Water utilities aim for water conservation by means of onsite treatment and reuse.
Until fairly recently, the burden of water conservation was placed squarely on the shoulders of the consumer. Onsite reuse and onsite treatment were activities performed by the customers using the water, not the utilities supplying the water. However, technological advances and changing economics have altered this situation. Nowadays, the local water utility may be the region’s largest single recycler of water. This transition is similar to the one that occurred at the start of the industrial age when small cottage industries were replaced by larger, centralized factories. Economics of scale drove this change, and it is partially doing so again. Furthermore, increased urbanization (for the first time in history, there are more humans living in cities than in the countryside), and resultant high-population densities require the use of large centralized water sources. The other factor pushing the transition to reutilization of water by utilities is the looming global water shortage. Previous methods may not be sufficient to meet this demand.
Supply and Demand
What is water demand, and how is it calculated? Disregarding the water required for the proper functioning of the local ecosystem, water demand usually refers to out-of-stream and out-of-groundwater extractions needed for human activities (domestic use, irrigation, industrial activities, energy production, etc.). Though the methods of calculation may vary somewhat from place to place, water planners and utility managers usually consider current and project population, historic per capita use rates, wealth demographics of their customer base, economic activity and growth projections, and seasonal fluctuations. The net projected demand is applied to available water supplies (measured in terms of peak production in gallons per day), return flows from water-treatment and septic facilities, and known reserves of water supply when assessing demand.
So how much water do we use? The startling conclusion of a recent United States Geological Survey (USGS) report clearly shows that the water conservation, reuse, and recycling efforts, such as those described in this article, have been highly successful. From “Estimated Use of Water in the United States in 2005” (USGS, Circular 1344):
Estimates of water use in the United States indicate that about 410 billion gallons per day (Bgal/d) were withdrawn in 2005 for all categories summarized in this report. This total is slightly less than the estimate for 2000, and about 5% less than total withdrawals in the peak year of 1980. Freshwater withdrawals in 2005 were 349 Bgal/d, or 85% of the total freshwater and saline-water withdrawals. Fresh groundwater withdrawals of 79.6 Bgal/day in 2005 were about 5% less than in 2000, and fresh surface-water withdrawals of 270 Bgal/day were about the same as in 2000. Withdrawals for thermoelectric-power generation and irrigation, the two largest uses of water, have stabilized or decreased since 1980. Withdrawals for public-supply and domestic uses have increased steadily since estimates began.
In other words, despite an increase in population of nearly one-third since 1980, the US is using less water now than during the peak years of the late 1970s. The actual numbers breakdown is shown in Table 1.
Much of our improved efficiency comes from the increased use of centralized water distribution systems (aka, “city water”). Sixty-two percent of Americans got their water from public suppliers in 1950. By 2005, this figure had risen to 86%, with most of the rest of America obtaining water from groundwater sources. It is these types of centralized water supply systems that can benefit the most from the water reuse and recycling methods described in this report.
As impressive as these overall water conservation trends are, we still have a lot of potential for further improvement. With a population of 308 million, the total daily fresh water needs of the average American comes to about 1,330 gallons. The public use portion of this amount comes to about 143 gallons per capita per day (with California utilizing 135 gallons per capita daily). With a population of almost 22 million, Australia’s public use comes to only 34 gallons per capita per day (“The 2008 Water Report” Queensland Water Commission). Australia has similar lifestyle and standard of living, but only 25% of the water usage. America has made an excellent start, but still has a long way to go. The increased use of these technologies can help us get there.
Means and Methods, From Low-Tech to High-Tech
Membrane Bioreactor (MBR) technology combines microfiltration, allowing the separation of liquids and solids with a suspended growth bioreactor (activated sludge) treatment system. When treating domestic wastewater, it can achieve quality levels sufficient for discharge into coastal, brackish, or surficial bodies of water. Initially developed in the 1960s, two MBR configurations have been designed. The first is the internal configuration. In this setup, the membrane units are immersed in, and integral to, the bioreactor, often, with submerged filtration units. The second is the external (or side stream) configuration, where the membranes are separate units whose operations require an intermediate pumping step. A relatively small operating footprint and recent technical advances have made MBR technology cost-effective, compared to conventional activated sludge processes.
An integrated microfiltration combined with a reverse osmosis membrane (MF/RO) is another approach for recycling water and is specifically designed for the desalination of brackish water from substandard sources. Though brackish water and seawater have been processed by desalination plants, most of these processes have utilized comparatively cumbersome, inefficient, and cost-prohibitive technologies, such as cartridge filtration, sand and mixed media, clarification, and air flotation. Recent advances in the design of integrated microfiltration and reverse osmosis (RO) systems make desalination a viable alternative as a water source for large populations. In general, this approach involves the use of microfiltration during pretreatment followed by RO. The pretreatment removes most of the particulate matter suspended in the brackish water, increasing the efficiency, reducing the operational cleanup frequencies, and prolonging the operational lives of the RO units. Overall productivity is increased as both operational and capital costs are reduced.
The first step in the process, microfiltration, a process that removes particulates from wastewater by means of passing the water though a membrane with micro pores, ranging in size from 0.1 to 10.0 micros in diameter. The second step, RO, is similar to microfiltration, but utilizes a semi-permeable membrane and high water pressure that forces the water to cross the membrane barrier, trapping impurities on the up gradient side. Microfiltration utilizes straining of particles, while RO uses a diffusion mechanism to remove solutes. Superior performance can be achieved by integrating microfiltration, instead of utilizing traditional pretreatment methods. This, plus the use of spiral-wound RO membranes, results in a cost-effective desalination method. From high-tech approaches, we now go to low-tech, natural techniques. It has long been known that natural wetlands can be used to treat wastewater discharges. Only in the past decade or so have we been constructing manmade wetlands for the primary purpose of treating effluent. Constructed wetlands are designed to utilize natural processes (root absorption and uptake, filtration, treatment by microbes, etc.) to treat wastewater. However, they are engineered to maximize the effectiveness of these operations that occur in nature in a more haphazard fashion. Constructed wetlands often serve a double purpose, treating wastewater, while simultaneously creating or restoring wetland habitats for wildlife.
Constructed wetlands are classified according to the life form of the filtration system’s dominant large aquatic plant. Hydraulically, there are two main types of constructed wetlands: subsurface flow systems and free water surface systems. The first results in flows occurring below the water surface, primarily through a permeable medium (soil, roots, etc.). By keeping the treated water below the surface, odors are eliminated and the overall effect is more aesthetically pleasing. The systems that the subsurface flows pass through are submerged structures such as root zones, rock and red filters, and vegetated beds. Embedded in porous media (typically, sand, gravel, and crushed rock) these structures are designed to allow hydraulic flows through them. Free water surface systems are designed to more closely mimic natural wetlands with water flows occurring primarily above the ground surface at shallow flow depths. While less efficient at wastewater treatment than subsurface flow systems, free water surface systems maximize the potential habitat benefits of the site.
While not a technology itself, water credits are an important financial mechanism for the promotion of water recycling and reuse. Since the early George W. Bush administration, industrial polluters have had the option of purchasing water credits to offset their effluent discharges. Similar to carbon credits and modeled on earlier Sulfur oxide (SOx) and Nitrogen oxide (NOx) credits used to reduce acid rain, water credits utilize economic incentives to fight groundwater and surface water pollution. This policy uses market forces and financial incentives to achieve water-quality goals. Under the water credits system, industrial and agricultural operations discharging effluent to surface waters and publicly owned wastewater treatment plants can meet their requirements under the Clean Water Act, by purchasing offsetting credits from facilities that have exceeded their water-quality requirements. Credits must be purchased from facilities sharing the same watershed as the purchaser.
This article will highlight some of those districts and discuss what led to the decision to include onsite water treatment and reclamation as part of their overall plan, the challenges they faced in the planning and/or implementation process, and their plans and hopes for the future.
Tempe is located in the East Valley section of the Phoenix Metropolitan Area. It is home to 170,000 people. It is also home to the Kyrene Reclamation Plant, one of the largest operating membrane wastewater-treatment facilities in North America. The facility’s 2.55-acre site was too small to accommodate a conventional design for a reclamation plant utilizing standard biological nutrient removal (BNR), secondary clarifiers, granular media filters, and disinfection. So membrane filtration was utilized out of necessity.
Utilizing GE technology, the membranes used at the Kyrene facility are manufactured by ZENON from hollow porous fibers, each with billions of microscopic pores small enough to block the passage of most suspended particles. A cassette consisting of a bundle of these fibers is submerged into the processing tanks and a vacuum applied to create enough negative pressure to draw permeate (wastewater) into the fibers. The particles accumulate on the surface of the fibers, and are removed continuously by a stream of bubbles and periodically by water backflows and chemical cleaning.
These improvements have more than doubled the facility’s operating capacity from 4.5 million to 9.0 million gallons per day (11.7-million-gallons-per-day peak-flow capacity). As one of the largest MBRs in North America, the Kyrene facility produces recycled water that exceeds the state of Arizona’s highest standard for quality. The discharge from the facility is directly utilized in non-potable applications, such as cooling systems and golf course irrigation, and is indirectly utilized by injection into underlying aquifers. This earns water credits and can be used to access additional water supplies during times of drought.
For all these efforts, the City of Tempe received a GE Ecomagination Leadership Award. This award is given to the top 1% of GE Water & Process Technologies’ customers who demonstrate significant environmental and economical performance improvements. “We have to make use of every water resource that we possibly can,” says Don Hawkes, Tempe’s water utilities manager. “The GE ZeeWeed membranes provide the flexibility to take the wastewater that is generated by all of the people who live here and turn it into a commodity that is marketable and usable.”
The rainy Pacific Northwest is not the first location that comes to mind when discussing the need for water reclamation. But here, the City of Olympia, led by the LOTT Alliance (a nonprofit organization that manages wastewater treatment for the cities of Lacey, Olympia, and Tumwater), has taken an innovative low-tech approach with the use of constructed wetlands and groundwater recharge basins for wastewater treatment and reclamation. For over a decade, the alliance has been operating a decentralized reclamation and reuse program. Their long-range plan is to produce the highest quality of reclaimed water, as defined by the State of Washington’s department of Health and Ecology. Their decentralized approach has been driven by state-imposed discharge limitations at their central wastewater treatment plant at Puget Sound.
LOTT currently produces 1.0 million gallons of water each day at its Budd Inlet Treatment Plant in Olympia. An additional 0.75 million gallons per day is produced at LOTT’s Hawke Prairie Reclamation satellite facility. LOTT plans to expand both sites in the near future and add a second satellite facility.
Operations at the Budd Inlet Treatment Plant feature a continuously back-flushing sand filtration system. The reclaimed water is of such high quality, it can be utilized in Olympia itself instead of being discharge to local marine waters. At the Hawke Prairie Reclamation facility, reused water circulates through a series of five constructed wetland ponds. The water from the ponds is filtered by over 225,000 wetlands plants and flows to a rectangular groundwater recharge basin. The accumulated water permeates through the bottom of the basin and into the underlying aquifer. The scenic beauty of the wetlands system, with its park-like setting, walking trails, and bird habitats, is a welcome side-benefit. MWA architects were enlisted by the cities of Lacey, Olympia, Thurston, and Tumwater, to create a master plan and design the architectural signature for future water reclamation buildings.
A neighbor of Tempe, Peoria is a major suburb of Phoenix. With a population of over 108,000, it experienced a growth boom through the 1990s that carries on to the present day. This growth drove the need for the largest capital project in the city’s history, the Butler Drive Water Reclamation Facility. When it was completed in 2008, it was the largest MBR facility in the US. The facility has a discharge capacity of 10.0 million gallons per day (13.0-million-gallons-per-day peak discharge).
This year, the Butler Drive Facility was honored by naming it a Public Works Project of the Year by the American Public Works Association (APWA). Its operation generates significant water credits that will allow the City of Peoria to extract an equivalent amount of water from the aquifer.
The Butler Drive facility, designed by Black & Veatch, utilizes standard upfront pretreatment of the effluent (bar and fine screen, grit removal, biological nutrient removal) prior to the MBR stage. Following the MBR, the effluent is further treated with ultraviolet disinfection. The treated wastewater can now be reused either by pumping to the New River–Agua Fria River underground storage project for groundwater recharge, or by directly using it in non-drinking-water applications such as landscape irrigation.
Manila, the capital of the Philippines, is but one of 17 cities and municipalities in the Manila Metro area. Rapidly growing, Manila now ranks as the 11th-largest metropolitan area and fifth-largest urban area. With more than 1.6 million inhabitants, the city of Manila’s population growth has put a severe strain on the city’s infrastructure, including its water-supply system. Water supply is managed by Maynilad Water Services Inc., the West Concessionaire serving the Manila urban area (Manila Water Company Inc. is the East Concessionaire), a publicly owned and operated water-supply utility company. Maynilad serves the western half of the Manila metropolitan area with its 8.9 million people.
A partial solution to Manila’s growing water-supply problem has been put forward by the Pall Corporation of East Hills, NY: 100-million-liters-per-day capacity Pall Aria integrated a MF/RO membrane Facility, which can provide drinking water for 800,000 of Manila’s inhabitants. A global leader in water filtration, Pall Corporation’s $14.7 million system utilizes integrated MF/RO membrane water-treatment technology. It is the first large-scale membrane filtration plant in the Philippines. Water production is scheduled to begin in 2010.
Utilizing a low-maintenance, energy-efficient system, Pall’s Manila plant will process brackish water from Laguna de Bay Lake, which is subject to severe seawater ingress and algal blooms, as well as exceptionally high turbidity, due to tropical storm patterns. An existing concrete intake system will be reengineered to divert water from the lake to the Pall system. “The Maynilad water project will be the first world-class membrane plant in the Philippines, with the potential to expand capacity in the future,” says Francis McKeever, vice president, Pall Water Processing Asia/Pacific. “This installation will combine Pall’s membrane systems and expertise with low-cost engineering and execution from an indigenous partner, to affordably deliver membrane technology to this burgeoning area.”
Future prospect for applying this technology remain bright, “The Maynilad plant will establish a model for the large-scale application of membrane filtration technology in Southeast Asia. Increased migration to cities places enormous stress on municipal water systems throughout the region, and creates an opportunity for Pall to provide systems that can be upgraded as the population increases,” says Jeff Seibert, president, Pall Water Processing.
“Water, water everywhere, but not a drop to drink.” This is a quote from The Rime of the Ancient Mariner, by the poet Samuel Taylor Coleridge, and it is the complaint of every sailor lost at sea surrounded by water that is vast, abundant, and undrinkable. Mankind is entering an era where every drop of fresh water will be precious. Soon, mankind will be like a sailor lost at sea, surrounded by a globe with 70% of its surface covered with water, and some way has to be found to economically utilize this vast resource.
Ocean water has an average salinity of 35 parts per thousand, consisting of chloride, sodium, sulfate, magnesium, calcium, and potassium. Advanced RO systems can reduce salt content to less than 500 parts per million. Seawater must also be pretreated to remove organics, algae, and fine particles than can clog a RO membrane. MF membranes differ from RO membranes, in that they are made of more chemically resistant materials that allow them to be cleaned more aggressively. As a result, fouling of pretreatment membranes is typically not a problem. The need for pretreatment makes the two-stage MF/RO system the current choice for extracting potable water from seawater. An efficient operation can treat 1,000 gallons of seawater and, by an advanced desalination process, produces 300 to 500 gallons of pure water.
“Waste not, want not” is an old saying, not from any poem, but no other statement could be truer about water usage and reutilization. Though the concept of “toilet to tap” is a hard sell for most water consumers (and most voters), the need for more efficient water recycling demands that wastewater be recycled more directly, more efficiently, and on a larger scale than ever before. Wastewater has to be considered a new resource, one we have only begun to exploit. Both seawater and wastewater represent new opportunities for water recycling, and necessary sources of water we are going to need in the future. To quote Hawkes, “Water is our most precious resource. Without water, there is no life. So it is vital for people, not only in the Southwest, but everywhere, to be judicious in their use of water.”
Author's Bio: Daniel P. Duffy, PE, writes frequently on the topics of landfills and the environment.