SMALL FLOW WASTEWATER TREATMENT TECHNOLOGY FOR DOMESTIC AND SPECIAL APPLICATIONS 1083
and evapotranspiration beds. Direct discharge of onsite treat-
ment system effluent is also a disposal option if an appropri-
ate receiving water is available and if the regulatory agencies
permit such discharges. Various onsite disposal methods are
listed below. Detailed discussion may be found in Ref. 2.
Typical design of septic tank and intermittent filter are shown
in Figure 1. Typical trench, seepage pit and mound systems
are shown in Figure 2.Subsurface soil absorption systems
trench and bed
seepage pit
mound
fill
artificially drained systems
electro-osmosis *
Evaporation systems
evapotranspiration and evapotranspiration—
percolation
evaporation and evapopercolation ponds
Surface water disposal
outfall in stream
outfall in lakeAerobic SystemSeveral types of household treatment systems are available
which utilize aerobic stabilization of organic wastes. Most
systems are designed for continuous flow.
The raw wastewater enters an anaerobic tank where
solids are settled and partially digested. The liquid enters an
aerobic compartment. Air is supplied either by mechanical
aerators or by diffusers. The bacterial action thus produced
is similar to that in an activated sludge plant. The solids in
the aerated liquid are settled into a separate tank. This tank
most commonly has a sloping bottom to return the settled
sludge into the aeration tank by gravity. System components
of such units are schematically shown in Figure 3.
Some of the manufacturers’ variations in the aerobic system
include (1) absence of anaerobic digestion tank, (2) different
methods of aeration, (3) packed bed media, (4) trickling filter,
(5) rotating biological contactor, and (6) use of tube settlers for
increasing the sedimentation rate. 2,12 Many of these variations
are shown in Figure 4.
The effluent from an aerobic system is generally better
than that from a septic tank. Manufacturers claim BOD and
suspended solids removal of about 90%. The effluent from
aerobic systems has lower clogging effect on soil absorption
system. If the system operates properly, the effluent is suit-
able for surface drainage. The disadvantages of the aerobic
system are higher operating costs, susceptibility to shock
loading, and variation in effluent quality. The design criteria,operational characteristics and cost data on aerobic systems
are extensively available in the literature. 2,3,13,14Constructed WetlandsConstructed and natural wetland rely solely on natural
process to treat wastewater and are most often used for sec-
ondary treatment. In a single-residence system, for exam-
ple, a septic tank generally provides partial treatment. The
effluent flows to the wetlands where it is distributed into
the system. Organic matter is stabilized by microorganisms
attached to the plant roots. Aquatic plants deliver oxygen,
provide shade, metabolic nutrients and surface area for
microbiological growth.
Constructed wetlands are designed either as a “discharge”
system or as an “non-discharge” system.^1 Two basic design
approaches exit for constructed wetlands. These are developed
by (1) Tennessee Valley Authority (TVA) for systems less
than 75,700 L/d, and (2) EPA for larger municipal wastewater
treatment plants. The design equations are based on hydraulic
loading, organic loading, and Darcy’s equation. The design
procedure for constructed wetland can be found in Refs. 1,
15–17. The suggested residential wetland design details are:
length 12.7 m, width 4.3m, depth 0.3 m, detention
time 3d, and hydraulic loading criteria 1.3 m^2 /L·d.
The aquatic plants are generally chosen from indig-
enous species of Typhaceae (cattail family), Cyperaceae
(sedge family), Gramineae (grass family), Scirpus validus
(softstem bulrush), or Phragmites australis (giant reed).
Care should be taken to avoid plants that “choke out” each
other, or those eaten by animals. The choice of vegetation is
dependent upon wastewater characteristics, solar radiation,
temperature, aesthetics, wildlife desired, indigenous species,
and the depth of constructed wetlands. Vegetation harvesting
may be necessary when it becomes too dense, cause obstruc-
tion to the natural flow and create anaerobic conditions.^1SMALL ESTABLISHMENTSSmall establishments generally include motels, restau-
rants, stores theatres, clubs, camps, rest areas, institutions,
apartment houses, small factories, subdivisions, small
communities, etc. Disposal of wastewaters from these
establishments in suburban areas certainly poses serious
problems. Construction of public sewer systems may not
be economically feasible to convey the wastewaters to an
existing treatment plant. Since the flows are relatively large,
often septic tanks and subsurface absorption fields may not
provide safe disposal, causing a very serious public health
hazard or pollution of ground or surface waters.
Package treatment plants were first introduced about
thirty-five years ago to treat wastes from small establish-
ments. During this time, there has been a rapid commercial
growth in package plant industry. The package plants are
prefabricated in the factory and, in most cases, completely
assembled prior to delivery to the plant site. Most of the
plants are made of steel but many concrete plants are also* Electro-osmosis is a technique used to drain and stabilize slowly
permeable soils during excavation. A direct current is passed through
the soil which draws the free water in the soil pores to the cathode.C019_002_r03.indd 1083C019_002_r03.indd 1083 11/18/2005 11:06:45 AM11/18/2005 11:06:45 AM