Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
4.3 Treatment (Topic c)
4.3.1 Small scale and community scale technologies
In the following discussion, small and community scale is defined as from 1
to 5000 Population Equivalents.
Swales, wide shallow grassy ditches, are used to direct stormwater flow
around a surface structure, and provide some flow attenuation, filtering, and
evapotranspiration due to the grass growth, as well as some minimal storage.
Ponds are used to contain stormwater flow, hold flows until pollutants are
biologically broken down or adsorbed onto particulate matter, and often allow
water to infiltrate into groundwater gradually through the pond bottom.
Sedimentation and filtration ponds may also be lined to facilitate removal of
accumulated sediments, usually every two to four years. Ponds and swales are
also effective in removing sediments from storm discharges, but are land
Ponds are also used to treat runoff from parking lots. These systems may also
include upstream filtration provided by grass plots underlain by sand and gravel
filters with an underdrain system feeding the holding ponds.
Natural, enhanced and artificial (constructed) wetlands are also used to
treat stormwater runoff before discharge to the environment. Application is
similar to that discussed under wastewater treatment below. Water may be
diverted through a natural wetland system to take advantage of adsorption
phenomena in the removal of sediments, and inorganic contaminants (metals).
These natural systems may be enhanced to optimize the level of treatment and
Stormwater treatment can also be achieved in systems consisting of rock berms
and alternating strips of native grasses and stiff grass hedges constructed on
slopes. The rock berms are intended to catch coarse sediment and floatables, and
convert piped flow to sheet flow down the slope. Native grasses provide
screening, vegetative filtering and infiltration. Coarse hedge forming grasses
provide flow attenuation to prevent flattening of filter grasses. Mowing and
trimming of the site removes nutrients accumulated in the vegetation. This type
of system has been applied in Austin Texas as part of a new subdivision, and
requires less space than ponds.
Where space is at a premium, or where the groundwater table is high, radial
flow filter cartridges installed in manhole structures have been used. One
manufactured type, the StormfilterTM, has been installed in more than 130 sites
in the U.S., including shopping centers, restaurants, highway runoff, hospitals,
and single retail outlets [http://www.stormwatermgt.com/products.html]. Runoff
from impervious surfaces such as roads and parking lots are treated by humic
filter media composed of pelleted composted deciduous leaves. Up to 90% removal
of solids, 85% removal of oils and greases and over 80% removal of heavy metals
are claimed for such systems.
Small versions of vortex grit and floatables removal systems, such as the
Downstream Defender (Storm King) or Vortex Separator (US EPA Design Criteria),
have been used as a pretreatment stage for discharges prior to discharge to a
gravel leachfield [http://www.hil-tech.com/dwnstrm.html]. Generally, there has
been little detailed testing of the benefits derived from these large scale
vortex flow devices. Research data indicates these vortex style systems
typically remove only the coarsest fraction of the suspended solids, whereas
toxic contaminants are more typically associated with the finer particulate
Catchbasin inserts have also been used to provide runoff treatment from
debris control using mesh bags and screens, to oil and heavy metals removal by
filter absorption. Rigorous catchbasin cleaning schedules become very important
to maintain the drainage capability of the system. Simple oil traps such as
submerged outlets on catchbasins are also used. The use of polypropylene sponges
are recently being commercially promoted for use in catchbasins to absorb oil
contained in the runoff. The hydrophobic oil containing sponges are periodically
collected and processed.
Best Management Practices (BMPs) are encouraged on industrial and
construction sites to prevent stormwater contamination at site. They include
covering piled materials, roofing equipment storage and maintenance areas,
covering drum storage systems, routine cleaning of paved surfaces, use of
sedimentation basins and straw filters to trap runoff associated sediments
migrating from the construction site, and regular inspections of potential
problem areas. Drain blockers can be used in maintenance and equipment wash
areas. On construction sites, erosion and subsequent large solids loads to the
stormwater collection system can be controlled using methods such as Ditch
DamsTM, a proprietary product composed of wire mesh and geotextile moulds which
are filled with dirt and ready-mix cement on site [http://www.gullywasher.com/ditchdam.html].
Individual toilet systems
Pit toilets are generally used on remote, low density private and public
recreational properties. Chemical toilets, whose contents are periodically
pumped out by vacuum truck and transported to a central treatment facility, are
widely used under temporary circumstances such as construction sites. Originally
mainly used for remote recreational property, composting toilets (eg. Dowmus
Composting Toilet illustrated in Figure 4.1) are now often used where a
conventional septic system has resulted in water pollution, as well as for
parks, beaches, highways, military installations, and golf courses [http://www.dowmus.com/home.html].
Imported from Sweden, North American modifications have included computer
control functions, including positive mixing of the contents of the tank (Toilet
Tronic, BC) and foam flush with an insulated composting tank for arctic regions
(AlasCan Composting Toilet) [http://www.alascanofmn/index.html]. Incinerating
toilet technology is used most often in mobile offices, boats, cottages cranes.
Figure 4.1: Diagram of the Dowmus Composting Toilet