| Cover Page | Apr. 30, 2020 |
| Cover page. | |
| Document Type | 8-K |
| Document Period End Date | Apr. 30, 2020 |
| Entity Registrant Name | Energy Recovery, Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-34112 |
| Entity Tax Identification Number | 01-0616867 |
| Entity Address, Address Line One | 1717 Doolittle Dr. |
| Entity Address, City or Town | San Leandro |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94577 |
| City Area Code | 510 |
| Local Phone Number | 483-7370 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $0.001 par value per share |
| Trading Symbol | ERII |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001421517 |
| Amendment Flag | false |
