Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
b. Overland flow
Overland flow systems aerobically treat wastewater, providing secondary
treatment and nitrogen removal. Pretreatment includes at least screening. Recent
practice adds a short detention time aerobic lagoon upstream for solids removal
and addition of dissolved oxygen. Public access to the application site is not
allowed. The technology is mainly used in the southeastern and southwestern U.S.
In cold weather climates, winter storage is required as treatment efficiency,
especially nitrogen removal, is adversely affected by cold temperatures.
c. Wetland systems
Wetland systems are used to treat primary and secondary effluent, especially
when discharging to a sensitive environment locations. Typical wetland plants
include cattails, reeds, rushes, bulrushes and sedges. The plants are not
usually harvested, though in some systems they may be periodically burned off to
maintain system hydraulics. Operation is possible in cold climates, although
storage may be required and the effect of freezing must be allowed for.
Constructed free water surface (FWS) wetlands can have a secondary purpose as
wildlife habitat, although mosquito control is an issue. FWS wetlands are more
common in the U.S. than subsurface flow wetlands. Subsurface flow wetlands have
been used for residential sewage treatment in Canada since the early 1990s, but
only since about 1995 in the US.
Natural wetland systems can and have been adapted for wastewater treatment
but are more likely to be a final disposal point, since adaptation of the
natural wetland may harm the local ecosystem.
d. Tank systems
Communities may have a single septic tank system followed by a large
leachfield (i.e.: Taylorsville, California). Large septic tanks are used
upstream of mechanical treatment, such as rotating biological contactors in
community sized installations. Imhoff tanks have been used for small communities
in the past (U.S. EPA survey in 1988, flows of about 0.95 MLD, 415 facilities)
however they are now prohibited in some states. Operational problems include
odour problems and production of odorous foam and sludge.
e. Mechanical treatment plants
All of the technologies in use for single family dwellings are available in
larger scale for use in small communities. The usual refinements include
positive sludge and scum removal from clarifiers, and sludge holding tanks
which, if aerated, also act as mesophilic aerobic digesters. Installation of
rotating fine screens upstream of the mechanical treatment plant decreases
buildup of floatable material.
Multiple tank systems with recycle, such as the Biogreen system imported from
Japan, are claimed to be capable of up to 50 percent nitrogen removal.
In this size range, recirculating intermittent sand filters are used rather
than the single pass systems used for individual homes.
Sequencing batch reactors (SBR) are economically competitive over 150 m3/d,
and have been used with success in many locations throughout the US and Canada.
SBR's have computer control allowing easy manipulation of react, settle and
decant times to allow for changing inlet conditions.
Remote monitoring of community-scale plants by telephone and by satellite is
available and becoming more common, allowing a central monitoring station by a
manufacturer to contact a local operator and assist with maintenance. Remote
monitoring of single family dwelling systems is possible, but is not generally
done due to the high cost of equipment and telephone lines.
Besides secondary gravity settling, other solids separation technologies used
at this scale include dissolved air floatation, and membrane filtration.
Advances in dissolved air floatation include improved air entrainment using
venturi devices which enable equipment manufacturers to use shallower tanks than
conventionally possible. These systems, such as the Hydroxyl Systems Inc.
Positive Floatation Mechanism (PFM), are also finding application as primary and
secondary solids clarifiersE and research suggests that the highly aerobic
sludge cap at the surface results in accelerated solids destruction and denser
sludge in such applications (Figure 4.3).
Figure 4.3: Hydroxyl Systems Inc. Positive Floatation Mechanism (PFM)
Membrane filtration technology operates either downstream of high solids
concentration activated sludge secondary treatment, or within the aerated
bioreactor of such systems. Effluent may be either forced by pressure through
wound tubular membranes, or drawn by light suction through the membranes using
vacuum pumps. Nominal pore sizes are typically small enough to filter out
bacteria, but cracks or breaks in the membrane may permit bacteria to cross over
the membrane barrier. An example of a commercially available membrane used in a
wastewater treatment membrane-bioreactor process is the Zenon Environmental Inc.
ZeeWeedŽ membrane system illustrated in Figure 4.4 [http://www.zenonenv.com/zeeweed.html].
Computer controlled pressure variations and/or periodic backwashing help to keep
the membranes clean. Nitrification is generally good due to the very long sludge
ages achieved in membrane systems, but denitrification and overall nitrogen
removal depends on the system design and presence of anoxic/anaerobic reactors
within the process. Phosphorus removal I membrane systems typically requires
Figure 4.4: Zenon Environmental Inc. Membrane Bioreactor System
And ZeeweedŽ Membrane
Physical/chemical treatment using chemical flocculation was practiced in the
past to treat wastewater with widely varying flows and loads, especially high
loadings. Greater understanding of biological wastewater treatment, complex
operational and maintenance requirements, and more stringent requirements for
chemical sludge disposal has resulted in fewer installations of this type.
However, there are still instances where biological wastewater treatment may not
be practical - for example, where a mobile unit requires instant treatment
without a startup time allowance. Electro-flocculation has been applied to
industrial processes in the past, and is now being developed by companies in
Canada, and the US, for use with wastewater treatment.
Figure 4.5: Solar Aquatics Treatment System (lager
One technology (developed in Massachusetts by Solar Aquatics, and illustrated
in Figure 4.5) combines activated sludge and constructed wetlands in a package
plant located inside a greenhouse.
f. Odour control
Especially for small community installations located near homes, odour
control can be an issue. Technologies used include activated carbon filtration,
chemical scrubbing and biofiltration. Biofilters for odour control are above
ground piles of composting material (often woodchips) with spray wetting systems
and underdrains to collect filtrate. Modular biofilter systems consisting of
stackable trays have also been developed.
g. Grease traps
Simple grease trap tanks have been generally used on wastewater discharges
from restaurants, laundromats and service stations to prevent plugging of the
downstream treatment system. A change to use of low temperature soluble
vegetable oils rather than animal fat has resulted in these traps being less
effective. Solutions have included the NIBBLER, a combination of attached and
suspended growth secondary treatment technology developed in Oregon, and enzyme
additives [Nothwest Cascade-Struth, Puyallup, Washington WA 98373; ph: 800
4442371]. Although the technical basis for such additives is questionable, some
users of additives have reported good results.