Well, heat capacity of water is 1 calorie (4.18J) per gram. The interwebs hand-wavily say it takes about 25 BTU/hr to cool 1 square foot in a commercial HVAC sy

Well, heat capacity of water is 1 calorie (4.18J) per gram. The interwebs hand-wavily say it takes about 25 BTU/hr to cool 1 square foot in a commercial HVAC system. 1 BTU is about 1kJ, so every 1 square foot of cooling (per hour) could theoretically be offset by 25,000J / 4.18J = ~5980g of water being heated 1 degree C. Density of water is 1g/mL, so it takes approximately 6L per hour per square foot if you assume that you don't raise the temperature of the water more than 1 degree C. They're talking about 50 million square feet with this system, so that's 300 million liters per hour, or about 80 million gallons per hour, or 1.3M gallons per minute. For context, the third water tunnel project in NYC was supposed to deliver 1.3B gallons per day, or about 903,000 GPM assuming constant flow. That's a big pipe. I tried to find actual citations of their flow rate, but haven't found anything yet. Regardless, I am skeptical of the claim that they aren't returning warmer water to the lake. I'm not worried about it (lake Ontario is gigantic), but I don't believe the article. (edit: 1BTU is the energy required to heat 1lb of water by 1 degree F at maximum density, so you could work it out directly from the bizarro-world units of HVAC systems.)