Access to safe drinking water is critical for the health of individuals and households. However, one of the biggest global challenges of the twenty-first century is the lack of access to clean, pure drinking water for the vast majority of the population. A recent study suggests that 2.1 billion people still lack access to safely managed drinking water services (WHO/UNICEF 2017) with the largest proportion coming from third world countries. The sustainable development goals of 2015 have not failed to capture this growing need with goal number six focusing on availability and sustainable management of water and sanitation for all.
In Kenya, arid and semi-arid lands (ASAL’s) occupy over 80% of the country’s landmass which is home to about 36% of the population. It is estimated that 41% of our population still relies on unimproved/untreated water sources from rivers, dams, ponds, lakes, shallow wells and boreholes. This high percentage can be attributed to depletion of surface waters, poor infrastructure for government water supply systems coupled with a continuously and rapidly growing population and water demand. Rural-urban migration has also caused a rapid explosion in urban population and without a proportionate expansion of the government water supply systems, urban residents are gripped with the reality of water shortages. The continued depletion of surface water has forced many real estate developers to search for alternative solutions from sub-surface sources such as shallow wells and boreholes, which in most cases are highly mineralized, with dissolved substances exceeding the maximum recommended limits. This exposes consumers to potential health risks.
The purest form of water exists as rainfall before it recharges surface and subsurface water sources. As the rainfall hits the ground, it then becomes susceptible to; biological contamination by bacteria, viruses, protozoa and other micro-organisms pre-existing in the water bodies, and chemical contamination by minerals embedded in the soil and rock stratum as the water percolates through the soil structure. This results in subsurface aquifers having lightly to heavily mineralized water, depending on the chemical characteristics of the surrounding rock structure.
WHO has set international standards for the maximum recommended limits for each parameter in drinking water and locally in Kenya, KEBS has also set local standards for drinking water that is acceptable for consumption without any negative health effects. This awareness has led to increased research in the field of water treatment to develop more efficient, cost-effective and sustainable treatment techniques.
Broadly, water treatment can be described as any process that aims to improve the quality of water to make it suitable for particular end use. These processes have evolved over time, with new developments in research leading to more advanced technologies ranging from disinfection, filtration, ultra-filtration, ultraviolet disinfection, ozonation, reverse osmosis and desalination systems, just to name a few. In this context, we aim to delve more on reverse osmosis systems for water purification that can now be run fully or partially on solar power
Reverse osmosis is among the finest levels of filtration available. By definition, it is a water treatment technology that utilizes a semi-permeable membrane to remove dissolved ions, molecules and larger particles from water. The RO membrane generally acts as a physical barrier to all dissolved salts, organic and inorganic molecules with a molecular weight greater than 100 and achieves rejection rates between 95% – 99% depending on a number of factors such as the membrane type, feed composition, temperature and system design.
The heart of the RO is the membrane(s) itself. In the natural osmosis process, saline water applies an osmotic pressure through the membrane to the feed water line, which has to be overcome and exceeded in order to push water in the reverse direction. This is achieved by placing a ‘high-pressure pump’ on the raw water feed line which applies a force sufficient to overcome this osmotic pressure. Raw water through any RO plant is separated into two streams of water once pumped through the membrane; the clean water stream in which all contaminants have been removed, and the concentrated stream in which all dissolved substances are rejected. The percentage recovery of a reverse osmosis plant is dependent on the design and capacity of the system and a well-designed system should achieve a recovery of between 50% – 75% for fresh/brackish water and 30% – 50% for seawater.
Reverse Osmosis systems have numerous and varied applications ranging from desalination of seawater or brackish water for drinking purposes, food and beverage processing, wastewater recovery, industrial process water and purification of drinking water for domestic home use. They have been tested in their reliability to provide excellent water quality with consistent results; and with proper maintenance, the system will provide good service over long time periods. Water being a requisite resource requires its supply to be guaranteed with a reliable and renewable source of energy and one set back of the larger plants which require high-pressure pumps of larger capacities, is in the high operational costs caused by electricity consumed to power the system components. With an increasing global initiative to go green and embrace more energy efficient systems, institutions have and are investing heavily in research to increase the efficiency of water treatment plants and integrate renewable energy sources such as solar.
The utilization of solar energy to drive water treatment plants is a potential sustainable solution to the world’s water scarcity problem. In recent years, much effort has been devoted to creating and developing innovative technologies in the field of solarized water treatment technologies and the future seems bright.
In Kenya, solar energy is an abundant and widely untapped resource whose estimated daily insolation is 4-6KWh/m2. The use of solar energy in PV (photovoltaic) systems for lighting, water heating and solar water pumping is rapidly gaining popularity due to its availability, reliability, efficiency and quick payback periods. This paradigm shift has offered many advantages to numerous individuals and communities who are now able to significantly cut down on operational costs which translates to an overall reduced cost of water per unit. Solar RO plants offer a great economic advantage in that they reduce RO operating cost by up to 50% compared to grid or diesel generators. Solar can operate RO plants at 100% daily design capacity for 6 – 8 hours and if required can use AC power as back up at night in hybrid mode (Solar & AC)
In conclusion, several factors need to be considered when considering solar reverse osmosis systems as an ideal solution for providing drinking water. The site location, system capacity, availability of reliable grid power and more importantly, the capital cost vs operational cost which translates to the total system cost are critical factors that need to be considered, as they determine the overall cost of water per unit. The general and traditional mindset has been to focus on the capital cost of the system as the sole determinant to buying water treatment equipment, however, a more holistic approach that considers the total cost calculated over the system design life provides a more realistic measure and offers long term returns and benefits. Solar powered water treatment plants are gaining widespread popularity and with the proper design, will provide a real solution to a real problem faced in many parts of our country.