I haven't found detailed information on exactly what kind of filters the Ullrich plant uses, but their website says that they are "gravity filters". These are often sand filters, which consist of graduated layers of gravel and sand media, with the coarsest media on the bottom and the finest media on the top. The treated and clarified water is pumped into the top of the filter and flows by gravity through the media. The media filters the fine particles, including bacteria, that cause turbidity out of the water.
The filtered particles build up on the media and have to be removed by backwashing the media. This is done by shutting off the flow into the filter, gently pumping filtered water upward through the media. The water with the solids cleaned off of the media is pumped out as waste and treated further before being discharged or otherwise used. If the backwash water is pumped too rapidly up through the media, it will stir and mix the media, greatly reducing the efficiency of the filter. A valve that froze in the wrong position could do this, as could a mistake by the operator or other failures. Other types of filter media could also be used, with similar need to be backwashed. Any problem with cleaning the filter or damage to the filter could cause the turbidity problems.
I wonder what kind of filters are used, what happened to them, and how much greater than 5.0 NTU the turbidity got.