CAPÍTULO VIII LEGISLACIÓN Y DOCUMENTACIÓN
8.9 Formas De Pago
8.9.4 Crédito Documentario o Carta de Crédito
Examples of internal water recycling by some of the largest industries in South Africa are discussed in this section. Table 8.2 gives a list of industries where direct water recycling with minimal treatment is practised.
Sasol
Sasol is a very large petroleum and chemical manufacturing industry that uses vast amounts of fresh water. Sasol has a very complex water system that employs a high degree of reuse and recycling. In most cases advanced treatment technology is used to treat the water to a suitable quality for reuse and recycling. Examples include the use of reverse osmosis (both tubular RO and spiral RO) as well as electrodialysis reversal (EDR) to desalinate mine water as well as some high total dissolved solids (TDS) industrial streams. The low TDS permeate is reused while the brine is further concentrated by means of evaporative processes before final disposal. One of the interesting examples of reuse is the use of so-called stripped gas liquor as cooling water.
The Sasol One factory produces synthesis gas from coal, together with oxygen and steam as main feedstock. As a by-product of the gas production, a liquid is formed. This gas liquor is treated to separate the hydrocarbon phase (oils and tars) and the solid phase from the aqueous phase. The aqueous phase is further treated by means of liquid-liquid extraction process to remove the bulk of the phenolic compounds from the water, as well as distillation and stripping to remove the solvent and to strip the bulk of the ammonia from the water. Stripped gas liquor (SGL) is produced and then transported to the water treatment section where the SGL is used.
In the original Sasol design SGL was transported directly to the Sasol One sewage treatment works. This sewage treatment works uses trickling filter technology to treat a variety of industrial waste water streams together with domestic sewage from the Sasolburg town. The final effluent from the sewage treatment works is used to transport ash in the Sasol One ash transport system (Riedel et. al., 2004).
Table 8.2: Examples of industrial use of secondary effluent with no or little tertiary treatment (Odendaal 1998).
Industry Purpose Treatment Quantity
AECI (chemicals manufacturer), Modderfontein
Gas washing in ammonia from coal process. Thereafter used on a cascade basis and finally for ash slurry
Sasolburg Ash transport - Biofiltration 12 000 m3/d General Hide
(tanning), Harrismith
Hide processing and dilution of final, high TDS effluent for irrigation
Cooling and rinsing of steel
fabrications - Activated sludge - Sand filtration - Chlorination
15 m3/d
Beatrix Goldmine,
Welkom Gold reduction works and
slurrying to slimes dams - Activated sludge
- Chlorination 3 000 m3/d Kelvin power station,
Johannesburg Cooling - Biofiltration and nutrient
removal activated sludge 35 000 m3/d Cape Fellmongering,
Port Elizabeth Process water (completely replaced potable water for
potable water) - Orbal activated sludge
- Sand filtration 800 m3/d
During 1998, a project was implemented to utilise SGL as make-up in a cooling system, rather than direct treatment in the sewage treatment works. Two process cooling towers and a section of the cooling water reticulation system were converted to run on SGL instead of the normal cooling water blend. In the cooling system, phenol and ammonia was biologically reduced. The cooling system concentrated the SGL before the blow-down was treated in the sewage treatment works. The load (hydraulic and chemical) to the sewage treatment works was significantly reduced by this action. This system proved to have a positive impact on the water discharged from Sasol One to the Vaal River through the reduction of the COD, ammonia and phenol concentrations and the volume of final discharge. A further positive impact was a reduction in the amount of water abstracted from the Vaal River as cooling water make-up. Raw water abstraction was reduced by approximately 9 Ml/d while the discharge was reduced by approximately 6 Ml/d. The project also had a number of negative impacts, mainly related to increased levels of fouling in heat exchangers.
Eskom
Eskom is the national body responsible for generation of electrical power in South Africa and uses very large quantities of fresh water mainly for evaporative cooling purposes at its inland power stations. These recycling systems necessitate blow-down of some of the recycled cooling water to maintain the concentration of chemical species in the re-circulating water within predetermined limits to prevent fouling of condenser systems. The blow down containing relatively high TDS levels cannot be discharged to the water environment.
An acceptable way of disposal is to use the blow down to convey ash to the ash dumps. In a ‘wet ashing’ system, the ash is removed from the boiler in the form of a wet slurry and
conveyed to the ash dam. The water that is used to convey the ash percolates through the ash in the ash dam and collects in ash water recovery dams. The water is then pumped back to the power station to repeat the cycle of conveyance. Wet ashing is a partly consumptive use of blow down since about 60% of the water used is trapped in the ash and 40% becomes available for reuse.
Where dry ashing systems are used, other means of disposal of blow down are required. On a dry ash system, water is used to condition the ash to form hydrated minerals. Thus, although consumptive, less water is used than in a wet ashing system. The excess blow down therefore has to be managed in another manner. The only feasible alternative for Eskom to treat the blow down is by means of desalination technology. Eskom had already initiated research into desalination of cooling water blow down in the early 1980s and installed the first full-scale desalination plant in the mid 1980s at the Lethabo power station. This tubular reverse osmosis system had a capacity of 9000 m3 per day and produced high quality permeate that was reused in the power station. The tubular RO plant has since been replaced by a spiral RO system. Soon after the initial desalination plant was installed at the Lethabo power station, an electrodialysis reversal (EDR) plant was installed at the Tutaka power station for the same purpose producing about 6 Ml/d of high quality desalinated water for reuse.
Mondi Paper Mill
In 1994 Mondi Kraft, a leading international supplier of pulp, paper and board products, installed a system for the treatment of “black liquor” effluent streams generated in the production of pulp (Odendaal, 1998). The system was designed for the integrated pulp and liner-board facility at Piet Retief. The decision was motivated by a need to conserve water, to reduce operating costs and minimise the impact of the mill’s effluent stream on the environment. The effluent treatment process was the result of more than 3 years of on-site pilot plant and laboratory studies.
The treatment plant currently treats 1 700 m3/d, with facilities to upgrade to 2 400 m3/d.
The treatment process consists of three stages:
• tubular ultrafiltration for the removal of suspended solids and organic compounds with high molecular weight;
• ion exchange for the removal of low molecular weight compounds;
• reverse osmosis, whereby the majority of remaining organic materials and dissolved salts are removed;
The effluent stream is treated until the water is sufficiently clean for reuse in the factory.
The final concentrated waste, in the form of salt cake, is produced in saleable form.
Columbus Stainless Steel Plant
The Columbus Stainless Steel factory at Middelburg produces various effluent streams during the manufacturing and beneficiation process, and has taken proactive steps to comply with the discharge requirements laid down by the Department of Water Affairs and Forestry.
The effluent streams have been categorised into two broad streams, namely strong and weak effluents. After pilot trials, it was decided to treat the weak effluents stream with the aid of reverse osmosis. This effluent stream derives mainly from contaminated site run-off water and ion exchange regeneration effluents, and has a flow rate of 3 300 m3/d. The first reverse osmosis plant with a capacity of 1 500 m3/d was commissioned during July 1994, and a second with a capacity of 1 800 m3/d in May 1995 (Odendaal, 1998).
Pretreatment consists of screening, coagulation, sand filtration, pH adjustment with sulphuric acid and cartridge filtration. The permeate is reused in the factory as process water and the brine is further concentrated in an evaporator-crystallizer.
8.7.6 Irrigation reuse
Policy relating to the reuse of treated or partially treated wastewater for irrigation in South Africa differs from that of many other countries. In general the disposal of purified effluent by means of irrigation is not allowed. The requirement by law is that the treated effluent must be returned to the source from which the water was abstracted to allow use by down stream users. Exceptions include the irrigation of public parks and sport fields that would otherwise require abstraction of water from available freshwater supplies and in coastal areas where wastewater would otherwise be lost to sea. Temporary exceptions are given to irrigate (dispose of) wastewater where it is technically and economically impossible to meet the effluent standards for discharging to water courses.
The South African Department of Health formulated guidelines with respect to the public health effects of irrigation with treated municipal wastewater. In general, no irrigation with treated sewage effluent is allowed on crops that may be eaten raw.
The city of Cape Town has embarked on an ambitious project to determine the potential to increase the reuse of treated wastewater from its municipal wastewater works which would otherwise be lost to sea. A comprehensive study was undertaken to establish the potential for using treated wastewater to replace municipal water for irrigation of golf courses and other public areas such as parks. The results from the study indicate that a potential exists to replace 170 Ml/d of municipal water by treated effluent for irrigation purposes (City of Cape Town, 2006).
Cape Town has a growing population of around 3.2 million and suffered from a severe drought and extensive water restrictions in 2004/5. This gave rise to investigations to improve water management, to investigate desalination of seawater to supplement water sources and to investigate the increased use of treated effluents for applications where fresh water could be replaced.
More than 60% of the water consumed in Cape Town ends up in 20 treatment works across the city from where the treated effluent is discharged back to the environment and eventually to the sea. A fraction of this water is already reused for irrigation of golf courses and by a limited number of industries. The total existing daily summer reuse is about 30 Ml/d, which is about 7% of the total effluent available.
The city recently undertook a project to investigate requirements for the refurbishment and expansion of wastewater treatment works and infrastructure for distribution. This resulted in a number of works being upgraded, such as Bellville, Parow and Kraaifontein.
The largest project being undertaken at present is at the Potsdam treatment works which will increase the treated effluent available for reuse to 24 Ml/d.
Some of the existing users of treated effluent from the city’s treatment works include:
• Milnerton golf course;
• Sappi Paper;
• Sports fields in Milnerton and Table View;
• Table View beachfront dune project.
New users on the reuse scheme include:
• Chevron refinery;
• Kynoch Chemical;
• Agricultural use.
8.8 CONCLUSIONS
Indirect water reuse is taking place in practically all countries in the region where abstracted water that is partially derived from treated return flows is used for domestic purposes, gardening and irrigation. In the semi-arid countries of the region, the runoff in most streams and rivers during the dry season consists to a large degree of return treated or partially treated wastewater.
Most of the countries in the southern part where water resources are limited rely on indirect reuse for sufficient water to meet their requirements. South Africa specifically is highly dependent on return flows to meet its water needs.
South Africa has a large number of direct reuse schemes, mainly in industries that experience difficulties to treat their effluents to the required standards. Many industries therefore recycle effluents for internal use and often employ advanced treatment that produces water of such high quality that it could be reused for high quality applications.
8.9 REFERENCES
Basson, M.S. (1998) Overview of water resources availability and utilisation in South Africa. DWAF Report P RSA/00/0197. CTP Book Printers, Cape Town.
City of Cape Town (2006) The City of Cape Town brings treated effluent to the forefront. Press release.
Department of Water Affairs and Forestry (2004) National Water Resources Strategy. DWAF Pretoria.
Gisclon, A. , McCarley, S. and McNally, K. (2002) The Durban water recycling project – the vision becomes reality. Proceedings Biennial Conference of the Water Institute of Southern Africa, Durban.
Madyiwa, S. (2006) Peronal communication. Department of Agricultural Services, PO Box CY 639, Causeway, Harare, Zimbabwe.
Madyiwa S (2007) Department of Natural Resources, Harare, Zimbabwe. Personal communication.
Odendaal, P.E., Van der Westhuizen, J.L.J. and Grobler, G.J. (1998) In: Wastewater reclamation and reuse. Edtor T.K. Asano. pp1163 - 1191 Technomic Publishing Co. Inc., Lancaster, USA.
Gleick, P.H. (1998) The World's Water 1998-1999. Island Press, Washington, DC.
Riedel, K.J., Gubuza, D.M., Booysen, M.G., Edwards, W. (2004) Use of industrial effluent as process cooling water. Proceedings WISA conference, Durban.
Tole, P. M. (2006) Personal communication. Kenyatta University, PO Box 83844, Nairobi, Kenya.
Water Sewage and Effluent (2006) Closing the loop? Brooke Patrick Publications. Johannesburg.
Water Wheel (2006) Mine water becomes a commodity. Water Research Commission, Pretoria.
©2008 IWA Publishing. Water Reuse - An International Survey of current practice, issues and needs by B. Jiménez et al. ISBN: 9781843390893. Published by IWA Publishing, London, UK.