Manufacturing-Processing RAW SUGAR FACTORY WASTES AND THEIR CONTROL

Transcription

1 Manufacturing-Processing RAW SUGAR FACTORY WASTES AND THEIR CONTROL E. R. Hendrickson Environmental Engineering, Inc. Gainesville, Florida and F. A. Grillot, Jr. F. C. Schaffer and Associates, Inc. Baton Rouge, Louisiana ABSTRACT This paper reviews the; problems confronted by the raw sugar mill in complying with local, state and federal environmental regulatory agency requirements. The mill's environmental problems are broken down into 3 basic categories-air, solids and ~ater. Each of these ~ategories is studied ~nd pr~- II.' sented separately. The basic research for the air study was accomplished In \j Florida during 1968 and The basic research for the solids and water,) study was accomplished in Louisiana during The objectives of the research were to determine the nature of the problem, the seriousness of the problem, its. effects on the ecology, and the solution of the problem to balance the economic needs of the mill with the regulation requirements of the environmental agencies. The research in the air study found that the sugarcane industry, along with other natural and manmade sources, occasionally may be the cause of nuisance levels of particulate matter and dustfall. However, the observed air quality in the area is, far from being detrimental to the health or property of man. In the solids study, it was found that the solids produced by the industry must eventually be contained. or disposed of by methods other than discharge to streams or waterbodies. The water study showed that after some in-plant modifications to reduce total water consumption, the state of the art is such that ultimate wastes can be treated and released into streams and waterbodies well within the requirements of the various' agencies. However, the critical determination will be in developing a balance between ecology and economy. INTRODUCTION The interest and attention devoted to air, water, solids, noise, and nuclear pollution has reached such a level in the US that it is discussed almost every day at all levels of management and government. The waste materials.from sugar production are included in many of these discussions. Factory owners, factory managers, employees, engineers, and various departments of local, state and federal environmental agencies are all working feverishly together toward a common end-the reduction and/or elimination of ecologically detrimental wastes from the sugar farm, raw mill, and refinery. In this effort a very close line must be drawn between ecology and economics, between panic and progress. This paper 1544

2 E.' R. HENDRICKSON, F. A. GRILLOT, JR will present a general insight into the problems of the sugar farmer and raw sugar mill and their relation to the overall environmental problem. AIRBORNE WASTES In the harvesting of sugarcane and processing of raw sugar, various materials may become airborne and result in reduced quality of the ambient air. Where such practices are carried out, cane field burning and the use of bagasse as fuel appear to be the major contributors. Cane field spraying; odors from processing, molasses storage, and liquid waste disposal; and fugitive dust from fields and unpaved areas appear to be minor contributors. In Florida, cane field burning and the use of bagasse for fuel are both practiced and have come under the scrutiny of the Florida Department of Air and Water Pollution Control. The allegation has been made by the agency that these 2 practices create local nuisance conditions and contribute extensively to the air pollution in urban areas as much as 50 miles distant. Ambient aitstudies of limited scope plus theoretical emission and dispersion calculations conducted during 1967 and 1968 by personnel of the Palm Beach County Health Department led to the contention that the sugar industry was a major contributor to polluted air in southeast Florida. These calculations led the air pollution control authorities to conclude that the industry was responsible for more than 25% of the total air pollution and more than 95% of suspended particulates. Based on sampling for suspended particulates at 3 stations, they concluded that the level of air pollution in the sugarcane growing area was substantially higher than in other areas of Palm Beach County and that the levels increase significantly during the harvesting season. Based on the observations which were made, the state pollution control authorities served notice on the 9 operating mills that they were in violation of existing regulations. This action set off a round of source evaluation and the development of an extensive air quality improvement program on the part of the Florida Sugar Cane League, Inc. The source sampling revealed that all boilers in the industry were exceeding the existing process weight limitations of the state regulations when burning bagasse. The industry contended that this regulation should not be applied toa bagasse combustion source since it originally was developed for application to metallurgical furnace fumes. The application to bagasse boilers represents an arbitrary extrapolation from a completely different type of source, and it is not related' to reduction of some undesirable condition in the environment. The size ofparticles emitted from bagasse furnaces is such that they are largely non-respirable, and the majority of the weight of the particles fall out on farmland owned by the sugarcane growers. These preliminary observations raised doubt as to the validity of the official claims. The Florida Sugar Cane League, Inc. retained Environmental Engineering, Inc. and David B. Smith Engineers, Inc. as a joint venture to plan and conduct an air quality improvement program. The program included continuation of the bagasse boiler emission evaluation, pilot plant studies of devices for reduction of particulate emissions from cane field burning, and an ambient air survey for particulates and sulfur dioxide. Concurrently, the Florida Agricultural Experiment Station conducted an evaluation of alternatives to cane field burning.

3 1546 MANUFAGTURING-PROCESSING 1(li Bagasse Boilers Bagasse is a fuel of varying composition and heat value, depending on such factors as season of the year, moisture content, variety of cane, and soil type upon which grown. As an average, it will contain about 50% moisture by weight and I lb of wet bagasse will produce about 2 lb of steam. Burning bagasse is difficult at best but does eliminate what would otherwise be a significant solid waste disposal problem. Almost all bagasse-burning boilers utilize fuel oil. for supplemental fuel, and some mills are considering the 'abandonment of bagasse as a primary fuel where there is an alternate use for the bagasse. The nature of the fuel and of the 'boilers results in the emission of fly ash consisting of partly and incompletely burned-bagasse. The density of the fly ash is somewhat less than that of water. A typical size distribution is shown in Table 1. It should be Table 1. Typical particle siie di'stribution analysis of bagasse-burning boiler stack emissions. Size range (microns) No. percentage based on Martin's diameter noted that particles of about 10 microns or greater will settle readily by gravity. Thirty-eight percent of the particles by number fall into this category. During the season, 35 tests of boiler stack emissions were made on 26 boilers at 8 mills. During the season, 53 tests were made on 26 boilers at 8 mills. The results of these tests are shown in Table 2. Table 2. Bagasse boiler emissions during Bagasse emission values Maximum Minimum Average Solid fuel delivery rate, Ib/hr (dry) Particulate emission rate, Ib/hr Particulate emission rate, Ib/1000 Ib (dry feed) , , , loa Solid fuel delivery rate.vlhrhr (dry) Particulate emission rate, 'lb/hr Particulate emission rate, Ib/l,OOO Ib (dry feed) 39, ,5 6, , The fly ash characteristics mentioned lpreviously make the dry collection of these particles especially difficult. Mechanical collectors (banks of smalldiameter dry cyclones) are presently installed on the boilers at 4 of the mills.

4 E. R. HENDRICKSON, F. A. GRILLOT, JR The removal efficiency was observed to vary from a low of 21 to a high of 63%. By proper design, installation, operation and maintenance, the dry cyclone can be expected to remove significant quantities of fly ash from bagasse-burning boilers. Since the removal efficiencies of the installed dry collectors were not satisfactory to the state abatement agency, a pilot plant wet scrubbing unit of 2500 cfm capacity was test operated on 2 boilers in early The unit was equipped to operate as a cyclonic scrubber or a venturi scrubber so that the particulate removal efficiencies of up to 98% were observed for the venturi scrubber and up to 94% for the cyclonic scrubber. Both would meet the existing process weight emission standards. However, it is generally acknowledged that the present code cannot reasonably be applied to bagasse-burning power boilers and will likely be replaced with a more realistic code on particulate emissions. It is therefore prudent to question the need for installing wet scrubbers, with the attendant problems of treating and disposing of the scrubbing liquid. A more realistic approach may be the reduction of particulate emissions from the boiler furnaces through internal modifications to improve cbmbustion, and the proper design and operation of dry cyclones. Cane Field Burning Burning of the standing cane prior to harvest is practiced in Florida. In addition to improving the efficiency of hand cutting and reducing the amount of trash carried to the mills, burning reduces the need for pesticides and rodenticides, reduces injuries, and reduces the quantities of solid wastes to be disposed of. Mechanically cut cane is often burned for one or more of these reasons. The smoke plumes, however, are a possible source of nuisance and annoyance. Several attempts were made to measure the rate of particulate emission from burning cane fields by aircraft sampling. Transverse and longitudinal paths were followed in each plume. The airplane was equipped with an optical particle counter with which it was possible to measure the number of particles in several discrete size ranges from microns. The data collected were inconclusive. General observations were that a quantity of large particles fall out on the cane fields as soon as they are released from the thermal updraft. Some large particles were carried to several thousand feet before settling began. The upper air plume contained quantities of particles of micron and sub-micron sizes.. The rapidity with which the volume of the plume changed made it impossible to obtain significant concentration traverses. Ambient Air Su.rvey Working in close cooperation with state and county regulatory agencies, the Florida Sugar Cane League developed and implemented a massive ambient air study covering the entire Florida sugarcane growing and processing area. The study, which started in September 1969 and continued through May 1970, encompassed pre-season, season and post-season conditions. The major objective of the ambient air study was to evaluate the air quality of the area, including rural as well as populated centers. It also was intended to estimate the influence of sugar harvesting and mill operations on the air quality.

5 1548 MANUFAGrURING-PROCESSING The ambient air survey involved a meteorological network, a sampling network for suspended and settleable particulates, a sampling network to monitor sulfur oxides, aircraft sampling, a sub-system for reporting time and location of field burning, and a correlation program. Thirty-four ambient air sampling sites were selected in the tri-county area. These initially were located on a grid network which was later modified by field investigation. Final location depended on access, site conditions, security, and availability of electricity. Eight of the sites were in urban areas, located on the rooftops of buildings, and 26 were in rural areas on power poles. All 34 stations were equipped with dustfalliruckets to measure the settleable particulate concentration accumulated during a 30-day exposure. The 8 urban stations plus 16 of the rural stations were equipped with high-volume samplers which, collected suspended particulates during a 24-hour period. The Hi-Vols wereoperated every 4th day. At the 8 urban stations, sulfation rate was measured over a 30-day period. Settleable particulate matter usually is considered to be that fraction greater than about 10 microns. Although the measurement procedure is crude, the results usually are considered to be a gross measure of the general dirtiness of. a community. Geometric mean values for all stations during the entire period of the survey ranged from a low of 5 tons/mi 2/month to a high of 22 tons/mi 2 / month. The average was less than 10. Morphological examination of collected material revealed that the amount of burned plant material in the samples taken during the burning season ranged between 11 and 47 weight percent. The category of burned plant material includes not only particulates resulting from cane processing operations, but also particulates from any vegetation' which is burned intentionally or accidentally. The levels of.settleable particulate matter observed are generally acceptable in residential areas. Very little data relating to harmful effects as measured by settleable particulates can be found. The category of suspended particulate matter is defined, to some extent, by the method of collection and analysis, but generally is in the size range of about 0.1 to less than 100 microns. Suspended particulates, in appropriate concentration, have been related to human health effects, materials damage, and visibility interference. During the burning season the geometric mean concentrations for all 34 states ranged between 22 and 70 p.g/m 3 The range of concentrations for the non-burning season was between 26 and 100 p.g/m 3 The geometric mean concentration of all stations operating during the burning season was 39 p.g/m 3, and during the non-burning season it was 44 p.g/m 3 When all data were evaluated for both burning and non-burning conditions, the geometric mean concentration for the sampling period was 41 p.g/m 3 This value would probably have been reduced if the sampling period had included the summer months, because data from the Palm Beach County Health Department indicates lower concentrations during this period. Morphological examination of the samples taken during the burning season resulted in observations of weight percent burned plant material. During the non-burning season the range was 4-14 weight percent. According to the criteria document of the National Air Pollution Control Administration, the minimum levels at which adverse effects have been noted include: p.g/m 3 in the presence of sulfation rates exceeding 30 mg/em"/mo, at which concentration increased death rates for persons over 50 may occur; p.g/m 3 in the presence of sulfur dioxide and moisture, at which corrosion of steel and zinc panels occurs at an accelerated rate; at about 150

6 E. R. HENDRICKSON, F. A. GRILLOT, JR p.g/ms,' when relative humidity is less than 70 percent, visibility may be reduced to as low as 5 miles. All values except that for visibility are annual geometric means. The concentration for visibility is a 24-hour observation. The maximum observed sulfation rate outside of West Palm Beach was 3.2 p.g SOs/cm2/day and the minimum was The maximum geometric mean of the sulfation rate that occurred during the sampling period was 0.51 p.g SOs/ cm2/day, and the minimum value was 0.16 p.g/sos/cm 2/day. This is an indication of an extremely low level of sulfur dioxide exposure, far below that causing any adverse health effects even in the presence of particulate matter. SOLID WASTES In the hauling of sugarcane from the harvest field and in processing it at the raw sugar mill, various solid wastes are created. From the field to the mill, improperly designed or overloaded cane carriers can scatter considerable quantities of cane stalks along public roadways. These scatterings are not only unsightly to the distinterested traveler, but can even become a serious safety hazard where quantities left on heavilytraveled public roads cause them to become slick, creating braking difficulties with automobiles. Rainfall then washes the sucrose from crushed stalks into roadside ditches, causing high BOD levels which deplete the dissolved oxygen. In addition, this cane spread over the roadway rep' resents a loss of production to the cane farmer as he receives credit only for the cane that is weighed into the mill. Therefore, there are 2 incentives here for properly designed cane hauling equipment; one is to reduce the visible and runoff pollution along the public roadway; and the other is to increase the yield of the cane processed from the farm. This loss may be from %. Once the cane is in the mill, there are 3 primary solid wastes created: bagasse, fly ash, and filter mud. The first 2 of these were discussed in detail in the airborne wastes section of this paper, so we will deal only with filter mud here. All harvested cane will carry with it some quantity of dirt and soil from the field. This is especially a problem with mechanically harvested cane. Part of the soil can be removed by washing the cane. The removal of the remaining mud from the raw juice is accomplished by the utilization of clarifiers and rotary vacuum filters. The quantity of filter mud which must be disposed of will vary from mill to mill and with mechanically harvest cane it will constitute 60-70% of the BOD disposal requirements of a mill and as high as 90% of the suspended solids. Stated in simpler terms, filter mud can have a BOD requirement of approximately the same as a population of 40,000 people/l,ooo tons of cane ground/day. This quickly leads to the conclusion that filter mud must be impounded and stabilized. This material in some cases may be beneficial in adding nutrients and humus to the soil. WATERBORNE WASTES It is the wastes that become waterborne in the mill and factory that offer one of the greatest challenges to the factory manager and the environmental engineer, for it is here that considerable quantities of monies, both capital and operating, can be wasted, saved, or properly invested. Waterborne wastes

7 1550 MANUFACTURING-'--'--PROCESSING from a sugar mill can be defined within the following categories: cane wash water; condensate; condenser water; evaporator boilout; and yard drainage. Cane Wash Water Not all mills wash their cane prior to processing but those that do have an inherent problem with disposal of this water as the volume is usually large, approximately 1.6 million gal/dayji,ooo tons of cane ground. Cane washing is usually required in those areas where mechanical harvesting is practiced or where harvesting must be completed during the rainy season. The BOD, COD and total solids content of this water is widely variable, depending on specific mill conditions, but in general is high, as shown in Table 3. Primary and secondary Table 3. Typical characteristics of mill waste solids and liquids, on the basis of 1,000 tons cane ground/day. Flow BOD Population COD Total solids Point (MGD) (Ib/day) equivalent (Ib/day) (lb/day) Cane wash water 1.6 3,200 18,900 7,600 53,800 Condenser water ,000 1,600 1,000 Filter mud - 6,800 40,000 16, ,200 Fly ash ,900 5,900 Bagasse fiber , Total ,790 63,020 28,60q. 400,100 treatment of these wastes are practiced in some mills now and in all probability will be required in all mills in the next few years. This type of treatment, when combined with recycling, does not have a high operating cost but initial investment/ton of cane can be expensive. Condensate Condensate is classified as that water that is accumulated from condensation in the various heaters, exchangers, vacuum pans, steam traps, etc., throughout the mill. It is very important water in that it is usually very pure and contains little or no contamination, unless the mill is operating at above design capacities, in which case considerable sucrose losses and condensate contamination can occur. Considerable effort is and should be made to conserve all quantities of this water, as it is needed throughout the mill for maceration water, boiler feedwater, dilution of molasses, washing of sugar in the centrifugals, and other pure water uses. Because of its purity this water should be conserved with utmost priority.. Condenser Water Condenser water is defined as that water flowing from the condensers. It contains the water that is circulated through the condenser plus the condensed vapors-mainly from the evaporators and vacuum pans. This water is either once-through water or water circulated from a closed system. In properly con-

8 E. R. HENDRICKSON, F. A. GRILLOT, JR trolled evaporators, it remains low in sucrose content and is generally hot when it comes off the barometric legs. It is generally cooled by some method before recirculating. An important consideration here is that many mills are operating considerably above their design.rates, which increases the opportunity for sucrose carryover due to high internal velocities of the equipment and ineffective mist eliminators and catchalls. Factories falling in this category should give careful consideration to frequent or continuous analysis of this stream as sucrose losses not only increase pollution problems but reduce plant efficiencies and sugar recovery. Sugar losses can approach 1 lb of sugar/ton of cane in extreme cases. This water also should be conserved and kept pure for additional plant reuse, although there should be no objection to discharge to streams with sufficient flow to minimize thermal pollution if close control over sucrose carryover is practiced. This can be accomplished with proper operation and control and with cooling towers or spray ponds arid properly designed recirculation systems where stream volume is insufficient to dissipate the thermal pollution. Evaporator Boilout All mills must boil out the evaporators on a periodicbasis determined by the amount of fouling experienced. This boilout is usually accomplished by water, acid and caustic washings. The effluents cannot be drained directly into the natural streams or waterbodies because of their effect on marine life and waterfowl. Therefore, this material must be impounded or properly neutralized and treated before release. Fortunately, the volumes of these streams are small and, therefore, treating costs are not generally high. Factory Floor Washings and Yard Drainage This material must be impounded. Adequate factory floor and equipment drainage facilities-and segregation of floor washing from condenser water effluent-are necessary to prevent possible pollution of adjacent public streams. Additionally, proper equipment maintenance and supervision will minimize losses in recoverable sugar... Adequate yard drainage facilities are necessary. Mangled cane lying in the yard can contribute to sizeable economic losses to the factory. Additionally, in the event of rain the runoff poses a pollution problem. Measures should be taken to minimize the volume of cane dropping to the truck and tractor route pattern in transfer to the feeder table or to the yard. Also, the yard and the adjacent vehicle route should be thoroughly cleaned on a prescribed routine schedule.