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Water stabilisation through a limestone mediated sidestream process
Author(s)
Tsotsi, Ciciyela Mlungisi
Date Issued
2001
Type
Thesis
Publisher
Cape Technikon
Abstract
Approximately 40% of the surface waters of South Africa, and the surface waters
of Lesotho, are soft and acidic with low calcium, alkalinity and carborrate species
concentration. Distribution of such waters results in aggressive attack of cement
concrete pipes and linings, and corrosive attack of metal pipes. valves, etc.
within the distribution network. The impact of such attack is usually significant,
and usually includes the loss of water, the need for expensive repairs and
deterioration in drinking-water quality.
Water conditioning to prevent aggression and/or corrosion (termed stabilization)
is conventionally achieved by the addition of lime (Ca(OH)2) and carbon dioxide
(C02). Although this process is well documented and understood, it has a
number of drawbacks which include the use of troublesome lime, high chemical
operating costs, and the need for high quality white lime which is currently in
short supply in South Africa. An alternative stabilization process is stabilisation
via contact with limestone (solid calcium carbonate), which has been shown to
have many advantages over Iime-mediated stabilization. However, the use of
limestone mediated stabilization has hitherto been limited to smaller water
treatment works as a result of the large contact tanks required. Use of limestone
for larger water treatment works was made viable by the development by CSIR
of the limestone mediated Sidestream Stabilisation Process (SSP).
The SSP consists of taking a sidestream of approximately 2 - 5% of the
unstabilised water, dosing high levels of C02, and then contacting the COr
acidified stream with limestone. The acidified sidestream takes up considerable
amounts of calcium carbonate (CaC03), increasing the alkalinity and the calcium
concentration.
of Lesotho, are soft and acidic with low calcium, alkalinity and carborrate species
concentration. Distribution of such waters results in aggressive attack of cement
concrete pipes and linings, and corrosive attack of metal pipes. valves, etc.
within the distribution network. The impact of such attack is usually significant,
and usually includes the loss of water, the need for expensive repairs and
deterioration in drinking-water quality.
Water conditioning to prevent aggression and/or corrosion (termed stabilization)
is conventionally achieved by the addition of lime (Ca(OH)2) and carbon dioxide
(C02). Although this process is well documented and understood, it has a
number of drawbacks which include the use of troublesome lime, high chemical
operating costs, and the need for high quality white lime which is currently in
short supply in South Africa. An alternative stabilization process is stabilisation
via contact with limestone (solid calcium carbonate), which has been shown to
have many advantages over Iime-mediated stabilization. However, the use of
limestone mediated stabilization has hitherto been limited to smaller water
treatment works as a result of the large contact tanks required. Use of limestone
for larger water treatment works was made viable by the development by CSIR
of the limestone mediated Sidestream Stabilisation Process (SSP).
The SSP consists of taking a sidestream of approximately 2 - 5% of the
unstabilised water, dosing high levels of C02, and then contacting the COr
acidified stream with limestone. The acidified sidestream takes up considerable
amounts of calcium carbonate (CaC03), increasing the alkalinity and the calcium
concentration.
Additional information
Thesis (MTech (Chemical engineering))--Cape Technikon, Cape Town, 2001
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