A disinfectant concentration measuring subsystem comprising: (a) a reservoir configured to contain a reagent solution, wherein the reservoir comprises an open end, the open end comprising threading ; (b) a supply conduit and a return conduit ; (c) a reservoir cap configured to couple with the open end of the reservoir, wherein the reservoir cap comprises: (i) a static member configured to couple with the supply conduit and the return conduit , and (ii) a moving member associated with the static member, wherein the moving member comprises threading complementary to the threading of the reservoir, and wherein the moving member is configured to rotate relative to the static member to couple with the open end of the reservoir; (d) a first pump configured to pump the reagent solution from the reservoir through the supply conduit ; (e) first mixing chamber in fluid communication with an outlet of a liquid distribution system by which said mixing chamber receive a disinfectant solution; (f) a valve in communication with the pump and the return conduit , wherein the valve is a selector valve and is configured to selectively pump reagent solution back into the reservoir via the return conduit , wherein the selector valve is in fluid communication with the first mixing chamber; and (g) a concentration analysis assembly that is operable to receive the disinfectant solution and the reagent solution after their mixing in the mixing chamber. (a) a reservoir configured to contain a reagent solution, wherein the reservoir comprises an open end, the open end comprising threading ; (b) a supply conduit and a return conduit ; (c) a reservoir cap configured to couple with the open end of the reservoir, wherein the reservoir cap comprises: (i) a static member configured to couple with the supply conduit and the return conduit , and (ii) a moving member associated with the static member, wherein the moving member comprises threading complementary to the threading of the reservoir, and wherein the moving member is configured to rotate relative to the static member to couple with the open end of the reservoir; (i) a static member configured to couple with the supply conduit and the return conduit , and (ii) a moving member associated with the static member, wherein the moving member comprises threading complementary to the threading of the reservoir, and wherein the moving member is configured to rotate relative to the static member to couple with the open end of the reservoir; (d) a first pump configured to pump the reagent solution from the reservoir through the supply conduit ; (e) first mixing chamber in fluid communication with an outlet of a liquid distribution system by which said mixing chamber receive a disinfectant solution; (f) a valve in communication with the pump and the return conduit , wherein the valve is a selector valve and is configured to selectively pump reagent solution back into the reservoir via the return conduit , wherein the selector valve is in fluid communication with the first mixing chamber; and (g) a concentration analysis assembly that is operable to receive the disinfectant solution and the reagent solution after their mixing in the mixing chamber. <EOS>