GALVESTON, Texas (AP) — Experts blame a faulty valve at a wastewater treatment plant for the release of more than 135,000 gallons of partially treated sludge into Galveston Bay.

The Galveston County Daily News ( ) reported Wednesday that city workers say the valve was supposed to automatically close during an aeration process. The valve remained open for about 20 minutes Sunday night before a worker noticed and closed it.

Contractors working on Galveston's $75 million main wastewater treatment facility are trying to determine whether the issue was mechanical or software failure.

The sludge was released west of the Pelican Island Bridge, in the Galveston Ship Channel. Galveston officials have advised people to avoid swimming and fishing in the area. Authorities say there's no apparent immediate harm to wildlife.

State environmental regulators are investigating the release.