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TO: |
Mayor and Members of Council |
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FROM: |
Jerry Klaus, General Manager, Waterworks |
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PREPARED BY: |
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DATE OF MEETING: |
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SUBJECT: |
Evaluation of Household Food
Waste Disposers and their Effects on the Municipal Sanitary Sewers |
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RECOMMENDATION:
THAT the report entitled ‘Evaluation of
Household Food Waste Disposers and their Effects on the Municipal Sanitary
Sewers’ be received;
AND THAT shredding residential organic waste
and disposal of such waste in the municipal sanitary sewers with food waste
disposers continue to be banned in the Town of
PURPOSE:
The purpose of this report is to analyse the
use of domestic food waste disposers within the Town of
BACKGROUND:
Food waste disposers (FWDs) are mechanical
devices designed to shred or pulverize organic food waste, mix that waste with
water, and then discharge the water and waste mixture into the sanitary sewers
for ultimate treatment and disposal at the sewage treatment plant. These devices, also called garbarators or
Insinkerators™, are normally installed in the drainage pipe directly below the
kitchen sink. FWDs are designed to
provide a convenient means of disposing organic waste because the wastes are
removed from the home with minimal handling by the user. Organic waste generated during food
preparation, or clean-up, is pushed or sucked into the device, shredded, and
then carried away to the sanitary sewer with water. The user does not store the waste for later
pick-up and disposal.
The use of FWDs has been banned in the Town of
The Corporation of the Town of
“6. No person shall discharge or deposit or permit to be discharged or deposited…into (the)
sewer system or sewage works…any matter or quantity of matter…which may or may tend to obstruct the sewer…without
limiting the generality of the foregoing, any of the following:
(m) any garbage whether shredded or not, including shredding by any
mechanical means;”
Representatives of the manufacturer of FWDs met
with Town Staff to discuss the potential use of food waste disposers in the
Town of
The manufacturer provided Town staff with
several reports related to FWDs, as well as a list of municipalities in the
·
Survey of municipalities in
the Greater Toronto Area regarding their policy toward the use of FWDs
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Analysis of the effects FWDs
would have on water consumption, By-Law compliance, operation of municipal
sewers, and sewage treatment costs
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Comparison of FWD to Mission Green organic waste collection
DISCUSSION:
1.
Survey of Ontario Municipalities
The Town of
2.
Effects of FWDs on Sanitary Sewers and Sewage
Treatment Costs
Water Consumption and Sewer Loading
FWDs will increase the amount of
water used by an average of 4-litres/capita/day. If all current apartment units were equipped
with FWDs, that would result in the additional use of 41,000 m3 (41,000,000 litres) of water per year. This is equivalent to the total water used by
between 100 and 140 new housing units.
Sewer Use By-Law Compliance
FWDs add organic matter and
suspended solids to the existing sanitary sewerage flows. Staff collected samples of sanitary sewage to
determine the typical sewage strength in both low and high density residential
developments. From this data and the
typical contribution of biological oxygen demand (BOD) and total suspended
solids (TSS), it was determined that sewage discharges from high rise
developments will exceed the Town and Region of York Sewer Use By-Law limits
when more than 65% of the apartment units are equipped with FWDs. When sewage strength exceeds the By-Law
limits, additional costs for wastewater treatment are incurred and additional
costs for sewer maintenance can be expected.
Operation of Municipal Sewers
Food waste disposers shred the
organic waste and mix it with water, allowing the solids to be carried in
suspension in the liquid sewage. The
shredded waste may have higher density than water and therefore will tend to
settle out of the liquid. This settling
effect is counter-acted by the turbulence and velocity of the water in the
sewer which can re-suspend the settled material and carry the solids
downstream. Approximately, 16-20% of
solids generated by FWDs will settle out and would require re-suspension by
turbulence in the sewer to be transported.
When organic material settles in
the sewers, there is increased production of hydrogen sulphide gas. This gas not only creates odour problems, but
it can be fatal even in small concentrations.
Hydrogen sulphide gas also leads to increased corrosion and
deterioration of the sewer system. This
in turn can lead higher maintenance costs and higher replacement costs. Settled material must be flushed out of the
sewer on a regular basis to avoid the production of gas and odours and to avoid
blockages.
Sewer system operators in municipalities
with large numbers of FWDs report that most of the sewer blockages occur in the
sanitary sewer connection from the building to the mainline sewer. The connections are often laid with minimal
slope and the pipes are prone to structural failure. These factors create conditions that lead to
blockages. The building owner is
responsible for the cost of clearing the blockages and repairing the connection
pipes.
Sewage Treatment Plant Costs
The organic material that enters
the sewage system from FWDs eventually reaches the sewage treatment plant where
mechanical, biological and chemical processes remove the organic material from
the wastewater before it is disinfected and discharged back into surface
waters. If all other existing apartments
are equipped with FWDs, the additional cost for sewage treatment would be
approximately $665,000 per year. As the
average strength of the raw sewage increases, the treatment costs also
increases. Wide spread use of FWDs will
increase the raw sewage strength. If
FWDs are widely adopted, and the sewage strength increases above the current
values, the treatment cost will increase and the sewage treatment plant may
require expansion at additional cost.
The final by-product of sewage
treatment is the leftover sludge of settled solids, microbial biomass. This sludge is dewatered at the treatment
plant and disposed of by land-spreading or incineration. Since the sludge is contaminated with non-organic
material, such as metals and biologically resistant chemicals land-spreading
and incineration have environmental impacts that must be carefully
managed. Adding organics from FWDs
ultimately increases the volume of sewage sludge generated at the plant which ultimately
must be disposed of by incineration, land-spreading, or other means.
Comparison
of FWD to
FWD and source separated organics
(SSO) collection system have different waste diversion potential. The quantity
of food waste generated annually by households in multi-family residence that
is suitable for disposal in a FWD is approximately 125kg per household per year
or approximately 60% of the organic waste generated in multi-family residential
units. In
Cost analysis and comparison of
various alternative methods for disposing of organics indicate that home composting
is the least expensive option while FWDs are the most expensive. Based on the information available, the use of
FWDs is 1.5-2.0 times the cost of SSO for disposal of kitchen waste.
The installation cost of a typical
household FWD unit is estimated to be about $600. This assumes a simple
installation that can be completed by a plumbing contractor only. The cost of installing FWDs in all of the
existing apartments within the Town will be approximately $5,340,000. FWDs
require an under-sink electrical supply which few, if any, homes and apartments
posses. Installation of an under-sink
electrical outlet would add to the cost of the FWD installation. Also, in some
households the vertical space beneath the kitchen sink is insufficient for a
FWD installation. In these cases, the
cost of a FWD installation would be many times greater than that of a simple
installation.
CONCLUSIONS:
Staff recommend that the Town
continue its current practice of prohibiting the disposal of shredded organic
material in the sanitary sewer system. Calculation of the total costs to dispose
organic waste by means of a FWD and treatment of the waste at the sewage
treatment plant indicates that the FWD method is 1.5-2.0 times more expensive
than SSO programs, such as Mission Green. These figures do not include any increased
costs associated with sewer maintenance and rehabilitation that may be required
due to the increase in organic matter deposits, oil and grease build-up,
hydrogen sulphide production, and additional sewage flows.
Mission Green represents a
significant existing investment by the Town and its residents. Expansion of Mission Green organics collection to multi-unit buildings and
private condominiums should be encouraged.
FINANCIAL IMPLICATIONS:
There are no financial implications
resulting from the adoption of the recommendation of this report.
ATTACHMENTS:
Attachment”A” Technical Memo
– Investigation of Food Waste Disposers
Attachment “B” Letter dated
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General Manager, |
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Peter Loukes, P.Eng. Director of Operations and Asset
Management |
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Jim Sales Commissioner of Community and
Fire Services |
Q:\Commission
Share\Operations and Asset Management\Reports\2005\Waterworks\Food Waste
Disposer\Evaluation of Household Food Waste Disposers (Report).doc
