Biological wastewater treatment?

In a wastewater treatment study using sequencing batch reactors i was supplying system with a C:N:P ratio of 100:5:2 and there was sludge bulking problem.. after i increased the c:n:p ratio to 100:5:40 and i observed no bulking anymore. what is the mechanism here? what solved bulking problem here? pH change? or more phosphorus? why?.. thanks