11 WATER TREATING SYSTEMS-NAMELY, AERATION APPARATUS, ACID AND BASIC NEUTRALIZATIO APPARATUS, COAGULATION APPARATUS, CLARIFICATION APPARATUS, SEDIMENTATION APPARATUS, FILTRATION APPARATUS, ABSORPTION APPARATUS, SLUDGE COLLECTION APPARATUS, SLUDGE DISPOSAL APPARATUS, SLUDGE TREATMENT APPARATUS, PROCESS CONTROL APPARATUS, SOLD INDIVIDUALLY AND IN UNITS OF TWO OR MORE AND INCLUDING COMPONENTS OF EACH APPARATUS; PRESSURE, GRAVITY AND VACUUMFILTERS; ION EXCHANGE WATER SOFTENER APPARATUS AND LIME COAGULATION SOFTENING APPARATUS; AND SEWAGE TREATMENT PLANTS-NAMELY, AERATION APPARATUS, ACID AND BASIC NEUTRALIZATION APPARATUS, COAGULATION APPARATUS, CLARIFICATION APPARATUS, SEDIMENTATION APPARATUS, FILTRATION APPARATUS, ABSORPTION APPARATUS, SLUDGE COLLECTION APPARATUS, SLUDGE DISPOSAL APPARATUS, SLUDGE TREATMENT APPARATUS, PROCESS CONTROL APPARATUS, SOLD INDIVIDUALLY AND IN UNITS OF TWO OR MORE AND INCLUDING COMPONENTS OF EACH APPARATUS