Safe Use of Recycled Wastewater in Ulaanbaatar, Mongolia

Introduction The Millennium Challenge Corporation (MCC), a novel development institution of the United States, is assisting the Government of Mongolia (GOM) to address the imminent water shortage in the country's capital city, Ulaanbaatar. In addition to investing in expanded water supply and an advanced water purification system, the program is introducing industrial wastewater recycling for the first time in the country's history. The Wastewater Recycling Activity (WRA) includes construction of a new Wastewater Recycling Plant (WRP) along with conveyance pipelines to provide high-quality treated wastewater to be used by water-intensive industries, such as Ulaanbaatar's two Combined Heating and Power Plants (CHPPs), known as CHPP 3 and CHPP 4. Such an arrangement will alleviate approximately 50,000 cubic meters of water per day of demand from Ulaanbaatar's water supply aquifers. Objectives It must be demonstrated that the use of this recycled wastewater in the cooling water loop at the CHPPs is a safe, established practice, and will not negatively impact the performance of the CHPPs. In service of this objective, scale-forming potential of the current well water and the proposed future recycled wastewater are analyzed and presented. Furthermore, we investigate the potential changes in scale formation with and without the use of anti-scalants and corrosion inhibitors, alongside an analysis of dosing requirements and resultant estimated capital and maintenance costs. The implications for improved operational regimes, with increased cycles of concentration and decreased blowdown water, are explored. Anti-scalant technology is fairly new in the Mongolian context, and as such a review of the successful, safe, worldwide use of these products is put forth. Overall, the session's objective is to motivate and support the beneficial re-use of the recycled wastewater - in conjunction with anti-scalants - to demonstrate the value of this arrangement for improved power plant operations and improved water security for Ulaanbaatar. Status The program is being implemented by the Millennium Challenge Account - Mongolia (MCA-M) - a local special-purpose entity established for administering the grant and managing the implementation. The program is approximately 3 years into its 5-year grant performance period, which entered into force in April, 2021. Designs for the conveyance pipelines and modifications at the CHPPs for acceptance of the recycled wastewater are complete, and construction for these improvements is underway. The Wastewater Recycling Plant is a design-build project; construction is well underway and designs are approximately 90% complete. A combination of both Mongolian and international consultants and contractors are supporting the MCC and MCA-M in implementing more than $300 million of water and wastewater infrastructure construction. Methodology The presentation begins with a primer on a traditional power plant's cooling cycle and a review of the existing and predicted water quality for the system's makeup water. Additionally, we discuss the legacy of scaling issues at the CHPPs and the existing source and fate of the cooling water, as the CHPPs reuse excess blowdown water for non-potable services like fire suppression. A theoretical understanding of scale-forming potential using the Langelier Scaling Index is cemented before analyzing scale-forming potential under different operating regimes (with and without anti-scalant, and at a variety of cycles of concentration). The effect on blowdown water quantity is investigated as well. Dosing requirements, costs, and health and safety concerns are addressed, including a presentation of the prevalence and safe operation of this arrangement at power plants across the United States. The presentation concludes with a summary of recommendations, painting a picture of how the recycled wastewater can fit into and even improve the power plant's existing operations. Findings and Significance Water demand in the rapidly growing city of Ulaanbaatar already exceeds existing water production. By adopting recycled wastewater in the cooling loop of the CHPPs, Ulaanbaatar can alleviate 50,000 cubic meters per day of stress on its existing potable water sources. Furthermore, by adopting anti-scalant technology, the CHPPs can use the recycled wastewater to run a plant that is more efficient than it is even while using its water production wells - higher cycles of concentration and less excess blowdown are possible while simultaneously creating a greater degree of protection against scale formation, which can -- and has -- hampered operation of this essential piece of the city's infrastructure. Ultimately, we demonstrate that the use of recycled wastewater for power plant cooling water is a viable arrangement for Ulaanbaatar, and provides much-needed relief for a rapidly growing and water-stressed metropolis.