such as an activated sludge or trickling filter followed by a secondary settling tank. Equivalent
secondary treatment performance can usually be attained by physical-chemical processes.
Sedimentation. Clarification (settling).
Sewers. Lateral Sewer-One that discharges into a branch or main sewer and receives wastewater from
Branch Sewer-One that serves a small area and receives wastewater directly from sources or from
Main or Trunk Sewer-One that receives wastewater from many tributary branch sewers and serves a
Interceptor Sewer-One that receives wastewater from trunk sewers and branch sewers and conducts it
to the point of treatment or discharge.
Sludge. A concentrate in the form of a semiliquid mass resulting from settling of suspended solids in the
treatment of sewage and industrial wastes.
Sludge Conditioning. Treatment of liquid sludge, usually by heat treatment or addition of chemicals,
before dewatering to facilitate water removal and enhance drainability.
Sludge Dewatering. The process of removing a part of the water from the sludge to convert to a
semisolid form. Methods used include draining, pressing, vaccum filtration, pressure filtration,
centrifugation and others.
Sludge Incineration. The burning of dewatered sludge under sufficiently high temperature to oxidize all
organic components. The resulting residue is a sterile ash.
Sludge Stabilization. Any treatment including such operations as anaerobic or aerobic digestion which
converts sludge to a form which can be disposed of without a detrimental effect on the environment.
Sludge Thickening. Settling, air flotation, centrifugation or similar operations to decrease the water
content of the sludge yet maintain it in a fluid form.
Suspended Solids. Solids retained by filtering a sample of a water or wastewater stream. Retained
material is dried at 103C prior to weighing. Expressed in mg/L.
Total Solids. This dissolved and suspended solids content of a water or wastewater stream. Determined
by evaporating liquid and drying to a residue at 103C prior to weighing. Expressed in mg/L.
Toxic Material. Any material which inhibits normal biological processes in animals, treatment processes,
or the environment. Normally these are materials which cause such inhibition at low concentration levels.
Turbidity. A measure of fine suspended material (usually colloidal) in a liquid. Usually expressed in
standard Jackson turbidity units. In most cases, suspended material consists of fine clay or silt
particles, dispersed organics and microorganisms.
Volatile Solids. Solids, dissolved or suspended, which are primarily organic and exert the significant
portion of the BOD during stabilization. Expressed in mg/L.
Wastewater Inventory. A detailed listing of all wastewater sources including data on flow, temperature,
BOD, suspended solids and other parameters necessary to define quality.
Weir. A control device placed in a channel or tank which facilitates measurement or control of the water