While most water treatment is carried out using biological processes to separate organic contaminants, certain applications are better suited to reverse osmosis (RO), most notably desalination of seawater where RO is replacing more energy intensive distillation processes. Biological treatments excel at breaking down organic matter including sewage, food waste and certain industrial wastes, allowing the reuse of nutrients such as carbon, nitrogen and phosphorus. Biological processes require little energy and have a low energy and environmental impact. However, these methods can struggle to remove other contaminants such as salts, minerals and heavy metals, as well as some bacteria and viruses. RO allows water to be treated to a high level of purity, or the extraction of drinking or irrigation water from brackish or seawater sources. RO can also provide a more compact solution where space is limited. Biological treatments typically require large settling tanks, aeration systems and significant time for microbial processes to occur. RO systems can be located in compact industrial installations but have higher operating costs, largely due to the energy required to run high-pressure pumps.
Conventional RO systems use a high pressure pump to force water through a semipermeable membrane, essentially filtering water at a molecular level, allowing dissolved as well as suspended contaminants to be removed. The greater the concentration on the input side, the higher the pressure needs to be to reverse the natural direction of the osmosis. While filtering excludes particles based on size, the selectivity in an RO process is driven by differences in solubility or diffusivity. This means the process is highly affected by pressure, solute concentration and other environmental factors. One of the major disadvantages of such systems is a high level of water wastage. This occurs due to the need to prevent the membrane from becoming blocked by a layer of contaminant building up on the membrane. To prevent this accumulation, a reject stream containing an elevated concentration of contaminants is continuously discharged as wastewater. The ratio of permeate water produced to the feed water entering the system is known as the recovery rate and is typically between 40% and 60%. Higher recovery rates mean less wasted water but can increase cost and reduce water quality due to greater stress on the membrane and reduced effectiveness in removing contaminants.
This is where closed circuit RO systems come in. “A traditional reverse osmosis system treats the water with a single pass whereas with a CCRO system we take that water that is treated at the end of the reverse osmosis membranes and we return it to the head of the plant multiple times thus we’re treating that water on several cycles, we’re improving the efficiency of the system and reducing the amount of salt. While traditional reverse osmosis results in 75% efficiency, this new technology has the ability to be 95% efficient, increasing water quality and quantity,” says Joe Mauawad, general manager at southern California’s EMWD (Eastern Municipal Water District).
DESALINATION AND REVERSE OSMOSIS
There are almost 20,000 desalination plants operating globally, supplying only 0.7% of water needs but consuming 25% of the energy used by the water sector. Desalination is a very energy-intensive way to acquire fresh water, but this is improving. The average energy intensity has reduced from 20-30kWh/m3 in 1970 to about 3kWh/m3 today. This does vary significantly depending on the concentration of inorganic salts and organics, collectively measured as total dissolved solids (TDS). Drinking water has TDS<500mg/L, while below 1,500mg/L water is considered fresh and may be suitable for irrigation. Between 1,500 and 5,000mg/L water is classified as brackish, above this it is said to be saline, but seawater is has TDS>25,000mg/L. There is a very big difference in the amount of dissolved solids to be removed from brackish water compared to seawater.
RO becomes more difficult the higher the concentration of contaminants, while distillation is not really affected by the contamination level. For the most highly contaminated water, including seawater, distillation can provide the lowest energy consumption, especially multiple-effect distillation. As the concentration reduces, RO becomes more energy efficient, meaning it is a better choice for brackish water where it can achieve an energy intensity of less than 1 kWh/m3. A hybrid approach, with distillation and RO used together can often be most efficient. While RO requires significant energy for high-pressure pumps, this is still considerably lower than the energy needed to generate the heat and reduced pressure required for distillation processes.
Closed circuit reverse osmosis (CCRO) increases the recovery rate beyond 85% by recycling some of the concentrate back into the RO system, in a closed-circuit system. While there has been relatively little innovation in RO processes for some decades, CCRO may offer a significant advance. Conventional RO is a steady state process, with the membrane constantly bombarded with foulants such as scalants. CCRO operates in batches, with an internal recirculation loop running most of the time and periodic flushing cycles approximately every half an hour.
NEW POSSIBILITIES
“At first sight, it really looks like a traditional reverse osmosis but when you understand how it works, it really reveals unlimited possibilities,” according to Korneel Caron, CCRO business development manager at DuPont Water Solutions. “Whereas before, you have to say to your clients, ‘you can try to do this, but there might be a risk of frequent membrane replacement’ or frequent cleanings or we might try to have this high recovery.”
He credits two particular technological advances. First is membrane technology: “to make membranes less susceptible to fouling and scaling. To make them more energy efficient. To have a whole range of different membranes. To make them more water efficient, but still some limitations.” And the second factor is process advances.
With the development of reverse osmosis systems, in particular improved membranes, the capital and operational costs of desalination have been steadily reducing over the last few decades. Closed circuit reverse osmosis builds on this, allowing higher recovery rates, lower energy and improved membrane life. This is achieved through more sophisticated control of process parameters, such as pressure. While the equipment is initially more expensive than conventional RO plant, these costs should be recovered through reduced operating costs. Increased complexity does however bring additional maintenance challenges which may counteract some of these benefits.