and environmental friendly desalination of groundwater with RO. It is expected that in the future, the investment and maintenance costs for the PURO installation decrease due to increased experience in making such installations. Also, the application of PURO with higher capacity will reduce the maintenance cost of PURO. Acknowledgment The authors would like to gratefully acknowledge the support and generosity of Netherlands Enterprise Agency (RVO), Province of ZuidHolland, Oasen Water Company, Waternet Water Company, Logisticon Water Treatment, Drilling Company Haitjema B.V.and Delft University of Technology without which the current study could not have been completed.