Driven by space constraints, stricter limits, plants search for new solutions

As they seek to comply with ever more stringent discharge limits, municipal wastewater treatment plants (WWTPs) across the country frequently must contend with increasing space limitations that complicate efforts to expand or upgrade treatment facilities. This trend toward "doing more with less" is reflected in the growing interest in wastewater technologies that enable more advanced treatment without requiring WWTPs to increase in size, industry analysts say.

Stepping up Nutrient Removal

Nutrient limits, in particular, are the "real drivers" in municipal wastewater treatment technology today, said Al Firmin, senior vice president and wastewater practice leader with CDM (Cambridge, Mass.). No longer confined to a particular region, such as the Southeast, stringent nutrient limits are "pretty prevalent now," Firmin said.

"Almost everything we do on a municipal wastewater plant these days has either nitrogen or phosphorus requirements or both," he said.

Joe Husband, vice president and national wastewater treatment technology leader for the consulting engineering firm Malcolm Pirnie Inc. (White Plains, N.Y.), agreed. "What's driving our clients and what's driving the technology now," Husband said, are approaches that "will help us reduce nutrient discharges."

Chemically enhanced clarification methods also are being used to achieve "very low" phosphorus effluent levels, said Rod Reardon, a vice president with CDM. Because certain organisms require phosphorus during secondary treatment, the nutrient can only be reduced so much before the biological process is upset, Reardon noted. Therefore, additional reductions must occur after secondary treatment.

"Chemically enhanced clarification processes really have a role to play there, too," he said.

For example, a ballasted flocculation system recently was installed at a 530,000-m^sup 3^/d (140-mgd) WWTP in Syracuse, New York, to meet strict phosphorus limits, said William Sullivan, vice president of sales for Veolia Water Systems' Kruger Inc., (Pittsburgh, Pa.), maker of the Actiflo high-rate clarification system.

Finding Space-Efficient Options

The inability of many urban and suburban WWTPs to expand has put pressure on designers and vendors to find ways to work around space constraints. A common question among wastewater utilities is, "How do I do more in less space?" Husband said.

For many municipal WWTPs, the answer increasingly involves membranes, particularly in the form of a membrane bioreactor (MBR). Consisting of membranes immersed in an aeration basin, MBRs combine biological treatment with membrane filtration to produce high-quality effluent.

At the same time, MBRs obviate the need for secondary clarifiers and conventional sand filters, helping to reduce a plant's overall "footprint," said Deo Phagoo, director of wastewater treatment for Zenon Environmental Inc. (Oakville, Ontario), which manufactures the ZeeWeed MBR system. In areas where land and construction costs are high, MBRs have become "very competitive" with conventional activated sludge treatment, Phagoo said.

Historically, the high cost of membranes limited the use of MBRs primarily to wastewater reuse applications. However, lower membrane prices are prompting engineers to reassess this assumption.

"Over the last 10 years, the cost of membranes has come down by a factor of ten," said Glenn Daigger, chief technology officer for the water business group within CH2M Hill (Englewood, Colo.). When comparing MBRs with a conventional WWTP that conducts nutrient removal and effluent filtration, "increasingly we're seeing on a present-worth cost basis that a membrane bioreactor is more cost-effective," Daigger said.

Another option gaining in popularity among space-limited WWTPs seeking to improve treatment performance is the integrated fixed-film activated sludge (IFAS) process. The approach involves adding media to an aeration basin to provide surface areas on which biomass can grow and employing screens to retain the media within the basin. By supporting additional biomass, the IFAS process can "essentially double" an aeration basin's treatment capacity while also improving its ability to remove nutrients, said Jim Carroll, product manager for USFilter Corp., (Warrendale, Pa.), which manufactures an IFAS system known as the attached growth airlift reactor (AGAR).

Ultimately, a major benefit of IFAS is its ability to "get more biological treatment" out of a WWTP's existing aeration basin, said Cindy Wallis-Lage, director of the wastewater treatment technology department within Black & Veatch (Overland Park, Kan.). IFAS is ideal for "getting more work done in a smaller space," Wallis-Lage said.

Although IFAS has been used for years in Europe, the approach only recently has begun to catch on in the United States, Carroll said, and it is expected to continue to attract interest as growing numbers of WWTPs are required to upgrade their facilities.

Managing Peak Flows

Increasingly strict wet weather-related requirements, such as decreasing overflows from combined and sanitary sewers, are prompting many wastewater utilities to seek cost-effective ways to handle peak flows that accompany storms. In addition to the difficulties often faced in expanding treatment capacity, utilities frequently cannot afford to increase the storage capacity of their collection systems.

As a result, high-rate clarification systems that require relatively little space and can be activated when needed are "being looked at pretty seriously" by utilities to treat peak flows, said Terry Krause, vice president and wastewater treatment technology practice leader for Earth Tech (Long Beach, Calif.).

By using chemicals and microscopic sand particles or recycled sludge as ballast to enhance flocculation and sedimentation, high-rate clarification systems can accommodate much higher flows at shorter detention times compared to a traditional clarifier. Moreover, the systems typically require much less space than a traditional clarifier. Although they can be expensive, the systems only need to be operated during wet weather events, Krause said.

"The nice thing about them is that they are a physical-chemical operation, which means you can turn them on and off," he said.

Another advantage the systems offer is the ability to construct them in locations other than a WWTP, said Jim Kunz, a senior vice president with Earth Tech. With regulatory agencies requiring that wet weather flows be captured to a greater extent than ever before, "the cost of conveying the peak flows," Kunz said, "is putting a fair amount of pressure on solutions that don't require" transporting all flows during a storm to a treatment plant." For example, a high-rate clarification system can be installed at the location of a combined sewer's outlet to treat overflows before discharge, Kunz said.

The U.S. Environmental Protection Agency's recent decision not to finalize its regulations for wastewater blending created "ambiguity" regarding the practice, said Denny Parker, senior vice president and director of technology for Brown and Caldwell (Walnut Creek, Calif.). However, the need to accommodate peak flows that otherwise would overwhelm a WWTP's secondary biological processes remains, and different states may adopt different approaches to blending, Parker said.

"I think many communities will now look to technologies that allow compliance under a different set of scenarios," he said, including high-rate physical-chemical and biological treatment technologies.

Insulating Against Energy Costs

The high prices of oil and natural gas have sparked renewed interest in energy-efficient technologies. Although utilities have long sought to optimize plant performance, rising energy costs have made efficiency an even more important priority, said Fred Goldman, a principal with Kennedy/Jenks Consultants (San Francisco).

Trent Slovak, associate vice president with PBS&J (Miami, Fla.), has witnessed a similar trend. Because of energy costs, certain control technologies and high-efficiency electric motors are "becoming more popular," Slovak said.

Many WWTPs are now using dissolved-oxygen probes to monitor oxygen levels in aeration systems and optimize blower performance, Goldman said. "Overall, we're seeing plants paying more attention to energy."