Advances in wastewater collection and treatment

September 2009 » Products » WASTEWATER
New technologies promote sustainability, energy efficiency, and cost effectiveness.
Bob Drake

Wastewater collection and treatment arguably ranks among the most significant advances in public health in the United States. Yet today, the public is largely unaware of the civil infrastructure that has virtually eliminated diseases that spawned deadly epidemics in this country during the last century. Nevertheless, many aging municipal wastewater systems now need rehabilitation, replacement, or expansion, and new technologies and design tools are poised to take the wastewater industry to new heights in sustainability, energy efficiency, and cost effectiveness.

New technologies and design tools are poised to take the wastewater industry to new heights in sustainability, energy efficiency, and cost effectiveness.

Sustainability
The wastewater industry is addressing environmental issues not only in terms of effluent water quality, but also with regard for water scarcity and reuse potential. “The shortage of water appears to be pushing utilities toward membrane bioreactors (MBRs) and reverse osmosis treatment systems,” said Alec Mackie, marketing manager for JWC Environmental. And that impacts pretreatment systems. “These systems are sensitive to solids and simply must be protected by high-tech and reliable fine screens,” he said. “From hair to rags to plastics, the screens must get the material out or the MBR becomes a maintenance headache.”

According to Mackie, treatment plants are moving to finer inlet screens. “Many of the plant upgrades switch from bar screens with 1-inch openings to perforated plate fine screens with 1/4-inch or 1/10-inch openings,” he said. “We’re even working on smaller openings. We’re also seeing more plants in the 100 to 200 million-gallon-per-day (mgd) range install perf plate fine screens. They want to put a stop to all the rags and trash ending up in sludge.”

JWC’s high-flow fine screens allow treatment plants to capture and remove very small plastics, rags, and trash at the headworks, Mackie said. And the company is also shipping more internally fed Drumscreens — rotating screens that can remove hair and other small debris that can upset MBRs or other high-tech filtering systems.

AIRVAC Vice President Rich Naret noted that a lack of water in other countries and drastic water-saving measures have led to blockages in some aging gravity networks. “This is now starting to occur with aging gravity systems in the United States as well,” he said. “And, it very well could be an issue with new gravity systems as low-flush toilets and other water-saving devices become more common, requiring designers to consider the reduction in transport liquid and its effect on the hydraulics of the system.”

According to Naret, vacuum sewers are not affected by this reduction in transport liquid because they do not rely on gravity and the 2-foot-per-second (fps) scour velocity required to keep solids suspended. Instantaneous velocities within a vacuum sewer are in the 15-fps to 18-fps range, he said, making blockages virtually non-existent.

To help stem water shortages or to compensate for poor quality source water, recycling and reusing treated wastewater is gaining interest in many areas. “This driver is causing some wastewater systems to look at improving their processes to help handle this demand,” said Doug Johnson, director, marketing & business development, Emerson Process Management Power & Water Solutions. But water reuse faces an uphill battle. “While this is generally accepted for use in non-potable applications such as irrigation, there remains a stigma attached to ‘toilet-to-tap’ water reuse among the general population,” Johnson said.

Johnson noted two challenges to water reuse: changing treatment processes to produce water that is safe to drink in a reliable, secure manner; and educating the public on the safety, security, reliability, reduced cost, and improved availability that this new water supply strategy will bring.

Nevertheless, the number of water reclamation projects is increasing, according to Jack S. Cook, vice president, Water and Wastewater Solutions, Bentley Systems, Inc. “We are seeing more integrated water resource planning in the industry that looks to alternative water supply.”

In response, the integrated approach to planning and design is pushing design-build project delivery, Cook said. “Design-build project delivery is on the rapid increase, and supporting technology has to accommodate collaboration, managed content sharing, interoperation, and visualization of heterogeneous engineering information.”

Bentley delivers and supports collaboration servers that act as the engineering content hub serving design, contractor, and owner team workflows. According to Cook, its enterprise configurations are highly interoperable and customizable and can accommodate technology diversity that characterizes many project teams.

“Our technologies have been used successfully to host 4D project innovation laboratories that foster creativity and broaden contributor input into key treatment plant design decisions,” Cook said. “The performance gains that derive from the delta file transfer capabilities of ProjectWise V8i, combined with those in ProjectWise Navigator V8i and upcoming Dynamic Review for Models (currently with early adopters), will further enhance the offering.”

AIRVAC’s Naret echoes the trend toward an integrated approach. “We are beginning to see designers looking at the bigger picture of having an entire project — the collection system as well as the treatment plant — being greener than what they otherwise would have been, he said.

Energy efficiency
A large part of the big picture for wastewater collection and treatment systems is energy. “Rationalizing sustainable design and operations around carbon footprint is more important than ever,” said Bentley’s Cook. “The design teams must consider more design iterations around alternatives that impact energy utilization.

Pumping is an important factor in evaluating the energy use of a wastewater system. “For pump-dominant collection systems, Bentley has strong functionality for costing energy that can help plan operations that lower the carbon footprint,” said Cook. “The use of variable-frequency drive pumping for managing energy is increasingly important.”

AiRVAC is developing a valve monitoring system that will notify the operator in real time of any problem at any given valve pit. The monitoring system will also provide key system-wide data 24/7, allowing the operator to fine tune the system to optimize performance.

Bentley recently made available in SewerGEMS V8i exclusive technology for modeling the operation and performance of variable-speed pumping. This now consolidates analysis for wastewater collection master planning and design into Bentley’s premier application for hydrodynamic analysis of collection systems.

Of course, one way to reduce energy costs is to eliminate pumps. According to Naret, a vacuum sewer system can reduce energy use by replacing multiple lift stations with a single vacuum station.

Emerson Process Management’s PlantWeb automation architecture with the Ovation control system helps unify plant treatment operations with wastewater collection/SCADA systems to give operators and engineers districtwide visibility from a central control room.

But, consumption is not the only variable in the energy equation for wastewater treatment plants. “The cost of energy is having a fascinating effect on treatment plants,” said JWC’s Mackie. “We are seeing more and more facilities generate their own electricity by burning methane from sludge digesters. And why not? This byproduct is valuable and there are several ways to convert the biogas into electricity.”

JWC assists plants generate biogas by keeping the sludge digestion process flowing smoothly. Its screen systems remove plastics and trash with wedge wire or perforated plate panels and its grinders turn large solids into small pieces so sludge pumps and heat exchangers don’t clog.

“Here in California, we’re watching hundreds of plants turn on waste-to-energy systems,” Mackie said. “The Los Angeles County Sanitation Districts was recently ranked by the U.S. Environmental Protection Agency (EPA) as the largest municipal on-site green power producer in the nation. Who knew treatment plants could generate so much clean power? It’s an exciting trend — treatment plants can provide clean water and now clean power too!”

(See the EPA’s list of the top 20 organizations in its Green Power Partnership that consume the most green power on-site: www.epa.gov/greenpower/toplists/top20onsite.htm)

Cost effectiveness
But, energy cost is only part of a wastewater utility’s concern. “In the past, municipalities were willing to accept the trade-off between lower capital costs and higher operation and maintenance (O&M) costs provided the project was cost-effective from a life-cycle standpoint,” said AIRVAC’s Naret. “Now that vacuum sewer technology has been around for several decades and lower capital costs are a given in the proper application, municipalities no longer settle for just saving money upfront. They also want systems with the lowest possible O&M costs by having components that are dependable, reliable, operator friendly, and easily and inexpensively replaced.”

For example, vacuum stations have controls that allow the operator to monitor vacuum mains and equipment within the vacuum station. However, there has not been a practical and affordable method of monitoring each individual vacuum valve, Naret said. AIRVAC is developing a valve monitoring system that will notify the operator in real time of any problem at any given valve pit. The monitoring system will also provide key system-wide data 24/7, allowing the operator to fine tune the system to optimize performance. The net result will be smaller power bills and lower O&M costs.

Systems to optimize performance will be increasingly important as skilled plant operators, engineers, and managers retire, according to Emerson’s Johnson. He also cited a need to better integrate wastewater collection systems and treatment plant operations.

Emerson Process Management developed PlantWeb automation architecture with the Ovation control system to assist in this effort. According to Johnson, the systems provide wastewater operations managers with capabilities to better manage energy and chemical use through advanced automation strategies and protect capital equipment through predictive intelligence. By unifying treatment plant control systems with wastewater collection/SCADA systems in a single system, operators and engineers can achieve district-wide visibility and control from a single system that is flexible enough to adapt to new regulatory and reporting requirements.

However, success in meeting those stricter regulatory requirements will rely in part on the ability of wastewater utilities to afford these new technologies. “We’re very encouraged by recent surveys showing water and sewer rates are gradually rising,” said Mackie. “If local utilities can charge customers the true cost of water, then hopefully the industry can move away from the boom and bust of federal financing.”

Waste-to-energy in action
A gasification system, first approved by the city of Sanford, Fla., in 2008, began operating on May 21, 2009, at Sanford’s South Water Resource Center. The MaxWest system will gasify Sanford’s treated wastewater sludge to provide renewable green thermal energy to replace energy from natural gas for the city’s sludge dryer. The gasification facility is scalable and has sufficient capacity to meet Sanford’s expected growth during coming decades and also to serve as a disposal site for other nearby cities and private waste haulers.

It is expected to save the city millions of dollars in natural gas fuel costs. Sanford’s 20-year contract with MaxWest also provides long-term energy price stability. And, as the system grows, the opportunity to produce renewable green electricity is available.


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