November 19, 1998
To:Works and Utilities Committee
From:Barry H. Gutteridge, Commissioner, Works and Emergency Services
Subject:Drinking Water Taste and Odour Control Options
Purpose:
To provide information on options to eliminate or reduce the occurrences of an unpleasant
taste and odour in drinking water and our strategy to address this issue in the future.
Funding Sources, Financial Implications and Impact Statement:
There are no financial implications as a result of this report.
Recommendations:
It is recommended that this report be received for information.
Council Reference/Background/History:
The Works and Utilities Committee on September 9, 1998, had before it a report (dated
August 28, 1998) from the General Manager, Water & Wastewater Services, providing
information on the recent occurrence of an unpleasant taste and odour in drinking water and a
strategy to address the issue; advising that the taste and odour was caused by minuscule levels
of a compound produced by algae and higher than normal water temperatures, and that water
quality was otherwise not affected and the water remained safe to consume; further advising
that given the likelihood of increased frequency and duration of taste and odour occurrences in
future, the feasibility of alternative processes for taste and odour control at the water treatment
plants is being investigated.
The Committee received the aforementioned report; and requested the General Manager,
Water and Wastewater Services, to submit a further report to the Committee on:
(1)the options available to eliminate or reduce odour and other problems in the future with
regard to water quality;
(2)chemical analyses of water samples for the last ten years; and
(3)a description of the steps involved in making the water from Lake Ontario potable.
This report addresses issues relevant to taste and odour control. The other issues will be the
subject of a separate report.
Comments and/or Discussion and/or Justification:
Every year during late summer and early fall, the temperature of the raw water being drawn
into our water treatment plants, through intake lines one to three kilometres from shore,
increases to maximum levels and the growth of algae in the lake water heightens.
These naturally occurring phenomena do not in themselves bring about taste and odour
episodes. The cause of these episodes, which have been sporadic, is the formation of trace
amounts of compounds called Geosmin and Methylisoborneol (MIB) produced during the life
cycle of only certain species of algae.
Because of the relatively short duration and infrequent occurrences of past taste and odour
episodes, expenditures required for modifications to our water treatment plants to incorporate
a taste and odour control processes were not justified. The episode in August of this year
lasted two weeks and resulted in more than 1,400 public complaints. Given our experience
this year with increases in lake water temperature occurring earlier than normal and ecological
changes resulting in blooms of algae at lower depths, it is likely that the frequency, duration
and intensity of taste and odour episodes will increase in the future. Accordingly, we have
completed a preliminary investigation of processes which may be feasible for short-term and
long-term taste and odour control at our plants and have developed a strategy for addressing
taste and odour.
The following options for taste and odour control were reviewed:
1.Activated Carbon
Activated carbon as a powder or in granular form to replace anthracite (filter cap) or in
separate contactors, is used extensively to remove taste and odour compounds from water
supplies.
The primary raw material used for activated carbon is any organic material with a high carbon
content (coal, wood, peat, coconut shells). The carbon-based material is converted to activated
carbon by thermal decomposition in a furnace using controlled atmosphere and heat.
Activated carbon has a tremendous adsorptive capacity, an affinity for a wide variety of
dissolved organics and an ability to be custom-tailored to suit specific applications.
(a)Granular Activated Carbon (GAC) -Contactors
The process which is anticipated to be most effective in removal of compounds causing taste
and odour is GAC contactors following the conventional filtration process. This process will
also enable further reduction in the levels of trace organic compounds such as
trihalomethanes. GAC contactors are essentially a second set of filter beds. Therefore,
additional land space would be required at each of our plants. This would be problematic at
the R.C. Harris Plant. The estimated capital costs of installing this process at our four water
treatment plants is in excess of $100 million and its installation is expected to take at least six
years.
(b)Powdered Activated Carbon (PAC)
PAC is a very finely ground powder and is added directly to raw water early in the treatment
process. PAC requires minimal capital cost to feed, has the flexibility to be applied as needed
and can be implemented within short timelines.
Although the use of PAC for taste and odour control is effective for removal of low levels of
taste and odour compounds, it may not be effective for removal of high levels of compounds.
Bench-scale testing to determine its impact on filter performance and treated water quality
would be required. There may need to be modifications to other treatment processes such as
pre-chlorine. Use of PAC would also have a visual impact for the plants that discharge
backwash waters directly into the lake without any treatment.
(c)Granular Activated Carbon (GAC) -Filter Capping
GAC-Filter Capping involves replacing the existing anthracite (and if necessary, some sand)
in the existing filter beds with GAC. Because filter capping can be implemented readily as a
retrofit, it has significantly lower capital cost than post-filter GAC contactors and does not
require additional land. However, contact times associated with the GAC-Filter Capping
treatment suggest this may not be completely effective in taste and odour removal.
As in the case of PAC, bench-scale testing and some pilot testing would be necessary to
determine the overall performance of GAC, impact of GAC capping on filter effectiveness and
backwashing, GAC replacement frequencies and the overall impact on treated water quality.
2.Oxidation Processes
Odours associated with high concentrations of some types of algae can be removed by the use
of free chlorine (about 2 milligrams per litre) before adding ammonia. However, since
increase in chlorine can actually exacerbate the problems by algae die-off releasing more taste
and odour compounds in the water, this option is not recommended.
Other chemicals such as chlorine dioxide, potassium permanganate (to prevent algae and
slime growth in water intakes), and ozone can be used for taste and odour control. The use of
these chemicals could also impart their own taste and odour problems ("chlorinous" for
chlorine dioxide or "ozonous" for ozone) to the drinking water. These options would require
further investigation and testing to assess their viability.
3.Advanced Oxidation Processes (AOPs)
AOPs use combination of two oxidants and have been found to be more effective than ozone
alone. AOPs include hydogen peroxide-ozone (also called peroxone) process, ozone-Ultra
Violet (UV) process, ozone-elevated pH process, hydrogen peroxide-UV process. While many
of these processes are observed to be capable of destroying taste and odour compounds on
bench and pilot scale, there is insufficient operational experience on the performance of these
processes on a plant scale.
4.Biological Treatment
Biological treatment involves operation of the conventional filters so that microbial growth is
encouraged within the filter media. These bacteria biodegrade taste and odour compounds
either by changing their structure or by making these compounds less volatile.
Since ozone increases the biodegradability of natural organic matter (NOM) in water,
ozonation is often considered as a pre-treatment for biological processes. However, the net
result of ozonation-biological process could be the removal of compounds responsible for
earthy-musty odours and the creation of compounds responsible for fruity odours.
It must be noted that while a properly operated biological filter is capable of removing almost
all taste and odour compounds, biological instability of the process during operation can
create intense, undesirable odours. Extensive pilot testing is required before further
consideration of this process to ensure that the bacteriological quality of drinking water is not
affected.
5.Resource Control
Management and control of taste and odour problems through resource control can be defined
as the measures taken to prevent or alleviate the problem at the source as opposed to changes
at the treatment plant. Such measures include control of nutrient influx, algae control using
algicides, artificial destratification (i.e. mixing), biological control of nuisance algae, etc.
These measures are typically applicable to smaller lakes and reservoirs and are not suitable for
Lake Ontario.
6.Extension of Intakes
One of the options that warrants further investigation is the possibility of extending the
intakes so that colder (say 4oC) and more consistent quality water is withdrawn all year round.
It is expected that raw water at such low temperature would be free of algae and other agents
releasing taste and odour compounds. As part of the Deep Lake Water Cooling Project, a new
deeper intake has been proposed for the Island Filtration Plant. It is expected that the
pre-design study for this project will be completed by early 1999. A sampling program, as part
of this study, will provide information on the actual water quality at the proposed intake
location. Results to date confirm the absence of taste and odour compounds in the deep lake
water. However, additional analysis is required before final conclusions can be made. The
detailed design and construction of the facilities may take place over two to three years. Once
the pre-design study for the Island Plant is completed, the feasibility of extending intakes at
our Harris, Horgan and Clark Plants to draw water from deeper levels will be reviewed as an
alternative approach to process changes. This is a costly option as the proposed intake for the
Island is estimated to cost over $20 million. The Island intake is proposed to be 7.5 feet in
diameter and 2.6 kilometres in length. Existing intakes at the other plants are typically 10 to
12 feet in diameter and extensions may be 4 to 11 kilometres long depending on the plant.
Having assessed the options, we have developed the following strategy. A feasibility study by
consultants will be funded through available 1998 funds and completed by early 1999.
Pending completion of the feasibility study, we have allocated funding in our 1999 Capital
Works Program for the installation of additional facilities using activated carbon at our plants
for short-term taste and odour control.
This interim approach is based on the following criteria for selection of the treatment option:
(1)effectiveness of taste and odour control;
(2)effect on treated water quality;
(3)lake water quality impact;
(4)cost;
(5)available resources (facilities at the plants, personnel);
(6)ease of installation; and
(7)time required for installation.
The proposed approach includes replacement of existing anthracite with GAC in the filters at
the R.L. Clark and half of R.C. Harris Filtration Plants and facilities to feed PAC at the F.J.
Horgan and Island Plants. Since our objective is to install taste and odour control facilities as
soon as possible, installation of GAC contactors is viewed as a long-term soluton. Addition of
PAC at the Harris and Clark Plants is not proposed due to the absence of residue management
facilities at these plants and the impact of direct discharge of backwash waters into the lake.
The outcome of the feasibility study is not expected to alter the fundamentals of this interim
approach but may identify alternative combinations of GAC/PAC application. Should this
occur, adjustments to the program will be made. The total Capital cost for this option is
$7,022,000.00. Funds for this work have been included in the 1999-2003 Capital Works
Program submission.
This interim solution is expected to alleviate taste and odour problems based on the levels of
taste and odour compounds previously experienced in the raw water. Should the levels of such
compounds be higher, some problems may still be experienced, albeit at a reduced level.
There will be production capacity in the order of 1,800 million litres per day which will meet
anticipated demands during the months of August and September when taste and odour events
traditionally occur.
The other options discussed are long term and involve substantial capital expenditure.
Additional assessment is required as projects of that scope require an intensive study to
confirm that appropriate solutions are chosen. A long-term solution will be the subject of a
future report.
Conclusion:
Options to eliminate or reduce taste and odour in Toronto's drinking water have been
reviewed. A feasibility study will be undertaken to determine an effective interim solution.
Funding has been allocated in the 1999 Capital Works Program to implement an interim
solution at an estimated cost of $7 million, pending confirmation by the feasibility study.
Contact Name and Telephone Number:
Mr. H.A. Taniguchi, P. Eng.
Director of Water Supply Division
Phone: (416) 392-8220
Fax: (416) 392-3639
E-mail: htaniguchi@metrodesk.metrotor.on.ca
Michael A. Price, P.Eng., FICE
General Manager
Water and Wastewater Services
Barry H. Gutteridge
Commissioner
Works and Emergency Services
ADT/LBL/rah