By John Koch
Pioneering water reuse system
The small New Mexico village of Cloudcroft holds more than one important distinction: for example, at an elevation of 9,000 feet, it is home to the nation’s highest golf course. More importantly, the local population agreed to implement an integrated water conservation and indirect potable reuse project that uses advanced membrane technology to supplement the existing raw water source consisting of spring and well water with treated wastewater effluent.
Faced with a drought that necessitated trucking 20,000 gallons of water up the mountain each day during the peak summer tourism season, the 1,000 local residents quickly let go of any concerns about using recycled wastewater. From the state’s $10 million initiative to promote innovative water conservation, the village received $600,000 in 2004 to help fund the new $2 million water reuse system.
The state-of-the-art system employs a second-generation membrane bioreactor (MBR) and a gravity-fed reverse osmosis (RO) system to treat wastewater flows that ultimately exceed drinking water quality standards. For the most part, the treated effluent is discharged into a man-made reservoir rather than pumped into a larger body of water such as an aquifer, river, lake or ocean. What makes Cloudcroft unusual is that this reservoir serves as a raw water source for the town’s drinking water treatment system.
Essentially, Cloudcroft is noteworthy for implementing a system that shortens the distance – and traces an obvious path – from the wastewater treatment discharge point to the intake point of the potable water treatment system.
The benefits of water recycling are clear: the village’s water needs are met through an energy-efficient sustainable process that reduces water pollution. Yet the question arises: why don’t more water-strapped communities implement similar systems? The answer may have as much to do with public attitudes as it does with the science of water treatment.
In the old joke about four friends at a bar, the optimist says that the glass is half-full; the pessimist says that the glass is half-empty; the accountant says the glass is twice as big as it needs to be… the water engineer says the glass contains billions of molecules – fascinating pieces of history that have each taken a remarkable 4.4 billion year journey going back to the formation of planet Earth. The problem is, unlike the water engineer, most people do not want to think in much detail about the history of their drinking water.
At Cloudcroft, the concerns are understandable, as the origin of the drinking water is clearly identifiable, the intervening time is relatively short and the proportion of recycled water is relatively high. The purified wastewater constitutes up to 50 percent of the drinking water supply. The effluent from the reclaimed wastewater treatment plant (RWTP) is pumped into the reservoir, where it is mixed with well and spring waters. Prior to intake into the potable water treatment system, the reservoir water is stored an average of 30 days for natural treatment by diffusion and sunlight. The use of an artificial reservoir and the blending with well and spring waters, classifies the Cloudcroft integrated water treatment system as an ‘indirect planned potable reuse system’.
While the public has general concerns about water reuse, water engineers recognize that wastewater also contains pathogens and other so-called emerging pollutants of concern (EPOC) including pharmaceutically active substances, endocrine disrupters and personal care products. For these reasons, it was felt that a multiple membrane barrier solution was a good choice.
Livingston Associates, of Alamogordo, N.M. performed the engineering design for the project. The key elements of the system are the MBR and RO membranes. These membranes, which will be installed in the reclaimed wastewater treatment plant, will make the effluent discharged into the reservoir safe for human consumption. The integrated water treatment process also includes an ultrafiltration (UF) system to treat the reservoir water (a mix of RWTP effluent plus well and spring water), an increasingly common treatment method for treating surface water.
Second generation MBR
The project involves the conversion of the original wastewater treatment plant (WWTP) to an MBR process. The MBR is designed for an average flow of 100,000 gallons per day (GPD), with room for an additional 100,000 GPD in the future. The pre-existing 200,000-gallon equalization basin (EQ) is being retrofitted for the MBR process. It will be divided into two compartments: a 100,000-gallon basin for flow equalization and the remaining 100,000-gallon basin for the MBR.
Raw wastewater influent will enter the system and pass through a one-mm rotating drum screen located at the existing headworks. The screened influent will flow by gravity to the EQ basin, before being pumped into the anoxic basin. From there, the flow enters the aeration basin to receive aerobic treatment and then enters the four membrane chambers that house the proprietary submerged membrane modules.
The MBR system will produce a high quality effluent with a turbidity of typically less than 0.2-NTU (or 1.0-mg/L TSS). The filtrate will be disinfected with chloramines and pumped to a new 75,000-gallon water storage tank at the RWTP site.
The technology is a second-generation submerged MBR system that employs hollow fibers. A key advantage of the system is the use of a single header with hollow fibers that are fixed only at the bottom. The sealed upper ends of the fibers are allowed to float freely; this eliminates the build-up of hair and fibrous materials that can clog the upper end of membrane fibers in MBR designs that employ both a top and bottom header. Solids and particulates, including bacteria, are rejected by the membrane and remain on the outside, while permeate is drawn through the membrane to the inside of the fibers. Outside-to-inside technology such as this provides optimal solids management and a high flow-rate, while using up to 50 percent less energy than other MBR systems.
Another advantage of this design is the introduction of air scour at the center of the fiber bundle, right where it is needed. Compressed air creates bubbles that shake the membranes and scour the outside of the hollow fibers, removing accumulated debris. The unique air scour design is a tremendous improvement over older technology, since it minimizes sludging around the membrane and reduces energy consumption.
The high-strength fibers in these new modules also overcome the fiber breakage problems typical of first generation systems that utilize non-braided fibers. The free-floating tips of the hollow fibers in the single-header design also place less mechanical stress on the fibers compared to double-header designs.
Unlike flat sheet membranes that do not support backflushing, the new modules resist fouling and maintain flux by introducing a small portion of the filtrate back through the fiber pores from the inside-out at timed intervals. The hollow fibers provide significantly higher membrane surface area and therefore higher filtration capacity within the same module footprint, compared to flat sheet membrane designs.
Pure water from RO
The MBR is the first step in a multiple physical-barrier approach to reclaimed water repurification. The high-quality MBR permeate will be pumped uphill into a 75,000-gallon storage tank. From there, some of the water will be diverted for non-potable reuse (i.e., to irrigate the golf course and high school athletic fields).
Each day, 100,000 gallons will flow downhill about 2.5 miles to the water treatment facilities (WTF) that house the RO system. The force of gravity produces approximately 175 psi of residual pressure at the terminus of the four-inch waterline – the pressure required to operate the RO system.
The RO system is a single train, three-stage, one-pass system with five pressure vessels set up in an 2:2:1 array that contains high-rejection, low-pressure thin-film composite membranes that have been successfully utilized in a number of reuse applications. They have been shown to be effective in rejecting many emerging contaminants while achieving water recovery of about 80 percent.
The RO system will produce an average of 80,000 gpd of permeate, with a total dissolved solids (TDS) content of about 50-mg/L from a feed quality of around 1,000 mg/L TDS.
Permeate from the RO system will receive peroxide and UV disinfection and will be discharged into a 1,000,000-gallon lined and covered reservoir. From there, the reservoir water will flow into a 750,000-gallon covered and lined reservoir, where it will blend with existing spring and ground waters. A portion of the RO permeate will be used for aquifer recharge during times of low water demand.
The concentrate from the RO process will be diverted to a 250,000-gallon open and lined reservoir along with UF backwash water. This water is to be used for road dust control, construction, snow making for the ski area, gravel mining operations, forest fire fighting and other beneficial purposes.
High quality, safe drinking water
The final stage of the integrated water treatment is the ultrafiltration (UF) of reservoir water containing RO permeate, well and spring water. Each day, approximately 180,000 gallons of blended water will be treated through the UF system. The permeate from this system will be filtered by granular activated carbon (GAC) prior to receiving additional disinfection using sodium hypochlorite. The disinfected water will then go into the water distribution system.
Because the high-quality (low total dissolved solids) water from the RO process is to be used for blending, the overall water quality in the distribution system is expected to improve when Cloudcroft begins using reclaimed water.
It just goes to show that it is not where the water has been that counts, but where it is going. The integrated membrane system and its multiple physical-barriers provides protection that will give the residents and tourists in Cloudcroft the confidence to enjoy high-quality, high-tech water as pure as a mountain stream.
About the author
John Koch is MBR Technical Director, Koch Membrane Systems. He can be reached at 414-771-1124 or by e-mail at [email protected].
About the products
The MBR and RO membranes used in Cloudcroft are from Koch Membrane Systems GmBH. The MBR utilizes the PURON™ submerged membrane module. PURON is a trademark of Koch Membrane Systems GmbH and is registered in Austria, Belgium, China, France, Germany, Italy, Luxembourg, Mexico, Netherlands, Spain, Saudi Arabia, Taiwan and the United Kingdom. The RO treatment train utilizes Magnum® 8822HR membranes. Magnum is a registered trademark of Koch Membrane Systems, Inc. in the United States and other countries.
About the company
Koch Membrane Systems, Inc. has been a global leader in separation and filtration products for more than 35 years. A designer and manufacturer of state-of-the-art membrane cartridges and elements, as well as complete membrane systems, Koch Membrane Systems products are specified for the most demanding applications. To enhance membrane performance, KMS offers a line of antiscalants and cleaning chemicals, and provides a wide range of maintenance and technical service programs. The company has supplied membranes for more than 15,000 systems installed around the world, serving the food processing, life sciences, and general manufacturing industries, as well as providing potable water and wastewater treatment technologies for communities of all sizes.