Investors with an interest in Pollution Control stocks have likely encountered both Donaldson (DCI) and Casella (CWST). But which of these two stocks offers value investors a better bang for their buck right now? We'll need to take a closer look.We have found that the best way to discover great value opportunities is to pair a strong Zacks Rank with a great grade in the Value category of our Style Scores system. The proven Zacks Rank emphasizes companies with positive estimate revision trends, and our Style Scores highlight stocks with specific traits.Donaldson and Casella are both sporting a Zacks Rank of # 2 (Buy) right now. The Zacks Rank favors stocks that have recently seen positive revisions to their earnings estimates, so investors should rest assured that both of these companies have improving earnings outlooks. But this is just one factor that value investors are interested in.Value investors analyze a variety of traditional, tried-and-true metrics to help find companies that they believe are undervalued at their current share price levels.The Style Score Value grade factors in a variety of key fundamental metrics, including the popular P/E ratio, P/S ratio, earnings yield, cash flow per share, and a number of other key stats that are commonly used by value investors.DCI currently has a forward P/E ratio of 21.87, while CWST has a forward P/E of 75.91. We also note that DCI has a PEG ratio of 2.08. This popular metric is similar to the widely-known P/E ratio, with the difference being that the PEG ratio also takes into account the company's expected earnings growth rate. CWST currently has a PEG ratio of 5.36.Another notable valuation metric for DCI is its P/B ratio of 6.52. Investors use the P/B ratio to look at a stock's market value versus its book value, which is defined as total assets minus total liabilities. By comparison, CWST has a P/B of 10.54.These are just a few of the metrics contributing to DCI's Value grade of B and CWST's Value grade of D.Both DCI and CWST are impressive stocks with solid earnings outlooks, but based on these valuation figures, we feel that DCI is the superior value option right now. Zacks’ Top Picks to Cash in on Artificial Intelligence This world-changing technology is projected to generate $100S of billions by 2025. From self-driving cars to consumer data analysis, people are relying on machines more than we ever have before. Now is the time to capitalize on the 4th Industrial Revolution. Zacks’ urgent special report reveals 6 AI picks investors need to know about today.See 6 Artificial Intelligence Stocks With Extreme Upside Potential>>Want the latest recommendations from Zacks Investment Research? Today, you can download 7 Best Stocks for the Next 30 Days. Click to get this free report Donaldson Company, Inc. (DCI): Free Stock Analysis Report Casella Waste Systems, Inc. (CWST): Free Stock Analysis Report To read this article on Zacks.com click here. Zacks Investment Research