Note: One lesson from the later part of this post is plan on a hybrid of natural gas and digester gas to run the fuel cell. Don’t think you will run only on the sewage produced methane. And the manufacturer, fuel cell says this too.

In many applications digester gas production volume is variable. In such applications, the plant can be designed to operate with automatic blending with natural gas. Over 30% of FuelCell Energy's Direct FuelCell^® (DFC^®) installations operate on renewable biogas.

economist.com has article on sewage (waste) treatment plants as a source of energy.

Renewable energy

The seat of power

Dec 30th 2009
From The Economist print edition

Better sewage treatment is the latest thing in clean energy

Illustration by David Simonds

WHERE there’s muck, there’s brass—or so the old saying has it. The cynical may suggest this refers to the question of who gets what, but thoughtful readers may be forgiven for wondering, while they are recovering from the excesses of Christmas in the smallest room in the house, what exactly happens when they flush the toilet.

The answer is encouraging. Less and less waste, these days, is actually allowed to go to waste. Instead, it is used to generate biogas, a methane-rich mixture that can be employed for heating and for the generation of electricity. Moreover, in an age concerned with the efficient use of energy, technological improvements are squeezing human fecal matter to release every last drop of the stuff. Making biogas means doing artificially to faeces what would happen to them naturally if they were simply dumped into the environment or allowed to degrade in the open air at a traditional sewage farm—namely, arranging for them to be chewed up by bacteria. Capturing the resulting methane has a double benefit. As well as yielding energy, it also prevents what is a potent greenhouse gas from being released into the atmosphere.

MSNBC.com had an article in 2004 on fuel cells and methane for power generation.

Poop power? Sewage turned into electricity

Fuel cells and waste sludge mix to power treatment plant

Miguel Llanos

Reporter

msnbc.com

updated 9:31 a.m. PT, Mon., July 19, 2004

RENTON, Wash. - It's not as neat as spinning straw into gold, but what Greg Bush gets to do in the world of sewage treatment is pretty magical: making electricity from what's flushed down the sewer. And he does it using fuel cells, technology that's cleaner and more efficient than traditional power generation.

…

How it works
The largest project of its type in the world, the process goes like this: Biodegradable solid waste is sent to large tanks, called digesters, that provide a home for three to four weeks. There bacteria eat away at the waste, releasing methane gas and further reducing the amount of solid waste.

James Cheng / MSNBC.com

Four large digester tanks sit behind the fuel cell power plant at the wastewater treatment plant in Renton, Wash.

The fuel cells mentioned by MSBBC in King County are no longer in use. But, here is the executive summary of the results, published in Apr 2009.

Final Report, King County Fuel Cell Demonstration Project

Issued April 2009

Executive Summary

Increasing energy costs, more stringent air emission regulations, and an interest in exploring emerging energy technologies prompted King County, Washington, to search for new and innovative ways to provide electricity for its wastewater treatment plants. In June 2004, the county began a two-year demonstration of a fuel cell power plant to be fueled by gas produced through anaerobic digestion of solids produced at its South Treatment Plant. The project was the first application in the country to use digester gas to fuel a molten carbonate fuel cell.

…

King County’s fuel cell power plant was sized to produce 1 megawatt (MW) of electricity and was designed to capture waste heat from the fuel cell exhaust and return it to the treatment plant. Two project goals were established:

Demonstrate that the molten carbonate fuel cell technology can be adapted to use anaerobic digester gas as a fuel source.

Achieve a nominal plant power output target of 1 MW using either digester gas or natural gas.

Both goals were achieved during the two‑year demonstration period. A number of secondary objectives (performance goals) were also met.

The final pdf is here.