Approximately 59% of additional capacity will involve tertiary or further treatment, and the remainder will involve secondary treatment only, predominantly in emerging markets.

Table 1 shows the projected increase in water reuse by market.   

Population growth is fueling strong political support for water reuse, especially in the United States, Europe and Australia.  Growing environmental concerns about discharges are a factor when considering water reuse versus seawater desalination as a fresh water source.  Key markets like China, the Middle East and North Africa have limited wastewater infrastructure for water collection and treatment.  Building new wastewater infrastructure is expensive, and this fact dramatically increases the availability of wastewater for reuse.

Economic Benefits of Water Reuse

When compared to desalinating seawater, water reuse delivers some cost savings.  For example, the lower salinity of wastewater reduces energy consumption for reuse compared to desalination.  The energy cost varies depending on the treatment technologies used for treating wastewater for reuse.  Wastewater typically contains a lower level of total dissolved solids (TDS) than seawater or brackish waters; thus, a reverse osmosis (RO) water treatment system can operate at a lower pressure and a higher water recovery, saving money on energy costs.  Biological treatment, such as a membrane bioreactor (MBR) treatment system, provides stable feed water quality to the downstream RO system, compared to variable quality surface or coastal water that can be influenced by storm events, red tide or algae blooms. 

Table 2 illustrates the energy cost by process or treatment technology. 

Typical Water Reuse Applications

Secondary wastewater effluent is ideal for a number of applications including: surface irrigation of orchards and vineyards; non-food crop irrigation; wetlands, wildlife habitat and stream augmentation; industrial cooling process; groundwater recharge of non-potable aquifer; and restricted landscape impoundment.

Tertiary/Quaternary treated wastewater effluent is used for food crop irrigation, landscape and golf course watering, vehicle cleaning, unrestricted recreational impoundment, toilet flushing, industrial process water, and indirect potable reuse (groundwater recharge of potable aquifer and surface water augmentation).

Reuse water is often designed for an intended purpose, and not all of it is produced to the highest standards of purity. Therefore, some non-potable users may require a certain amount of additional treatment.  While reuse water for dust suppression in a coal yard or as an ingredient in cement will need very little treatment beyond basic secondary treatment, wastewater that is reused in a boiler feed application will require advanced treatment that is higher than the potable water standard. Because reuse water for crop irrigation and lawn watering can also be used as a fertilizer, this application may require that the rich nutrients be left in the reuse water.

Key Technologies for Water Reuse

There are a number of technologies for treating wastewater for reuse.  The technologies chosen depend on the intended use, as illustrated in Figure 1.



MBR technology is currently used worldwide to treat water for reuse, and these plants are growing in size.  The North Head Plant in Sydney, Australia, installed in 2005, provides 0.5 MGD (2 MLD) of utility water.  Installed in 2006, the Martin Way plant in Olympia, Washington, provides 2 MGD (8 MLD) of groundwater recharge water, while the Beixiaohe plant in China, installed in 2007 at the Olympic Park Village, provides 17 MGD (64 MLD) of irrigation and utility water; it is currently the largest MBR reuse plant in the world.

Located on a small tract of land in a scenic and ecologically sensitive area, the North Head Sewage Treatment plant needed to reduce consumption of limited potable water.  The solution was a water reclamation plant featuring a MemJet MBR to treat 0.5 MGD (1.9 MLD) of municipal wastewater.  The compact plant fit on the existing property with minimal impact on the surrounding area.  Ninety nine percent of the potable water previously consumed was now conserved for the growing population, and the compact design saved civil and installation costs while preserving the area’s scenic beauty and ecology.

Three months before the start of the Beijing Olympic games, the Beixiaohe wastewater treatment plant needed to treat wastewater for reuse.  An MBR system is meeting that need, processing 90 percent of the city’s wastewater, with 50 percent of that being used for recycle and reuse.  The technology fit into the plant’s existing space, doubled the plant’s capacity, and was installed and started up in time to provide recycled water for fountains and lakes in the Olympic park.

MBR systems are favored when the plant size is less than 20 MGD (76 MLD), when new construction is required or the plant capacity needs expanding, when nutrient removal is necessary, or when there is a new requirement for odor control at the plant.

Ultrafiltration (UF) is another popular technology for producing water for reuse.  UF membranes offer a verifiable physical barrier, the ability to adapt to changing conditions, and fully automatic operation.  They often can provide the lowest cost per treated volume of wastewater.  Two types of UF membranes: submerged and pressurized, offer distinct advantages. 

Submerged systems use membrane modules immersed in the feed water.  Filtrate is drawn through the membranes, purifying the water.  An open tank configuration allows visual inspection and simplifies membrane installation and removal.  Submerged systems are suited for retrofit of conventional filtration basins, as capacity can be more than doubled in the same space.  Finished water quality is improved without the need for chemical pretreatment.

In pressurized systems, feed water is pumped through membrane modules contained in pressure vessels. The modular “building-block” configuration simplifies the design, installation and operation.  The compact design reduces the overall system footprint, allowing up to 4.5 MGD (17 MLD) of treated water capacity on a single skid. Advantages of UF membranes for reuse include high quality water, multiple reuse options, low maintenance, and long-term proven performance.

Submerged membrane systems are being used worldwide for municipal and industrial reuse applications – for example, at a golf resort on the Monterey Peninsula in California and at the West Basin Water Recycling facility in El Segundo, California.  At West Basin, lime softening and sand filtration of secondary effluent had proven to be too costly and ineffective in preventing RO membrane fouling. From 1997 through 2002, pressurized membrane filtration systems were phased in to supply high-quality water for local refineries as the conventional treatment was phased out. In 2004, submerged filtration systems were added for a total capacity of over 30 MGD (113 MLD).

Membrane filtration is favored over MBR for larger plants -- greater than 20 MGD (76 MLD), as pretreatment to RO, and for retrofitting existing wastewater treatment systems, as opposed to new construction or expansion.

Conclusion

Population growth in water-stressed regions of the world is driving the popularity of water reuse, which has significant economic advantages over alternative water sources.  Membrane bioreactor and membrane filtration are key technologies for advanced water reuse, and as these technologies continue to evolve and improve both in performance and economy, they will no doubt increase in popularity.

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