For the past several years, some landfills are experiencing a new phenomenon: black solids that are making their way onto these sites and into leachate and gas collection systems, clogging equipment and impairing management and extraction of leachate and gas.  Referred to as “black goo,” no one is positive of where it comes from or even exactly what it is.

But scientists studying these gummy solids say it seems to be a polymer-like material that tends to stick together. It’s showing up on wetter landfills and is reported to be especially problematic where operators accept wastewater treatment plants’ sludge; though with multiple changes in the waste stream, there’s a lot more to learn.

It’s become an issue for specialty contractors doing leachate collection system cleanouts and especially for some landfill operators; they grapple with black goo and its effects as part of their operations.

With funding from the Environmental Research & Education Foundation, Craig Benson, a landfill engineer at The University of Wisconsin-Madison, is trying to definitively determine what these sticky, black solids are and where they come from. In consecutive study phases he and his team will work to figure out how it moves through waste and into sumps and pumps; determine how to best flush it out; and ultimately how to prevent problems from happening in the first place.

Black goo was first observed at wet landfills that have conventional leachate collection systems but that also have dual-phase extraction systems to dewater waste that include pumps for liquid and a vacuum to pull gas.  

“It was getting into pumps and jamming them, and we had to rebuild them regularly. We started to ask, what is this stuff? Where is it coming from?” Benson says.

How it’s moving through waste will be an important question for the Wisconsin University team to answer because landfill experts can alter pathways through which it moves or processes that affect mobility once they understand this. Benson will look specifically at how mobile it is in the pore space in waste and what makes it more or less mobile.

“For example, is it temperature? We know polymers that bind water molecules, like those in baby diapers, vary with temperature. Does it have to do with the chemistry of leachate, specifically is it characteristic of younger or older leachate that has different chemical makeup?” Benson says.

But the first step is to identify the material’s basic molecular makeup, leveraging tools to study different elements in the goo to understand its content.

While the team thinks it’s a polymer, they are working to confirm this, or to know whatever else it could be made of, counting on this information for clues to where it comes from.

The level of gooiness is different between facilities. At some landfills it begins to harden like concrete. In others it stays gooey and pliable. But in all cases, it attaches to pumps and check valves and settles in sumps. It settles anywhere water stalls, notes Nelson Breeden, region engineer at Waste Connections, based out of Woodlands, TX.

“If it keeps moving it does not seem to be a problem. We try to move it by running leachate transfer pumps to recirculate water through pipes, but there’s only so much you can do.

Hydraulic jetting works to move it through pipes or wherever it builds up. But it does not permanently fix the problem,” he says.

Progressive Environmental Services has a solution that has worked at one of Waste Connections’ facilities. But due to the product’s acidity, it’s not for other sites where the company installed protective covers that contain limestone because it breaks down the limestone.

“I think there are a lot of different potential causes but, based on the limited chemical tests we’ve done, it seems to be all over the place.

The way it behaves and where we have most problems seems to point often to polymers that wastewater treatment plants use to make sludge dryer. If you are running it through a pump, it does not have time to coagulate and bring particles together. So, if polymers are present in stagnant water, particles would join, then behave like goo,” Breeden says.

Greg Werner, a principal at CEC, confers that dewatering liquids are likely the most common path whereby black goo makes it to landfill.

It’s captured through the dewatering process and gets mixed in with other leachate. This is when CEC’s industrial wastewater group first sees it, so they are brought in on the receiving end when they treat leachate before it’s discharged to a water body or a publicly owned wastewater treatment plant.

“When we have challenges in treatment plants, we have to make adjustments. Do we know for sure if black goo is the culprit? No.  If it’s already mixed in once it gets to the landfill’s treatment plant, we do not physically see it. It’s masked by leachate,” he says.

It can impact the process of creating sludge and the performance of treatment equipment.

“We are trying to treat compounds in leachate, but when goo is mixed in, conventional methods that may have worked for years are not as effective because it has different characteristics. So, we have to use alternative methods, once we see a performance problem [that could be tied to goo],” Werner says.

The best course is to adjust the treatment strategy. Werner references two main approach types: one is to enhance or adjust existing chemical or biological processes in order to treat goo characteristics, which is applicable when it is blended with other materials.

An ideal way, though less often an option since the material is rarely seen, is to physically remove it from the stream before it gets into the treatment process. In this scenario engineers may be able to employ separation or a filtration method to protect the equipment.

Waste Connections continues working to stay on top of these tough black solids on the front end (before leachate is treated) at the few wetter facilities where it’s shown up.

“Leachate collection system pumps cut on and off based on the water level in the sump. So, you have stagnant time where polymers can pull together and stick to pumps and begin to build up. If the line clogs you have to jet it out and may have to cut the line and replace it. To avoid this, we continually flush it through, if possible.

Otherwise, you can build water head in your landfill and have compliance issues. So, if the pump is not operating it’s all hands-on-deck to clean it and get it back into operation,” Breeden says.

Benson and his team in Wisconsin are currently collecting samples from landfills around the country.

“To understand what goo is and where it comes from you need diversity of samples. Any given sample will be somewhat unique, so we are collecting as many as we can and looking for common threads. Ideally, we want to get it from everywhere that it is showing up,” Benson says.