By George Hawkins
Situated at the southernmost tip of Washington, D.C., on the Potomac River, the District of Columbia Water and Sewer Authority's (DC Water) Blue Plains plant is the world's largest tertiary treatment facility. Within three years the 370-mgd facility will be renowned for another reason as well — as the first operation in North America to use thermal hydrolysis to pretreat sludge for anaerobic digestion and the largest such plant in the world.
The move dovetails with an agencywide energy audit that was completed in December (see sidebar, page 44) as part of DC Water's overall effort to voluntarily reduce its carbon footprint.
Built in 1938 and expanded and modified several times over the years, Blue Plains now serves 1.1 million residential and commercial customers on a retail basis and processes wastewater on a wholesale basis for suburban jurisdictions in Maryland and Virginia.
Over the last decade, we've invested almost $1 billion improving processes to meet nitrogen and phosphorous mandates related to Chesapeake Bay water quality improvements, as well as improving plant automation to streamline operation. The advanced degree of wastewater treatment we have at Blue Plains requires significant power usage, about 32 to 35 megawatts (MW), making Blue Plains the largest power consumer within the District.
ONLY 300 PROJECTS TO GO
Audit identifies hundreds of money-saving opportunities.
After establishing a sustainable energy policy in 2008, the District of Columbia Water and Sewer Authority (DC Water) retained engineering firm MWH to evaluate consumption operationwide and lay the groundwork for implementing conservation measures.
For one year, from December 2009 through December 2010, MWH consultants interviewed utility employees at all levels and conducted prioritization workshops with key staff members. They collected and analyzed operational data on pumps; electric, heating and air conditioning, and lighting systems; and water systems, including items like overall treated volume and opportunities to reduce water consumption. They evaluated water use by the authority. They considered renewable energy options.
The firm identified 250 projects — upgrading to more efficient lighting, HVAC improvements, building envelope improvements, and resetting or installing automated controls — managers can implement immediately to save a total of more than $500,000 annually. DC Water would recover the costs of these low and no-cost measures over a period of 12 to 18 months based on the total amount saved.
For example, the utility uses more than 1 megawatt of electricity annually to illuminate facilities. Converting to LED fixtures and installing motion sensors could save $100,000.
The firm identified 40 more capital-intensive projects that could save more than $3 million annually — the top-tier projects will save an annual $2 million. If implemented, the utility would recover the costs invested to achieve these improvements over a period of 12 to 18 months. They include:
— Ernest Jolly (email@example.com) is energy manager of DC Water; Robyn McGuckin (firstname.lastname@example.org) is director of resource efficiency management for MWH; Bert Wellens (email@example.com) is technical manager and energy engineer for MWH; and Richard Atoulikian (firstname.lastname@example.org) is vice president and project manager for MWH.
For many years the plant has processed its sludge with lime to produce a Class B fertilizer material that's given to farmers, foresters, and mine reclamation projects. Rather than destroying solids, the lime stabilization process increases volume because calcium oxide is added to the sludge to stabilize it. About 65 truckloads of biosolids leave the plant every day. Although this program has been a major success over the years, the large biosolids production volume must be trucked to fields and locations at long distances (often more than 100 miles away), resulting in high operating costs and high truck fuel usage, which elevates carbon emissions associated with plant operation.
DC Water recognized the need for a better biosolids processing system nearly a decade ago. After evaluating a large variety of options at that time, the authority selected anaerobic digestion because it is well proven, destroys half of the sludge, and creates a major energy product (methane gas) that has great value. In developing our plans further in recent years, we recognized that we needed to produce a biosolids product that was Class A (pathogen-free) and that had maximum flexibility for multiple reuse markets. And, we needed a digestion process that could fit within our very small available footprint. We had tracked the development of the thermal hydrolysis process for many years, but in 2006 we began a more concerted effort to confirm the process could be of major value for DC Water.
Saving money while making money
Space is an ever-increasing challenge at Blue Plains. We have a 6-acre plot within the plant that can be used to accommodate new biosolids processing, but the rest of the 153-acre facility is already spoken for with projects that include enhanced nitrogen removal and a long-term control plan to reduce combined sewer overflows.
Because thermal hydrolysis of sludge is a new process that has not yet been implemented in North America, DC Water conducted thorough evaluations that included several years of pilot testing with our material. We also investigated many of the 20 operating plants overseas, and confirmed successful operation with process operators and owners. By 2009, we completed our due-diligence and assessment work and were ready to proceed with detailed planning, design, and implementation. We found that the thermal hydrolysis process developed by Cambi has by far the most experience, especially with large plants such as Blue Plains.
Thermal hydrolysis greatly improves the efficiency of anaerobic digestion by feeding digesters at much thicker solids concentrations, thus cutting the tankage requirements in half. This is a huge benefit for Blue Plains, which has such limited space. The process also produces exceptional quality Class A material that has low odor levels largely because of a higher degree of stabilization. Also, with thermal hydrolysis, digestion produces more methane gas, thus allowing even more power to be produced.
Thermal hydrolysis uses heat (320° F) and pressure of about 100 psi to kill bacteria and other organisms, including pathogens. This makes the solids more readily degradable within anaerobic digestion. The thermal hydrolysis “cooking” process takes place in vessels where the temperature and pressure are raised by adding steam. The temperature of the sludge is then reduced to that required for stable anaerobic digestion (about 100° F).
Next, the methane gas produced during digestion will be burned in combustion gas turbines that are connected to power generators. The hot turbine exhaust gas is used to produce medium-pressure steam (about 175 psi) that is used in the thermal hydrolysis cooking process.
Using thermal hydrolysis and anaerobic digestion for biosolids will generate approximately 13 MW of electricity, saving about $10 million/year on plant energy costs. The process also offers a reliable source of backup power for critical plant processes during commercial power outages.
Another major benefit of the thermal hydrolysis pretreatment process is that final dewatering of the biosol-ids produces a high-solids-content cake material (about 30% solids). Additionally, the dewatering can be accomplished with low-energy machines such as belt filter presses. With the high organic destruction of solids during digestion, coupled with the improved dewatering, the daily volume of final biosolids product will be cut dramatically at Blue Plains, saving another $10 million/year in trucking and related costs for handling the biosolids.
Starting in 2014, we will realize these savings from trucking less product, as well as savings from reduced plant power purchases.
Program implementation will involve several construction projects from 2011 to 2014. The biggest project is a design-build procurement for the Main Process Train, which includes the thermal hydrolysis and digestion facilities. An extensive system of performance guarantees and start-up activities will help ensure smooth commissioning of the new facilities and allow transition from the existing Class B process to the new approach.
—Hawkins (george.hawkins@dcwater. com) is general manager of the District of Columbia Water and Sewer Authority (DC Water) in Washington, D.C.