Orthophosphate is added into drinking water distribution systems (DWDSs) to reduce lead levels, but its effects on monochloramine decay and microbial activity are underexplored. The main purpose of this study was to determine the effect of orthophosphate addition and increasing monochloramine levels on monochloramine decay in a phosphorus-limited full-scale DWDS using batch tests. The study also evaluated nitrification potential at selected sites in this DWDS after seven years of orthophosphate application, comparing them with findings from a previous study conducted before its introduction. This comparison provides new insights related to orthophosphate's effects in a full-scale DWDS. In the batch test, samples were divided into unprocessed and microbiologically inhibited groups to observe chemical and microbial monochloramine decay over three weeks. The results suggest that orthophosphate may increase microbiological monochloramine decay. Below certain monochloramine levels, defined in previous research as a critical threshold residual (CTR) (0.4–0.7 mg Cl2/L), an accelerated decay rate of monochloramine occurred. The study noted higher CTR values in post-orthophosphate samples, suggesting the need for higher minimum monochloramine levels in the DWDS to prevent accelerated decay when using orthophosphate. Nitrite formation during the batch test was more pronounced in post-orthophosphate samples, suggesting that orthophosphate may enhance nitrification potential.