The average operational life of a wastewater treatment (WWT) facility is estimated at somewhere between 20-25 years. And because most of America's WWT facilities date back to the early seventies, municipalities have spent the better part of the past two decades upgrading and, in some cases, replacing aging facilities.
Though these upgrades require a significant investment of time and money, a host of organizations are experiencing increased efficiency, lower operating costs, and ongoing compliance with federal regulations.
How do they do it? Many of these facilities have integrated anaerobic digesters into their operations, giving them a viable and cost effective means of cultivating and capturing valuable methane gas and converting it into energy.
In this article, we will explore how the WWT industry has evolved, where the industry is heading, and how an organization like yours can implement WWT strategies that save you money and even generate revenue.
The first organized move to centralize and treat wastewater dates to 1948, when Congress passed the Federal Water Pollution Control Act, a comprehensive statute aimed at, "restoring and maintaining the chemical, physical and biological integrity of the nation's waters." In 1972, the Act received a comprehensive overhaul and was re-christened the Clean Water Act (CWA). Since it was passed into legislation, the CWA has been amended on a near annual basis, often with particular attention to the treatment, disposal, and management of wastewater.
Today, conventional wastewater treatment relies on the same three stage process that it has used since the early-seventies. For the uninitiated, the process goes a little something like this:
• In the primary treatment stage, incoming wastewater, or influent, is held in quiescent basin where heavy solids can settle to the bottom while oil, grease and lighter solids float to the surface. The settled and floating materials are removed and the remaining liquid may be discharged or subjected to secondary treatment.
• During secondary treatment dissolved and suspended biological matter is removed. Secondary treatment is typically performed by indigenous, water-borne micro-organisms in a managed habitat.
• The final stage is tertiary treatment, which is most often defined as anything beyond primary and secondary treatment. Treated water is sometimes disinfected chemically or physically (for example, by lagoons and microfiltration) prior to discharge into a stream, river, bay, lagoon or wetland, or it can be used for irrigation. If it is sufficiently clean, it can also be used for groundwater recharge or agricultural purposes.
The organic solids that settle out of wastewater are collectively and informally referred to as sludge. In compliance with CWA standards, this sludge is commonly treated until relieved of bacteria and other hazardous contaminants before being incinerated, or deposited in sanitary landfills.
Increasingly, WWT facilities are deviating from common and traditional wastewater treatment and sludge disposal practices by integrating more advanced, sustainable methods, such as activated sludge, and biological digestion processes.
Large-scale activated sludge systems are biological treatment processes in which a mixture of sewage and activated sludge is agitated and aerated. The activated sludge is subsequently separated from the treated sewage by settlement and may be re-used. A notable example of this practice is seen at Milwaukee Metropolitan Sewer District in Milwaukee, Wis., where, since 1926, the city's wastewater sludge has been processed to produce Milorganite brand fertilizer.
Aerobic digestion is the most widely used stabilization process in plants with average flows less than 5 million gallons per day. Its main benefit is that it reduces the mass of sludge, making disposal easier and less costly. The process is driven by micro-organisms that feed on organic materials to stabilize them, and reduce biological oxygen demand and suspended solids in the wastewater.
Anaerobic digestion is a complex biological process in which biodegradable organic matters are broken-down by bacteria into biogas consisting of methane, carbon dioxide, and other trace amount of gases. The methane portion of this biogas can be used to generate heat and electricity.
To date, one of the largest examples of anaerobic digestion at work is California's Point Loma Wastewater Treatment Plant, a facility that serves a 450-square-mile area near San Diego, Calif., and has a capacity of 240 million gallons per day.
The plant uses eight anaerobic digesters that break down the organic solids removed from the wastewater, producing methane gas that is collected, cleaned, and piped to an on-site gas utilization facility. The gas is then used to provide space heating and cooling.
The methane produced by the digesters also fuels two internal-combustion reciprocating engines that run generators with a total capacity of 4.5 megawatts. Heat produced by the operation of the engines is used to heat the digesters for optimum performance in the generation of gas. The generated electricity runs process pumps, lights, and computers. Since 2000, the city of San Diego has saved approximately $3 million annually in operational energy costs.
Following this example, the city of Dubuque, Iowa recently broke ground on its plan to construct a $64 million wastewater treatment facility, graduating from an outdated incineration process to anaerobic digestion. Jonathan Brown, manager of Dubuque's Water Pollution Control Plant estimates that the present facility spends about $400,000 per year on electricity and another $250,000 per year on fuel oil. However, with the integration of anaerobic digesters, Brown anticipates that the plant's energy costs will quickly and steadily diminish to zero.
"This will no longer be a waste product," said Brown. "It is a resource that we will be using to generate heat and electricity, and the production of a usable material either for composting or a soil amendment. It's a pretty exciting concept."
However, Point Loma and Dubuque are something of a rarity in the national mindset of wastewater treatment. As of 2007, there were more than 30,000 WWT facilities in the United States. The EPA estimates only 200 of those facilities were using methods of anaerobic digestion. When put into sustainable context, that's a large amount of untapped energy going to waste.
"In 2005, the Lawrence Berkeley National Laboratory determined that the energy potential in municipal wastewater in the United States is estimated to be as high as 7.20 million kilowatt hours of electricity," said Melvin W. Cook, Founder, CTO of Filtration Dynamics, a recent innovator in the realm of wastewater filtration technology. "This endless supply of energy is greater than the 2007 production of the Hoover Dam and Glen Canyon Dam combined. "
Filtration Dynamics is a California-based start-up that hopes to define the future of wastewater treatment. The company's Resource Recovery Plant Concept is designed to filter municipal wastewater to EPA standards with an inherent synergy that will simultaneously produce and generate a maximum amount of methane and electricity to achieve net electrical energy.
Unlike current anaerobic digestion processes, the company's filtration technology isolates and concentrates organics by separating and filtering them out of the water and putting them into an anaerobic digester to produce methane. The difference is the Resource Recovery Plant Concept is more efficient, more cost-effective, and more sustainable. According to Filtration Dynamics, the Concept will pose a multitude of benefits, including:
• Costs 50% less than the average WWT facility upgrade
• Reduces operation and maintenance costs by 25%
• Requires 80% less space to operate (50' x 50' per MGD)
• Operates indoors and around the clock
• Provides modular scalability for the future
• Eliminates sewage sludge and related costs
• Qualifies for state and federal rebates and carbon and energy credits
"Right now, most technology providers have a different understanding of the needs of the industry," said Cook. "The mindset almost seems limited to sewage sludge. They don't seem to understand the concept of energy."
In its current testing form, the system is best suited to the 15,610 WWT facilities handling less than five million gallons of wastewater per day- the same facilities whose flow rates fall in the EPA's lower economical limit for installation of Combined Heat and Power systems.
The company's Resource Recovery Plant Concept has already generated a great deal of interest. In 2006, Filtration Dynamics was a finalist in the Clean Tech Open, a competitive initiative to find, fund and foster ideas that address today's most urgent energy, environmental, and economic challenges. Most recently, the concept earned the company a place as one of the top vote-getters in the General Electric (GE) Ecomagination Challenge, a competition that invites businesses, entrepreneurs, innovators and students to share their best ideas on how to build the next-generation power grid. The winners of the competition will be announced Nov. 16.
"Right now, we're looking for the right opportunity to prove the concept," said Cook. "We have the information. We know it's viable. And we know that no one has seen what we're offering."
Until innovations like Cook's become a reality, municipalities will still be left to evaluate whether current anaerobic digestion systems are the right choice for their communities. When considering an anaerobic digestion system, the United States Department of Energy recommends that organizations keep in mind these simple rules of thumb:
• A typical WWT facility processes 1 million gallons per day (MGD) of wastewater for every 10,000 in population served.
• Anaerobic digesters are generally used when wastewater flow is greater than 5 MGD.
• For each MGD processed by a plant with anaerobic digesters, the available biogas can generate up to 350 kWh, EPRI.
• The heating value of the gas produced from the anaerobic digesters is nominally 60 percent that of natural gas (1000 Btu per cubic foot), but with maximum digestion and proper cleanup can be increased to as much as 95 percent.
• Project viability is more probable for facilities that are within 15 miles of a wastewater treatment plant.
After considering these guidelines, evaluating your current facility, and exploring the availability of applicable technology, you might find yourself in a position to generate additional revenue, lower your operating cost and decrease your ecological footprint.
For more information about Filtration Dynamics, visit their GE Ecomagination profile.
Interactive Diagram of Wastewater Treatment - "Go with the Flow" - Water Environment Federation