August 2011, Vol. 23, No.8

Solar Magic

news art

Greenhouse turns wastewater solids into organic fertilizer

It may not be possible to make a silk purse out of a sow’s ear. But the Natchez, Miss., Wastewater Treatment Plant (WWTP) may be doing something even better. Its engineers have found an innovative way to turn wastewater solids into Class A biosolids using only the power of the sun.

The transformation was made possible thanks to a desire to rid the city of an environmental headache, coupled with an opportunity to build a low-cost, solar-powered greenhouse, according to David Gardner, Natchez city engineer. The solution is one that other communities facing high solids management costs might want to consider.  

A new solution to an old problem  

The Natchez WWTP, which serves a population of 22,000, produces about 1.5 dry Mg (1.6 dry ton) of solids from the 9500 m3/d (2.5 mgd) of wastewater it treats. For decades, it has stored these solids in two nearby 1.4-ha (3.5-ac) lagoons that together can house a roughly 20-year supply.

“Every 15 to 17 years, the lagoons start getting full, and we have to do something,” said Gardner. Historically, that “something” meant pumping the solids from the lagoons and transporting them by truck 16 km (10 mi) north to a Mississippi Department of Environmental Quality (MDEQ)-approved land farm, where they were injected into the ground.

“We last emptied the lagoons 14 years ago at a cost of about $1.5 million,” said Gardner. “It was costing us a fortune, especially when you consider that 90% of what we’re hauling away is water.”

But if the Natchez WWTP could somehow reduce the pathogens in its solids to below detectable levels and also achieve Class A certification by the MDEQ, the resulting biosolids could be land-applied. The WWTP could even bag and market the Class A biosolids to the public for use on lawns and gardens.

That tantalizing possibility drove Gardner and the city’s engineers to brainstorm stabilization processes they might use. “If we could achieve Class A certification, we know farmers and others who will make the trip here to get it from us, instead of us paying to haul it 10 miles [16 km] away,” Gardner said. “That’s what led us to the idea of the solar-powered greenhouse.”


How it works 

These days, Natchez puts its solids through a belt filter press and sends the resulting water back to the plant for treatment.

Workers then haul the solids 60 m (200 ft) to the plant’s new greenhouse, where the solids are spread over a concrete floor to a depth of 150 to 200 mm (6 to 8 in.). Monitors inside the greenhouse help keep the indoor environment at optimal drying conditions.

“Our system requires very little human intervention,” said Gardner. “We key in the [solids’] initial moisture content, and the computer does the rest.” If the humidity inside is high, for example, exhaust fans automatically direct humid air outside.

Meanwhile, a robotic tiller travels up and down the length of the greenhouse floor to agitate the solids, taking its cues from an automated system that tracks moisture content.

The time it takes for the solids to dry depends on the temperature inside the greenhouse. One recent batch took 10 days.

As the solids dry, their volume is reduced. “For every 100 lb [45 kg] put into the system, we produce about 3 lb [1.3 kg] of organic fertilizer,” said Gardner. That’s about a 97% reduction.

Once the drying process is complete, the utility uses a tractor to scoop the rich, odorless, dirt-like material — now about 12 mm (0.5 in.) deep — from the greenhouse. For the time being, it is being stored in a grain house, while the city awaits test results and clearance from the MDEQ.

Gardner said the city intends initially to make the fertilizer available to farmers and landscaping companies, but doesn’t count out the possibility of branding and marketing it to the public down the road.


The payoff

In financial terms, the solar greenhouse project was a no-brainer, said Gardner.

Planning, design, and construction costs were paid with a $5 million loan from the state revolving fund, 80% of which has already been forgiven. The City of Natchez will pay the remaining $1 million loan back at an interest rate of 1.75% over the next 20 years.

That comes to $2500 a month, or $30,000 a year, compared to the hundreds of thousands of dollars it cost the city to store and, eventually, transport and dispose of the solids. That’s before factoring in any revenue the city may eventually generate through fertilizer sales.

The city’s new approach to solids management is doing more than creating a marketable product, Gardner said. “By taking advantage of the sun’s drying capabilities, we’re reducing our operating costs by about $200,000 a year.” In fact, this project, in combination with other energy-reduction measures, enabled the plant to reduce its operating budget by 3% this year.

In addition to saving money, the greenhouse project also is relieving Natchez WWTP staff of some significant long-term headaches. Not only will the greenhouse be used to process the new solids generated each day, but it also eventually will eliminate the solids that have accumulated in the lagoons since they were last emptied. Within 22 or 23 years, Gardner predicted, the city will have emptied the two existing lagoons altogether.

 “It takes us out of the problem altogether,” said Gardner. “I look forward to the day when I won’t have to ever worry about the lagoons being full again.”


—Mary Bufe , WE&T


From treatment plant to cooling tower

Municipal wastewater is an abundant source of cooling water for thirsty power plants


Power plants are water hogs. With population growth and climate change putting a pinch on water resources, the power sector is being forced to look beyond surface or groundwater for its cooling needs. Increasingly, power facilities are partnering with wastewater treatment plants (WWTPs) to take advantage of abundant sources of municipal wastewater for cooling. Both power and water industry experts say they are benefiting environmentally and financially from the arrangement.

Each kilowatt of power generated by coal, natural gas, and nuclear thermoelectric plants requires on average 95 L (25 gal) of water to produce, according to Tom Feeley, a technical adviser with the U.S. Department of Energy (DOE) National Energy Technology Laboratory (Pittsburgh). The electricity industry is the second-largest user of water, responsible for 40% of all freshwater withdrawals in the nation, he said.

Although a relatively small amount of water is needed to create the steam that drives the turbines, vast quantities of water are required to cool the steam, explained Radisav Vidic, an environmental engineer at the University of Pittsburgh. After the steam passes through the turbines, “engineers run it through a heat exchanger with cold water on one side that condenses the steam so it can be sent back to the boiler,” he said.  

About 43% of U.S. power plants still employ “once-through” cooling in which water is pulled from a river or lake, chills the steam, and then is dumped back, heated, into the waterbody, Vidic said. However, concerns about heat pollution, loss of aquatic life, and water shortages have effectively eliminated once-through cooling as an option. New water-cooled units typically employ closed-cycle cooling that sends the heated cooling water into a tower where airflow brings the water temperature down so that it can be reused four to eight times to condense the steam.


The potential of wastewater

“I hear anecdotes from people at power plants that siting new plants is a challenge in part because it’s hard to find cooling water,” said Sujoy Roy, an environmental engineer in the Lafayette, Calif., office of Tetra Tech (Pasadena, Calif.).

With demand for electricity projected to grow by 30% from 2008 to 2035, power generators are turning to treated municipal wastewater due to its abundance and uniform quality, Vidic said.

Vidic and his colleagues have estimated that about 50% of existing power plants could obtain all their cooling needs from WWTPs located within a 16-km (10-mi) radius, and if the radius is extended to 40 km (25 mi), 76% of existing power plants could draw water from wastewater treatment facilities. More than 80% of proposed power plants could rely on reclaimed water from treatment plants within a 16-km (10-mi) radius.

Roughly 60 of the nation’s 5400 power plants use reclaimed water for all or part of their cooling needs, according to DOE. The thirsty states of Florida, California, Texas, and Arizona have the most facilities using reclaimed water. But states not typically associated with water shortages, such as Massachusetts, Mississippi, and Wisconsin, also host power plants taking advantage of wastewater for cooling. Although the practice has been around since the 1960s, the number of power generators using reclaimed water didn’t begin to take off until the last 20 years and now is a growing trend, said Robert Goldstein, a principal scientist with the Electric Power Research Institute, an electric industry research organization (Palo Alto, Calif.).

However, treated wastewater is of a lower quality than typical freshwater sources and exacerbates the technical challenge of controlling corrosion, scaling, and biofouling at cooling towers, said Dave Dzombak, an environmental engineer at the University of Pittsburgh. Elevated nutrient levels can feed the growth of bacteria, fungi, and algae that form a slimy coating on the heat exchangers, lowering efficiency. The precipitation of salts, known as scaling, can clog pipes. High levels of phosphate and ammonia in reclaimed water corrode metals in the cooling system.

“What usually happens is that the power plant hires a consultant who recommends that the plant take the secondary effluent and provide tertiary treatment to obtain drinking water quality, which they have lots of experience with in power plants,” Vidic said. He and Dzombak are looking at alternatives to tertiary treatment, such as chemical dosing, which could bring down the cost of using effluent in cooling towers. “One of the treatment trains we’re evaluating would cost about $1/1000 gal [$0.26/1000 L] compared to $2.78 per 1000 gal [$0.73/1000 L] for drinking water for commercial use,” Dzombak said.

Biofouling can be controlled with sodium hypochlorite or monochloramine, Vidic said. Polymeric antiscalants such as polymaleic acid or agents such as tetra-potassium pyrophosphate can prevent scaling. Tolytriazole is a commonly used chemical inhibitor that has curbed corrosion in tests with municipal wastewater, he said.

However, WWTPs that employ advanced secondary treatment probably would not have to reconfigure their process, said Gregg Eckhardt, a senior resource analyst at the San Antonio Water System. Since the 1960s, the WWTP has supplied reclaimed water for cooling to the City Public Service Energy power plant. “It has been a great option for San Antonio,” he said.

“I think this is definitely the way to go for power plant cooling,” said Brad Hans, plant supervisor at the Terry Bundy Generating Station in Lincoln, Neb. Since it was commissioned in 2003, the public utility has been using reclaimed water from the city’s WWTP 3.2 km (2 mi) away. The arrangement saves money, is good for the environment, and hasn’t required big changes at the power plant, he said. “Scaling is not a problem because we use pH control and dispersants to keep scalants in suspension,” he said. The plant employs stainless steel or plastic pipes, and keeps the water moving to avoid corrosion. High ammonia concentrations drove problems with biofouling until a new National Pollutant Discharge Elimination System (NPDES) permit required the WWTP to drop the levels.


Mankato leads the way

Two problems were solved at once when the energy company Calpine Corp. (Houston) teamed up with the city of Mankato, Minn., to reuse municipal wastewater effluent for power plant cooling. Calpine’s new gas-fired power plant, which opened in 2006, found a reliable source of cooling water. And the city renewed its NPDES permit, meeting a stringent new limit on phosphorus loading to the Minnesota River.

“In order to achieve a 1-mg/L total phosphorus limit by 2015, we were looking at installation of $10 million in new phosphorus removal technologies when Calpine approached us,” said Jim Bruender, superintendent of Mankato’s 42,000-m3/d (11.2-mgd) wastewater plant.

Calpine planned to build a 350-MW power plant that eventually would double in size, said Derek Cambridge, a project director at Black & Veatch (Overland Park, Kan.). The company’s water needs for its closed-cycle cooling system would grow from 11,700 to 23,500 m3/d (3.1 to 6.2 mgd) once the plant was fully built. The company ruled out the use of groundwater or surface water, in part because of the cost of potable water. In addition, if the company used surface or groundwater, it would have to obtain a permit to directly discharge waste cooling water to the Minnesota River. A stiff state standard requiring zero discharge of phosphorus from new sources made this option unattractive, Cambridge explained.

However, if Calpine used wastewater for cooling, it could send its waste cooling water back to the treatment plant, making the city responsible for its eventual discharge to the river. Mankato accepted Calpine’s offer to build a water reclamation facility at the city’s WWTP, Bruender said. The $20 million facility operates in two stages. All of the plant’s effluent runs into the first stage, which combines coagulation, flocculation, and sedimentation to drop phosphorus concentrations below 0.4 mg/L. At this point, any water not needed by the power plant is sent to the chlorine contact tanks for disinfection and then to the Minnesota River.

Water needed for power plant cooling flows to the second treatment stage, which ensures that effluent meets California Title 22 Standards for Water Reuse. These standards, considered the best in the industry, protect public health from any pathogens that might escape from water evaporating out of the cooling towers, Bruender said. Depending on power plant needs, roughly 3800 to 11,000 m3/d (1 to 3 mgd) flows to a disk filter that removes solids that harbor bacteria. After filtration, the water is chlorinated and pumped about 2470 m (2700 yd) to the power plant.

To control scaling and corrosion on pipes and structures within the plant, Calpine adds a number of chemicals, including sodium bromide to prevent biofouling, said David Jacobs, plant manager for Calpine Mankato Energy Center. After circulating through roughly four cycles, the used cooling water is sent back to the WWTP. Calpine has promised the city “that any water they return to us will meet the pretreatment standards we require from all industries discharging to our plant,” Bruender said. The power plant also has pledged that it will not cause the treatment plant to violate its NPDES permit.

Both partners have saved money on the deal. Mankato avoided the capital cost of a new treatment facility. The city spends less than $500,000 a year in operation and maintenance costs. But after 20 years, Mankato can begin charging Calpine for the cooling water. Calpine estimates that it saves about $1.5 million a year by not using surface or groundwater for cooling. Bruender said the environment is a clear winner since the arrangement prevents the extraction of large amounts of surface or groundwater and protects the Minnesota River from nutrients that drive algal blooms. “That’s why we entered the agreement — to lead the way and be stewards of the environment,” he said.


—Janet Pelley , WE&T

Addressing the real problem

The wastewater and nonwoven fabric manufacturing industries work together to address clogged collection systems

When toilets back up and pump stations malfunction, it is often the wastewater agency that gets blamed, although the flushing habits of its customers may be the true culprit.

In May, Northern Ireland Water (Belfast) was fined £2000 ($3200) plus court costs for releasing untreated wastewater into a local waterway. The discharge was traced back to one of the agency’s pumping stations, but Northern Ireland Water said that “inappropriate items” flushed down toilets by residents caused the pumps to malfunction.

Those inappropriate items include “cotton buds, [diapers], sanitary items, household wipes, and condoms, which regularly make their way through the [collection] system and block the pumps at the pumping stations,” said a spokesperson of Northern Ireland Water in a May 4 press release. “Whilst NI Water has responsibility for the [collection] system, everyone in Northern Ireland can help reduce pollution incidents. The advice is simple: Only toilet roll and human waste should be flushed down the toilet.”

The Norwalk (Conn.) Water Pollution Control Authority experienced backups at several of its pumping stations in 2001 because of mechanical problems as well as fats, oils, and grease issues, but more importantly because of the flushing of inappropriate items by customers, said Timothy Newton, senior operations specialist at CH2M Hill (Englewood, Colo.), who was working with the agency at the time on its collection system.

“We were losing a lot of time and energy with the ragging of the pumps,” Newton said.

A computerized maintenance management system had calculated how often some parts in the collection system would need to be replaced, “but because of the ragging, we had to replace [parts] more often than we had anticipated,” Newton said. “The ragging was putting stress on the motors and wearing out the impellers.”

Newton said the biggest cause of the ragging were diapers and feminine hygiene products coming from apartment buildings and multifamily homes. “Those were the population concentrations,” he said.

To combat this, the project team located the ragging “hotspots” and increased the size of the impellers at these locations and added a much larger grinder to pumps farther down the line. Also, in places where there were ordinances against flushing inappropriate items, property owners had to pay for the cleaning of laterals whenever they got clogged.

“They started to educate the renters so that they wouldn’t have to pay these fines,” Newton said.


Developing guidelines

To prevent backups, education is key, but educating the public is a continual challenge. Those in the wastewater and nonwoven manufacturing industry are trying diligently to educate customers on what to flush and what not to flush, and manufacturers on how to properly create and label flushable products.

Though the nonwoven manufacturing industry produces almost 138 million Mg (152 million ton) of fabric annually, only 1% of that fabric is flushable moist toilet tissue products, said Steve Ogle, director of technical affairs at the Association of Nonwoven Fabrics Industry (INDA; Cary, N.C.). Ogle also said that less than 52 m2 of materials found in the screens of one wastewater treatment plant (WWTP) daily are flushable.

“The thousands of pounds of products WWTPs are extracting are nonflushable,” Ogle said.

Ogle said INDA wants to do a joint education program with the Water Environment Research Federation (WERF; Alexandria, Va.) “to teach what consumers should not flush and to educate manufacturers on how to make sure their products are assessed accurately before they put them on the shelf.” INDA and WERF also are talking about possibly collaborating on a national study that would address flushable and dispersible products.

INDA has worked with WERF in this area before. The association and its European sister organization, The European Nonwovens Trade Association (EDANA; Brussels), largely based its guidelines on how manufacturers can test their flushable products on a 2003 WERF report, “Protocols to Assess the Breakdown of Flushable Consumer Products.” At the request of Procter & Gamble Co. (Cincinnati), WERF created a peer review committee that examined the manufacturer’s research and its overall approach for assessing the fate and compatibility of consumer products in the wastewater disposal system.

“The ultimate goal of this peer review was to develop a scientifically sound document that describes the approach, current methodologies, and quality assurance that support the flushability of consumer products,” according to the executive summary of the report.

INDA worked for 4 years on its “Guidance Document for Assessing the Flushability of Nonwoven Consumer Products” with the assistance of 31 nonwoven manufacturers. In 2008, INDA put the document before a peer review committee composed of INDA members, wastewater industry professionals, and WERF staff. (The guidance was updated again in 2009 to include a pump test.)

The 31 companies that participated in the guidance document’s creation signed a code of practice stating that they would implement the guidelines. EDANA also uses the guidelines, but in both associations, following these guidelines is completely voluntary.

“These are not standards, and we never intended for them to be interpreted as standards,” Ogle explained.

For those who chose to follow the guidelines, NSF International (Ann Arbor, Mich.) offers a certification program. It launched the program at INDA’s 2010 World of Wipes International Conference in Chicago.

“Obviously, there are so many products in the marketplace claiming to be flushable,” said Tom Bruursema, general manager of drinking water and wastewater treatment units at NSF.  “Many consumers are confused about those claims.”

According to the company Web site, lab technicians at NSF complete lab testing of flushability using INDA’s guidelines, and NSF conducts a manufacturing facility inspection in which representatives review product information and packaging, conduct a walkthrough of the production facility to inspect the manufacturing process, and may request to review documents such as purchasing records.

So far, no manufacturers have completed the program, Bruursema said.

“That’s not to say that there aren’t products that we’re looking at,” Bruursema said. There are manufacturers who have started the certification process but have yet to complete it.

“This still falls within the timeline we had expected since we only debuted the program last year,” Bruursema said.

Where they fall short

Though INDA and nonwoven manufacturers may have good intentions, much of the testing on the flushability relies too heavily on how these products make their way through wastewater treatment plants, not collection systems, argued Nick Arhontes, the director of the Facilities Support Services department at the Orange County Sanitation District (Fountain Valley, Calif.).

“Treatment plants are traditionally heavily designed, and you have a lot of good bar screen and rag removal systems,” Arhontes said. But collection systems don’t have these mechanisms, he said, “and when agencies focus on water reduction and you start to limit water use, there’s less water getting into the sewer” to transport wastewater.

Arhontes also takes issue with the labeling on some of the nonflushable products.

“That’s a standing concern,” Arhontes said. “The labeling and the information on some of these boxes is so small you can’t read it, so I think more work has to be done to improve labeling and then improve public education once the labeling is dealt with.”

Some California lawmakers tried to push for stricter label requirements through legislation. Last year, state Sen. Jared Huffman introduced a bill on the topic, but it languished in committee.

“Essentially, the near-final draft of the proposed legislation looked for improved labeling and it looked for improved testing,” Arhontes explained.

According to the language of the bill, this testing must be conducted by a third party, and documentation must be shown that substantiates the validity of claims that the product would dissolve quickly in a similar manner to toilet tissue.

Arhontes said so far the bill has not been brought back before the California General Assembly.


—LaShell Stratton-Childers , WE&T


An innovative partnership

The City of Santa Paula, Calif., partners with the private industry to build an energy-saving WWTP


It has been more than a year since the completion of the new Santa Paula (Calif.) Water Recycling Facility, an energy-efficient, 15,900-m3/d (4.2-mgd) wastewater treatment plant (WWTP).

Not only did the facility come on-line 7 months early, but it also has exceeded expectations in its energy efficiency and cost savings, according to Marian Clayton, director of marketing at PERC Water, a private water recycling company that built, manages, and operates the facility.

The initial expectation was to reduce energy consumption by 15% compared to the city’s original WWTP, but “that was before we had actually run the facility,” Clayton said. “Now that the facility has been up and running for almost a year, we found that it has been on average more than 35% more energy-efficient than we anticipated.”

According to a March 2010 PERC Water press release, because of the facility’s energy-saving technologies, such as its membrane scouring system and lighting design, the city saved on average more than $10,000 a month during the first 7 months of operation.


A unique problem with a creative solution

The City of Santa Paula built its new water recycling facility because it was mandated by the state to replace its 70-year-old WWTP. The plant had “reached the end of its useful life and accrued more than $8 million in compliance-related fines,” according to a December 2010 PERC Water press release.

“The city, for some years before, had tried to procure the [new] project through a design-bid-build scenario,” explained Bob Nespeca, vice president of asset management at PERC Water. With this delivery method, the city would have supplied all the funding, hired an engineering firm to design plans, and later had contractors bid on the project, with the project going to the lowest bidder. But because of financial constraints and the sinking economy, “the city just came to the realization that they did not have the financial wherewithal to complete the project, nor did it have the staffing to do so,” he said.

So the city came back to the marketplace with a modified approach known as a design-build-operate-finance contract, Nespeca explained. This is how PERC Water, along with the independent infrastructure-funded firm, Alinda Capital Partners LLC (New York), became involved.

The Santa Paula facility is the first of its kind to be built under California Government Code Sec. 5956,which permits the use ofprivate investment to build, operate, and maintain public infrastructure. The City of Santa Paula created a public–private partnership with Santa Paula Water, which is an alliance between PERC Water and Alinda. Under the partnership, Santa Paula Water agreed to finance construction of the facility through private equity as well as operate it.

This unique project delivery method has proven to be a success for all parties involved, Clayton said.

“The way the deal works is for 30 years the facility is owned by a private company and is paid for by the city through a service fee,” Nespeca said. “That service fee covers the operational costs of the facility, capital replacements, and the capital reimbursement of the original construction. So at the end of the 30 years, the city becomes — with no money down — the owner of the facility.”

Clayton said the new delivery method has taken some getting used to for everyone involved.

“This was something that was new that hadn’t really been done with this type of infrastructure, and it hadn’t been done before in California,” Clayton said. “So the biggest struggle with this project has been educating and continuing to educate people about the [design-build-operate-finance] structure.”


Smooth sailing

Though adapting to the new delivery method was challenging, the construction itself went a lot smoother.

Clayton said construction began in July 2008, was completed by December 2009, and the facility was at full flow by May 13, 2010.

“The date we had contracted to have it up and running was Dec. 15, 2010,” Clayton said. “So we finished 7 months in advance.”

The new facility includes an energy-saving lighting system that incorporates natural lighting, LED lamps, mercury-vapor exterior lights, and electronic ballasts for fluorescent lamps, light sensors, and automatic dimming devices. Also, because membrane scouring and aeration usually account for nearly half of a WWTP’s power consumption, the company chose energy-efficient equipment for these systems.

Nespeca said the team decided to go with a different blower than it had originally chosen for the project. “It was an upgrade,” he said. “It was just more efficient equipment that burned less power.”

The planners also decided to incorporate green infrastructure. The decorative water feature in front of the facility is actually a stormwater retention basin, Nespeca said.

This type of green infrastructure is “sort of a trademark of all projects we do,” Nespeca said.

Nespeca said PERC Water also hopes to eventually incorporate solar energy at the facility by installing solar arrays near the plant.

“This land is available because the city had anticipated a much larger footprint [for the plant],” Clayton explained.

So far the wastewater treated at the facility is being sent into the ground through percolation ponds, Nespeca said. Santa Paula’s effluent is California Title 22 certified so it can be used for various industrial uses, he explained. Nespeca added that the city is in the process of designing a reuse system that will include storage tanks and pumps. The reuse system will use the water to irrigate citrus and avocado groves throughout Ventura County.


—LaShell Stratton-Childers , WE&T


©2011 Water Environment Federation. All rights reserved.