Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
Wastewater technology for individual homes ranges from primary treatment in a
septic tank, separation followed by ground disposal, to small package mechanical
wastewater treatment plants. In some environmentally sensitive areas, especially
those depending on tourist revenues such as the Florida Keys, nutrient removal
is an important issue.
There is a performance standard for individual household wastewater treatment
systems, referred to as the ANSI/NSF Standard 40. This standard was developed by
the National Sanitation Foundation (NSF International) in Ann Arbor Michigan (a
not-for-profit organization) on behalf of the American National Standards
Institute (ANSI) to verify manufacturer's claims regarding performance. It is
a voluntary certification testing program, which many Health agencies throughout
Canada and the US have come to rely on. Testing under this standard is carried
out in both Canada and the US at three locations: 1) Chelsea, Michigan; 2) North
Saanich, British Columbia; and 3) Baton Rouge, Louisiana. Testing is carried out
over a 26 week period, and involves monitoring treatment performance under
standard loading and stress testing conditions.
a. Septic systems
Approximately one-quarter of all homes in North America are served by septic
tank systems. Most tanks are constructed of concrete or fiberglass. Two
compartments tanks are favoured due to more positive prevention of solids
carryover to a drainfield. Tanks used as pretreatment before small diameter
piping collection are fitted with filter vaults.
There are many different kinds of biofilters which mimic the action of
wastewater degradation in unsaturated soil, and are generally used where local
soil conditions or development density preclude disposal in conventional septic
tile fields. In addition to low BOD5 and TSS in biofilter effluents, fecal
coliform counts are often significantly reduced, although this is not typically
relied upon for final disposal due to the possibility of filter
short-circuiting. The filters described below are usually used for single family
dwellings but have also been applied for small home clusters. Effluent is
generally nitrified, with approximately 50 percent removal of total nitrogen,
but with the exception of peat filters, biofilters are not generally effective
for phosphorus removal.
Intermittent sand filters are tending to be designed at lower loading rates
and using finer media, to improve fecal coliform removal. Required piping,
filter cloth, pumping equipment and controls can be purchased in a kit form for
construction and installation by knowledgeable installers.
The Waterloo BiofilterTM (Figure 4.2), developed at the University of
Waterloo, uses an absorbent plastic foam media and positive fan ventilation to
allow loading at ten times the rate of an intermittent sand filter [http://oceta.on.ca/profiles/wbsi/bi.filter.html].
Nitrogen removal can be effected using recirculation, or by passing the effluent
through a free draining sawdust box. Phosphorus removal is effected by passing
the effluent through a ferric iron mineral filter. The Waterloo Biofilter has
been installed in several locations in Ontario, one in Massachusetts, and one in
Upflow filter systems intermittently dose septic tank effluent below a bed of
media, allowing solids to settle and decompose below the bed, while liquids rise
through the media and treated effluent overflows over the top. The Glendon
Biofilter, developed in Washington State operates in this fashion and the
distributors claim it achieves a 10/10 (BOD/TSS) effluent. This system is
typically designed for single home applications.
Peat filters treat septic tank effluent by gravity distribution by
conventional leachfield piping over a buried peat filter. Sphagnum type peat is
used. At least thirteen peat filter systems are in operation in Ontario, ranging
from single family systems to schools. The filters have proven effective in
reducing phosphorus, and BOD5 at appropriate loading levels. Nitrogen reduction
is on the order of 40 to 60 percent.
Figure 4.2: Waterloo Biofilter System
c. Mechanical package plants
Single family dwellings have historically been supplied with either simple
activated sludge plants or rotating biological contactors in integrated single
tank systems. Newer technology combines both suspended and attached growth systems
(moving bed biofilm reactors) with either fixed or floating media submerged
in a suspended biomass. The combined systems are theoretically more resilient
to shock flows and loads. These systems effect some nutrient removal, mostly
in terms of nitrogen removal.
Solids separation is usually by clarifier, generally oversized by large
system design standards to allow for wide flow variations, with no positive scum
or sludge removal. Solids buildup is pumped out by vacuum truck at regular
intervals and hauled to central treatment facilities. Biofilters have also been
used with some success to improve effluent solids quality. Other types of solids
separation systems, (such as ultra-filtration membranes) are available, but are
generally not cost effective in single home situations; they are discussed in
the following section.
Where single home systems discharge to a surface water body, disinfection is
used. Options include chlorination/dechlorination using solid puck systems,
single lamp ultraviolet disinfection systems, and ozonation. Ozonation is
available as part of a package treatment plant developed by Hydroxyl Systems
Inc, based in British Columbia, Canada [http://www.hydroxyl.com].
Small communities use a variety of technologies from simple pond systems to
more complex mechanical treatment plants.
a. Pond systems
Pond systems are popular in rural areas with available land area but low
operational expertise. In 1984, there were 5298 stabilization ponds in use in
the United States, with a further 2783 planned (Water Environment Federation;
see 4.10 Sources of information).
In Canada, almost 9% of the population surveyed in 1996 with central sewage
treatment was served by stabilization ponds. The actual number may be higher as
communities with less than 1,000 persons were not surveyed ([http://www.ec.gc.ca/water/en/manage/vse/e-datab.htm]
Environment Canada). Shallow aerobic ponds are used mainly in the southern part
of the United States. Anaerobic ponds are generally used for strong industrial
wastes rather than for municipal wastewater. Facultative ponds are the most
common, and are in use in all climatic regions. Algae growth in these types of
ponds can contribute to seasonal high solids concentrations in the effluent.
Where wastewater temperature are sufficiently high enough during the summer,
near complete nitrogen removal can be achieved (i.e. up to 95%).
In 1984, there were 1368 aerated lagoons in use, with a further 1494 planned
(WEF). Aerated lagoons are often designed as several smaller ponds in series. A
few more recent systems have provided high mixing in the first cell, followed by
lower levels of mixing in subsequent cells to help prevent algae production.
Integrated lagoon systems combine facultative primary cells with fermentation
pits acting as upflow anaerobic digesters followed by a shallow aerobic lagoon,
with recycle back to the facultative lagoon. A system in St. Helena, California,
has been operating for 27 years.
Lagoon discharges may be controlled to protect the receiving environment,
discharging only when receiving stream conditions are satisfactory. Some
lagoons, located where evaporation exceeds precipitation, can be designed as
Lagoon designs are tending towards more positive pond lining, including
geosynthetic clays and geomembranes. Odour from anaerobic ponds is controlled
using floating covers with gas collection.
Plant harvesting, a form of smaller polishing pond treatment practiced in
warmer climates (air temperatures above 7 deg. C) involves growth and harvesting
of floating aquatic plants such as water hyacinths and duckweed. Water hyacinth
systems are in place in Headlands, Alabama, and San Benito, Texas. They have
been used for nitrogen removal in Florida. Phosphorus removal is not practiced.