I run a well used school pool and have posted on here before. We use a mixture of trichlor pucks and liquid chlorine. Last year the pool ran pretty well. I had to increase the water replacement when we got to higher CYA levels but that worked out okay all things considered.
This year, things went well for the first five weeks or so, but then went a bit haywire. The first sign of trouble was the filter pressure going up 3-4 days after the last backwash. (I've never seen the filter pressure go up before.) Other signs were increasing CC and the pH going up with it. In this pool the two things always seem to go hand in hand. Strangely, the chlorine consumption didn't seem to be affected.
When this happened, I closed the pool and raised the chlorine level to 25PPM for two nights (CYA=50). I didn't have a FAS-DPD kit at that point, so I used a dilution method with a photometer, so my testing wasn't super accurate, but I don't think it was terrible either. The CC went up the first night, but then came right down over the next day, virtually to zero. (I've never seen zero CC with a photometer).
We reopened and everything seemed okay for several days and then the CC and pH started to go up again. So this time, I purchased a FAS-DPD kit and took the pool up to 30PPM (CYA now about 45 because I'd added water and vacuumed to waste). The CC was never more than 0.5, even on the first evening of shocking. I kept the pool at this level for 48 hours and brushed and vaccumed when I could. We don't have any ladders or lights to worry about, and I left the vacuum and brush in the pool to clean them. The CC went to zero pretty quickly. I can't be 100% sure about the overnight test because there was daylight on the pool for four or five hours before I could get to test it in the morning, but it only fell by 3.5PPM with those hours of daylight included.
We reopened when the chlorine dropped below 10. But by the end of the first day of use, the pH has gone up, the CC is up, and now the pool is starting to look ever so slightly milky. I don't think it's calcium clouding as the water is fairly balanced (TA is 90, CH is 230, and pH 7.6). There was still 8PPM of FC in the water by the end of that day, which seems like plenty.
I'm getting a bit fed up now. I thought I'd reached the right kind of shock levels to kill even mustard algae and maintained them high enough to prevent it coming back. Have I overlooked something in the shocking process? The only other thing I can think of is that the chlorine isn't keeping up with the demands made on it due to the number of bathers, and the filter isn't keeping up either. I think most people on this forum run residential pools, but your thoughts would really be appreciated.