How to Provide a Sustainable Water Supply in Africa Using Reverse Osmosis System with Specialized Pre/Post Treatment
Sea Water Desalination is a specific process involving molecular separation via membrane technology to reduce the dissolved salt and mineral content of sea water to a level suitable for human and animal consumption, commercial/industrial and irrigation uses. This process involves three water streams namely sea water as the intake source, Product / Permeate water of potable drinking water quality with low salinity and Reject/Concentrate water of high salinity. The process is relatively straight forward and works on the principle of Reverse Osmosis System.
The input to the Sea Water Desalination system is sea water which is drawn from the oceans or a coastal beach well source. This water then undergoes a process known as pretreatment. The pretreatment process involves filtering of both macro and micro particles through specific pre-filtration, followed by dosing of a specialized antiscalant formulation to prohibit the formation or buildup of mineral scales which lead to fouling conditions in the reverse osmosis process. Once the pretreatment is complete, the water is fed into the layers of the Reverse Osmosis System membranes to remove salt and other molecular impurities.
The Brine/Reject or concentrated salt water is typically collected in a separate storage tank and discharged to the ocean. The Product/Permeate or potable water then is treated in a post treatment process involving Calcite pH adjustment and disinfection by UV light and chlorine to maintain residual disinfection in the water distribution system. Additionally, certain minerals and salts can typically be added to the treated water to “adjust” the taste of the treated water to meet local customer taste preferences and comply with local norms. However, for process water requirements in certain industries, the system will have to be designed according to the requirements of the specific industries and hence, the decision on how and where to use the desalinated water will be based on a consultation with the client to understand their internal operations. This treated water can then be stored and distributed as required.
Sea Water desalination is a leading technology of choice for water treatment based on two principal reasons: Production of high quality treated water and an ongoing trend of increased production with decreasing CAPEX and OPEX costs. According to research conducted by Global Water Intelligence, the typical CAPEX break down for Sea Water desalination project is shown below:
The trend of decreasing costs associated with the typical Sea Water Desalination system is primarily due to an increase in technological advancement: membrane performance under difficult conditions and higher recovery of energy linked to the purification process.
The materials of construction of most Reverse Osmosis Membranes are cellulose acetate and polyamide. Today, sea water desalination systems are designed, engineered and custom built based on a specific water analysis provided by clients to meet their specific water needs. Custom built commercial/industrial sea water desalination systems can incorporate cost effective and low maintenance energy saving design elements. These include incorporating membranes with specialized nanotechnology; the nano composite is incorporated into the membrane material providing for higher element flux rates and salt rejection percentages. These membranes typically produce 20% more water while typically using 20% less power consumption versus a comparative sea water RO membrane for given water flow rate. This allows for a reduction of the system footprint. Moreover, utilizing this approach can provide a lower capital outlay, operating and maintenance costs and is capable of performing effectively in multiple applications such as Oil & Gas Facilities and Oil Platforms, Power Plants, Commercial/ Residential Coastal Developments, Hotels / Resorts, and in Cities/ Coastal Villages with varying salt water feed TDS levels from 10,000 ppm up to 42,000 – 45,000ppm.
Nano fiber prefiltration in Reverse Osmosis System
Advanced nano fiber prefiltration technology provides significant reduction in biological and colloidal fouling, a major issue in membrane systems. Membrane fouling is one of the primary causes of reverse osmosis system failure and associated increased operating and maintenance costs. Operational cost savings can be observed through the reduction of membrane cleaning frequency and extended membrane useful life with higher membrane performance duration. Moreover, this technology ensures higher system flow rates with reduced power consumption due to the significant reduction in colloidal fouling thereby, providing higher water production while reducing operational costs.
In combination with specialized media, the prefiltration process allows for higher particulate prefiltration efficiency in the sea water desalination system and reduced backwash water demands.
An efficient environmentally friendly intake and discharge solution reduces both the overall costs associated with the sea water desalination process while protecting and preserving the coastal ecosystems throughout the world.
Strategies that handle the RO concentrate
There are two strategies that can efficiently and effectively handle the RO concentrate/reject water of high salinity. The first strategy is an optimized discharge system solution with low capital and O&M costs that is specifically designed to discharge the high salinity concentrate water without any harm to coastal marine life through highly efficient offshore ambient water mixing.
The second strategy involves a specialized zero liquid discharge solution with a higher initial capital cost outlay, but relatively low O&M costs of around 1-2%. Utilizing a zero liquid discharge approach that can be powered by hybrid renewable energy can increase the overall potable water production from the typical 35-45% system recovery to around 85% from a sea water source. The remaining 15% of this concentrate stream would be used for dry salt production that can be sold for commercial purposes generating a small offsetting revenue stream.
Additional trends involve utilizing hybrid renewable/conventional power combination to power larger community and municipal scale system installation to provide lower operating cost and enhance environmental sustainability.
Challenge
Facing challenges of clean water shortages in North Africa, something had to be done to increase clean water supply to supply the growing needs for drinking water in coastal communities.
These communities were feeling the effects of water scarcity, but the coastal location provided ample access to sea water. Environmental protection measures for marine life needed to be considered in this desalination approach.
Solution
Knowing the water demands were increasing, Genesis Water Technologies was engaged by a local partner to design, engineer and build an optimized sea water desalination solution. This system solution needed to be environmental sensitive to the local marine environment.
Genesis Water Technologies requested a water analysis to analyze the composition of the sea water source that would need to be treated.
Upon reviewing the analysis, the TDS levels ranged from 40,000-43,000 mg/l.
Further discussion involved whether the system solution could be capable of zero liquid discharge.
This approach was possible and feasible, however, in this particular solution; the utilization of an optimized brine discharge system was designed due to initial capital cost budget limitations.
Genesis Water Technologies engineered and designed a custom built GWT series modular desalination system. This system includes an advanced open sea water intake system and outfall designed to drastically minimize any potentially effects to the marine environment from its intake and brine discharge. The water from the intake system is pumped to an inlet water storage tank. From the water storage tank, the water is processed through a multi-stage filtration process and then into a sea water reverse osmosis system. The sea water desalination system was designed using a patented process of GWT DLP series nanofiber prefiltration and nano composite membrane technology to remove colloidal organics, biofouling, and dissolved salts. Following this process, the water is remineralized to improve taste and is sent to treated water storage tanks. The water is disinfected post tank for water distribution.
Results
The system solution is expected to be implemented in phases in over the 1-2 years, with initial water quality exceeding all US EPA and WHO standards for potable drinking water. This system solution will provide a safe source of potable water for many years to come to meet the water needs of the area with the ability to add capacity as needed into the future.
