Google, Salesforce, H&M and other brands have turned to unlikely allies to help them clean up their carbon pollution: sewage treatment plants and paper mills. The companies joined an $80 million plan to take CO2 out of the atmosphere, though the strategies they’re using have yet to show whether they can have a meaningful impact on climate change.

They’re paying $32.1 million to a startup called CREW that aims to trap carbon dioxide emissions produced at wastewater treatment facilities. And $48 million will go to another startup called CO280 that retrofits pulp and paper mills with controversial carbon capture technologies. The two agreements were facilitated by a carbon removal initiative called Frontier that’s led by led by Stripe, Google, Shopify, and McKinsey Sustainability on behalf of those founding companies and other brands trying to meet their own sustainability goals.

Companies are increasingly looking for ways to try to cancel out the damage caused by their greenhouse gas emissions. They’ve funneled millions into startups building new-fangled industrial plants that filter CO2 out of the ambient air or seawater. Frontier’s latest announcement shows they’re also open to backing even more novel tactics for drawing down carbon dioxide.

“We do need to be looking at a lot of different kinds of approaches,” says Wil Burns co-director of the Institute for Responsible Carbon Removal at American University, who is also part of an assessment committee for Frontier. “Some of these approaches still remain extremely expensive, notably direct air capture, so we’re looking for approaches that potentially are less expensive.”

The first generation of industrial facilities built over the past decade or so to filter CO2 out of the air — called direct air capture — cost companies including Microsoft upwards of $600 per ton of captured carbon. The deals Frontier just brokered come out to around $447 per ton of CO2 removal by CREW (for a total of 71,878 tons), and $214 per ton for CO280’s services (for a total of 224,500 tons).

That’s still well above the $100 per ton that industry leaders often say they’re targeting. And for a company like Google that was responsible for 14.3 million metric tons of carbon dioxide pollution last year, you can see how prohibitively expensive the technology still is.

While CREW’s strategy is more expensive, Burns says he’s particularly excited about its potential. The idea is to capture carbon dioxide that otherwise would have been released by microbes that break down organic waste in water treatment tanks. To do this, CREW adds alkaline minerals to the tanks. Those minerals react with the CO2 microbes produce, trapping it in water as bicarbonate. Eventually, the bicarbonate travels with the treated wastewater out to oceans, which are natural sinks that keep CO2 out of the atmosphere.

CO280, on the other hand uses technologies initially developed by the fossil fuel industry to capture CO2 emissions from smokestacks before they can escape into the atmosphere. These kinds of devices have been added to industrial facilities and power plants in the past, and can collect CO2 that companies might then shoot back into the ground to push out hard-to-reach oil reserves.

CO280 takes a different approach by adding carbon capture devices to facilities that burn “black liquor,” a bi-product from pulp manufacturing that’s used to generate heat and power. The devices are supposed to capture the CO2 from burning black liquor so that it can be permanently stored in underground wells. Since the fuel is made from trees, the process essentially sequesters CO2 that those trees drew in through photosynthesis during their lifetimes.

To be sure, there are still major concerns about how effective carbon capture technologies are as a way to mitigate climate change. They use a lot of energy, which generates its own greenhouse gas emissions. There are also additional emissions from cutting down trees and transporting wood for paper mills, and it can be difficult to ensure that tree plantations are managed sustainably.

Companies buying carbon removal services also get flak from environmentalists concerned that it’s a distraction from more critical efforts to transition from fossil fuels to clean energy. At the end of the day, the only surefire way to stop climate change is to prevent the greenhouse gas emissions from fossil fuels that are causing the crisis in the first place.