CONSOLIDATION AND TRANSFER OF LIMESTONE MEDIATED STABLISATION TECHNOLOGY FOR SMALL TO MEDIUM SCALE USERS
Report No 1026/1/02
BACKGROUND AND MOTIVATION
A significant proportion of the surface waters of Southern Africa characteristically have low total alkalinity (0-25 mg/L as CaCO3), low calcium (0-25 mg/L as CaCO3) and low pH (4.0 to 7.0). Furthermore, virtually all of the groundwaters of the southern and eastern fringes of South Africa (up to approximately 200 km inland) have similar characteristics. In addition, surface waters of the southern region have variable colour due to the presence of complex Natural Organic Matter (NOM) constituents; the presence of which can further lower pH to as low as 4.0.
These soft, acidic waters attack distribution systems through being aggressive (to cement concrete) and corrosive (to metals). Such attack can have significant implications relating to lost water, distribution network rehabilitation requirements and undesirable change in water quality; accordingly, in larger, formal water supply systems, water conditioning processes are used to stabilise the water thereby ameliorating aggressive and corrosive attack.
Conventionally, stabilisation of soft, acidic waters is achieved via the addition of lime and carbon dioxide. However, for smallholdings, farms, country hotels and smaller municipalities, stabilisation via lime dosing (with or without carbon dioxide), is problematic, requiring skilled, well- trained staff and expensive equipment. Furthermore, stabilisation with lime is expensive; often comprising about half of the overall chemical cost of treating brown waters. For small volume water users, stabilisation is not readily implementable and the costs of corrosion can be significant. However, an alternative means of stabilising soft, acidic waters by contact with limestone (CaCO3) is considered in this report.
Limestone based stabilisation has been shown to be an attractive means of mitigating against both aggression and corrosion, principally because of simplicity of operation, and significantly reduced chemical costs. This project aims to assist in the increased use of limestone mediated stabilisation by providing clarification on constraints and providing implementation guidelines.
These guidelines are based on data and observations from operational systems or field/1aboratory based investigations.
AIMS AND OBJECTIVES
- Groundwater Stabilisation/Iron Removal Systems
Completion of Research, Development and Implementation (RDI) relating to the use of small scale stabilisation/iron removal systems for groundwater; and in particular, determination of recommended maximum levels of dissolved iron and manganese in the raw feed, and scaling up of the system from 50 m3/day to the order of 1 ML/day.
- Surface Water Stabilisation Systems
Completion and consolidation of RDI for medium scale (2-40 ML/d) limestone stabilisation systems for surface water; and in particular, to determine the recommended upper colour limit in the raw water feed to the units, and to consolidate existing data on these units.
- Assess Degree of Protection Afforded by Partial Stabilisation
Assessment of the relative degree of protection that limestone mediated stabilisation affords to common conduit materials, namely iron, copper and cement, and to assess the respective costs of protection against the degree of protection afforded.
- Technology Transfer
The effective transfer of small to medium scale limestone mediated treatment technologies via practical design guidelines for both small and medium scale limestone mediated systems.
The research programme was designed to address the objectives, thereby necessitating a combination of field based test procedures, and collection of data from existing limestone contactors and municipal distribution networks. Data collection, documentation and interpretation was implemented.
- Data was collected from various groundwater stabilisation systems utilising limestone. Results were obtained from Umgeni Water from their initiatives to use a Spraystab system. A workshop was held to determine optimum ways for upgrading capacity of groundwater treatment systems.
- Small field units were installed and monitored at three locations with a range of colour (10 mg/L Pt to 450 mg/L Pt). A large number of operational limestone contactors in the Western Cape were visited to consolidate existing data on these units.
- Corrosion data was collected from two municipal networks, and the protection afforded by partial corrosion assessed. Aggression trials were run by exposing concrete samples to waters with a range of stabilisation characteristics.
- A guideline entitled "A Guideline for the Stabilisation of Waters with Limestone" was developed.
SUMMARY OF RESULTS AND CONCLUSIONS
With this project, limestone mediated stabilisation technology was consolidated, using information gathered through experience of CSIR and various water suppliers that implemented the technology. A number of operational limestone contactor units that treat surface waters have been visited. In all cases the limestone contactors improved the aggressive/corrosive qualities of their feed waters, and in most cases, significantly so. Information gathered from these visits was assessed and used to set out guidelines for the design and operation of limestone contactors. In particular, where shortcomings in the design or operation of a system led to inefficient or sub-optimal performance of the process, these shortcomings were documented, to prevent these mistakes from being repeated. Final design and operational details will be captured in a guidelines document, which will be used to transfer limestone stabilisation technology to potential users.
Particular issues relating to the implementation and design of limestone contactor systems for surface waters, which were identified at the planning stage of the project, were:
- the upper limit for colour in the feed water of surface water limestone contactors, and
- the effect of limestone mediated, partial, stabilisation on the reduction of aggressive attack of cement materials and corrosion of metals in reticulation networks.
Conclusions re the above two issues are given below.
- Upper limit for colour in the feed water:
- The following conclusions were reached w.r.t. limestone stabilisation of coloured surface waters:
- Limestone contactors appear to be capable of partially stabilising acidic coloured waters, with colour ranging from 50 to 450 mg/L Pt, quite readily, if the limestone bed is regularly flushed to remove fines and other loose deposits. It should, however, be noted that partial stabilisation of highly coloured waters should be practically limited to smaller limestone contactors. This is a result of the fact that the limestone bed will eventually become "blocked". At this stage, flushing alone will not clean the bed, and the limestone would need to be replaced. Removal and replacement of limestone for a large contactor would be costly and time consuming. In smaller limestone contact systems, replacement of limestone would not be as problematic.
- Since such effective flushing is not achievable on larger municipal units that treat volumes of I ML/day or more, the maximum limit for colour in the feed water to the larger municipal units is set at 30 mg/L. This limit is based on experience on existing municipal units.
- For smaller contact units in which the whole limestone bed can be replaced with relative ease, a higher colour level can be tolerated, bearing in mind that with higher colour levels, the flushing requirement increases.
- Regular flushing (or possibly backwashing in the case of small contactor units), according to a fixed routine, is essential to maintain performance. The cleaning frequency depends on the colour and iron load imposed on the reactor. The insoluble organic and turbidity load will also affect the performance, if regular flushing is not carried out.
The effect of partial stabilisation on cement aggression
Laboratory aggression tests were initiated, to test the effect of limestone partial stabilisation on the rate of aggressive attack on cast cement concrete samples. The experiment was repeated with fully stabilised water (i.e. with slightly positive calcium carbonate precipitation potential, achieved by means of chemical dosage), so that the effect of partial stabilisation can be compared to that of full stabilisation.
Wrt aggression mitigation, it has been shown that:
- The unstabilised water is very aggressive towards cement concrete, leaching out calcium and total alkalinity from the samples at a significantly greater rate than both the partially stabilised water and fully stabilised water.
- Comparison of the partially stabilised water with the fully stabilised water indicates that:
- In both cases, greater dissolution of alkali's existed. The total alkalinity curve includes the dissolution of all alkali's (i.e. calcium and other alkali's), whereas the calcium curve only records dissolution of calcium mineral. Therefore, total alkalinity is the more important measure of the rate of aggressive attack.
- At the end of the experiment, calcium mineral dissolution had slowed down significantly for the fully stabilised water and was slowing down for the partially stabilised water.
- Whilst the trendlines of "total attack" (leaching of calcium and other alkali's from the cement matrix-indicated by the total alkalinity curves) for both the partially stabilised and fully stabilised waters indicate that a decreasing rate of attack was occurring, the curves had not yet settled down at the end of the experiment. Results, however, clearly indicate that the rate of attack for both the partially stabilised and fully stabilised waters is very similar (constant difference -delta total alkalinity) and that the curves are tending towards settling. This result suggests that the relative degree of protection against aggressive attack offered by effective limestone mediated partial stabilisation is similar to a fully stabilised water.
The effect of partial stabilisation on corrosion
- Data was collected from the Bredasdorp and Stellenbosch reticulation networks, to determine the effect that limestone mediated, partial stabilisation has on iron and copper corrosion in municipal water distribution networks.
- It has been shown that partial stabilisation, using limestone contactors, can significantly reduce corrosive attack of iron and copper by soft, acidic waters. Data collected from the Bredasdorp network showed that maintaining a calcium carbonate dissolution potential of below 6.0 mg/L and pH above 7.5 throughout the distribution network effectively eliminated incidences of
high rates of iron corrosion. Similarly, maintaining the pH above 7.1 throughout the network eliminated incidences of high rates of copper corrosion.
- Data collected from the Stellenbosch network showed copper corrosion was influenced by variable efficiency of partial stabilisation/pH adjustment, and that at pH's above 7.3, copper corrosion was terminated.
- It has been shown that partial stabilisation with limestone can achieve the stabilisation levels described above (a calcium carbonate dissolution potential below 6.0 mg/L and pH above 7.5), hence, limestone contact is a viable and attractive means of corrosion prevention for the many small holdings, farms, country hotels and rural villages receiving soft, acidic waters.
- Special consideration is needed for the stabilisation of groundwaters. The Spraystab unit, developed during the previous WRC project, has been shown to be effective for iron removal for small scale users (1 to 3 m3/h), where iron is less than 3 mg/l.
- Importantly, the Spraystab system is able to achieve iron removal with a total retention time of about fifteen minutes; i.e. considerably less than the two to three hours of a conventional system.
- During this project, groundwater treatment and/or iron removal sites were visited. Information gathered from these visits was assessed and used to set out guidelines for the design and operation of Spraystab groundwater treatment systems.
- Two particular issues relating to the implementation and design of the Spraystab system that were addressed in this project are discussed below.
Treatment capacity of the Spraystab system
- It was found that the present Spraystab system can be up-sized to treat approximately 5 m3/h (if the feed's iron concentration does not exceed 1 mg/L). Dimensions are given in the report for the upgraded design.
Upper limits for iron and manganese removal with the Spraystab type system
- The upper limit for iron in a water to be treated with the Spraystab system is set at 4 mg/L. However, the iron load affects the maximum capacity to which the system can be upgraded. If iron in the feed water exceeds 1 mg/L, the maximum capacity to which the Spraystab can be up-sized is 2.5 m3/h.
- Scaling up of the process to handle larger flow rates than mentioned above is feasible, but not in the single unit configuration as was developed for the smaller units described above. Larger systems would use separate aeration units, incorporating settling tanks in the case of water containing iron, followed by upflow reactor vessels, limestone contact units and multi-media filters or membrane filters. These larger systems have to be designed to suit the requirements of the specific application.