On-site Assessment of the Suitability of Limestone Mediated
Stabilisation at Makwane Water Treatment Plant
1
TERMS OF REFERENCE
This investigative report was commissioned by N. D. Basson, Manager: Scientific Services of
Goudveld Water on 08 June 2000, for the on-site assessment of Limestone Mediated
Stabilisation technology at the Makwane Water Treatment Plant, QwaQwa, Free State
Province.
This report is to contain inter alia:
•
•
•
•
A brief description of the CSIR test-rig and the operation thereof.
Data collected from the operation of the test-rig.
An assessment of the suitability of limestone mediated stabilisation for fullscale operation.
A costing comparison of the full-scale implementation of stabilisation with
Bredasdorp limestone versus lime and sodium carbonate at Makwane.
The on-site study was carried out by P F de Souza, K C Jason and T P Manxodidi, Cape
Water Programme, Environmentek, CSIR during the period 19 June 2000 to 23 June 2000.
2
ACKNOWLEDGEMENTS
The authors would like to thank the following persons who contributed towards the success
of the project:
•
Danie Traut and Fanus Weyers of Goudveld Water for making the necessary
site arrangements, providing information with regards the treatment and
supply of water in QwaQwa, and assisting with on-site troubleshooting.
•
Kenneth Jason and Thabisa Manxodidi for their field test work including onsite measurements and titrations of chemical determinants.
•
Mike Louw and the staff of the CSIR Analytical Laboratory for chemical
analyses of samples.
3
BACKGROUND
3.1
Introduction
Water sources for municipal supplies in Southern Africa range over a broad spectrum of
chemical quality, resulting from the region’s complex geographical, hydrological and
geological characteristics. At one end of the water quality scale, a large portion of waters are
very soft, typically with low conductivity (5-50 mS/m), low total alkalinity (0-25 mg/L as
CaCO3), low calcium (0 - 25 mg/L as CaCO3) and low pH (4.0-7.0). At the other end are
waters with high concentrations of dissolved solids (including calcium, magnesium, sodium
chloride, and sulphate ions) with high total alkalinity and variable pH, and where no oxygen
is present, high concentrations of dissolved iron, manganese and possibly carbon dioxide. Inbetween these, waters with virtually any combination of chemical quality may be found.
Most waters will require some form of treatment before discharge to distribution systems.
Conventional water purification of soft waters usually comprises clarification using floc
agents such as ferric chloride, ferric sulphate and aluminium sulphate, and disinfection via
chlorination. Both processes further depress pH and total alkalinity prior to release of the
purified water into the distribution network. Figure 1 shows the approximate distribution of
soft, acidic waters in South Africa.
Figure 1: Approximate distribution of soft, acidic waters in South Africa
Goudveld Water - Assessment of Limestone Mediated Stabilisation
In most Southern African municipal water distribution systems, approximately 90% of the
pipes are either composed of a cement-type material or are cement-lined iron pipes. The
remainder of the pipes are metal or UPVC-type pipes. This arises for two reasons. Firstly,
cement-type pipes are cheaper than metal-type, and secondly, cement material does not
undergo redox reactions in an aqueous environment and is therefore not subject to corrosion.
Nearly all distribution network storage reservoirs are concrete. However, concrete and
cement-lined structures containing or transporting water are vulnerable to aggressive attack
by soft waters, and numerous examples exist of the significant cost of lost water (originating
from pipe bursts and leaks), pipe and reservoir rehabilitation, and decrease in water quality
resulting from aggressive and corrosive attack of distribution networks. This is, in particular,
the case for smaller towns not provided with water by water boards, and where conventional
treatment processes are often found to be problematic for small water treatment works.
The financial cost of such aggressive and corrosive attack on distribution networks and water
systems in private households in Southern Africa is substantial.
Brief South African examples indicate the severity of typical related problems.
Lost water
In many smaller municipalities water lost from pipe bursts and leaks is unquantified, yet
recognized as being higher than acceptable. It is not unusual for lost water to be as high as
50% of treated water, and the associated cost can be substantial. For example: in the Durban
townships of Kwamashu, Umlazi and Ntazuma, total water losses from service reservoirs and
the reticulation network was “found to be around 50% of the total bulk volume of water
purchased from Umgeni Water” prior to rehabilitation. The cost of lost water amounted to
about twelve million Rand in 19921.
Repairs
The cost of reservoir and pipe rehabilitation can be significant for water supply authorities.
For example: in Stellenbosch, prior to the installation of stabilization units ending 1997, an
average of 30 sections of pipe were replaced per month at an average cost of about R5,000
per section2. Similarly, Umgeni Water had to carry out expensive repairs to their Durban
Heights Reservoir in 1990, to overcome a leak of 300 m3/day as a result of aggressive attack3.
1
Deighton, ME and Vawda, MS (1992) Water Management in Durban’s townships-stopping the snowball.
Paper presented at the Young Water Engineers Conference, 5-6 March 1992, Midrand.
2
Hartenzenberg, PSJ (1993) Private communication
3
Hakin, WD and Crawford, CE (1992) The use of a geo-membrane in a potable water reservoir to prevent
concrete attack. Paper presented at the Young Water Engineers Conference, 5-6 March 1992, Midrand.
Goudveld Water - Assessment of Limestone Mediated Stabilisation
At consumer level, corrosive attack can necessitate expensive replacement of pipes and
geysers.
Potability
Corrosive attack of metal conduits and valves in household and municipal water reticulation
systems results in raised level of dissolved metals in household water. In many cases
dissolved metal levels resulting from corrosion exceed SABS 241-1999 Specification for
Water for Domestic Supplies requirements. Moderately raised levels of copper and iron can
lead to stomach complaints, staining of baths, laundry and hair. Severe cases make the water
unfit for human consumption and/or other household use.
These examples of the impact of aggressive and corrosive attack reflect a general problem
that exists throughout areas where soft, acidic waters are found. To overcome these problems,
some understanding of aggression, corrosion and effective water conditioning is essential.
3.2
Soft Water Aggression and Corrosion
3.2.1 Saturation State with respect to Calcium Carbonate
In terrestrial waters, the carbonate system is the dominating one to such an extent that other
weak acid/base systems are usually neglected. The carbonate system in water is comprised of
the species molecularly dissolved carbon dioxide, CO2aq, carbonic acid, H2CO3, and the ionic
species bicarbonate, HCO3-, and carbonate, CO32-, and the water species, H+ and OH-. The
relative concentrations of the dissolved species are governed by chemical equilibrium, and it
is the interaction between these species that controls the pH in natural terrestrial waters.
Furthermore, consideration needs to be given to inter-phase equilibrium, i.e. water brought
into contact with the gaseous phase (carbon dioxide in the air) or a solid phase (calcium
carbonate).
For soft, acidic waters, the solubility of the mineral CaCO3 is of importance. In this context it
is necessary to obtain both a qualitative description of saturation (i.e. whether the water is
saturated, undersaturated or supersaturated, with respect to CaCO3), and a quantitative
description of the saturation state (i.e. the mass of CaCO3 that will dissolve in/precipitate
from the water). Use of non-quantitative methods such as the Langelier Saturation Index and
the Aggressiveness Index should be avoided, as their values are often misconstrued as being
of quantitative significance. The Calcium Carbonate Precipitation Potential (CCPP)
provides an accurate prediction of the amount of solid calcium carbonate that will precipitate
from or dissolve into a water.
Goudveld Water - Assessment of Limestone Mediated Stabilisation
The CCPP defines the mass of CaCO3 to be precipitated from a water to attain saturation with
respect to CaCO3. For example, a water with a CCPP of 35 mg/L as CaCO3 will precipitate
35 mg/L CaCO3 , to reach chemical equilibrium. In doing so, the pH, Alkalinity and calcium
levels of the water will decrease, whilst the CCPP will decrease eventually to zero.
Conversely, a water with a Calcium Carbonate Dissolution Potential (CCDP) of 10 mg/L
as CaCO3 will dissolve 10 mg/L CaCO3 to reach chemical equilibrium.
Consequently these parameters, CCPP or CCDP, give both a quantitative, and qualitative
description of the saturation state of a water. The CCPP/CCDP may be experimentally
determined with the Marble Test (see Standard Methods, 1985). However, this method is
both labourious and prone to error, and for these reasons, theoretical methods, based on
equilibrium chemistry, are preferable. Theoretical determination of the saturation state and
the CCPP/CCDP, using equilibrium chemistry, is carried out very easily by using an
interactive computer programme such as STASOFT. For detailed description of the use of
either of these procedures, the reader is referred to the South African Water Research
Commission publications: Softening and stabilisation of municipal waters4, and Chemical
Conditioning of Low and Medium Salinity Waters: STASOFT Version 3.05. In addition, an
updated version of the STASOFT package (STASOFT Version 4.0) is currently being
developed.
3.2.2 Aggressive attack
Soft acidic waters attack cementitious material by leaching free lime, calcium aluminates and
silicates out of the cement matrix. Where the chemical characteristics of the water are such
that it is undersaturated with respect to calcium carbonate, calcium carbonate precipitates,
formed on the outer surfaces of the cement mass by the reaction of free lime (Ca(OH)2) and
carbon dioxide in the air or water, will dissolve. Under such conditions, progressive leaching
of calcium minerals will occur, thereby damaging the integrity of the material, and may result
in the eventual failure of the structure. Such attack is termed aggressive attack and such
waters are called aggressive. Changing the chemical characteristics of the water such that it
does not dissolve CaCO3 from the cement matrix can prevent such aggressive attack.
4
Loewenthal, RD, Weichers, HNS and Marais, GvR (1986) Softening and Stabilization of Municipal Waters.
Water Research Commission, Pretoria.
5
Friend, JFC and Loewenthal, RE (1992) Chemical Conditioning of Low-and Medium Salinity Waters:
STASOFT Version 3. Water Research Commission, Pretoria.
Goudveld Water - Assessment of Limestone Mediated Stabilisation
3.2.3 Corrosive attack
When water is being conveyed, or used for any purpose, interaction with metal components
of the water distribution and household systems occurs. The most commonly utilised metals
in these systems are iron (municipal and household) and copper (mainly household).
Corrosion of these metals principally results from oxidation and reduction reactions at sites
on the metal-water interface. Depending on the characteristics of the water and metal, the
reactions may give rise to continuous dissolution of the metal into the water, (corrosion), or
precipitation of stable minerals onto the metal surface, thereby reducing the areas of active
electro-chemical sites and the rates of reactions, even eventually stopping the corrosion
completely (passivation).
3.3
Aggression/Corrosion Mitigation
3.3.1 Full Stabilisation
Aggression Mitigation
In order to prevent aggressive attack of distribution networks, it is important to alter the
chemical characteristics of the water so that it is saturated with respect to CaCO3 prior to
distribution in a reticulation system. Under such conditions, initially the dissolution process
continues with the more soluble free lime being leached from the outer surface of the cement
paste. Dissolution of free lime results in supersaturation with respect to CaCO3 at the cement
surface. Concomitant precipitation of CaCO3 takes place, sealing off the uncarbonated
cement surface from the bulk water body, and thereby preventing further dissolution. To
guard against the development of undersaturated conditions resulting from carbon dioxide
generation by biological activity, a slight degree of supersaturation is desirable.
Æ
To prevent aggressive attack by soft, acid waters a CCPP of 1 to 4 mg/L is
usually recommended.
Corrosion Mitigation
Control of corrosion in low pH, low alkalinity waters may take several forms including
chemical addition and selection of materials resistant to corrosion. The corrosivity of a water
depends mainly on pH, carbonate balance and oxygen levels within the water. However,
other factors influencing the corrosion process are: the presence of chlorides and sulphates;
velocity of flow; temperature; and the presence of micro-organisms.
Goudveld Water - Assessment of Limestone Mediated Stabilisation
Guidelines for iron corrosion passivation are as follows6:
Guideline 1:
The bulk water should be saturated, or slightly supersaturated, with
respect to CaCO3.
Guideline 2:
Calcium and Alkalinity values should not be less than 50 mg/L (as
CaCO3).
Guideline 3:
Waters be regarded as potentially corrosive when either the chloride or
sulphate levels exceed 50 mg/L.
Guideline 4:
Design conduits in reticulation systems to maintain a velocity in excess
of 0.2 m/s, preferably > 1 m/s, and avoid dead ends. Where these
conditions are not likely to be satisfied, utilise cement type or plastic
pipes.
Guideline 5:
The dissolved oxygen in the water should be greater than about 4 mg/L
(as O2)
Iron corrosion passivation for soft acidic waters requires extremely high chemical doses and
is, hence, both expensive and impractical. Consequently, pipes should be lined with cement
mortar, and the water treated to be non-aggressive.
With regards to copper corrosion passivation, soft, acidic terrestrial waters passivation is
achieved by the formation of a protective copper oxide layer which is generally readily
achievable by ensuring pH greater than 7.1.
Æ
To prevent corrosive attack by soft, acid waters it is recommended that
iron pipes should be lined with cement mortar, and a CCPP of 1 to 4
mg/L is usually recommended.
From the above it is clear that the principal means to prevent aggressive and corrosive attack
by soft, acidic waters is the chemical conditioning, or stabilisation, of the water. Stabilisation
is usually achieved via the addition of either lime (to increase calcium and Alkalinity levels)
or sodium alkali’s (to increase Alkalinity), and carbon dioxide (to increase carbonate species
and adjust pH).
6
Loewenthal, RD, Weichers, HNS and Marais, GvR (1986) Softening and Stabilization of Municipal Waters.
Water Research Commission, Pretoria.
Goudveld Water - Assessment of Limestone Mediated Stabilisation
3.3.2 Partial Stabilisation
Whilst conventional full stabilisation via the addition of lime and carbon dioxide, or sodium
alkali’s and carbon dioxide, is well documented and understood, control of the process is
expensive and requires well-trained staff and reliable equipment. For the rural small volume
user, such stabilisation processes are not feasible, and the costs of aggression/corrosion can
be significant. An alternative approach is partial stabilisation.
Partial stabilisation has been shown to be effective in preventing cement aggression, copper
corrosion and greatly reducing corrosion of any ferrous material in the water system. Results
obtained from limestone stabilisation units installed at sites in South Africa where there were
previously considerable corrosion problems, and typically severe cases of “blue” and “red”
water, are given in Table 1. Partial stabilisation effectively eliminated the “blue” and “red”
water problems. Table 2 shows water quality results within the distribution network of
Bredasdorp, South-Western Cape. Samples were collected directly after limestone
stabilisation and within the reticulation network. The CCDP of all the samples was
acceptably low, and pH values were all greater than 8.3, indicating the effectiveness of the
limestone stabilisation process in providing a well buffered, partially stabilised water.
Table 1: Limestone treatment of surface water
Prior to Limestone Stabilisation
After Limestone Stabilisation
Cold Tap
Hot Tap
Cold Tap
Hot Tap
Calcium as Ca (mg/L)
0.5
0.7
5.5
5.4
Alkalinity as CaCO3 (mg/L)
0.8
11.8
12.8
14.2
PH
4.7
6.5
7.0
7.1
Conductivity (mS/m)
6.5
7.0
6.7
6.4
159.0
20.5
9.08
8.43
Copper as Cu (mg/L)
0.1
14.9
< 0.02
< 0.02
Iron as Fe (mg/L)
14.9
16.9
< 0.05
< 0.05
CCDP as CaCO3 (mg/L)
Goudveld Water - Assessment of Limestone Mediated Stabilisation
Table 2: Bredasdorp reticulation network data
PrePoststabilisation stabilisation
Determinant
Network
#1
Network
#2
Network
#3
pH (field)
5.1
8.8
8.9
9.2
8.3
Ca (mg/L CaCO3)
11.7
32.8
32.5
36.0
32.3
Alkalinity (mg/L
CaCO3)
0.8
15.5
17.8
19.8
16.3
CCDP (mg/L CaCO3)
52.7
1.9
1.4
-0.6
2.9
Turbidity (NTU)
0.47
0.3
n/a
n/a
n/a
n/a:
Not analysed
The above tables show that partial stabilisation using limestone contactors can significantly
reduce corrosive attack of iron and copper and aggressive attack of cement/concrete pipes.
4.
STABILISATION OF SOFT, ACIDIC WATERS USING LIMESTONE
4.1
Conventional Stabilisation
The guidelines set out above (in section 3.3.1) for limiting aggression and corrosion can be
met in part through good design, and in part through chemical treatment. The principal
treatment component of measures to prevent corrosive and/or aggressive attack by soft, acidic
waters (which are usually low in sulphate and chloride species) is the chemical conditioning,
or stabilisation, of the water.
Several methods are available for attaining the desired slightly supersaturated water quality
of 1 to 4 mg/L as CaCO3. These include inter alia dosing of sodium alkali’s (such as sodium
carbonate, sodium hydroxide or sodium bicarbonate) and the conventional approach of using
lime and carbon dioxide. By far the most commonly used process for the stabilisation of soft,
acidic waters is the latter, in which stabilisation is achieved via the addition of lime
(Ca(OH)2), to increase calcium (Ca2+) and Alkalinity levels, and the addition of carbon
dioxide, (CO2), to add carbonate species and adjust pH.
Whilst such stabilisation is well documented and understood, control of the process requires
well-trained staff and reliable equipment which are both seldom available in the many small
towns and communities receiving such waters. Hence, in many cases only lime is dosed, such
that pH is adjusted from low levels to more desirable levels of, say, 8.0, thereby providing a
Goudveld Water - Assessment of Limestone Mediated Stabilisation
partially stabilised water. Even so, for a smaller municipal installation, stabilisation using
lime, (with or without carbon dioxide), remains notoriously problematic and difficult to
control. Anecdotal accounts by Goudveld Water confirm that due to these operational
problems, lime dosing seldom occurs on a continuous basis during normal Water Treatment
Plant operation at Makwane. The following figures show the current lime dosing system
employed at Makwane Water Treatment Plant.
Figure 2: Lime dosing system at Makwane Water Treatment Plant, QwaQwa
Furthermore, lime and carbon dioxide mediated stabilisation is expensive, usually comprising
more than half of the chemical cost of treating water results from stabilisation. In addition,
the increasing limited availability of high quality (white) lime locally will result in increased
operating chemical costs at water treatment facilities, as lime will need to be sourced outside
of South Africa.
Goudveld Water - Assessment of Limestone Mediated Stabilisation
4.2
Partial Stabilisation with Limestone
The first attempts to stabilise municipal water were made using limestone. The first
documented application occurred in 1906 in Frankfurt, Germany, when domestic water was
treated by contacting it with a marble bed. It was shown that with such treatment, corrosion
of the distribution network was significantly reduced. However, the availability of the more
readily soluble lime resulted in the use of limestone falling from favour, and by the 1930’s,
the application of limestone was essentially discontinued. Nevertheless, limestone
stabilisation was further investigated in the Netherlands in the 1960’s and a number of pilot
studies were carried out. In South Africa, a limited number of small limestone units were
installed in the 1960’s and 1970’s, but these fell into disuse; apparently as a result of poor
treatment efficiency and hydraulic failure resulting from both poor design and the use of
inappropriate limestone. Although there are few documented cases of full-scale use of the
technology, it would appear that there has been recent renewed interest in limestone mediated
stabilisation and that a number of small plants have been installed in the USA and Canada.
In South Africa, Cape Water Programme, CSIR, reinitiated research into limestone
stabilisation in the early 1990’s. The development process has included:
•
•
•
•
•
•
Identification of limestone deposits, and experimental assessment of relative
suitability of various limestone deposits, and optimisation of particle size distribution
for fixed bed contact process.
Interaction with limestone mine and chemical suppliers to ensure commercial supply
of water treatment grade limestone pebbles.
Kinetic modelling of limestone dissolution rates.
Pilot plant verification of fixed bed process and process design considerations, and the
marketing thereof to civil engineering consultants and town engineers.
Experimental determination of reactor sizing for full-scale implementation.
Process design, trouble-shooting and commissioning.
The limestone used to-date in South Africa is the commercially available limestone pebbles
from Bredasdorp, South-Western Cape. The Bredasdorp deposit is a sedimentary deposit of a
porous, friable nature. The cation content of this limestone is (by mass) 96 % calcium, 1.7 %
silica and 1.3% magnesium. Iron and manganese are present at less than 0.1%. Thus, the
Bredasdorp limestone deposit can be classified as a high calcium (and low magnesium)
limestone. The limestone used has a grading of +12 mm -15 mm. The following figures
Goudveld Water - Assessment of Limestone Mediated Stabilisation
show various aspects relating to the mining and supply of Bredasdorp limestone, marketed
and supplied as “Aquastab Pebbles”.
Figure 3: Bredasdorp limestone quarry
Figure 4: Bredasdorp limestone mine
Goudveld Water - Assessment of Limestone Mediated Stabilisation
Figure 5: Bredasdorp “Aquastab Pebbles”
3.3
Basic Process Description
In the limestone contact process, the aggressive raw water is contacted with limestone
pebbles in a fixed bed reactor (see Figure 6 below).
Access lid
Maximum level
Overflow
Recharge level
Limestone bed
Outlet
Flush outlet
Distribution system
Inlet
Overflow
Figure 6: Configuration of fixed bed limestone contactor
Goudveld Water - Assessment of Limestone Mediated Stabilisation
The raw water is passed through a distribution manifold, or a false bottom, and percolates in
an upward flow direction through the limestone bed. The natural CaCO3 dissolution driving
force of the water (reflected by the CCDP) is used to take up calcium and carbonate species
by exposing the water to graded particles of solid limestone (CaCO3). In this manner,
Alkalinity, calcium and pH can all be increased to effect partial stabilisation. Typically, a
water with CCDP of 25 mg/L CaCO3 will take up close to 25 mg/L CaCO3 if sufficient
contact time is allowed to reach chemical stability; in doing so pH, Alkalinity and calcium
levels of the water naturally increase to levels closely similar to those of a fully stabilised
water.
Table 3 lists the full-scale limestone contact stabilisation units installed to-date in the
Western Cape.
Table 3: Operational limestone contactors in South Africa
Flow Rate
(ML/day)
Unit Location
Raw Water Characteristics
Franschhoek
0.2
Groundwater
Franschhoek WTW
2.5
Chlorinated, mountain catchment water
Jonkershoek,Stellenbosch
2.5
Chlorinated, mountain catchment water
2
Blend of groundwater and surface water
Napier
Montagu
3.8
Treated dam water
Montagu Extension
5.6
Treated dam water
Porterville
4
Bredasdorp
4.8
Rozendal, Stellenbosch
6
Chlorinated, filtered, mountain catchment water
Treated dam water
Chlorinated, mountain catchment water
Villiersdorp
3.5
Surface water and mountain catchment water
Wellington
10
Chlorinated, filtered, mountain catchment water
Idas Valley, Stellenbosch
18
Chlorinated, filtered, mountain catchment water
Installation and operation of the above units has shown that partial stabilisation with
limestone contactors has significant advantages over the traditional use of lime and carbon
dioxide. These include inter alia:
•
Limestone is significantly cheaper than lime. For example: in the Western Cape,
Goudveld Water - Assessment of Limestone Mediated Stabilisation
limestone costs approximately SAR 160/t vs. SAR 900/t for white lime (2000 prices).
•
No carbon dioxide is used. pH is controlled naturally at desirable levels as the water
approaches chemical equilibrium.
•
The process requires little operator skill.
•
Lime dosing equipment, which is generally problematic on small water treatment
plants, is not required.
•
No risk of alkali overdosing.
•
Significantly reduced overall operating costs.
Figures 7 & 8 show a number of operational South African limestone contactors, varying in
size from 2 ML/day to 6 ML/day. Figure 9 shows the outlet of a limestone contactor and the
high quality (low turbidity) final product water.
Figure 7: Jonkershoek, Stellenbosch: 2 ML/day
Goudveld Water - Assessment of Limestone Mediated Stabilisation
Figure 8: Rozendal, Stellenbosch: 6 ML/day
Figure 9: Limestone contactor outlet
By definition, limestone mediated stabilisation will never lead to a CaCO3 supersaturated
water. Nevertheless, such partial stabilisation significantly reduces the aggressive and
corrosive characteristics of the water, making the water essentially non-aggressive to cement
Goudveld Water - Assessment of Limestone Mediated Stabilisation
concrete, non-corrosive to copper, and significantly less corrosive to iron. Importantly,
observations at operational units have shown that partial stabilisation is effective at producing
a well buffered, stable water that retains these characteristics through the distribution
network.
5.
PILOT PLANT TESTS – ON-SITE EXPERIMENTAL ASSESSMENT OF
LIMESTONE STABILISATION AT MAKWANE WATER TREATMENT
PLANT
5.1
Description of the Test Rig
The apparatus to carry out the limestone stabilisation tests is shown below in Figure 10.
Figure 10: CSIR limestone stabilisation test rig
Goudveld Water - Assessment of Limestone Mediated Stabilisation
The apparatus comprises three identical vertical columns constructed from clear UPVC. The
columns each have an overall height of 2000 mm, and an internal diameter of 155 mm. The
columns are connected in parallel to a manifold that is pressurised by a feed pump. A
diaphragm valve controls the flow through each column and the flow rate is measured by a
rotameter. The flow rate to each column can be varied from 12 L/hr to 125 L/hr. The water
enters through the bottom of the column, passes through a false bottom into the limestone and
travels upwards through the limestone. Each column has three sampling ports located in the
limestone bed at approximately 440 mm, 875 mm and 1445 mm bed depth. The fourth
sampling port is located in the clear-well at the outlet of the column. Each column was filled
with “Aquastab pebbles”, the commercially available Bredasdorp limestone with sizing “-15
mm + 12 mm”.
5.2
Methodology
After setting up the apparatus and connecting to the “raw” water supply, the reactors were
allowed to run at high throughput to flush fines to waste. The period of flushing was
determined by the amount of fines present. Thereafter test runs were carried out. Samples
were taken for testing on site. CSIR staff tested for pH, Alkalinity (via both titration to pH
4.5 and Gran Function), calcium and temperature. Control samples were also collected for
cross checks at the CSIR analytical laboratory in Stellenbosch.
Figure 11: In-situ pH measurement at a sample port on the limestone test rig
Goudveld Water - Assessment of Limestone Mediated Stabilisation
Figure 12: On-site Calcium and Alkalinity titrations
During each run the three reactors were operated at three different flow rates to provide total
bed retention times of about 2 to 35 minutes. From the results of the analyses the degree of
stabilisation was determined using the STASOFT computer package of the South African
Water Research Commission. CCDP’s were calculated using measured temperatures.
5.3
Test Site – Makwane Water Treatment Plant
5.3.1 General
The Makwane Water Treatment Plant is the only water treatment facility in QwaQwa treating
mountain catchment water originating from the nearby Metsi Matso Dam. The raw water is
soft and acidic and therefore aggressive/corrosive to concrete/metal conduits.
The treatment process at Makwane Water Treatment Plant incorporates initial stabilisation
via brown lime addition, clarification by coagulation (using aluminium sulphate as the
coagulant), flocculation and settling, followed by filtration using rapid sand filters. The
treated water is then disinfected using chlorine gas before entering the local reservoirs and
surrounding network. At present the plant treats some 5.5 ML/day serving approximately 80
000 consumers in the QwaQwa area.
Goudveld Water - Assessment of Limestone Mediated Stabilisation
Figure 13: Makwane Water Treatment Plant, QwaQwa
In addition, the Fika Patso Dam supplies approximately 55 ML/day to the remainder of the
approximate 600 000 inhabitants of QwaQwa. Information provided by Goudveld Water
indicates that this mountain catchment water is also soft and acidic, requiring stabilisation to
prevent aggressive/corrosive attack.
The Makwane and QwaQwa networks are primarily comprised of asbestos-cement pipes
while older sections of the network often contain iron pipes. Network rehabilitation
incorporates replacement with either asbestos-cement or PVC piping systems. The network is
therefore susceptible to aggressive attack if stabilisation is not practised.
5.3.2 Raw and Treated Water Quality
Historical raw water data obtained from Goudveld Water indicates that the raw water’s
chemical characteristics vary considerably with seasonal changes (See APPENDIX A). For
example, pH varies between 6.5 to 8.6, calcium between 3 and 26 mg/L as Ca, Alkalinity
between 9 and 26 mg/L as CaCO3, and turbidity between 0.6 and 4.5 NTU. The raw water
therefore appears to be moderately aggressive. However, as aluminium sulphate and chlorine
are added, the pH and Alkalinity of the water is lowered. Accordingly, even though a raised
pH and Alkalinity is often recorded in the raw water, the addition of such chemicals will
depress the pH and Alkalinity, thereby making the water aggressive/corrosive. At the time of
the site visit, raw water into the plant and treated water quality prior to stabilisation (and
therefore the feed into the limestone test rig) was as shown in the following table.
Goudveld Water - Assessment of Limestone Mediated Stabilisation
Table 4: Raw water and treated water quality(prior to stabilisation) at Makwane test site
Determinant
Raw water
Treated water
pH (CSIR lab)
n/a
n/a
pH (CSIR field)
6.34 - 6.71
6.28 – 6.55
7 – 10
5–9
6.8
4.8 – 8.0
Calcium (field) as Ca mg/L
1.6 – 2.8
1.6 – 4
Calcium (lab) as Ca mg/L
1.4 – 1.5
1.7 – 3.0
Electrical Conductivity mS/m
2.0 – 2.4
2.4 – 3.0
CCDP
13.0 – 20.0
12.0 – 24.7
Iron as Fe mg/L
0.10 – 0.12
0.06 – 0.11
Turbidity NTU
0.50 – 0.57
0.54 – 0.60
Alkalinity (field) as CaCO3 as mg/L
Alkalinity (lab) as CaCO3 as mg/L
5.4
Field Test Results
The test results are given below in Figures 14 and 15, and Table 5 (the raw data that was used
to draw the graphs are also given in tabular form in APPENDIX B).
Figure 14 shows the increase in pH and decrease in CCDP with limestone contact time.
Figure 15 shows the increase in calcium and Alkalinity. A rapid decrease in CCDP with
limestone contact is evident, from 24.7 mg/L to 2 mg/L CaCO3 in 10 minutes. The pH value
increased from approximately 6.4 to 9.0 over the same period. Calcium and Alkalinity values
increased from approximately 9 and 8 mg/L CaCO3 to approximately 17.5 and 17 mg/L
CaCO3, respectively. Table 5 shows how partial stabilisation using limestone does not
significantly increase the turbidity of the final treated water.
Goudveld Water - Assessment of Limestone Mediated Stabilisation
25
10
20
9
8.5
15
pH
8
7.5
10
7
6.5
5
6
CCDP (mg/L as CaCO3)
9.5
5.5
5
0
0
5
10
15
20
25
30
35
Retention Time (minutes)
pH
CCDP
Figure 14: Test Results at Makwane Water Treatment Plant: CCDP and pH at various sampling points,
representing change with limestone contact time
20
18
mg/L as CaCO3
16
14
12
10
8
6
4
2
0
0
5
10
15
20
25
30
35
Retention Time (minutes)
Calcium
Alkalinity
Figure 15: Test Results at Makwane Water Treatment Plant: Calcium and Alkalinity at various sampling
points, representing change with limestone contact time
Goudveld Water - Assessment of Limestone Mediated Stabilisation
Concerns regarding increases in turbidity are addressed in the following table, which shows
that no significant increase in turbidity, as a result of passing the treated water through the
limestone bed, occurs. In fact, some reduction in turbidity can occur as the limestone bed acts
as a roughing filter.
Table 5: Turbidity comparison of raw, pre-stabilisation (treated water) and post-stabilisation water
Raw
Pre-stabilisation
Post-stabilisation
Turbidity (NTU)
0.50 - 0.57
0.54 - 0.60
0.46 - 0.57
These results are visually confirmed in Figure 9, which shows a clear low turbidity water
having passed through a limestone contactor.
5.5
Discussion
The product water from the Water Treatment Plant at Makwane is soft, and aggressive and
corrosive and would benefit from stabilisation. With the characteristics of this water one can
expect corrosion and aggression related problems, and this is supported by anecdotal
accounts by Goudveld Water which describe problems being experienced in the distribution
network as a result of attack of the network. The relative high unavailability of quality white
lime for stabilisation is also problematic, and currently brown lime is used at Makwane for
pre-treatment pH adjustment. This situation is far from ideal as brown lime cannot be used
for final stabilisation/pH adjustment, as it will increase the turbidity, iron and manganese of
the final treated water. (In addition the use of brown lime for pH adjustment of the raw water
prior to treatment, affects downstream treatment processes.)
During the trial period the limestone stabilisation process was shown to be capable of
bringing about effective partial stabilisation within a retention time of about 10 minutes.
CCDP was effectively reduced to about 2 mg/L as CaCO3, and pH increased to desirable
levels of about 9.0. Importantly, the tests showed the ability of the system to successfully
handle fluctuations in raw water quality, such as those that occurred during the trial period,
and is known to occur during normal operation.
Transportation of Bredasdorp “Aquastab Pebbles” from Stellenbosch, Western Cape resulted
in the presence of significant amounts of limestone fines in one of the four limestone bags.
The possible implications of this are discussed in section 7.
Goudveld Water - Assessment of Limestone Mediated Stabilisation
It would be useful to investigate and consider the availability of suitable local Free State
limestone deposits.
6.
COST COMPARISON OF LIMESTONE STABILISATION – MAKWANE
WATER TREATMENT PLANT
Further to the experimental verification of the limestone process, investigation was also made
into the cost savings of limestone stabilisation versus both lime stabilisation and/or sodium
carbonate stabilisation for the Water Treatment Plant at Makwane.
Comparative costs were calculated by creating a cost model. A comparison was carried out
using Trans Hex (white) lime. Trans Hex lime was previously used at Makwane for
stabilisation purposes, but due to the unavailability of white lime, they are no longer supplied.
Goudveld Water is currently unable to source a suitable supplier of white lime for the
Makwane Water Treatment Plant, and therefore Dwala brown lime is currently being used for
stabilisation. The use of brown lime is, however, not ideal, as brown lime cannot be used for
final stabilisation and pH adjustment, as it will increase the turbidity, iron and manganese of
the treated water. In addition the use of brown lime for stabilisation of the raw water prior to
treatment, affects downstream treatment processes. For this reason a cost comparison of
brown lime is not included. A white lime cost comparison was conducted to serve as a
reference should an alternative source of white lime become available. It must, however, be
noted that the cost comparison presented can be regarded as conservative, as stabilisation
with white lime would require that the lime be imported at greater cost than used for the cost
comparison calculations.
Figure 16: Trans Hex white lime and Dwala brown lime
Goudveld Water - Assessment of Limestone Mediated Stabilisation
The following assumptions were used:
Plant type and water consumption
The stabilisation unit considered is a standard limestone contactor treating water at a rate of
5.5 ML/day. The water considered is that of Makwane as per the on-site tests carried out
during this investigation. The period considered is a ten-year period.
Chemical demand
Chemical dosage requirements, used in comparative cost calculations, were for the treated
water prior to stabilisation. Chemical dosage requirements were calculated using STASOFT
to provide a water with a CCDP of, or close to, zero. Approximate chemical dosages for the
Makwane raw water, at the time of the site visit was found to be approximately 22 mg/L
calcium carbonate, 10.7 mg/L white lime (presuming 88% purity as available Ca(OH)2), and
46 mg/L sodium carbonate.
Capitol costs
Capital costs for a 6 ML/day stabilisation installation were approximated using South African
prices as follows:
Limestone Contactor:
Lime doser:
Sodium carbonate doser:
R 500 000
R 200 000
R 140 000
It should be noted that the limestone contactor price given is for reinforced concrete. Use of
fibreglass could reduce this significantly.
Running costs
The running costs associated with stabilisation include the following:
•
•
•
•
Chemical costs
Routine labour costs
Preventative and line maintenance equipment costs
Preventative and line maintenance labour costs
With regard to the above the limestone contactor process has significant advantages over the
other processes. This is due to the minimum use of working parts, reduced need of operator
control and control systems, and the inherent durability of the limestone process. In order to
Goudveld Water - Assessment of Limestone Mediated Stabilisation
quantify this substantial advantage with regard to running costs, it was necessary to make a
number of assumptions using engineering judgement. Wherever doubt existed in these
assumptions, the conservative alternative was used. The following assumptions were used:
Chemical costs
To calculate chemical costs over the period of ten years starting from 2000, 2000 prices were
used as supplied by P&B Lime and Goudveld Water. A comparison was carried out using
Trans Hex lime as previously used by Makwane for stabilisation purposes. Trans Hex lime is
no longer available, but the price thereof (R1260/ton) was felt to be more reasonable than the
presently available imported lime (>R1500/ton). Dwala brown lime is currently only used for
pre-treatment pH adjustment and is not suitable for final treated water stabilisation, and
therefore a cost comparison was not conducted.
These prices were conservatively escalated. Lime and sodium carbonate were escalated at 8%
per annum. Limestone was escalated at 4% per annum.
Routine Operator costs
In order to take into account the significant variance in operator requirements between the
three different systems, operator costs over a ten-year period were calculated. A 2000 salary
of R50 000 per annum was assumed, and escalated at a conservative 5% per annum. It was
accepted that the operator would need to spend varying amounts of time on routine operation
(i.e. mixing solutions, checking dosing volumes, pH’s, etc.) for the three systems. For the
lime unit an average figure totalling 1 day a week was used. For the sodium carbonate, half a
day a week and for the limestone, 1 day a month.
Preventative and line maintenance equipment costs
In order to take into account the significant variance in system durability, preventative and
line maintenance equipment costs were calculated. These were calculated as a percentage of
the original stabilisation plant capital costs as given above.
Annual maintenance costs were as follows: 7.5% of stabilisation plant cost for lime, 5% of
stabilisation plant cost for sodium carbonate, 1% of stabilisation plant cost for limestone.
These prices were escalated at an annual rate of 10%.
Preventative and line maintenance salary costs
In order to take into account the significant variance in system durability, preventative and
line maintenance labour costs were calculated. These costs are separate from the routine
operating labour costs. Preventative and line maintenance salary costs were calculated using
Goudveld Water - Assessment of Limestone Mediated Stabilisation
the same salary figures as above, with the same escalation. It was presumed that the plant
operator would use 20% of his time on Water Treatment Plant maintenance. Of this 20%, he
would use varying percentages for maintenance of the different stabilisation units. Lime
stabilisation would take up 50%, sodium carbonate would require 25% whilst the limestone
process would require only 2% of his plant maintenance time.
A comparison of the running costs for the three alternate stabilisation options is given in
Table 6.
Goudveld Water - Assessment of Limestone Mediated Stabilisation
Costs comparison: Trans Hex lime
Table 6: Cumulative cost comparison between various stabilisation options over ten years
CHEMICAL COSTS
2000 prices
Escalation p.a.
Cumulative cost
Lime
Sodium carbonate
Limestone
R1260/ton
8%
R450,512
R2500/ton
8%
R3,842,818
R370/ton
4%
R222,384
Carbonation costs
Not applicable
N/A
Total cumulative chemical cost
R450,512
R3,842,818
R222,384
LABOUR
2000 salary
Salary increment p.a.
R50 000
5%
R50 000
5%
R50 000
5%
Routine operating
Total time per week
Cumulative cost
2 days
R202,398
1 day
R101,199
0.25 days
R25,300
20%
50%
20%
25%
20%
2%
Cumulative cost
R70,839
R35,420
R2,834
EQUIPMENT MAINTENANCE
Capital costs stab unit (1997)
R200,000
R140,000
R500,000
Equipment cost escalation
% maintenance of capital cost
10% p.a.
7.5% p.a.
10% p.a.
5% p.a.
10% p.a.
1%
Cumulative cost
R332,910
R174,778
R83,227
R1,056,659
R4,154,215
R333,745
Preventative and line maintenance
Total WTP maintenance % of annual salary cost
% of WTP maint for stab unit maintenance
TOTAL CUMULATIVE COSTS
TOTAL CUMULATIVE SAVINGS
Limestone over lime
Limestone over sodium carbonate
Note:
R722,913
R3,820,469
Graphs of all the escalated costs are shown in Appendix C
Goudveld Water - Assessment of Limestone Mediated Stabilisation
Total costs relating to stabilisation
2000 to 2010 - Trans Hex lime
600
Total cost in kR
500
400
300
200
100
0
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
Year
Lime
Sodium carbonate
Limestone
Figure 17: Cost comparison: total stabilisation costs
Comments:
The above table and graph (and the graphs of APPENDIX C), show:
•
•
•
Stabilisation with sodium carbonate is considerably more expensive than
stabilisation with either lime or limestone.
The preventative and line maintenance related running costs of limestone are
considerably lower than those of lime, and sodium carbonate.
When using the Trans Hex lime, the chemical cost of lime stabilisation is twice
the cost of stabilisation using limestone. Taking into account the total
cumulative running costs over a period of ten years, it is clear that the cost of
limestone stabilisation is considerably lower than lime stabilisation. In
addition, it must be noted that the cost comparison presented can be regarded
as conservative, as stabilisation with white lime would require that the lime be
imported at greater cost than used for the cost comparison calculations.
Goudveld Water - Assessment of Limestone Mediated Stabilisation
7.
DISCUSSION and CONCLUSIONS
The carrying out of short duration on-site limestone stabilisation tests have shown inter alia:
•
The product water from the Water Treatment Plant at Makwane is soft, aggressive
and corrosive, and requires stabilisation.
•
Presently, no stabilisation is practised, as only brown lime is available and dosed
ahead of any other treatment processes. The use of brown lime is not suitable for final
dosing of the treated water leaving the Water Treatment Plant at Makwane, as it leads
to increases in turbidity, iron and manganese.
•
The limestone stabilisation process was shown to be capable of bringing about
effective partial stabilisation with a retention time of about 10 minutes. CCDP was
effectively reduced to about 2 mg/L CaCO3, and pH increased to desirable levels of
about 9.0. Importantly, the tests showed the ability of the limestone system to handle
fluctuations in water quality, such as those that occurred during the trial period, and
which are often recorded during normal plant operation.
•
Transportation of Bredasdorp “Aquastab Pebbles” from Stellenbosch, Western Cape
by VW Kombi resulted in the presence of significant amounts of limestone fines in
one of the four limestone bags. The presence of excessive fines would be problematic
for full-scale operations, requiring an initial extensive flushing. Considering this, and
the fact that the cost of transporting limestone to QwaQwa is significant (1.4 times
limestone cost) it would be sensible to consider other possible limestone deposits
which are sourced closer to the QwaQwa area. Consideration should therefore be
given to the possibility of using local Free State deposits of limestone, as this would
lead to a significant decrease in transportation costs, thereby reducing operating
chemical costs. CSIR would be able to carry out a comparative investigation of
various limestones, to assess their suitability for stabilisation purposes.
•
Although the cost of transport decreases the financial savings that limestone
stabilisation is usually able to provide, it has, nevertheless, been shown that limestone
stabilisation is still significantly less expensive than lime mediated stabilisation using
white lime.
Goudveld Water - Assessment of Limestone Mediated Stabilisation
•
In making a decision as to whether to use lime or limestone stabilisation, the
important advantages of the latter for remote locations should be considered. These
include:
o pH is controlled naturally at desirable levels as the water approaches chemical
equilibrium.
o The process requires little operator skill, and handles fluctuations in water
quality by itself.
o Lime dosing equipment, which is generally problematic on small water
treatment plants, is not required.
o No risk of alkali overdosing.
•
8.
In addition, anecdotal accounts by Goudveld Water revealed that water originating
from the Fika Patso Dam is also soft and acidic, and would also benefit from
stabilisation. Due to the relative high cost and operational problems associated with
conventional stabilisation methods using lime and carbon dioxide dosing, the use of
an alternative such as limestone mediated stabilisation, should be considered. (CSIR
is currently developing a limestone-based process suitable for the 55 ML/day
required.)
RECOMMENDATIONS
It is recommended that:
•
Partial stabilisation with limestone be implemented at the Makwane Water
Treatment Plant.
•
Consideration should be given to the possibility of using local Free State
deposits of limestone. This would lead to a decrease in limestone fines and a
significant reduction in transportation costs, thereby reducing maintenance
requirements and operating chemical costs.
•
Consideration should be given to the use of a limestone-based process for
stabilisation of the Fika Patso Dam water.
Goudveld Water - Assessment of Limestone Mediated Stabilisation
32