RO Can Make Sense
The result of the RO reclamation process usually is pristine water that is much better than most of the water supplies in the country
Application of high-pressure membranes in wastewater reuse is gaining momentum rapidly as utilities look for ways to use high-quality reclaimed wastewater to supplant both non-potable and potable water supplies.
High-pressure membranes such as reverse osmosis (RO) have long been used for advanced water treatment, including seawater desalination and wastewater reclamation, to produce high-quality, low-salinity water for various non-potable water uses.
RO uses a semi-permeable membrane that allows water to pass through at a much greater rate than dissolved material. High pressure—in the range of 250-1,200 psi—is applied to overcome the osmotic pressure and to force water through the membranes. RO membranes have been proven to provide effective removal of total dissolved solids; organic compounds and pharmaceutical chemicals of high molecular weights; color; hardness; pathogenic elements including viruses and Cryptosporidium; endocrine disrupters; nitrates; and synthetic organic compounds such as pesticides and fertilizer.
Only a small fraction of sale molecules and low-molecular weight compounds—if present in the raw water—can penetrate RO membranes. The result of the RO reclamation process usually is pristine water that is much better than most of the water supplies in the country.
Applications and examples
However, despite the outstanding water quality produced by RO, the general public is not yet ready to embrace the concept of using reclaimed water directly as potable water. In the meantime, one of the most common applications for high-pressure membrane technology is groundwater recharge to replenish the aquifer or to prevent salt-water intrusion.
This process is referred to as indirect potable reuse. Water Factory 21 in Orange County, Calif., and the West Basin Water Recycling Facility near Carson and El Segundo, Calif., have been producing high-quality recycled water treated by advanced technologies for seawater intrusion barrier injection, with the majority of the injected water entering the groundwater and eventually becoming part of the water supply—hence the term “indirect potable reuse.”
As advanced treatment technologies become cost-effective, and as public acceptance increases, recycled water may be used to augment surface water supplies.
HDR, Inc., recently prepared a pre-design for a 4.5 mgd microfiltration/reverse osmosis plant for the city of San Diego where the goal is to reduce the salinity of the reclaimed water to make it more acceptable to end users.
The San Diego water repurification program would be the first example of planned, indirect potable reuse where purified water is discharged directly into a surface reservoir without percolation or injection into groundwater.
Even though the public may not be ready to drink reclaimed water, RO reclaimed water can be supplied through separate distribution systems for a wide range of applications including agricultural and landscape irrigation for a project in St. Petersburg, Fla. This same process was used for a wildlife habitat enhancement in the Orlando Wetlands Park in Orlando, Fla.; a recreational impoundment and groundwater recharge in Gainesville, Fla.; and a surface water augmentation as well as other indirect potable uses such as commercial toilet flushing in San Ramon Valley, Calif.
High-pressure membranes such as RO are essentially the final water-purifying step in a wastewater reclamation process. Prior to RO membrane treatment, conventional primary, secondary, and even tertiary treatments are usually employed to reduce organic and particulate loading to the RO process. The selection of pretreatment to RO should be carefully evaluated with an eye on optimum system operation.
In a recent Water Reuse Demonstration project in King County, Wash., HDR evaluated a combination of several emerging technologies, including fuzzy filters; ballasted sedimentation; biological aerated filters; and membrane bioreactors, that could serve as a pretreatment to RO. The study showed that managing membrane fouling is a key operational requirement for a successful RO implementation. Material that tends to cause membrane fouling should be reduced to the lowest possible levels.
Fouling materials in secondary effluent include microorganisms, fine particles, oils, grease and dissolved organics. Suspended solids or solids formed by precipitation of reduced iron or manganese species and high molecular weight cationic polymers used for coagulation, can also contribute to fouling and should be reduced as much as possible in a pre-filtration step by either conventional sand filters or low-pressure membranes such as ultrafiltration or microfiltration.
The RO product stream is usually disinfected using a combination of ultraviolet radiation and chlorine or chloramination prior to discharge to the receiving environment. Concentrate from RO membrane processes is regulated as an industrial waste and often requires treatment before disposal.
Costs for both construction and operation of RO membrane processes depend on many site-specific factors. A recent estimate of capital costs for a 5 mgd wastewater reuse treatment facility utilizing RO membrane technology at King County, Wash., was in the range of $2.80-$3.14 per gpd installed.
Membrane equipment typically accounts for 25% of the total construction cost, while storage and pumping facilities contribute about 25-30%. The average ratio of assigned personnel per 1 mgd of capacity was 3.2, and the average operation and maintenance cost reported was $1.28 per 1,000 gal of permeate produced. Power (30%) and membrane replacement costs (20%) constituted major parts of the total operation and maintenance cost.
RO produces the highest quality reclaimed water, which means that RO repurified water has the widest range of possible uses. But a decision to use RO means adding another step to the treatment process and bearing the additional costs. Before making that decision, a utility should carefully consider its short- and long-term water needs. If RO will be part of the treatment process some time in the future, the pretreatment processes should use low-pressure membranes or membrane bioreactors to facilitate easy incorporation of the RO process.
Small and large communities all over the U.S. face intermittent water shortages now, and they know that finding enough potable water for a growing population will be a difficult challenge for many years. Yet, pristine water is being used to flush toilets, water lawns and wash factory floors. One choice may be to learn to use reclaimed water to do these jobs.
Cities are becoming more aware of the need to increase their use of reclaimed water. For example, the San Diego City Council recently approved a $900,000 one-year study on the feasibility of increasing its use of reclaimed water. Water demand by San Diego residents is expected to increase 25% by 2030, according to Deputy Water Director Marsi Steiner.
As regulations to treat wastewater become stricter, and as the technology cost becomes more competitive with conventional technology, citizens, planners and decision makers are becoming more aware of what RO technology can do. With time, perceptions toward direct and indirect reuse will change and the application of high-pressure membranes in reuse will likely gather momentum.