Americans produce a lot of waste. Some of it gets thrown in the kitchen garbage can or hauled to the dump, while some of it gets flushed down the toilet or sent down the garbage disposal.
Like the stuff you throw in your garbage can, a lot of the solids in our wastewater stream eventually end up in the landfill where they emit greenhouse gasses and contribute to climate change.
However, in Grand Rapids, the city’s wastewater treatment plant takes the organic matter that comes through its sewage system and turns it into something useful: energy.
Essentially, they're turning poop into power.
The Rapid, the transit system that runs buses around Grand Rapids and its suburbs, has started converting buses over to engines that could run on compressed natural gas.
Steve Schipper, chief operating officer for the Rapid, said the move away from diesel was both environmentally and financially motivated. Natural gas produces fewer emissions and is less susceptible to huge price swings.
The Rapid has been driving on natural gas purchased from DTE for a while now. Now, some of that natural gas will be coming from the city’s sewage.
How does this work?
The Water Resource Recovery Facility is where everything that gets flushed down a Grand Rapids toilet or garbage disposal ends up.
Jared Grabinski, assistant environmental services manager at the facility, has played a big role in bringing the city’s $85 million biodigester online. A biodigester is a system that turns waste into a form of compressed natural gas.
Before the solid waste reaches the biodigesters, there is a series of processes to separate the waste they want from the waste they don’t.
“This is where the first part of the physical process of the separation for the wastewater comes,” Grabinski said, referring to the rotating screens that catch larger items that don't belong. “We'll strain out anything bigger than a quarter inch to our bar screens, and we will slow the velocity down enough to where sand, grit, shells, dirt, rocks will fall out of the process.”
Then, the facility pumps all the wastewater that made it through those first screens into deep cement tanks outside. They then slow the velocity of the water coming through to about one foot per second.
“These passes are for grit removal. That's where we slow the velocity enough for that heavier stuff to just drop out of suspension,” said Grabinski. “Heavier stuff, grit, sand, seashells, rocks, anything that's big enough or small enough to get through the screens, but heavy enough to settle out.”
The rest of the water goes on to the next phase of treatment where they slow the water down again. Slow-moving bars skim any oils or scum off the surface.
“At this point, the wastewater will go to the actual meat and potatoes of the treatment process, the biological treatment,” said Grabinski.
Once they have separated all of the grit and grease, there's a bunch of organic material mixed in with the water. That’s what they call “primary effluent.” They mix it with what’s called “return-activated sludge” to create “mixed liquor.”
“So the food and the bugs are mixed together and then they begin that treatment process all over again,” said Grabinski.
The staff ensures that the right balance of microorganisms and waste for the biodigester. They add oxygen to keep the bugs growing.
“That bubbling action provides mixing and contact between the food and the bugs, and it also gives the biology air to respirate… to make more of themselves,” Grabinski said. “The cells will keep replicating as the use of those pollutants in the water.”
The water that’s going to get sent back to the Grand River gets separated out and then sanitized with UV light.
“And over here is our final settling tank. So this is where the bugs, now that they're happy, they're full,” Grabinski said. “They produce a zoogleal slime and they stick together.”
Now that the bugs are in their flocks eating the waste and there's clean water heading to the river, it can finally go to the biodigesters.
The Biodigesters
Biodigesters are three gigantic cylinders on the property that hold 1.4 million gallons each.
Much like your stomach, the inside of the biodigester is a warm, humid environment where microorganisms digest the things that come in. As those bugs eat away at the organic material, they produce gas. A big component of that biogas they’re creating are methane gas and carbon dioxide.
“You can look down to the digester. Basically, it's just swirling black bubbles… The bubbles is the gas coming out of the solution and the mix pumps just mixing it and turning it around” said Grabinski.
Instead of this waste being sent to a landfill and sending all the biogas into the atmosphere, the biodigester is capturing it. Then it goes through a series of processes that will turn it from this mix of methane, carbon dioxide, and other substances to just pure methane, which is what the city can then sell to DTE.
How the methane gets filtered out from the biogas mix
“We run it through a triple membrane system,” said Russ Lewis, an operations supervisor at the facility. “It has to go through a condensation removal and it goes through a siloxane VOC removal cylinders to remove any of those. And then it goes through the triple membrane system that removes the CO2 and the gas that comes through with the CO2. We run it through another section and it scavenges what little bit we can of that off through another set of membranes.”
Screens monitor every step of both the wastewater treatment and the biodigester systems. They track how fast the water is flowing, report the temperature and pressure readings inside of the biodigesters, and monitor the pump stations they have scattered throughout the city.
Once that gas is all cleaned up and chemically identical to the compressed natural gas that’s coming from other sources, it gets fed into a DTE pipeline.
This is what can be used to power the buses.
Uriah Thompson, a Grand Rapids resident of 46 years, has been using the bus for his daily commute and had never heard about the biodigester helping fuel the Rapid buses.
“I know the Earth is being polluted every day. Every day. So I'm fine with the natural way,” said Thompson.
