Decentralized solar powered membrane filtration: Opportunities in desert environments

Water availability in desert environments remain a grave concern – and has always been a matter of survival. With climate change this need will be further exacerbated as temperatures rise becomes more severe and water resources more scarce. Renewable energy powered membrane filtration – or desalination – cannot increase the quantity of water. The advanced treatment can however make water that is unfit for consumption potable as is illustrated in Figure 1 at the example of a site in Tanzania, East Africa.

Decentralized – or autonomous – treatment systems are typically small, do not rely on infrastructure and need to be robust to withstand operation in the harsh environment that characterizes deserts. Providing the energy for operation, specifically the operation of the high pressure pump required for nanofiltration or reverse osmosis, renewable energy is an obvious choice. In deserts where solar irradiance is typically abundant, photovoltaics or solar energy are suitable so long as sand storms and dust as well as excessive heat can be managed.

The research on directly coupled renewable energy powered membrane filtration system of the Schäfer-Richards teams began in Australia two decades ago. Brackish groundwaters were treated in the Australian outback that contained a wealth of contaminants ranging from salts through to natural uranium. The direct coupling means that the renewable energy resource is neither converted nor stored, resulting in a fluctuation of energy with solar irradiance, while water is stored for periods of shutdown (bad weather, nights).

Treating brackish water is a significantly more economic option to seawater desalination, because the salinity and hence the pressure requirements are lower. Contaminants that occur in brackish groundwater, in addition to salinity, may include nitrates, arsenic, fluoride, uranium and many more. Organic matter contents is bound to increase with climate change, while micropollutants, such as pesticides, are increasing in occurrence globally. Nanofiltration and reverse osmosis can remove many of these contaminants, while the generation of a concentrate remains a challenge in remote areas. Evaporation ponds may be a viable option in desert environments. The formation of inorganic scaling through salt precipitation is a further operational challenge.

This lecture intents to provide an overview of renewable energy powered nanofiltration/reverse osmosis and inspire a discussion about the suitability of such technologies in the desert context. Issues may span from typical water contaminants to the operational issues such as extreme temperatures.