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
