We're a retail pool chemical store in the western New York region and experience homeowners developing chlorine demand upon opening their pool in the spring. Why is this so prevalent at this time of year? Thanks
Sometimes CYA gets broken down over the winter by soil bacteria growing in the water. When that happens, one of the breakdown products is ammonia. Sometimes the ammonia out gasses, but if it hangs around it takes a significant amount of chlorine to break it down.
See this thread for more technical info including a link to the degradation pathway and to papers describing a variety of bacteria that can break down CYA. As noted in this paper, "Cyanuric acid is degraded by a large number of soil bacteria and fungi". This includes the very ubiquitous Pseudomonas. For every 10 ppm CYA that is degraded, it breaks down to around 3 ppm ammonia that can take up to 30 ppm FC cumulatively added to eliminate.
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