1082SMALL FLOW WASTEWATER TREATMENT TECHNOLOGY FOR
DOMESTIC AND SPECIAL APPLICATIONS
Recent concern regarding water pollution and general
worldwide public awareness of the problem associated with
pollution has resulted in increased pressures on all waste dis-
chargers to provide effective treatment and disposal of their
waste streams. Small subdivisions, motels, resorts, mobile
homes, watercraft, railroads, and the like, have not escaped
these pressures, although in many respects their total waste
contribution to the environment may be small. However, as
a result of the public awareness of pollution and increas-
ing regulatory pressures, there has been a rapid commercial
growth in recent years in plants designed and built specifi-
cally for such applications. In this article the state of the art
of small flow wastewater treatment systems developed for
such special applications is presented.INDIVIDUAL HOMESThe 1990 census figures indicate that there are more than
25 million onsite residential wastewater treatment systems
in the United States, often because wastewater collection
sewers are not available.^1 These systems include a variety of
components and configurations. Among the most common
systems are the anaerobic and aerobic biological treatment.
In recent years constructed wetlands have also been investi-
gated for home application. However, no treatment systems
as yet has been entirely satisfactory either to homeowners or
to health officials. 2,3Anaerobic SystemSeptic Tank Septic tank is the most commonly used individ-
ual waste disposal system. The US Environmental Protection
Agency estimates that there are approximately 18 million
housing units in the US that use on-site wastewater and
disposal systems. This is about 25% of all housing units.
Additionally, about one-half million new systems are being
installed each year. 2,3
A septic tank consists of a tank in which wastes are
accumulated and digested under anaerobic conditions. The
effluent from septic tanks is malodorous, and the bacterial
count is often quite high. Subsurface absorption field is nec-
essary to absorb the effluent from the septic tank. Capacity,
hydraulic design and soil conditions are most important fac-
tors influencing the septic tank performance. A detailed dis-
cussion on septic tank design, performance, and economics
is available in several publications. 1 – 7A conventional septic tank removes about 40–50 percent
TSS. In recent years many improvements have been made
in the design of septic tank, gravel filter, and soil absorption
trenches that enhance their performance significantly. An
important variation in conventional septic tank design is cur-
rently being manufactured. The Ruck system requires wash
waters be separated from sanitary and kitchen wastes. The san-
itary and kitchen wastes are thus held in an upper compartment
for a longer period of anaerobic digestion, providing more con-
centrated treatment to the sanitary wastes. 3,8 The wash water
is treated in a lower compartment for shorter periods where
effluent from the upper chamber is mixed. The system is sold
as a package unit in a fiberglass housing. A soil absorption
system is also included in this package which is designed to
make the system independent of the natural soil characteris-
tics. Andreadakis reviewed the performance of an on-site treat-
ment and disposal system using a septic tank, gravel filter, and
soil absorption trenches.^9 BOD 5 and TSS removal efficiencies
averaged over 92 and 93 percent respectively, and up to 70
percent nitrogen was removed due to nitrification followed by
denitrification. 9,10 Many researchers believe that the reduction
in hydraulic loading by water conserving devices will improve
the performance of on-site treatment and disposal systems.^11
Intermittent Sand Filters The main purpose of the inter-
mittent sand filters is to reduce the BOD 5 and TSS prior to soil
infiltration. Currently, many intermittent sand filters are used
throughout the United States to treat wastewater from indi-
vidual homes. The process is highly efficient, and requires a
minimum of operation and maintenance. Intermittent sand
filters are beds of granular material underlain by graded
gravel and collecting tiles.^2 Uniform distribution is normally
obtained by dosing or flooding the entire surface of the bed.
Recirculation has also been used. The filters may be buried
or may have some free access.
Disposal Methods Under favorable conditions the efflu-
ent from treatment devices are safely disposed of by (a) sub-
surface absorption, (b) evaporation, and (c) discharge into
surface waters. The subsurface soil absorption is usually the
best method of wastewater disposal from homes because
of its simplicity, stability and low cost. Partially treated
wastewater is discharged below ground surface where it
is absorbed and treated by the soil as it percolates into the
ground. Nearly one-third of the homes in the United States
dispose of their wastewater in this way.^9
Evaporation systems utilize techniques to evaporate the
effluent without infiltration. The system utilize evaporationC019_002_r03.indd 1082C019_002_r03.indd 1082 11/18/2005 11:06:45 AM11/18/2005 11:06:45 AM