What is CCRO? / Understanding CCRO
Closed Circuit Reverse Osmosis (CCRO) shares many similarities with traditional RO systems, yet it distinguishes itself by incorporating dual operational modes. In CCRO, the reject stream is recycled back into the incoming feed stream while consistently producing permeate. This is continued until the desired recovery percentage is reached (or another desired set point) at which point it transitions to the plug flow mode of operation. This phase is where the brine is purged from the system. During plug flow, the water flows through membranes and the more pressure applied, the greater the plug flow.
When a water volume equivalent to that of the systems volume has passed, the brine valve closes and the system returns back into the closed circuit mode of operation. The shift from closed circuit back to plug flow happens when one of the control setpoints is achieved. These include conditions such as reaching a certain pressure at the inlet to the membranes or after a total volumetric recovery has been achieved. As soon as either of these triggers is met, the brine valve reopens, initiating the plug flow mode within the system.
Benefits to using CCRO for water desalination
Fouling Resistance – The independently controlled crossflow supplied by a circulation pump efficiently washes the membranes which results in lower concentration polarization and reduces the effects of scaling and fouling. As the salinity throughout the CCRO process cycles from the feed water salinity to that of the most concentrated brine, biofilm formation and scale precipitation can be disrupted and even reversed.
The cycle time of the CCRO process is much shorter than the induction time for precipitation of most sparingly soluble salts. This contrasts sharply from the steady-state conditions in traditional RO systems, which maintain nearly constant concentrations throughout their membrane arrays for months or even years. Also because recovery can be easily manipulated, CCRO processes can be tuned to maximise recovery if the concentration of scaling salts or other feed water properties change.
Adaptable – Recovery is achieved in time, crossflow is adjusted using a circulation pump and permeate flux is controlled by the speed of a high pressure pump. Under this unique configuration, CCRO systems can operate with a lower lead element flux and higher crossflow velocity, which provide better control of fouling and scaling than traditional multi-stage RO systems.
Recovery – CCRO is able to achieve higher recoveries, sometimes up to as much as 98%.
Reduced Waste – CCRO reduces water waste, 50 to 80% less water therefore is more sustainable
Reduced Costs – CCRO reduces water costs as water can be recycled. Reduces wastewater costs as there is less water going down the drain that needs to be paid for. Good for limited water supplies.
Cleaning – As CCRO systems are effectively a single stage array, they require a much shorter cleaning time and are a lot easier to clean all at once. This in turn saves labour costs for chemical mixing, chemical consumption for cleaning as well as a shorter downtime for your plant.
Biofouling – Two features of the CCRO system create an inhospitable environment for microbes. The pressure in the filtration loop is constantly cycling from low to high. Salinity is constantly varying as the wastewater gradually becomes more concentrated and then is flushed. These two features deprive microorganisms of the environmental stability they’d need to survive, resulting in less biofouling.
Membrane – Due to the fact that recovery is achieved in time as opposed to space, CCRO plants can use shorter membrane arrays compared to traditional RO systems. This provides a better flux distribution and also reduce the lead element flux.
Energy Consumption – CCRO saves 5 to 40% specific energy compared to traditional RO technologies. This is because a variable pressure pump only ever provides as much pressure as is needed for optimal recovery in a given cycle. Furthermore, because the single stage array uses fewer, shorter membranes vessels, the starting pressure is much lower.
Flexibility – CCRO systems are much more flexible than other forms of water technologies. They are able to save up to three different setpoints which trigger when the concentrated brine is purged from the system. These setpoints can be values for volumetric recovery, the pressure of the high-pressure pump or the internal conductivity of the water. By using multiple setpoints and parameters, CCRO systems can adapt to variations in feedwater quality or permeate requirements at the touch of a button.