This article is part of our Rising Stars Portfolio series.

Last month, I purchased shares of Clean Harbors (NYSE: CLH) for my Rising Star portfolio, attracted by its waste cleanup initiatives and interest in sustainable solutions in its own business. But even as that stock reaches new highs, I'm growing equally intrigued by several recent investments from one of the company's rivals.

Clean Harbors shares hit a new 52-week high Wednesday as the company continued the acquisitive strategy it kicked off with its January purchase of Badger Daylighting. This time, Clean Harbors announced its $167 million buyout of Canada's Peak Energy Services. According to the company, the acquisition should be a done deal in the second quarter, and will immediately add to its profits.

Although the purchase suggests that Clean Harbors is moving to the production end of the industry, an Investor's Business Daily article pointed out that the company can better target shale producers, who could use its services if regulators push through more stringent rules about the industry's generation of contaminants and waste. The Peak acquisition reinforces one of the strengths that led me to buy Clean Harbors in the first place: its ability to build relationships with companies to which it can later upsell other services.

However, Clean Harbors isn't the only cleanup company catching my eye. Rival Waste Management (NYSE: WM) has made several impressive investments of its own. Alongside fellow investors such as Kleiner Perkins and France's Total (NYSE: TOT), Waste Management recently ploughed $22 million into Agilyx, which works on technology to transform waste plastic into a synthetic crude oil.

Entering this budding, intensely green market niche is just one of Waste Management's intriguing recent moves. Among other ventures, it's bought into Enerkem, a company aiming to turn garbage into fuel. Enerkem recently ranked No. 42 on Fast Company's list of 50 Most Innovative Companies, alongside household names such as No. 41, Cisco (Nasdaq: CSCO), which is developing an "open energy" smartgrid, and No. 45, General Electric (NYSE: GE), which pursues its "ecomagination" mission through more efficient trains, airplanes, and automation.

With the Department of Energy estimating that biomass could provide 40% of our energy needs, start-ups like Enerkem and Agilyx offer fascinating possibilities. How cool would it be to divert our plentiful waste from dead-end landfills into productive energy? I admire Waste Management's shrewd investments in exciting and potentially lucrative companies like these.

As of this writing, Clean Harbors has already gained about 6.8% since its purchase for my Rising Star portfolio, so I'm hardly suffering from buyer's remorse. However, I'm also starting to wonder whether Waste Management is greener and more forward-thinking than I previously realized. With its investments in innovative and eco-friendly ventures, it's earned a definite spot on my watchlist.