(a) Septic tank. A septic tank is a device that receives raw
wastewater from a single residence, several residences, a hotel, or an institution. The
septic tank retains the wastewater long enough for the settleable residue to accumulate
on the bottom. The effluent is discharged through an overflow pipe.
(b) Imhoff tank. An Imhoff tank is a two-story tank that serves a small
installation or community. It is a further refinement of the septic tank principle whereby
the wastewater enters the upper chamber and, as it passes slowly through, solids settle
to the lower chamber. A detention period of 2 to 3 hours in an Imhoff tank will reduce
nonfiltrable residue by 45 to 60 percent. Two types of Imhoff tanks are shown in
Figures 2-11 and 2-12 of lesson 2.
(c) Separate settling tank. The separate settling tank is the most
common device used for primary settling at large installations and municipal wastewater
systems. It may be circular or rectangular, having a sloped hopper bottom for the
accumulation of solids. A 2-hour sedimentation period will usually effect removal of 50
to 60 percent of the suspended matter and 30 to 40 percent of the BOD (see para
1-5b(2)). With the addition of chemicals to effect coagulation, as in water treatment,
sedimentation may remove as much as 75 to 85 percent of the suspended matter and
50 to 70 percent of the total BOD.
(4) Sludge removal. Removal of sludge is required to ensure continued
efficient operation of primary treatment equipment. Sludge from septic tanks is normally
pumped into commercially operated tank trucks and disposed of by burial or discharge
into municipal wastewater systems. Sludge from the bottom compartments of Imhoff
tanks is pumped on a periodic basis, as digestion is completed, into sludge drying beds,
tank sludge lagoons, or some similar type of disposal facility. Sludge from primary
settling tanks is removed continuously, or frequently, to prevent septic conditions from
developing in the tank. It is pumped into sludge digesters for complete digestion (see
b. Secondary Treatment. The secondary wastewater treatment plant performs,
essentially, a biochemical process, although some of the same physical processes
employed in primary treatment are also involved. Wastewater that received only
primary treatment usually still contains 50 to 60 percent of the original wastewater
material, including all of the dissolved matter, most of the colloidal and finely divided
organic matter, and approximately as high a bacterial content as the raw wastewater.
This partly treated wastewater is highly putrescible and will pollute large bodies of
receiving water unless further treated. The objectives of secondary treatment are to
bring about a rapid oxidation and stabilization of this remaining organic matter, to
remove the resulting settleable solids, and to reduce the bacterial content. The
following processes are considered secondary treatment processes.
(1) Filtration. Filtration effects biological oxidation of organic material on
and in beds of stone or sand.