Pima County biogas plant returns to limited service after rupture and membrane fouling; contractor faces legal challenge

Pima County staff reported Dec. 18 that the wastewater biogas system is back in limited service after a multi‑month outage caused by equipment failures and defective contractor work.

During a director update, staff described a safety incident in which a cast‑iron vessel ruptured while nitrogen was being introduced during maintenance; no injuries were reported. Forensics found the plant’s membranes were roughly 30% fouled only a few years into service. Staff said a carbon‑filled vessel that is intended to remove oils and protect the membranes had not contained carbon — despite contractor and design‑firm sign‑offs — which contributed to early membrane failure.

County personnel said they replaced the fouled membranes and restarted the system, but are operating it on day shift only while retraining staff and troubleshooting higher siloxane values detected in the gas stream. Staff expect to resume 24/7 operation in the new year once checks and training are complete. A pending legal claim against the contractor was mentioned; staff said restoring full output would strengthen the county’s position in that dispute by showing the system can meet design performance.

Committee members and staff noted the outage reduced biogas revenue during the fiscal year; in the presentation staff referenced prior revenue expectations from the gas plant and said restoring full operation should help recover some of that lost income.

What’s next: staff said they will provide operational updates at future meetings and monitor the contractor legal process; no final dispositions were recorded in the meeting.