1. Scope

Sugar is the only food extracted from two different plants - the sugar beet (beta vulgaris) and the sugar cane (saccharum officinarum) - which grow in different regions. Competition between these two international cash crops only arises in small border areas where both are considerably below their physiological optimum, usually latitude 25 to 38 degrees north. The main sugar beet growing areas are located in the temperate climate of Europe and North America in regions with average mid-summer temperatures of 16° to 25°C and an annual rainfall of at least 600 mm, but sugar beet is also grown in the sub-tropics in the winter months. Irrigation is essential where the rainfall is less than 500 mm. Beet thrives best on deep loamy soils with a neutral to weakly alkaline reaction, and in intensive farming requires adequate mineral compound fertilisation. Since beet can only be grown in the same field every fourth year to ensure a healthy crop (avoiding, for example, beet nematode, the main cause of the disease known as beet sickness), the catchment area of a sugar beet factory is very large. The vegetation period is generally five to six months, with yields in a temperate climate ranging from 40 to 60 t/ha, and in the sub-tropics averaging 30 to 40 t/ha. The sugar content ranges from 16% to 18%. Sugar cane grows in tropical lowland climates, and is farmed almost exclusively between latitude 30° south and 30° north, and particularly between the north and south 20°C isotherms. Besides intensive sunlight, a rainfall of at least 1,650 mm or irrigation is essential. Heavy, nutrient-rich soils with a high water capacity are preferred; pH values in the weakly acidic to neutral range are best. Nutrient requirements are high due to the huge mass production. Pest and disease attack have been reduced by resistance breeding and plant development, with biological pest control playing an increasingly important role. Sugar cane is suitable for monoculture and is indeed mainly grown as such. Plant cane is generally harvested after 14 to 18 months, and the new growth (ratoon) after 12 to 14 months. Yields are from 60 to 120 t/ha; the sugar content is on average 12.5%. Harvest quantity and sugar content decline as stocks age, with the result that the total useful life does not normally exceed 4 to 5 harvests.

Sugar factories are agro-industrial centres which contract out the cultivation of their raw material or, alternatively, grow it themselves, have their own energy and water supplies and large, varied workshop complexes. The plant installed is designed to handle a single natural raw material. Where used for direct processing of the harvest, the seasonal processing period coincides with the period of use of the sugar production plant. New plants process between 5,000 and 20,000 t daily (24 hr), although in order to handle 10,000 t/day, sugar production plants must have an appropriate infrastructure. The production plant should be situated wherever possible in the centre of the cane or beet growing area, it should be close to water and should be connected to the public railway and road networks. The by-products arising during sugar manufacture - molasses, sliced beet and cane bagasse - are either used or processed further in the plant, or alternatively form the raw materials for other industries.

· Harvesting, storage and cleaning of the raw material

Sugar beet is harvested almost exclusively mechanically while sugar cane in contrast is harvested largely manually (cutting of stalks). The raw material is then transported to the factory by rail or road. Exceptionally, sugar cane is transported by water. Sugar beet can be stored for one to three days, depending on the temperature and method of storage, whereas sugar cane cannot be stored and must be processed immediately after harvesting or in any case no more than 12 hours later; sugar losses of up to 2%/24 hr are possible. Sugar beet is always washed before processing, but sugar cane is usually only washed if it has been harvested by machine.

· Cutting, crushing and extract purification

Sugar beet is chopped in slicing machines, and sugar is extracted from the slices in the countercurrent with water at 60 - 70°C in an extraction plant; the water is then removed mechanically and before drying the extracted beet is usually mixed with up to 30% molasses, normally made into pellets and used as animal feed. Because of their residual sugar content (approx. 0.8%), the slices - after drying - can be used as silage (preserved by fermentation) and as an agricultural feedstuff.

Sugar cane is prepared by revolving knives, crushers and/or shredders and then the juice is extracted in four to seven rollers in line or is extracted like sugar beet in a diffuser. A fibrous residue (bagasse) with a low sucrose content is produced at a rate of 25 to 30 kg/100 kg cane. The fibre content is approx. 50%/bagasse.

· Extract purification

Beet and cane are processed in a very similar way after the raw juice has been extracted. The raw juice is purified mechanically and chemically. First fibre and cell particles are removed mechanically, then the juice is purified chemically by precipitation of some of the nonsucrose substances dissolved or dispersed in the juice, and the precipitate is then filtered off. In the beet sugar industry, repeated precipitation with calcium carbonate has proved successful, an operation in juice purification where lime and carbon dioxide are introduced into the juice at the same time. Synthetic flocculants, in particular polyelectrolytes, improve particle agglomeration and reduce sedimentation times in the decanter from the normal 40 - 60 minutes to 15 - 20 minutes. In the cane sugar industry, simple liming (defecation) is usually employed as the extract purification process, lime/sulphur dioxide treatment (sulpho-defecation) being less common and lime/carbon dioxide treatment rare. The decantate is then finely filtered for a second time and goes directly to the evaporation station. The sediment or sludge concentrate (approx. 25 to 30 kg/100 kg raw material) is usually separated in rotary vacuum filters into filtrate and filter sludge/cake (approx. 3 to 6 kg/100 kg raw material), the filtrate returned to the process and the filter sludge separated.

· Evaporation and crystallisation

The clear juice (from 12 to 15% dry matter/dry sugar content) is continuously concentrated by multiple stage evaporation until it has a dry content of 60 to 70%, each stage of this process being heated with the steam (steam-saturated air released when the clear juice is concentrated) from the previous stage. In the boiling process, more water is removed from the concentrated juice (syrup) in boilers operating at an approx. 80% vacuum. The juice is boiled at a lower temperature than normal because of the low pressure in the equipment, thus preventing any discoloration due to caramelisation. When a certain ratio of sugar to water (supersaturation) is reached, crystals form. By adding more syrup and evaporating more water, crystallisation continues under controlled conditions until the required crystal size and quantity are obtained. The boiling process is then complete and the resulting boiled mass, now called massecuite, is drained from the vacuum pans into crystallisers. As the massecuite is constantly cooled, the supersaturation changes, causing the sugar crystals to grow once again. The massecuite is then transferred from the crystallisers into centrifuges, in which the crystallisate is separated from the syrup, leaving behind the yellowy brown raw sugar. The centrifuged syrup is boiled to form a massecuite once again and the crystallisate obtained from it is centrifuged. The syrup produced from the centrifuging is called molasses. If, when the massecuite is centrifuged, the crystallisate is cleaned of the residual syrup still attached to it by a water and/or steam jet (affination), white sugar is extracted in just one process from beet or cane. In refining (recrystallisation), a plant-intensive technology, raw sugar and poor quality white sugar are dissolved, and then decoloured and filtered by the addition of activated carbon or bone char, or ion exchange resins. Refined sugar, which meets the most exacting requirements in the sugar processing industry, is extracted from the subsequent crystallisation process. The quantity of molasses produced ranges from 3 to 6% of the raw material fed in, depending on the technological quality of the raw material and the end product. The sugar content of the blackstrap molasses is around 50%.

· Storage

The sugar extracted is cooled and dried before storage or packaging. It can be stored loose, packed (1 kg) or bagged (50 or 100 kg). The essential factor for proper storage is a relative humidity of around 65% in the store. This is approximately the point of equilibrium between the absorption and the release of moisture from the sugar crystals.

2. Environmental impacts and protective measures

Typical environmental impacts caused by sugar manufacture are due to:

- wastewater from beet and cane washing, the boiler house (boiler blow-down water) and extract purification in the evaporation and boiling station (excess condensate and purification water), refining (regeneration water from the ion exchange resins), the manufacture of alcohol, yeast, paper or chipboard (where molasses and bagasse are further processed in the plant), site cleaning and rainfall;
- emissions into the air from the boiler plant (flue gases from processes in which solid, liquid and gaseous combustibles are burnt), airborne substances (soot and flue ash), raw material processing, extraction, juice purification and juice concentration (ammonia) and from biochemical reactions of organic wastewater components in lagoons (ammonia and hydrogen sulphide);
- solid waste from raw material treatment (earth, plant remains), the steam generator (ash) and extract purification (filter sludge).

2.1 Cultivation, harvesting, storage and cleaning of the raw material

Because of the demands placed on it, the soil is heavily polluted due, in particular, to many years of single-crop agriculture (sugar cane), the main pollutants being:

- the fertilizer and pesticide feed,
- adverse effects on the natural cycle due to soil compaction and salination, dehydration and microorganism decimation.

Precautions in agriculture:

- growing not to be allowed in marginal soils
- examination of soils for chemical and physical properties, water-retention capacity, drainage properties and workability (important where crops need to be irrigated),
- fertilisation to be in line with crop requirements in terms of time and quantity,
- checking of pesticides for suitability for targeted pest control and establishing of a precise dosing concentration and quantity,
- observation springs to be made for the constant monitoring of groundwater and any changes to it.

The environmental effects caused by the harvesting and transport of the raw material are basically air pollution from the burning of sugar cane fields (flue ash) and contaminated access routes. Clarification agents containing lead (lead acetate solution) should no longer be used for the polarimetric sugar analysis of beet and cane extracts; instead the environmentally friendly reagents aluminium chloride or sulphate alone should be used.

Odours are rarely a nuisance in beet storage, occurring more frequently with sugar cane especially where it is stored for more than one day. Sugar beet is normally supplied with 10 to 20% moist dirt attached. In dry seasons, this percentage can be under 5% but can be over 60% in times of frequent and prolonged rainfall. Accordingly, the quantity of suspended solids in a flotation and washing operation, where some 750% water on one beet can be assumed, can range from around 7 to 80 kg per m3 water. The water pollution, after one operation, stands at around 200 to 300 mg BOD5/l, although this value can rise to over 1,000 mg/l if larger quantities of sugar and other beet components should transfer into the flotation water. Washing and flotation water are today kept in a closed circuit, continuously purified in basins with mechanical sludge removal and cleared of plant fragments using narrow-mesh sieves. The recycling of the flotation and washing water reduces the wastewater quantity to around 30 to 50% in the case of beet. This reduction in quantity means that wastewater treatment is at last a feasible option. The earthy sludge concentrate produced is deposited, wherever possible, in dips, boggy soils and lowlands which can then be put to agricultural use. The water is kept at a pH of about 11 using waste lime from lime kilns so as to prevent any odour nuisance due to microbial activity in the flotation water.

The burning of sugar cane before harvesting is still common practice, its only advantage being that it facilitates manual harvesting, as all the dry parts of the plants are removed by burning and the harvest volume is thereby considerably reduced. Cutting speed and thus earnings per worker are higher as the piece work pay is not based on harvested weight but unit of length/row. The drawbacks are the adverse effect on cane quality due to damage to the cell tissue and thus increased risk of infections at points of damage, destruction of organic matter, damage to the soil structure due to increased drying, increased soil erosion particularly on hilly sites, and finally air pollution in the form of fumes and flue ash emissions. Sugar cane field burning would therefore seem to be contraindicated for biological and ecological reasons.

The level of contamination of the harvested crop depends directly on the harvesting technique and soil and weather conditions during harvesting. Hand-cut cane can contain between 7 and 20% foreign matter, while the percentage weight when mechanically harvested is 3 to 5%.

If the cane is washed, one can estimate 3 m3 to 10 m3 washing water/t (fresh water, excess condensate, hot well water and recycled washing water).

If cooled excess condensate and hot well water is not fed into the circuit, it can all be used for washing instead of fresh water.

In this way, both the factory's water consumption and the pollutant freight of its wastewater can be reduced. Depending on the washing system, the BOD5 value can be anything from 200 to 900 mg/l. Foreign matter could be removed by pneumatic dry separation to obviate the need for cane washing. The sludge concentrate is treated and the odour nuisance prevented in the same way as in the beet sugar industry.

2.2 Raw material cutting, crushing and extraction

Noise nuisance is produced by the high-speed slicing equipment for sugar beet and in the whole area of mill extraction (sugar cane). Individual ear muffs are required. Dust is generated with particular intensity in the area of sugar cane intake and transfer to the mill tandem. No direct hazard to personnel is caused where these operations are automated.

The intermediate products of the sugar industry are ideal nutrient media for a large number of microorganisms. The whole range of activities, ranging from the preparation of the raw material through to crystallisation, also provides a promising culture medium. The risk of microbial contamination is particularly high in extraction, where not even the most stringent technical hygiene measures and optimum process management can obviate the need to use disinfectants. Repeated high-pressure steam disinfection at the points in the mill tandem most at risk (links in the chain and connecting elements) is only about 60% as effective as biocides. A chemical treatment can also be used during mill operation, while steam treatment can be effectively applied when the mill is not working. Major disinfection operations can lead to heavy sugar losses and are therefore not economically viable. The substance most frequently used to disinfect extraction plants is still formalin (approx. 35% aqueous solution of formaldehyde). It is added in batches at a concentration of around 0.02 to 0.04% in relation to the quantity of raw material processed. The formaldehyde concentration in the juice decreases constantly throughout the subsequent process stages. In the clear juice, levels are less than 1 mg/kg while only traces can be found in the syrup and concentrations of around 0.10 mg/kg in white sugar. It is nonetheless clear that, however it is used, formaldehyde is removed from the sugar to the extent that the technically unavoidable residues are rendered harmless. Traces of formaldehyde are also found in the condensates which are produced from evaporation and which are returned to the factory's water circuit. Formalin is controversial in the light of the carcinogenic effect attributed to it, but is still the preferred disinfectant in extraction. Alternative substances, e.g. thiocarbamate, quaternary ammonia compounds, cresol derivatives, hydrogen peroxide and the like, have all been tested over recent years. Their effectiveness as disinfectants when used in extraction plants is comparable to that of formalin. Thiocarbamate, cresol and hydrogen peroxide - like formalin - are also removed by the extraction water during the process, thus only traces can be found in the extracted slices. Quaternary ammonia compounds, by contrast, are irreversibly adsorbed or precipitated along with other organic substances during extract purification.

2.3 Extract purification

The filter sludge produced in sugar factories has a dry content of 50 to 60%, up to three quarters of which is in the form of calcium carbonate, depending on the juice purification process, the rest consisting for the most part of organic substances. In beet sugar factories, it is usually pressed to a dry content of at least 70%. Because of its phosphate and nitrogen content, it is used mainly as fertilizer and instead of lime for soil neutralisation. In cane sugar factories, the sludge is either passed on to farmers or spread directly on to the factory's own fields. The high protein content in the dry cane filter sludge (14 to 18%) means that it can be used as supplementary fodder in cattle farming. The solid content is separated in almost all cases by continuous filtration through rotary filters and secondary filtration using precoated filters. The quantity of wash water produced is so low that it can be disposed of with the filtrate.

The auxiliary substance most frequently used in extract purification is lime (CaO). Depending on the purification process used, consumption ranges from around 0.75 kg CaO (defecation) to 20 kg per tonne of raw material (two-stage calcium carbonate precipitation). Lime and carbon dioxide is obtained from limestone in the coke-fired shaft furnaces of beet sugar factories. It is not economical for cane sugar factories to produce their own burnt lime because they need such small quantities of it. The CO2-rich gas produced in lime burning consists of around 35 to 40% CO2, the rest being N2. If the oxygen supply is inadequate, carbon monoxide (CO) may be produced. Since the combustion temperature is below 1,200°C, no nitrogen oxides (NOx) are formed. The washing water (8 to 10 kg/kg limestone) produced during gas purification is not organically loaded. The crushing of burnt lime is associated with the generation of large quantities of dust, necessitating the use of dust separators. Breathing apparatus must also be worn by personnel working in the immediate vicinity of the lime kiln, particularly personnel involved in cleaning work.

2.4 Evaporation, crystallisation and sugar drying

Some 4 - 6 m3 cooling water/t raw material - depending on whether single or central condensation is used - is required to condense the steam from the last vessel of the evaporation plant and the evaporation crystallisers. In the cooling water circuit, the mixed condensate (hot well water) produced from the condensers (steam condensation) at 40 to 50°C must be recooled to 20°C maximum in cooling towers, grading houses or evaporation lagoons (mist formation) so that it can be reused. The level of pollution of the condensate produced is determined by the technical conditions in the evaporation and boiling station and the condensation plant. Organic pollution and sugar losses in the mixed condensate can be kept very low where correctly designed juice separators are fitted, with values of around 30 to 150 mg/l BOD5 in raw sugar factories (cane processing) and 5 to 15 mg/l in the beet sugar industry of today. Incrustations are removed from evaporator pipes and other hot surfaces by cleaning with an approx. 5% soda solution followed by a 2 to 5% hydrochloric acid solution. The acids and lyes used for cleaning can be neutralised and fed into the water circuit.

Sugar dust from sugar driers or defective dust-extraction systems can give rise to severe air pollution. This is not only a health hazard but, at a grain size of < 0.03 mm, also highly explosive if the dust/air mixture concentration is within the explosion limit (approx. 20 to 300 g/m3). A low dust level is 2 g/kg sugar. Dust is separated in dry electro-filters or in wet dust-extractors (scrubbers). If no dust separators are installed (e.g. older white sugar factories), breathing masks must be provided. The concentration must be kept low by means of adequate ventilation and precautions must be taken to prevent ignition of the explosive atmosphere (smoking ban, no repair work which produces frictional heat or sparks, installation of explosion-proof electrical systems) in order to minimise the risk of explosion.

2.5 By-product manufacture

Dry slices: the water obtained from the mechanical drying of the extracted beet slices is recycled in the extraction process. The slices are mostly dried in drum type driers (700 to 900°C) to a dry content of 25 to 90%. The mixture of combustion gas from natural gas or oil plus flue gas from steam production is used as the drying gas. A waste gas quantity of 1.5 to 4.5 m3/kg extracted slices - depending on the drying system used - must be expected. The dust content of waste gases from the drier depends, inter alia, on the quantity of molasses added and process management (temperature, holding time). The dust concentration in the untreated gas ranges from 2 to 4 g/m3. Similar dust concentrations may also arise after the cooling drum, the pelleting station, the bagging plant and the pneumatic conveyors. The total carbon concentration ranges from 300 to 1,200 mg/m3, depending on process management. The SO2 concentration after the slice drying plant depends, inter alia, on the fuel used, the drying process and slice composition. SO2 concentrations of up to 1,000 mg/m3 in the waste gas have been encountered.

Sugar extraction from molasses: more sugar can be extracted from the molasses if additional expenditure is allowed for operating costs and equipment. The regeneration- and wash water then produced is heavily contaminated and must be handled separately if the chloride concentration in the total wastewater exceeds 2,000 mg/l.

Biotechnical processing of molasses: the biotechnical recycling of beet molasses almost always takes place in different plants; in contrast, it often forms part of a sugar factory and also gives rise to high wastewater loads. Yeast, ethanol, citric acid and much more can be produced from molasses by fermentation. The waste load remaining from baker's yeast manufacture is around 156 kg COD/t molasses or 187 kg/t baker's yeast. The distillation residue (vinasse) which remains is highly diluted (aqueous) (up to 96% water) and can be used as cattle feed (approx. 50 l/day and animal). As these difficultly biodegradable substances cannot be removed by economically feasible treatment processes, the process wastewater from yeast manufacture has to be thoroughly biologically treated, and then recycled for agricultural use where possible. However, even after biological treatment, the process wastewater must not be discharged directly into the drains. Since the climatic conditions of sugar-cane-growing countries favour eutrophication processes, stringent requirements must be laid down with regard to wastewater purification.

2.6 Energy supply

Some 300 to 400 kg steam and 35 to 40 kWh electrical energy is required to process 1 t beet to white sugar. Every factory must be equipped with steam and electrical energy generation plants (heat/power linkage). Oil, gas and coal are used as primary energy sources and waste gas purification is essential where emission limits are exceeded.

The steam and electrical energy requirement for sugar cane processing (to drive the roller mill with steam turbines) stands at around 550 kg steam/t cane (raw sugar production) or 615 kg steam/t cane (white sugar production) and 35 to 40 kWh electrical energy/t cane. The mean calorific value of bagasse (approx. 50% moisture) is around 8,400 kJ/kg (mean calorific value of oil around 42,000 kJ/kg). The bagasse produced is sufficient to cover the factory's energy requirements. Incomplete burning of bagasse (water content > 50%) increases the emission of flue ash and carbon particles.

To start up the factory (start of campaign), other energy sources have to be used. If a refinery is also operated, it may also be necessary to back up the bagasse with other fuels. Maintenance firing is also essential where the plant is shut down for a prolonged period. A complete conversion to other energy carriers is essential where bagasse is used as a raw material for paper or chipboard manufacture.

2.7 Water management

There is no stage in sugar production where water in some quantity is not required. The technical water requirement of the sugar extraction process in a beet factory is around 20 m3/t beet. The amount of process water introduced can be reduced to 0.5 m3/t by the introduction of water circuits. For practical reasons, highly contaminated (transport, purification, regeneration, hot well water) and slightly contaminated (turbine cooling, pump cooling, seal and gas scrubbing water, condensate excess) water circuits should be kept separate, as water with a low level of pollution (in Germany < 60 mg/l COD or 30 mg/l BOD5) can be drained into receiving streams. In a well-managed factory, the quantity of highly contaminated wastewater produced can be reduced to 0.2 m3/t beet; it should not exceed 0.5 m3/t, beyond which the cost of wastewater treatment would then become uneconomic. Pollution rises in the course of the campaign, reaching COD values of 6,500 mg/l or BOD5 values of 4,000 mg/l and more. In sugar cane processing, large quantities of cane washing water (up to 10 m3/t) and mixed condensate are produced during steam condensation and raw sugar refining, which must be managed in a circuit system (large land areas required for evaporation lagoons, high investment costs for cooling towers). Cane washing water (260 to 700 mg/l BOD5), filter residue (2,500 to 10,000 mg/l BOD5) and washing water from decolorising carbon and ion exchange resins in refineries (750 to 1,200 mg/l BOD5) are all heavily contaminated. The purification water also includes wastewater required for cleaning the production areas and plant during and after the campaign, and for cleaning sugar transport vehicles. There are also juice and water overflows at plant breakdowns (clear juice, for example, has a BOD5 of about 80,000 mg/l) so that values of up to 18,000 mg BOD5/l can occur. Negligence is the main cause of excessive wastewater contamination. If a plant is carefully managed, this wastewater does not exceed values of 5,000 mg BOD5/l. Low organic pollution and sugar losses in the mixed condensate (30 to 150 mg/l) can only be achieved by the installation of separators in the steam pipes.

The aim of establishing water management in a sugar factory must be to eject or treat as low a quantity of polluted water as possible. Water recycling heads the list of measures to be taken inside the factory. Water management must be such that, once closed circuits are established, unpolluted or only slightly polluted water requiring no further treatment is discharged into the drains.

The treatment processes for wastewater which can be carried out in sugar factories are largely determined by local factors. The management of the wastewater and circuit conditions inside the plant have a major effect on plant size and the level of degradation which can be achieved.

Wastewater treatment begins with the mechanical removal of suspended particles followed by aerobic treatment. The simplest and by far the most desirable treatment method for the processing of organically polluted, concentrated sugar factory wastewater is its collection in a series of lagoons using an overflow system. The wastewater then purifies itself. The time required for adequate degradation of the wastewater in the lagoons is determined by the following factors:

- height of water level in the lagoon
- lagoon area
- subsoil below the lagoon/adequate sealing against the subsoil
- weather conditions
- external flows of water.

The lower the water level and the warmer the weather during the degradation processes, the faster the water in the lagoon is treated. Lagoon depth should not exceed 1.20 m in temperate climates, while 1.50 m is acceptable for subtropical/tropical zones (sugar cane growing). If there is a high level of evaporation, the content of the wastewater is concentrated; if there are external flows of water and rainfall is high, the lagoon wastewater is diluted. The treatment of wastewater with a concentration of 5,000 mg BOD5/1, as is usual in the domestic sugar industry, requires a constant degradation efficiency of 99% or more to reach a BOD5 value of 30 mg/l. At a lagoon depth of 1 m and a degradation period of 6 to 8 months, values of 100 mg BOD5/l can be achieved, which is equivalent to partially biologically treated water. In sugar cane growing areas, complete biological purification - a BOD5 of less than 30 mg/l - can be achieved within five or six months with good lagoon management. This long-term process of degradation in lagoons could solve the wastewater problem for the sugar industry if sufficient lagoon area were available. The sugar cane processing industry, in fact, generally has sufficient land area available and so uses the lagoon process almost without exception.

Organic substances are degraded by both aerobic and anaerobic processes. In the anaerobic stages, fermentation and putrefaction occur, thus the possibility of odour nuisances, primarily due to the formation of hydrogen sulphide and butyric acid, cannot be ruled out. This drawback can be overcome, however, by selecting suitable locations and by adequate additional aeration. During the activated sludge process, oxygen is introduced into the water in the form of air via an aeration system.

Small-scale continuous systems operate with a substantially higher microorganism density and a higher oxygen supply, and achieve a degradation level of around 90%. Atmospheric pollution is clearly higher at 2 to 7 kg COD/(m3/day) and the energy required for the air supply stands at around 3.5 kWh/(m3/day).

Anaerobic wastewater purification plants consist of large tanks (around 3,000 to 7,000 m3) in which anaerobic bacteria degrade the organic pollutants to form biogas (approx. 75 to 85% methane). This is particularly effective where wastewater is heavily contaminated. The organic content is 80 - 85% decomposed, with the remaining degradation taking place aerobically in the aeration system. The benefits of this process are that the methane gas can be used directly as an energy source to heat the tanks and that the problems of odour can be overcome; an additional factor is that less space is required than in the case of lagoon systems.

Compared to the large quantities of sludge produced in the flotation and washing water circuit, and in some cases occurring in the form of lime sludge from juice purification, the quantities produced in wastewater conditioning are very low. Recycling is as for filter sludge (see 2.3). The form of "large-scale wastewater processing" most frequently used is overhead irrigation or, rarely, basin irrigation. The preconditions for this are level undrained areas, deep soils with no tendency to silting and a low water table (> 1.30 m). During the passage through the soil, the following processes take place:

- mechanical filtration on the surface
- absorption of the dissolved substances by bacteria in the soil
- biological oxidation of filtered and adsorbed material by bacteria in the soil in the intervals between the individual doses of wastewater.

Basin-irrigation is carried out mostly on small parcels of land surrounded by earth walls (so-called retaining filters). Mechanisation is severely restricted by parcel size and the earth walls. Only crops which grow vertically and which are not sensitive to overdamming are suitable, e.g. tree crops and meadow areas.

Sprinkler irrigation is the most expensive biological treatment process. All settleable solids must be removed to minimise malfunctions in the sprinkler irrigation installations. The load should be intermittent and the quantity rained onto each area must remain small (< 500 mm/vegetation period - individual doses not exceeding 80 mm). If the wastewater is first treated in lagoons to at least 180 mg BOD5/l, drained areas can also be irrigated if the water table is suitably low. In addition to wastewater treatment in the soil, the wastewater recycled for irrigation also acts as a fertilizer.

3. Notes on the analysis and evaluation of environmental impacts

3.1 Emission-limiting requirements

Two types of requirement are imposed on sugar factories: general and special. The provisions regarding general regulations to limit emissions contain:

- emission values, which current technology can keep to admissible levels,
- emission-limiting requirements conforming to the state of the art,
- other requirements to protect against harmful environmental impacts by air pollution and
- processes for determining the emissions.

The following general requirements are imposed:

- reduction in the quantity of waste gas by enclosing plant components,
- registration of waste gas flows,
- circulating air management and process optimisation through more efficient use of waste heat,
- waste gases must be released so that they are freely discharged without obstruction in the general airstream,
- chimneys should be at least 10 m above the ground and project 3 m above the roof ridge, but should be no more than twice the height of the building,
- in the field of wastewater treatment, including lagoons, anaerobic degradation shall be eliminated by technical or structural measures as far as possible.

The special requirements (e.g. in Germany in accordance with the TA-Luft [Technical Instructions on Air Quality Control]):

- the drum inlet temperature in sugar beet slice drying plants must not exceed 750°C or equivalent measures to reduce odours must be applied,
- dust emissions in the damp waste gas must not exceed 75 mg/m3(f),
- where solid or liquid fuels are used, the sulphur content by weight must not exceed 1 % in the case of solid fuels related to a net calorific value of 29.3 MJ/kg, or equivalent waste gas purification must be carried out.

A crucial factor in all emission considerations is the emission load resulting from the quantity of waste gas discharged from the chimney, multiplied by the pollutant concentration. This concerns primarily the load from sulphur, nitrogen oxide, carbon monoxide and dust.

The following emission limits apply to furnace installations with a furnace heat output of < 50 MW:

Emissions	Unit	solid	liquid fuels	gaseous
dust CO Nox SO2	mg/m3 mg/m3 mg/m3 mg/m3	50 250 400 2000	80 175 300 1700	5 100 200 35

Source: TA-Luft [Technical Instructions on Air Quality Control]

The emission values relate to an oxygen content by volume in the waste gas of 3% for liquid and gaseous fuels. In the case of solid fuel, 7% applies where coal is used and 11% applies where wood is used.

Flue ash and soot are the main air pollutants where bagasse is used as fuel, but flue gases from bagasse do not contain any toxic substances. Where fuel oil is used in the cane sugar industry, a sulphur content of 0.5 to 1.0 % by weight in the fuel oil is permissible.

The main parameter of any biological treatment and in any watercourse is the biochemical oxygen demand (BOD). This is the quantity of oxygen in mg/l which is consumed by microorganisms at 20°C within a degradation time of five days. The chemical oxygen demand (COD), on the other hand, is the standard for the content of oxidisable substances found in water, i.e. the method covers not only biologically active substances but also inert organic compounds. It is essential to use the COD method (evidence provided using potassium permanganate or potassium bichromate) as a fast method of determining the level of water pollution.

In its guidelines for the cane sugar industry, the World Bank takes the view that three parameters are of fundamental importance when it comes to assessing sugar factory wastewater pollution with biodegradable substances and their impact on the environment:

· BOD5 for determining the oxygen-consuming organic material;

· TSS (total suspended solids mg/l) for establishing the total quantity of suspended matter (primarily inorganic substances from cane and beet washing water);

· pH as extreme pH changes are harmful to water fauna.

The minimum requirements regarding the pollution levels in wastewater to be released into bodies of water are based on the treatment processes normally applied in the various industries and must conform to the state of the art.

For sugar production and associated industries (including alcohol and yeast production from molasses) the following minimum requirements are specified in Germany (source: (1)):

A cm3/l random sample

COD mg/l mixed sample

BOD5 mg/l mixed sample

(TF) random sample

Seal and cond. Water

0.3

60

--

30

--

--

Other water

0.5

500

450

50

40

4

A = volume of suspended solids

TF = toxicity to fish, expressed as the minimum dilution factor of the wastewater at which all test fish survive under standardised conditions within 48 hours.

These values apply to random samples in the case of lagoons.

Due to local circumstances it may be necessary to limit other parameters for discharging into watercourses, e.g. temperature, pH, ammonia, chloride.

In the USA, the Environmental Protection Agency (EPA) has imposed limit values for cane sugar factories (raw sugar factories and refineries).

General limit values regarded as "best available technology economically achievable" (BATEA or BAT) are:

		BOD5	A
Raw sugar factory	(kg/t cane)
max.daily value		0.10	0.24
30-day				pH 6.0-6.9
mean		0.05	0.08
White sugar factory	(only mixed condensate)	(kg/t raw syrup)
max.daily value		0.18	0.11
30-day				pH 6.0-6.9
mean		0.09	0.035
Liquid sugar factory	(only mixed condensate)	(kg/t raw syrup)
max.daily value		0.30	0.09
30-day				pH 6.0 - 6.9
mean		0.15	0.03

With regard to sugar factories, the noise immission guide values are (in Germany) 60 dB(A) in the daytime and 45 dB(A) at night. Cane sugar factories are generally located in the centre of the growing area, very rarely in the vicinity of sizeable residential areas. The design of factories is light and open (due to the climate); cane is received and conveyed to the mill in the open air (large quantities of dust generated).

Noise emissions can be restricted by structural and acoustic measures, with housings provided around sources of noise and soundproofing.

Where the noise from certain tasks or areas of the factory cannot be restricted or insulated, personnel shall be issued with appropriate individual protective gear.

These include, in particular, tipping and bagging plants, cane handling and roller extraction, washing plants for the raw material and the centrifuge station. In the workshop area, they include mainly work at rotary machines with a diameter > 500 mm, sheet metal processing machines and drilling and punching machines. The acoustic power level in these areas ranges from between 80 and 130 dB(A). At values of > 85 dB(A), individual protective gear (ear plugs, ear muffs) must be worn. With acoustic pressure levels of > 115 dB(A), the combined use of both items is recommended.

3.2 Technology for the reduction of emissions and emission monitoring

Measures to prevent damage due to atmospheric sulphur dioxide immissions in flue gases comprise the retention of SO2 in desulphurisation plants (e.g. absorption in lime milk) and the use of low-sulphur fuels. The installation of a scrubber before the chimney inlet has proved successful in reducing the emission load in waste gases. As well as its action on dust, this scrubbing operation achieves an SO2 separation of some 30%. If sludge from calcium carbonate precipitation is used as the washing liquid, pure gas dust concentrations of less than 75 mg/m3(f) are obtained. At the same time the SO2 emission is reduced by 60 to 70 %. "Calcium carbonate precipitation scrubbing" is therefore a particularly good dust and SO2 separation method as it does not pose any additional wastewater or residue problems.

Dust emissions occurring inside the sugar factory are reduced with scrubbers or fabric filters and the pure gas concentration is less than 20 mg/m3. Dust levels are kept low in the same way during further processing stages.

In the cane sugar industry, the generally high proportion of flue ash necessitates flue gas purification measures. Older furnace plants can be easily fitted with wet or dry separators (cyclones: approx. 96% effective, more investment- and maintenance-intensive than wet separators). The water requirement figures for wet separation are approx. 0.025 m3 water/25 m3 gas.

Emissions and the temperature in the waste gas from steam generation and slice drying are measured and monitored by integrated, continuously operating measuring instruments. In the cane sugar industry, portable, manually operated equipment (e.g. Orsat apparatus) is used mainly to determine, for example, oxygen, carbon dioxide and monoxide. If state-of-the-art waste gas purification systems are installed in new plants, and if dust emissions are below 75 mg/m3, daily measurements with a portable unit are adequate.

Any odour nuisance due to ammonia emissions is largely suppressed in advance by closed-circuit systems.

In principle lagoons should be fitted with additional aeration equipment, and aeration rollers have proved extremely successful here. They should not be located in the immediate vicinity of a factory or residential area (upwind).

A number of processes can be used to measure rate of discharge, e.g. measurement of flow rate with an impeller device and integration via the discharge cross section, or direct determination with a measuring weir.

The mixed samples taken for wastewater assessment are analysed for BOD5 according to DEV (source: (5)) and for sludge deposits, COD and toxicity to fish according to DIN. The EPA has specified the analysis methods for the cane sugar industry in "Methods of Chemical Analysis of Water and Wastes". In the case of lagoons, random samples are adequate in view of the low fluctuations over time of wastewater composition and the long retention times.

Control services and control mechanisms should be put in place to check that environmental provisions are observed, e.g. environmental protection consultants. Their task would also involve checking that environmental protection installations are in good working order and are regularly maintained, and they would also be responsible for personnel training and making personnel aware of environmental issues. Medical care should be provided inside the works and for the local population.

3.3 Limit values issued to protect health

Substances for which maximum workplace concentrations (MAK values) or technical approximate concentrations (TRK) apply in Germany: mg/m3 Application/source

Ammonia 35 - Raw material treatment, extraction, juice purification, juice concentration, lagoons;

Asbestos dust 0.025 - Heat insulation, filter aids (diatomaceous earth);

Lead 0.1 - Laboratory: lead acetate solution for the clarification of juice samples for polarisation analysis;

Calcium oxide 5 - Milk of lime manufacture: juice purification, juice neutralisation, wastewater treatment; lime burning;

Hydrogen chloride 7 - Evaporator station: cleaning with dilute hydrochloric acid to remove incrustations (calcium carbonate);

Formaldehyde 1.2 - Disinfectants: at places in the production area at risk from microorganisms, mainly extraction;

Hydrazine 0.13 - Anticorrosives for boiler feed water (chemical oxygen bonding with hydrazine hydrate);

Carbon dioxide 9000 - Juice purification (calcium carbonate precipitation); lime burning;

Sulphur dioxide 5 - Made from sulphur in sulphur furnaces, juice purification (calcium sulphite precipitation), acidification of the extraction water, waste gases where fossil fuels are used;

Hydrogen sulphide 15 - Raw material treatment, lagoons;

Dust (generally) 6 - Cane receipt and crushing, slice and sugar drying, sugar bagging; storage of excess bagasse.

Synthetic flocculants do not create dust or irritate the skin when handled, and do not constitute any toxicological hazard. Carcinogenic substances and substances which are suspected of having carcinogenic potential are: asbestos dust, alkali chromates and lead chromate (laboratory reagents), formaldehyde, hydrazine, fumes from VA welding.

The lethal dose (LD50) of a 39% formaldehyde solution is 800 mg/kg body weight (oral: rat); according to the working materials ordinance Arbeitsstoff-Verordnung classified as "low toxicity" and labelled with the hazard symbol R22 ("harmful if swallowed!) (necroses of the mouth, oesophagus, stomach).

Measures: in principle toxic chemicals must always to be kept sealed; the wearing of rubber gloves is recommended for analysis work; vessels and instruments must be thoroughly cleaned; installation of effective extraction and ventilation systems.

4. Interaction with other sectors

Sugar is produced jointly by agriculture (crop growing) and industry (processing technology), and there are close links in the ecological and technical fields. The use of modern agricultural knowledge and methods in the growing of the raw material, particularly with regard to fertilizer and pesticide application, largely determines the technological value of beet and cane (all physical, mechanical, chemical and biological properties of the raw material). High-quality raw material facilitates the tasks of extraction and juice purification and this in turn is reflected in improved technological - and hence in the final analysis economic - performance of the factory (higher sugar yields).

Excess bagasse can be used for the additional generation of electricity for the national grid (power stations sector) or for briquette production (domestic fuel supply). Bagasse is also a raw material for the manufacture of hardboard, cardboard or paper (wood and paper sector). Molasses, as well as extracts from cane and beet, are used as the raw material for fermentation processes (fermentation technology and biotechnology sector). Sugar is processed in numerous branches of the food industry. Refined sugar can be used in drug manufacture (pharmaceuticals sector).

All sugar beet and some sugar cane processing factories are equipped with lime kilns for the production of calcium oxide and carbon dioxide, and there are thus parallels with the cement/lime sector.

There are also links with the water supply, wastewater treatment and solid waste disposal sectors generally.

5. Summary assessment of environmental relevance

The impact on the environment from the sugar extraction process and the processing of the by-products from it are manifold, but can be kept to a reasonable, and in part legally prescribed, minimum level by means of established methods and processes. In new beet sugar factories the proportion of costs required for installations to protect the environment stands at some 15 to 20 % of total investment costs, while the figure for cane sugar factories is 10 to 15 %.

The wastewater produced can be minimised by optimum design of internal water circuits and the use of established purification processes (lagoon degradation/biological treatment plants). The rational control of the process must prevent any sugar solutions entering water circuits. This not only reduces pollution but increases profitability. Dumps for filter residues and earth can be used for soil conditioning once the load has been degraded. The production of fuel in the form of biogas should be taken into account when planning new factories.

Emissions from power stations and drying plants can be contained with the treatment technologies which have now been developed. A large quantity of soot and ash must be expected in the waste gas, particularly where bagasse is used as fuel, and consequently installations to optimise the combustion process and waste gas purification must be provided.

The open design of factories in warmer climates appears to obstruct possible noise prevention measures, thus noise nuisance would seem to be avoidable only by siting factories at an appropriate distance from residential areas.

In principle the environmental impact caused by sugar factories can be minimised by current technology. In the preparatory phase, it must be ensured that the plant installed will continue to be both fully operational and fully used for many years. This calls for the training of specialists at a technical level who realise the need for regular maintenance work. Training projects for sugar technicians and workmen can be appropriately integrated in sugar factories.

Sugar factories contribute to the general economic development of a country, including the intensification of agriculture, infrastructural improvement, start of the general industrialisation of rural areas and job creation in agriculture and manufacture, all of which attracts the potential workforce in the surrounding area. This generally leads to the uncontrolled growth of local communities and overburdening of the infrastructure and public services. Settlements in the immediate vicinity of the site must therefore be prevented from the outset. To minimise detrimental effects at the earliest possible stage, close cooperation must be sought at planning stage with the relevant authorities on the regional development plan. Likewise, the affected population groups - and this includes women - should be involved in the decision-making process at all planning phases so as to resolve environmental problems which may arise, e.g. land-use conflicts.

6. References

(1) Achtzehnte Allgemeine Verwaltungsvorschrift indestanforderungen an das Einleiten von Abwasser in Gewer (Zuckerherstellung), January 1982.

(2) Autorenkollektiv, Die Zuckerherstellung, Fachbuchverlag Leipzig, 1984.

(3) Bronn W.K.: Untersuchung der technologischen und wirtschaftlichen Mchkeiten einer Abfallminderung in Hefefabriken durch Einsatz von anderen Rohstoffen anstelle von Melasse, Forschungsbericht, 1985.

(4) Davids, P. und Lange, M.: Die TA-Luft, Technischer Kommentar, Herstellung oder Raffination von Zucker, 672 - 678, Verlag des Vereins Deutscher Ingenieure, 1986.

(5) Deutsche Einheitsverfahren zur Wasser-, Abwasser- und Schlammuntersuchung, Fachgruppe Wasserchemie, 1979.

(6) Groeueranlagen-Verordnung, Dreizehnte Verordnung zur Durchf des Bundes-Immissionsschutzgesetzes, 1983.

(7) Hugot: Handbook of Cane Sugar Engineering, Elsevier Scientific Publishing Company, 1972.

(8) International Commission for Uniform Methods of Sugar Analysis, Report on the Proceedings of the 20th Session, 1990.

(9) Korn, K.: Harmonisierung von Umweltschutz und Kostenbelastung an Beispielen der deutschen Zuckerindustrie, Zuckerindustrie 12, 1987.

(10) Meade, G. P., Chen J. C. P.: Cane Sugar Handbook, John Wiley Sons, N.Y. 1985.

(11) National Institute of Occupational Safety and Health, Registry of Toxic Effects of Chemical Substances, 1984.

(12) Persche Mitteilungen des Instituts fdwirtschaftliche Technologie und Zuckerindustrie zu Fragen lternative Chemikalien zur Desinfektion und Reinigung von Sen in der Zuckerindustrie, 1991.

(13) Reichel, H. U.: Auswirkungen der TA-Luft und der Groeueranlagen-Verordnung auf die Zuckerindustrie, 1985.

(14) TA-Luft: Erste Allgemeine Verwaltungsvorschrift zum Bundes-Immissionsschutzgesetz, 1986.

(15) Technische Regeln fliche Arbeitsstoffe, Bundesarbeitsblatt, 1985.

(16) UNEP - Industry & Environment Overview Series, Environmental Aspects of the Sugar Industry, 1982.

(17) Untersuchungen esinfektionsmittel fen Einsatz in Extraktionsanlagen, Nickisch/Hartfiel/Maud, Zuckerindustrie 108, 1983.

(18) Zucker-Berufsgenossenschaft, Lbereiche in der Zuckerindustrie, 1978.

Appendices:

Appendix 1: Flow chart - raw sugar manufacture from sugar cane

Appendix 2: Flow chart - white sugar manufacture from beet

Appendix 3: Flow chart - refining raw cane sugar

Appendix 4: Water management in a beet sugar factory

Appendix 5: Wastewater control in a cane sugar factory

Flow chart - raw sugar manufacture from sugar cane

Flow chart - white sugar manufacture from sugar beet

Flow chart - refining raw sugar cane

Water management in a beet sugar factory

Wastewater control in a cane sugar factory