water supply for the City of Ann Arbor is obtained from a series of wells and, in cases of emergency, from the Huron River. Results of analyses of composite samples of the water from the wells and the river are shown in Table 1 . The standard method of operating the softening plant involves series flow. At such times as repair of the mechanical equipment or other emergency measures are necessary, however, the plant may be (and has been) operated with parallel flow during the dewatering of different portions of the regular equipment. There is, in fact, very little difference in the quality of the effluent resulting from the two types of flow, the only change being that the filters have to remove a greater amount of suspended solids with the parallel flow arrangement. During the past year, there have been the opportunities to operate under four different methods: (1) conventional, (2) split treatment, (3) split treatment and return of primary sludge, and (4) split treatment and return of secondary sludge. On the flow sheet (Fig. 2) these different methods may be followed in the succeeding discussion.