Project Overview

RVA was retained by the City of St. John’s to provide the design, tendering, contract administration and field services of a lime building extension at Windsor Lake Water Treatment Plant (WTP).

Windsor Lake WTP, has a rated capacity of approximately 70,000 m3/d plant, applies hydrated lime and carbon dioxide (CO2) for pH and alkalinity adjustment, prior to microfiltration membranes. Lime impurities and undissolved lime create operational and maintenance issues with the downstream membranes, requiring the City to lower the lime dose. As a result, the treated water pH and alkalinity targets were not met. Additionally, the existing hydrated lime feed system was outdated. 

Based on an assessment report completed by another firm, RVA evaluated pH and alkalinity adjustment chemicals and dosages for application at Windsor Lake WTP to achieve treated water stability (i.e. corrosion potential) similar to other St. John’s WTPs. The corrosion potential of the treated water was evaluated based on various corrosion indices and metal release models. A life cycle analysis was completed, which confirmed that the addition of hydrated lime and CO2 downstream of the microfiltration membranes was the most cost-effective solution to meet the target pH and alkalinity. The new 500 m2 building extension includes a complete new hydrated lime system comprising a 50-ton silo, volumetric feed, batch tanks, transfer pumps, day tanks, and feed pumps. The building also houses mechanical, electrical, and storage rooms. The building extension was designed to blend harmoniously with the existing treatment building façade and provide transitional access for operators. Aside from the new facility, the site plan and the existing CO2 system was modified. A new mixing/injection chamber was constructed around the existing treated water discharge pipe. The chamber is accessible from the existing building basement and provides a designated location for the hydrated lime and CO2 application and for dynamic mixing. A construction sequence was designed to maintain the continuous production of treated water while the plant transitioned into the new treatment sequence. Following the tender of the project at the end of 2020, RVA also provided engineering services during and post-construction.