From Organic Waste to Energy Independence

The Greater Lawrence Sanitary District (GLSD) preserves and protects the water quality of the Merrimack River by providing wastewater treatment services to six communities: Lawrence, Methuen, Andover, North Andover and Dracut, Mass­a­chu­setts, and Salem, New Hampshire. GLSD’s wastewater treatment plant (WWTP), located in North Andover, treats an average flow of approx­i­mately 30 million gallons of wastewater per day.

For many years, GLSD has been a leader in water reclamation and beneficial reuse of biosolids. To take advantage of state regulations and economic oppor­tu­ni­ties, GLSD has built an Organics to Energy Project that will allow the WWTP to convert both biosolids and organic food waste from local sources into clean power. At full-scale waste processing rates, this innovative project will allow GLSD to operate off the grid during many operating conditions, and could generate up to $2.5 million in annual power cost savings.

A Strong Partnership and a History of Leadership
GLSD and CDM Smith have been working in partnership for 40 years. After CDM Smith completed early studies to determine the optimum site for a new WWTP along the Merrimack River, GLSD engaged the firm to design and oversee construc­tion of its WWTP, which opened in 1977. Since then, generations of CDM Smith engineers have collab­o­rated with coun­ter­parts at GLSD to build and continually improve one of the nation’s leading water reclamation utilities.

“GLSD was one of the first utilities I worked with in my career,” says CDM Smith vice president Michael Walsh, PE, BCEE. “In the late 1990s and early 2000s, we worked with the District to move from sludge disposal to beneficial use of biosolids. The current Organics to Energy project takes GLSD to the next level of innovation, using biosolids and food waste organics to generate power and move towards net-zero energy use. It’s all part of GLSD’s industry leadership in adopting new tech­nolo­gies to improve the sustain­abil­ity, energy efficiency, and overall performance of plant operations.”

Blending the Best of Public and Private
Between 2000 and 2004, the District constructed a new solids train that includes anaerobic digesters, centrifuges and a thermal drying system that produces a pelletized fertilizer product. This upgrade moved GLSD from sludge incin­er­a­tion and landfill disposal to beneficial use of biosolids, and reduced cost while greatly improving the sustain­abil­ity of GLSD plant operations. 

This project was not only technically innovative, but also pioneered the strategic use of private financing and operations. After careful analysis of all project delivery options, GLSD decided to implement new sludge thickening, digestion and dewatering facilities under a traditional design/bid/build format, while implementing the thermal drying portion of the project under a design-build-operate (DBO) contract arrangement.

Under this approach, the DBO contractor has been responsible for operation of the thermal drying process and for marketing and distribution of the biosolids product. This private-public partnership (P3 approach) allows GLSD to own the entire facility and direct the aspects of operations that are core to its identity, while drawing upon private-sector risk-management skills for certain specialized elements of the operation. 

Seiz­ing Op­por­tu­ni­ties and Set­ting Prece­dents
This di­ges­tion sys­tem con­structed as part of the early-2000s solids train up­grade is now being ex­panded to allow for the ac­cep­tance of food waste or­gan­ics under the Or­gan­ics to En­ergy Pro­ject.

In 2014, Mass­a­chu­setts banned the land­fill­ing of com­mer­cial food waste. Or­ganic waste has roughly four times the bio­gas po­ten­tial of biosolids, and state agen­cies were eager to find util­i­ties will­ing to in­vest in tak­ing ad­van­tage of that po­ten­tial. GLSD’s pro­posal for a com­bined heat and power (CHP) sys­tem that would use bio­gas to gen­er­ate elec­tri­cal and ther­mal en­ergy gen­er­ated wide­spread sup­port. The Mass­a­chu­setts De­part­ment of En­vi­ron­men­tal Pro­tec­tion, the Mass­a­chu­setts De­part­ment of En­ergy Re­sources, the Clean En­ergy Cen­ter, and the power util­ity, Na­tional Grid, are each pro­vid­ing grants to sup­port im­ple­men­ta­tion of this in­no­v­a­tive pro­ject.

CDM Smith de­signed state-of-the-art bio­gas clean­ing sys­tems up­stream of the en­gines, com­bined with Se­lec­tive Cat­alytic Re­duc­tion (SCR) tech­nol­ogy on the ex­haust sys­tem down­stream of the CHP en­gines, which to­gether will re­sult in sys­tem emis­sions that com­ply with strin­gent state and fed­eral air qual­ity stan­dards. This tech­ni­cal im­ple­men­ta­tion is the first in the state. The pro­ject also in­volves de­sign and in­stal­la­tion of an in­no­v­a­tive or­ganic waste re­ceiv­ing and feed sys­tem ca­pa­ble of stor­ing over 200,000 gal­lons of high solids food waste slurry. The added ca­pac­ity of the di­ges­tion sys­tem and the co-di­ges­tion of biosolids and or­gan­ics food waste is ex­pected to gen­er­ate a nearly 250 per­cent in­crease in bio­gas pro­duc­tion, as com­pared to cur­rent lev­els.

The ad­di­tional bio­gas will be used to fuel two 1.6 MW co­gen­er­a­tion en­gines that, under full de­sign op­er­at­ing con­di­tions, will meet the power needs of the en­tire GLSD fa­cil­ity. Ad­di­tion­ally, dur­ing pe­ri­ods when power gen­er­a­tion ex­ceeds plant de­mand, ex­cess power can be re­turned to the grid under a “net me­ter­ing” con­tract that was ne­go­ti­ated with the util­ity, Na­tional Grid. As part of this agree­ment, ex­cess power re­turned to the grid will be used to off­set power costs at the Dis­trict’s in­flu­ent pump sta­tion, which is lo­cated at a sep­a­rate site. CDM Smith pro­vided a ro­bust elec­tri­cal de­sign to pro­tect the elec­tri­cal grid from over­load­ing or oth­er­wise in­ter­fer­ing with op­er­a­tion of the CHP sys­tem. When com­plete, the Or­gan­ics to En­ergy Pro­ject will be ca­pa­ble of gen­er­at­ing over 3 MW of clean, re­new­able power and pro­vide 10 MMBTU/hr of re­cov­ered heat from the co­gen­er­a­tion en­gines.

“GLSD is fo­cused on in­no­va­tion and is al­ways think­ing about what the next de­vel­op­ment will be, what the next op­por­tu­nity will be,” says pro­ject man­ager Ben­jamin Mosher, PE, BCEE. “They’re out in front, find­ing things that make both en­vi­ron­men­tal and eco­nomic sense.”