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Case Study: 1 MW to 3 MW Anaerobic Digester Upgrade in Only 6 Months!

By John D. Forcier, P.E.

This is the second in the 8-part series, State of the Advanced Bioeconomy 2018, prepared by experts at Lee Enterprises Consulting (LEC) who will be speaking at the ABLC 2018 conference in Washington DC, February 28 to March 2.


Due to the project location in central Maine, there is a limited construction period of May 15 through November 15. In order to increase the biogas production and the energy output from 1MW to 3MW, we needed to increase the off-site organics supply from 13,000 tons/yr. (35 tons/day) to 45,000 tons/yr. (125 tons/day). This would be a significant challenge with limited locally available feedstocks at that scale. Getting revised permits would also provide some extra challenges. These revised permits included revised solid waste and air permits, updating the farm’s nutrient management plan with additional off-farm feedstocks and the upgrade to the electrical interconnect.


We had the opportunity to prepare an accelerated CPM project schedule to pre-order critical long-lead items and to sequence the construction tasks utilizing a multiple contract approach. Besides increasing the electrical output from 1MW to 3MW, we increased the thermal output from 3.7MM btu/hr. to 10MM btu/hr. of hot water for process heat and heating of several nearby structures. We also added more organic by-products including increasing animal bedding by 60% and liquid organic fertilizer by 90%, saving $120,000/year more. While designing the new AD tank for the needed detention time, it was decided to utilize the economy of scale to oversize the AD tank size to provide 1,400,000 gallons of desired additional organic fertilizer storage at a minimal cost.


When the initial AD system was built in 2011, it included 2@ 26,000 gallon receiving tanks; 2@400,000 gallon AD tanks, 1MW CHP unit and a solid separator to make animal bedding. In 2015/2016, upgrades were made including adding a Scott Turbo Separator (for separation of Source-Separated Organics), a second 1MW CHP (as a spare) and 2@20,000 gallon receiving tanks. The 2017 upgrade included adding a 2,200,000 gallon AD tank, new Control & Gas Buildings, converting one existing AD tank to a hydrolyzer, adding the 3rd 1MW CHP and upgrading the electrical interconnect.

Accelerated CPM Project Schedule

In order accomplish a major AD upgrade in a short construction window, an accelerated CPM project schedule with a multiple contract approach was prepared utilizing Microsoft Project software. Crucial long-lead items were identified and were pre-ordered. These included the third CHP unit, the dual biogas membrane, the AD mixers and the electrical interconnect upgrade components. Preliminary designs were prepared for permitting and for Grant applications. Much of the process equipment and process piping was ordered directly. Early bid packages were issued including initial site work, AD concrete work, insulation and siding work. As the final design progressed, other bid packages were issued for the mechanical/process work, the electrical work and the remaining concrete work and site work. Then the remaining process piping and equipment were also ordered.

Pre-Establish New AD Biological Process

The most critical schedule items involved getting the new AD process biology established. Therefore, it was essential to complete the new AD tank construction including the concrete work, the embedded heating system, the dual biogas membrane and the new AD mixers as soon as possible. Since the new process hot water systems and piping had not been completed yet, a temporary hot water boiler was installed and connected to the new AD tank heat piping. Once the AD tank was constructed, sealed and heated, then the initial filling of the AD tank began, less than 3 1/2 months from the start of construction. As the tank level reached the mixer level and the tank temperature reached the intended temperature, more manure and food waste feedstocks were added to ramp up the biogas production.

Saving Time With The Prefabrication of Other Process Upgrades

The main product of the AD system is biogas, which is used to power the CHP units for power to the grid and for local power. Previously, the biogas was collected from the AD tanks and was piped to each of the CHP’s, where it was dewatered, chilled and compressed. For more efficiency and control, it was decided to construct a new Gas Building near the AD tanks and perform those process functions for preparing and distributing the biogas in that building. In order to save time, the Gas Building was prefabricated off-site by the CHP supplier. We also needed a new Process Building to house process pumps, the hot water heating systems, the motor control center, VFD’s and the PLC and SCADA system. Again, in order to save time, it was decided to have the Process Building prefabricated off-site by the CHP supplier. Meanwhile, the construction crew could be concentrating on finishing the yard piping, site electrical and other concrete and site work.

Final Start-Up

While the biogas production continued ramping up, the third 1MW CHP unit and the electrical interconnect upgrade components were delivered and installed. Finally, on November 15, 2017, the upgrade was fully connected to the grid with 1,106 scfm of biogas @ 64% methane producing 3.016 MWe and 10 MMbtu/hr. of thermal energy. This culminated an aggressive schedule of not only constructing the AD upgrade, but achieving full 3MW production in only 6 months!

About the Author: John Forcier, President of Forcier Consulting Engineers, is a member of Lee Enterprises Consulting, the world’s premier bioeconomy consulting group, with more than 100 consultants and experts worldwide who collaborate on interdisciplinary projects, including the types discussed in this article. The opinions expressed herein are those of the author and do not necessarily express the views of LEC.