UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington D.C. 20549 Form 10-K (Mark One) þ ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the fiscal year ended December 31, 2018 or ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the transition period from to Commission File Number: 001-34112 Energy Recovery, Inc. (Exact Name of Registrant as Specified in Its Charter) Delaware 01-0616867 (State or Other Jurisdiction o ...