A CRITICAL ASSESSMENT OF A MEMBRANE-BASED PACKAGE PLANT FOR SMALL-USER SYSTEMS WATER TREATMENT

Transcription

1 Presented at the WISA 2000 Biennial Conference, Sun City, South Africa, 28 May 1 June 2000 A CRITICAL ASSESSMENT OF A MEMBRANE-BASED PACKAGE PLANT FOR SMALL-USER SYSTEMS WATER TREATMENT P F DE SOUZA AND G S MACKINTOSH Cape Water Programme, CSIR Western Cape, P O Box 320, Stellenbosch, 7599 Tel: (021) Fax: (021) Pdesouza@csir.co.za 1. INTRODUCTION In almost all areas in South Africa that receive their water from major water boards, the quality of water supplied is generally high. The situation, however, is usually very different in the small towns and rural areas falling outside these supply schemes, where an acceptable water quality cannot always be achieved. Population sectors affected by this lack of acceptable quality potable water include: Smaller and isolated rural villages and communities, typically previously disadvantaged communities, including isolated rural schools and hospitals. Commercial agriculture (for use in wine cellars, etc). Tourism (for use in country and seaside hotels, game parks, etc). In poorer communities the impact of water-borne disease in South Africa is particularly significant, with diarrhea alone being responsible for some deaths per annum and 20% of all deaths in the one to five age. Inadequate water treatment is compounded by the lack of procedures to assess and manage drinkingwater quality. These matters are receiving significant government attention, and the Water Services Act (WSA) of 1997 and the National Water Act of 1998 herald some landmark changes in South African water law. These Acts address drinking-water quality by the introduction of Regulations governing provision of drinking-water by water service authorities. The WSA and the draft Regulations oblige water service authorities to progressively ensure efficient, affordable, economical and sustainable access to water services. Prefabricated package water treatment plants have a potential for addressing these needs in both disadvantaged and advantaged sectors. Some of the potential advantages of package water treatment plants include: Suitable capacity, flexibility to serve a range of smaller user capacity requirements. Rapid deployment, ability to be easily transported, installed and commissioned. Simplified operation, usually have simplified and semi-automated operation requiring minimal skilled operator attention. Treatment adaptability, flexibility to be configured to treat specific problematic water characteristics. However, in the past there have been difficulties associated with the use of package plants in South Africa. These problems usually result from: The package plant not being of suitable configuration to adequately, and non-problematically treat a varied range of waters. The package plant requiring excessive, and unexpected, operator input requirements. These difficulties are typically related to the following needs:

2 o o o Skilled/trained operator Maintenance/troubleshooting Back-up service Clearly an opportunity exists for suitably configured and user-friendly water treatment package plants. However, to-date in South Africa, limited implementation of package plants has been brought about. This is largely because conventional package plants utilise traditional treatment processes such as chemical pretreatment, coagulation, settling and dual-media filtration in a scaled-down manner. These unit processes, which are effective on large-scale applications, usually prove to be troublesome for small-user systems. Problems may also result from the plant not being suitably configured to treat a varied range of waters, and the unexpected requirement for excessive skilled operator input. A potentially attractive alternative is the use of a package plant incorporating membrane technology. This approach, whilst Ahigh tech@ in nature, often makes for a more robust system with reduced operator inputs. In this regard, CSIR has recently undertaken field trials of a compact package plant incorporating pre-filtration, flocculation, membrane filtration, adsorption and chemical disinfection. This paper will present the findings of the trials, and make critical comment on the suitability of membrane-based package plants for treating the waters of small-user systems in South Africa. 2. DIEHL & SCHWIERCZ S MWA 2000 The MWA series package plant is designed and manufactured by Diehl & Schwiercz Wasser-Gas- and Umwelttechnik, Germany and was developed to condition surface water as potable water to within applicable German and World Health Organisation (WHO) regulations/guidelines. The package plant operates on a multi-stage modular design principle and incorporates pre-filtration, membrane filtration, adsorption and disinfection. The basic operating configuration of the MWA 2000 package plant is illustrated Figure 1 overleaf. The MWA 2000 is a mobile water treatment plant designed to condition surface water as to comply with international water quality standards. The unit incorporates pre-filtration, flocculation, membrane filtration, adsorption and chemical disinfection.. The proto-type MWA 2000 plant constructed for testing in South Africa was developed to treat 2000 L/hr, and is enclosed in a shipping container. The plant is designed in such a way that various modules can be incorporated to treat specific problematic waters, for example aeration, ion exchange, reverse osmosis, electrochemical- and UV-disinfection, etc. Chemicals used during the trial period for water treatment included those required for aiding flocculation (PAC and polyelectrolyte), ph adjustment (soda ash) and disinfection (calcium hypochlorite). The concentrations of chemicals used varied according to the raw water characteristics and flow rate. In addition, calcium hypochlorite and citric acid were used as cleaning chemicals for the membranes. The plant is supplied with an operation manual that describes procedures required for start-up, routine operation, plant cleaning, plant stopping, plant shutdown and conservation. In general, plant operation is fairly simple following optimization of treatment procedures by a suitably qualified engineer. Thereafter, and subsequent to suitable training, the operator should be able to operate the plant, and undertake basic maintenance, and chemical replenishment. This should ideally be limited to 1 hour per day.

3 Key: 1. Raw water 12. SK filter 2. Coarse filter with float 13. Membrane filters 3. Pre-chlorination dosing point 14. Adsorption reactor 4. Suction vessel with funnel 15. Adsorption reactor 5. Suction pump 16. Chlorine dosing point 6. ph adjustment dosing point 17. In-line static mixer 7. Flocculant dosing point 18. Dosing pump 8. Dosing pump 19. Chemical storage vessel 9. Chemical storage vessel 20. Product water storage vessel 10. In-line static mixer 21. Product water 11. VF reactor 21 Figure 1: Basic Configuration of the MWA SITE SELECTION Water sources in South Africa range over a wide spectrum of chemical quality, resulting from the region s complex geographical, hydrological and geological characteristics. Accordingly, testing was to be conducted on four typically problematic waters. In choosing test sites, consideration was also given to a suitable range of potential user categories (e.g. rural and peri-urban areas, farming and tourism), and logistical issues. The following water types and test sites were selected for the trials:

4 Storage dam waters In the peri-urban areas of South Africa, many farms, smallholdings and tourism orientated facilities utilise water from irrigation supply schemes. In most cases, these waters originate from large storage dams and, as such, requires treatment prior to use. The Paradyskloof Water Treatment Works in Stellenbosch in the Winelands region of the Western Cape (~ 60 km from Cape Town) receives water originating from the Theewaterskloof Dam, Cape Town s largest raw water source, and tends not to be overly problematic. This water is soft and acidic with low Alkalinity. In addition, the water has reasonably low colour and turbidity, but is microbiologically poor. Due to its proximity to CSIR offices, this site was convenient for initial operator training. Mountain catchment waters with colour Mountain catchment waters in the Western Cape region and southern coastal region of the country are typically low in ph and Alkalinity, and often contain Humic and Fulvic organic acids that impart a brown colour to the water. Successful results on a coloured water will provide a strong indication that the plant is suitable for similar, non-coloured, mountain catchment waters. The second test site selected was Suurbraak, a small community in the Overberg region of the Western Cape (~ 250 km from Cape Town), with a historically problematic water quality. The raw water is soft and acidic, containing a high degree of colour and has a very poor microbiological quality. A soft, ferruginous groundwater Many small communities depend on groundwater for potable water supplies. Forty percent of groundwater in South Africa is soft and/or ferruginous. These waters typically have low ph and Alkalinity, and often contain dissolved iron and manganese. The water described above is typical of the water supply to many smaller communities in the Western Cape, e.g. wine farms, agricultural smallholdings, country lodges, etc. Camphill Village, situated near Atlantis in the West Coast region of the Western Cape (~ 55 km from Cape Town) make use of such a groundwater, which also contains a high concentration of dissolved iron. Due to these high iron concentrations, this water is problematic and not suitable for drinking-water without treatment. Turbid surface water Since the previous waters types are all relatively low in turbidity, the MWA 500 will also be tested on surface water with a high amount of turbidity from silt and/or clay, and with a higher total dissolved solids concentration than the previously mentioned waters. For the final test site, the plant was located near the Breede River at Kogmanskloof Pumping Station, approximately 13 km from Robertson in the Breede Valley region of the Western Cape (~170 km from Cape Town). The characteristics of this river water include a raised turbidity. 4. PLANT OPERATION 4.1 Chemical Treatment Various chemicals are used to treat the raw water. The selection depends on the local conditions (quality of raw water, treatment capacity required, local availability of chemicals, conditions of chemical supply and storage, etc). Chemicals that can be used for plant operation include: Flocculation aids (polyaluminium salts, aluminium sulphate, ferric salts, etc)

5 Precipitants (barium chloride, etc) ph adjustment (acid and alkaline solutions) Oxidants (hydrogen peroxide, etc) Disinfectants (calcium hypochlorite, sodium hypochlorite, etc). The concentrations of chemicals used will vary according to the raw water characteristics and flow rate. Chemicals used while operating the unit at the test sites included: Soda ash (ph adjustment) Calcium hypochlorite (oxidation and disinfection) Polyaluminium chloride (PAC) (flocculant) Polyelectrolyte (flocculant aid) In addition, membrane cleaning is to be carried out on a regular basis. Calcium hypochlorite and citric acid were used as cleaning chemicals for the membranes. In selecting the chemicals to be used for the trials, care was taken to ensure that these chemicals would be relatively easily available to plant users. The soda ash and calcium hypochlorite (HTH) used is that used in swimming pool water treatment. The flocculant used is a polyaluminium chloride blend, termed Ultrafloc U3800, manufactured by NCP in South Africa. The polyelectrolyte used, termed Magnafloc LT20, was utilised. 4.2 Plant Operation The plant is supplied with an operation manual that describes procedures required for: Start-up Routine operation (filtration) Plant cleaning (Cleaning 1 backwashing of filter and Cleaning 2 membrane cleaning) Plant stopping Plant shutdown and conservation (for both short and long periods) In general, plant operation is fairly simple requiring little but regular operator input if running at its predetermined optimum (approximately 1 hour per day). In order to initially optimize the plant an engineer/technician fully trained in the operating and maintenance procedures for the MWA 2000 will be required to set up the proper characteristics for the system. This will include determination of optimum flow, dosing rates and frequency of flushing, cleaning, etc. After this period a trained operator can successfully operate the plant (NOTE: By operator is meant an individual with reasonable technical proficiency, preferably with either a water treatment background or the ability to quickly acquire the fundamentals thereof). During training the operator will become familiar with the aforementioned operational aspects of the plant. In addition, the operator should, subsequent to training, be able to: Undertake basic maintenance, and Replenish the required chemicals when exhausted. A series of warning lights will assist the operator in deciding when the plant requires cleaning and when fresh chemical solutions should be made up. In addition, the plant contains protection systems which automatically shut the plant down should a problem arise while the operator is not present. For this reason the plant can therefore, in theory, be left alone for extended periods, and checked by the operator on, for example, a twice daily basis. 5. RESULTS OF FIELD TRIALS AT SELECTED TEST SITES Microbiological and physico-chemical samples were collected by the project team for analysis at the CSIR s Analytical Laboratory in Stellenbosch. Sample analysis to evaluate plant performance was based on SABS , which is similar to international drinking-water quality standards. The results obtained from

6 testing the prototype MWA 2000 at the various selected sites indicates that the MWA 2000 is effective in treating a wide range of water qualities. Typical results are shown in Tables 1 and 2, for operation at Suurbraak. Table 1: MWA 2000 Operation at Suurbraak: Physico-Chemical Results Date Determinant Raw Final 01/12/1999 Calcium as Ca (mg/l) Alkalinity as CaCO 3 (mg/l) 0 7 Iron as Fe (mg/l) 0.43 < 0.05 Aluminium as Al (mg/l) Dissolved Organic Carbon (mg/l) Electrical Conductivity 25 C) PH Total Dissolved Solids (mg/l) Turbidity (NTU) Colour (Unfiltered) (mg Pt/L) Colour (Filtered) (mg Pt/L) /12/1999 Calcium as Ca (mg/l) Alkalinity as CaCO 3 (mg/l) 0 4 Iron as Fe (mg/l) Aluminium as Al (mg/l) Dissolved Organic Carbon (mg/l) Electrical Conductivity 25 C) PH Total Dissolved Solids (mg/l) Turbidity (NTU) Colour (Unfiltered) (mg Pt/L) 250 < 10 Colour (Filtered) (mg Pt/L) 250 < 10 SABS Class 0 (Ideal) SABS Class 1 (Acceptable) Failure SABS Class 1 Table 2: MWA 2000 operation at Suurbraak: microbiological results Date Determinant Raw Final 01/12/1999 Heterotrophic Plate Count (per 1 35EC) Total Coliform (per 100 ml) Faecal Coliform (per 100 ml) /12/1999 Heterotrophic Plate Count (per EC) Total Coliform (per 100 ml) Faecal Coliform (per 100 ml) SABS Class 0 (Ideal) SABS Class 1 (Acceptable) Failure SABS Class 1 The above tables describes the change in water quality affected by MWA 2000 plant operation, and shows

7 that the plant was operating satisfactorily. Generally, the MWA 2000 showed that it could consistently provide a high quality product water satisfying South African and other international drinking-water quality guidelines. The plant was able to successfully improve a number of essential water quality parameters to satisfy SABS Ideal standards, while all parameters of the product water satisfied SABS Acceptable standards. (The exception being manganese removal from groundwater.) This indicates a well-designed and constructed treatment system. The only water treatment shortcoming in the system is the lack of ability for effective stabilisation, i.e. meaningful increase in Alkalinity, calcium and ph for corrosion mitigation. (This can be readily overcome by inclusion of a simple limestone contactor, or increased alkali dosing). Despite this good water purification ability, various significant operational difficulties were experienced. 6. FURTHER OPERATIONAL OBSERVATIONS 6.1 Operational difficulties A variety of problematic operational teething issues, generally related to electronic controls, were experienced. In brief electronic control problems included a range of electronic malfunctions that could not be rectified on-site. At times these electronic malfunctions prevented the MWA 2000 from operating in the semi-automated manner for which it was designed. Importantly, these electronic faults necessitated a significant increase in operator attention. Whilst such difficulties are not unusual for a prototype system, it is imperative that all of these issues be readily resolved prior to commercialisation. For the purposes of this investigative report, it should be noted that these problems can be overcome by both increasing membrane capacity and ensuring a robust automated operational system. 6.2 Operational Costs As it was not possible to operate the plant on a continuous basis, it is difficult to quantify the operational costs at this stage. Initial estimates for operational costs for the MWA 2000, based on operational experiences and optimum dosing requirements attained at the various test sites (as discussed in section 6) are summarised below. Chemical Costs Chemical costs vary according to dosing requirements of the particular raw water, duration of plant operation, and quantity of raw water to be treated. The following cost estimates for chemicals were used to calculate the chemical cost requirements. Calcium hypochlorite (HTH) R15/kg PAC R10/kg Polyelectrolyte R200/kg Soda ash R10/kg Citric acid R50/kg The following preliminary cost estimates for the various sites are based on a 20-hour operating day and a 30- day month, with the production of 27 m 3 per day of potable water to the consumer. For this potable water volume production approximately 7 m 3 per day of potable water would be used for plant flushing and cleaning processes (from field observations). The preliminary cost estimates are: Suurbraak R0.47/m 3 or R380/month Camphill Village R0.74/m 3 or R600/month Paradyskloof R0.21/m 3 or R170/month Breede River R0.31/m 3 or R250/month It must be noted that the bulk of the chemical costs arises from the cost of PAC, the flocculant used during the trial period. In addition, costs at Camphill Village are high as a result of the use of soda ash. The use of cheaper alternative chemicals to increase the ph, such as a limestone contactor, should therefore be considered.

8 Electricity Costs The cost of electricity varies from region to region within South Africa, but can considered to be in the order of R0.20/kWh. Assuming an average power requirement of between 3 5 kw (pumps, compressor) for MWA 2000 normal operation, the following resultant electricity cost, based on a 20 hour operating day, can be expected: Power 3 kw R0.44/m 3 or R360/month Power 5 kw R0.74/m 3 or R600/month The preliminary operational cost estimates are presented in the following table (Rand per m 3 of potable water produced to consumers). Electricity costs used are for the maximum assumed power consumption of 5 kw. Labour costs (operator s wages) take into account the cost of the operator to give 1 hours attention to the plant per day (based on CSIR technician rates). Table 3: Preliminary comparative operational cost estimates for operation of the MWA 2000 at various test sites within South Africa. Test Site Suurbraak Camphill Village Paradyskloof Breede River Chemical cost (R/m 3 ) Electricity cost (R/m 3 ) Labour cost (R/m 3 ) TOTAL (R/m 3 ) NOTE: These total costs do not take into account capital costs of purchasing the package plant or repair costs. The above table clearly indicates that a major component of the overall operational cost is as a result of labour. This emphasizes the need for minimal input by the operator. Should the operator be required to pay more frequent attention to the plant, the cost of producing potable water will increase considerably. It is therefore very necessary to minimize operator input by providing a robust automation system. In South Africa, water from local authorities is supplied on a tariff basis (based on the amount of water consumed), and varies from region to region. Comparative costs for potable water supply in Cape Town is approximately R2.40/m 3 and for Stellenbosch costs are approximately R1.80/m 3. These costs increase considerably for more isolated towns and may be as high as R5.00/m 3. Based on 1 hour a day operator input, the MWA 2000 is shown to have competitive operating costs. (This excludes capital cost which still needs to be determined). 6.3 Availability of Components for Local Manufacture of MWA Series Package Plant Critical inspection and review of the MWA 2000 plant components indicated that many of the components used in the prototype MWA 2000 package plant can be obtained, or manufactured, in South Africa. In most cases this would contribute towards a reduction in capital cost of the package plant, making it more price competitive and attractive to potential consumers. In addition, consideration can be given to opportunities to reduce construction costs. Areas for consideration include the housing container, and the hydraulic filter and adsorption reactors. 7. CONCLUSIONS In test trials based on a wide, but typical, range of problematic water qualities as faced within South Africa, the MWA 2000 showed that it could consistently provide a high quality product water satisfying South African and other international drinking-water quality guidelines. A suitably trained operator can successfully run and maintain the plant once optimisation has been completed. Ideally, operator inputs should be restricted to about 1 hour per day.

9 A number of operational problems relating to electronic control shortcomings were experienced. These problems can be overcome by both increasing membrane capacity and ensuring a robust automated operational system. Whilst the operational difficulties prevented continuous operation, chemical and electrical costs for water purification are competitive. The operational cost estimates indicate that labour is the major factor when determining the cost of potable water produced. In addition, capital costs of the plant can be reduced by use of available South African components, and less expensive materials for the housing container, the hydraulic filter and adsorption reactors.