New Mexico town tries toilet to tap

Cloudcroft, N.M. agrees to implement an integrated water conservation and indirect potable water reuse project that uses advanced membrane technology to supplement the existing raw water source, consisting of spring and well water, with treated wastewater effluent.

Cloudcroft, N.M. — The small New Mexico village of Cloudcroft holds more than one important distinction. At an elevation of 9,000 feet, it is home to the nation's highest golf course. But it is the cutting edge way that it is addressing a serious water shortage that really makes Cloudcroft stand out.

The local population agreed to implement an integrated water conservation and indirect potable water 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 in 20,000-gal. 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 to help fund the village's 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 system to treat wastewater flows that, after treatment, 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.

Public acceptance

The benefits of water recycling are clear: The village's water needs are met through an energy-efficient sustainable process that reduces water pollution, but 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.

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% of the drinking water supply. The effluent from the reclaimed wastewater treatment plant 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 a “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, 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 supplied by Koch Membrane Systems. 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 system to treat the reservoir water (a mix of reclaimed wastewater treatment plant effluent and 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 to a membrane bioreactor process. The membrane bioreactor is designed for an average flow of 100,000 gal./day, with room for an additional 100,000-gal./day in the future. The pre-existing 200,000-gal. equalization basin is being retrofitted for the membrane bioreactor process by being divided into two compartments: a 100,000-gal. basin for flow equalization and the remaining 100,000-gal. basin for the membrane bioreactor.

Raw wastewater influent will enter the system and pass through a 1-mm rotating drum screen located at the existing headworks. The screened influent will flow by gravity to the equalization 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 “Puron” submerged membrane modules from Koch Membrane Systems.

The membrane bioreactor system will produce a high quality effluent with a turbidity of typically less than 1.0-mg/L TSS). The filtrate will be disinfected with chloramines and pumped to a new 75,000-gal. water storage tank at the reclaimed wastewater treatment plant site.

Pure RO water

The membrane bioreactor 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-gal. 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 gal. will flow downhill about 2.5 miles to the water treatment facilities that house the RO system. The force of gravity produces approximately 175 PSI of residual pressure at the terminus of the 4-in. 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 used 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%.

The RO system will produce an average of 80,000-gal./day of permeate, with a total dissolved solids content of about 50-mg/L from a feed quality of around 1,000 mg/L total dissolved solids.

Permeate from the RO system will receive peroxide and UV disinfection, and will be discharged into a 1 million-gal. lined and covered reservoir. From there, the reservoir water will flow into a 750,000-gal. 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-gal. open and lined reservoir along with ultrafiltration 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.

Safe drinking water

The final stage of the integrated water treatment is the ultrafiltration of reservoir water containing RO permeate, well and spring water. Each day, approximately 180,000-gal. of blended water will be treated through the ultrafiltration system. The permeate from the UF system will be filtered by granular activated carbon 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.

And that 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.

TAGS: Wastewater