Rust Kills: Investigation into Plant Water Corrosion and Mitigation Strategies

Presentation Description: This presentation educates the water quality mechanisms that lead to equipment corrosion and common industry mitigation strategies to reduce tremendous increases in operation and maintenance demands.

A case study from the Orange County Sanitation District will be showcased. The Thickening and Dewatering (T&D) Facility at Plant No. 1 experienced accelerated corrosion in the piston cake pumps, lubrication pumps, and water piping. The T&D Facility uses plant water (PW), or disinfected secondary effluent, for lubrication and cooling, such as for Central Generation Facility generators and centrifuge cleaning cycles. During the design of the T&D Facility, the design team selected materials to handle the PW based on water quality analysis results in 2006. However, the PW chemistry has changed significantly over the last 14 years; the 2020 and 2022 sample results indicated an increase in corrosivity compared to that of 2006, and corrosion has been observed as early as 2007.

Water quality sampling was conducted to characterize the current water quality chemistry (pH, chlorine, ammonia, phosphorus, etc.) throughout Plant No. 1. PW was found most corrosive at the Plant Water Pump Station (PWPS), the location for chlorine dosing. PWPS had the lowest Langelier Saturation Index (LSI) at -0.37, indicating slightly corrosive but non-scale-forming water. As PW travels away from PWPS, pH increases, raising the LSI at remaining sample locations. These PW samples, with LSI between 0.0 and 0.5, are slightly scale-forming yet are still considered corrosive. This concludes that chlorine residual diminishes as water traveled further from the injection point at PWPS and that chlorine, an oxidizing agent, is the potential cause of corrosion at PWPS and at the T&D Facility, coupled with lowered pH, lack of ammonia, and low phosphorus concentrations.

Various equipment was evaluated for material compatibility, and specifications were updated to include stainless steel or non-metallic components/piping. Furthermore, operational treatment methods were considered to address current corrosion issues, such as raising the pH through the addition of caustic or use of corrosion inhibitors. It was determined that adding caustic to PWPS, and subsequently raising the pH, is the most reasonable and cost-effective alternative to effectively mitigate the observed corrosion.