SOUTH SIOUX CITY, Neb. (AP) — A report released by Bix Ox Energy says an ongoing sewer odor is not primarily because of wastewater from the company’s plant, but rather the below-code plumbing in dozens of South Sioux City homes.

According to the report released Tuesday, most of the homes reporting “rotten eggs” sewer odors either have insufficient or broken plumbing systems or are sharing a wall with other units that have below-code plumbing. Engineering firm Black & Veatch began testing last week for sulfur-related compounds in the homes that could be causing the odors.

Bix Ox converts organic waste into methane gas, and went online at Roth Industrial Park in September. Shortly after going online, many residents that share a sewer line with the park began complaining of the odor. About two dozen residents were displaced from their homes, and a dozen still remain in hotels.