October 22, 2004 | General

Wastewater Treatment Plant Builds Profit Center From Anaerobic Digestion

BioCycle October 2004, Vol. 45, No. 10, p. 31
Through creative management of digesters, Boston’s Deer Island Treatment Plant leverages biosolids programs to achieve renewable energy goals.
Kristen Patneaude and Jeff Reade

THE DEER ISLAND Treatment Plant (DITP) of the Massachusetts Water Resources Authority (MWRA) is the centerpiece of the $3.8 billion Boston Harbor Project, one of the largest public works projects in recent history. DITP services 2.5 million people, has a 1.27 billion gallon per day capacity and is the second largest wastewater treatment plant in the country. Like most modern treatment plants, DITP uses both primary and secondary wastewater treatment processes to treat sewage.
During primary treatment, heavy solids fall to the bottom of large wastewater clarifiers (settling tanks), forming primary sludge. In secondary treatment, oxygen is injected into the wastewater to promote the growth of naturally occurring microorganisms. The microorganisms feed on the organic matter in the wastewater, creating secondary sludge. The secondary sludge settles out at the end of the process, leaving clear liquid. The treated liquid is disinfected, dechlorinated, then discharged 9.5 miles out into the waters of Massachusetts Bay, where it mixes with ocean currents. Meanwhile, primary and secondary sludges are pumped into the plant’s egg-shaped digesters for further biological treatment.
Sludge digestion takes place in one of the first installations of egg-shaped digesters in the U.S. Each of the 12 digesters is 15 stories tall and holds three million gallons. Anaerobic digestion encourages growth of microorganisms, which break down the raw sludge into methane gas, carbon dioxide, solid organic by-products, and water.
The methane gas produced in the digesters is used in the plant’s on-site power generating facility, while the digested sludge (about 95 dry tons/day are generated) is thickened and barged to MWRA’s Biosolids Processing Facility in Quincy, Massachusetts, where it is dewatered, dried, and further processed into EPA Class A fertilizer pellets (see sidebar).
DITP operates 24 hours a day, seven days a week throughout the year, and has an annual operating budget of $41 million. Utilities, including electricity, fuel oil and water, are the second largest expense at DITP after wages, and account for 27 percent of the operating budget. Electricity is the largest utility consumed, with demand of 162,000,000 kWh/yr (18.5 MW). DITP is able to significantly reduce the cost impact of this electrical demand, however, through use of self-generation and recovery systems.
The primary energy source for DITP’s self- generation system is digester gas (DiGas). Containing up to 70 percent methane by volume, over 160,000 cubic ft per hour is generated, which has a heat value equivalent to 6.5 million gallons of fuel oil per year. When burned in on-site boilers, this is enough to satisfy DITP’s process and facility heat demands for all but the coldest months of the year. DITP’s boilers produce a high pressure/temperature steam, that is sent through a steam turbine generator (STG) that is part of a cogeneration cycle. The STG converts energy from the high-pressure steam to electricity and discharges lower pressure steam. The low pressure steam is then used in heat exchangers to produce hot water for the plant’s process and heating loads. The average output of the STG utilizing digester gas only is approximately 3 MW.
In addition to the STG, DITP has two 1 MW hydroturbine generators at the inlet shaft of the effluent outfall tunnel. A 9.5-mile 24-foot-diameter outfall tunnel transports effluent into the 100-foot deep waters of Massachusetts Bay. The generators produce electricity by capturing the potential energy of the wastewater effluent flow as it drops into the tunnel. Output of the hydro- turbines is effluent flow dependent, but averages approximately 0.7 MW.
The use of on-site generation results in significant avoided cost savings for MWRA. Using digester gas in the boilers satisfies plant heat demand for all but the four coldest months of the year and results in avoided fuel oil purchases of $3.9 million annually. Combined, the DiGas derived STG output and the hydroturbines offset DITP’s electricity demand by 20 percent, or $2.4 million annually.
The Massachusetts Renewable Portfolio Standard (RPS) was mandated as part of the electricity restructuring legislation of 1997 and is administered by the state’s Division of Energy Resources (DOER). The program is designed to encourage development of green power, as defined by DOER, by requiring that a certain percentage of power sold by retail suppliers in Massachusetts be derived from renewable resources. Producers of renewable power are issued one certificate for each megawatt hour (MWh) of power produced from a renewable (green) source. Suppliers that are unable to meet the minimum requirement of renewable power from their own sources, may purchase certificates from generators that have certificates in excess of their own requirements.
MWRA is eligible for participation in the RPS program for the DiGas derived portion of the STG output. Since MWRA is not a supplier of electricity, DITP is able to sell all of its RPS certificates to suppliers looking to fulfill their RPS requirements. DITP generates approximately 6,000 MWh (6,000 certificates) each quarter. The value of RPS certificates continues to increase, as the percentage requirement increases and the demand for renewable energy increases. Certificates generated in 2003 and 2004 have been trading between $30 to $48/MWh. MWRA has received over $1.6 million in nonrate revenue from the sale of its RPS certificates since December 2002.
Markets for renewable energy certificates are established or being developed in many parts of the country. Although certificate trading is formally recognized only in a few states; other states are moving toward similar programs.
Created in 1997, ISO New England is the nonprofit corporation responsible for the day-to-day reliable operation of New England’s bulk power generation and transmission system, and management of a comprehensive regional bulk power system planning process. One of several programs that ISO-NE administers during peak usage periods is the Price Response Program, a voluntary “load shedding” program that goes into effect on a day when regional market prices are expected to reach or exceed $0.10 per kWh. Participants are called on a voluntary basis to reduce their electricity load and/or self-generate. Participants are then compensated by the ISO-NE for the amount of load the participant reduced during the event.
In addition to the STG, DITP also maintains two combustion turbine generators (CTGs), each rated at 26 MW. These are oil fueled and are intended to provide back-up power to the process in the event of a regional power failure. When called upon by ISO-NE, DITP supplies its own electricity by running the CTGs and reduces to zero the electricity taken from the regional power grid. The electricity on the grid that was no longer needed to meet DITP load is diverted to other electricity customers, thus helping to meet regional demand and reduce power prices. During the last fiscal year (July-June), DITP received over $350,000 in non-rate revenue for participating in ISO-NE price events over the course of 250 hours.
DITP has also made modifications to various plant process operations to realize energy savings. These demand side management projects include reducing the on-line number of secondary reactor basins, placing final stage secondary mixers and aerators on timed cycles, reducing the number of cryogenic oxygen trains for certain months throughout the year, enhanced automation at the influent pump stations to reduce the lift of head required to pump wastewater to DITP, optimization of digester gas handling system, odor control process modifications (replacing use of wet scrubbers with carbon adsorption scrubbers), and installation of strainers on the final effluent line to allow use of final effluent in process operations. These projects have resulted in estimated combined energy and water savings approaching $2.9 million annually.
DITP has been purchasing electricity from competitive suppliers since November 2001. DITP’s electricity load is attractive to suppliers because it is relatively constant throughout the day and throughout the year. There is relatively little variation over the course of a typical 24-hour period unlike an office building that experiences higher demand during business hours. DITP’s peak power demand is driven by sewerage flows which are greatest following wet weather events, which typically do not coincide with hot weather, when New England experiences its peak power demand.
The DITP electricity supply contract structure divides the load into fixed and variable components. There is a fixed price for a 10 MW base block of power; the remainder of DITP’s load is purchased on the spot market. This structure provides MWRA with some budget certainty while also providing an opportunity to hedge the market with minimal risk. If spot market prices rise to high enough levels, DITP is able to reduce its energy consumption on short notice by running the CTGs, providing a valuable hedge during times when the supply of electricity is scarce and market prices are high. During the past 12 months, DITP saved over $2 million by buying electricity from a competitive power supplier as compared to the price of default service from the local utility.
Although there are few wastewater treatment and resource recovery facilities on the scale of DITP, there are opportunities among the vast variety of programs and operational strategies available to reward even the smallest of facilities, if they are willing to take the initiative to explore them.
Kristen Patneaude is a Project Manager at the Deer Island Treatment Plant in Boston, Massachusetts. Jeff Reade is the Deputy Director of Operations.
THE Massachusetts Water Resources Authority’s (MWRA) Deer Island Treatment Plant generates about 95 dry tons/day of biosolids. That material is transported to MWRA’s Biosolids Processing Facility (BSF) in Quincy, where it is dewatered and then processed in rotating, high-temperature dryers within the plant. The completion in 1991 of the BSF marked the end of sludge discharges into Boston Harbor. For the first time in the region’s history, the area’s two sewage treatment plants had a place other than the ocean to put the organic solids they remove from wastewater.
The drying process yields a small, hard granule that is approximately 60 percent organic matter. The pellets contain nutrients such as slow-release nitrogen, phosphorous, calcium, sulfur and iron. Most of the pellets are marketed in bulk by New England Fertilizer Company for large-scale agricultural use. A small portion is packaged and distributed as Bay State Fertilizer. The product also is purchased wholesale by golf courses and landscapers throughout New England and has been available locally through garden centers and nurseries since 1995. Many communities within the MWRA sewerage district use the fertilizer on their parks, athletic fields, and municipal landscaping.

Sign up