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2. Environmental impacts and protective measures
2.1 Cultivation, harvesting, storage and cleaning of the raw
2.2 Raw material cutting, crushing and extraction
2.3 Extract purification
2.4 Evaporation, crystallisation and sugar drying
2.5 By-product manufacture
2.6 Energy supply
2.7 Water management
3. Notes on the analysis and evaluation of environmental impacts
3.1 Emission-limiting requirements
3.2 Technology for the reduction of emissions and emission monitoring
3.3 Limit values issued to protect health
4. Interaction with other sectors
5. Summary assessment of environmental relevance
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%.
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.
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