(a) Absorption trenches (subsurface tile fields). These trenches are
perforated tile lines laid in gravel beds in loose, porous soil into which the effluent from a
septic tank is continuously or intermittently dosed. Oxidation takes place as the effluent
percolates through the gravel and soil.
(b) Intermittent sand filters. These are beds of underdrained sand on
which settled wastewater is applied. Oxidation takes place in the bed.
(c) Trickling filters. These filters are beds of stone where effluent from
primary treatment is intermittently or continuously distributed. Films of organisms that
form on the surfaces of the stones stabilize the solids by aerobic methods.
(a) Activated sludge process. This process accelerates aeration
whereby the effluent from primary settling is mixed with return sludge and agitated
continuously in the presence of oxygen. The activated sludge thus formed has the
property of absorbing dissolved organic material and converting it into stable
substances that will settle.
(b) Oxidation pond. The pond is a relatively large, shallow pond,
either natural or artificial, into which settled wastewater is discharged for natural
purification under the influence of sunlight and air.
(3) Secondary settling. Sedimentation is an essential step following
biochemical processes such as the trickling filter and the activated sludge process.
These biochemical processes do not remove organic material; they convert it to a stable
form which will settle out by sedimentation. The sludge from final settling tanks is
pumped into digestion tanks. In the activated sludge process, a portion is returned and
mixed with the settled wastewater entering the aeration tanks.
(4) Sludge digestion. The sludge which settles out during primary and
secondary sedimentation is about 95 percent water. The remaining 5 percent is highly
putrescible organic matter. It is normally pumped directly into covered tanks and
permitted to digest by anaerobic bacterial action. In the Imhoff tank and the septic tank,
the sludge settles to the bottom where it is digested anaerobically. Digested sludge is
withdrawn and discharged into sludge drying beds. These beds are usually provided
with underdrains to facilitate drying. Dried, well-digested sludge is quite stable and is an
excellent low-grade fertilizer.
c. Tertiary Treatment. A tertiary wastewater treatment plant performs those
processes that remove contaminants from wastewater that were not removed by
conventional primary and secondary wastewater treatment. The purpose of tertiary
treatment is to upgrade the effluent from conventional treatment processes and to
renovate wastewater to a level at which it can be reused directly. Very few wastewater
treatment plants in operation today employ tertiary treatment methods; however, the