TECHNICAL FIELD

The present invention relates in general to a method for treating liquid effluent discharged from palm oil mills to comply with the stringent discharge standards in environmentally sensitive areas, and in particular to a method for converting the liquid effluent to a solid biomass with significantly reduced moisture content to make it suitable for recycling either as a biofuel or as a biofertilizer.

BACKGROUND ART

The palm oil milling process generates 0.6 to 0.7 tons of liquid palm oil mill effluent (POME) per ton of fresh fruit bunches (FFB) processed. POME is a colloidal slurry containing water, oil, cellulosic fruit debris, sand and water-soluble dissolved components originating from palm fruits. It is non-toxic as no chemicals are added during the oil extraction process. It is made up of about 94 percent water, 1 percent oil and 5 percent solids. POME is characterized by a high biological oxygen demand (BOD) and has to be treated to discharge standards stipulated by the Department of Environment (DOE). It is widely acknowledged that a more effective method of treating POME than the widely used anaerobic/aerobic ponding system is needed to comply with the more stringent discharge standards in environmentally sensitive areas.

In today’s environmental and economic climate, a heightened awareness exists regarding the sustainability of our industrial processes. Companies are now required to focus on best practices to reduce, re-use and re-cycle natural resources as a regular part of doing business. Attempts to recycle POME to comply with the stringent discharge standards in environmentally sensitive areas have generally been unsuccessful. The high moisture content of the effluent, the large quantity to be dried, the high sand content and the stickiness of the product being dried has meant that drying of POME is generally an expensive, energy-intensive and difficult operation that cannot normally be carried out using only the solid wastes generated by the palm oil milling process as energy sources. A problem faced with drying POME is that it undergoes an intermediate viscous or sticky phase. This causes the product being dried to stick to the surfaces of the drier and to agglomerate, thereby, decreasing the efficiency of the drier, extending the drying time and, possibly, leading to the breakdown of the drier.

One solution to the effluent problem in palm oil mills is to use an evaporator system for removing water from the effluent, thereby reducing the amount of effluent discharged. The large quantity of suspended solids and gums in POME will lead to an increase in the viscosity of the product being evaporated. The higher viscosity may significantly reduce the flowrate and affect the rate of heat transfer. Scale formation on the heating surfaces due to the higher viscosity can also present a problem. Scale formation may also be the result of product bum-on due to the use of high steam temperature to increase the overall rate of heat transfer. The overall heat transfer coefficient will steadily decline and the evaporator must be shut down and the tubes cleaned. Unless the problems of high product viscosity and the tendency to form a hard scale on the heating surfaces of conventional evaporation systems are appropriately addressed, evaporation alone cannot be considered a viable solution to the effluent problem in palm oil mills. The product leaving the evaporation system will still have high moisture content.

In view of the inability of existing evaporation and drying systems to provide a satisfactory zero discharge solution to the effluent problem in palm oil mills, a new treatment method is required that can cope with the large quantity of POME discharged and its unique characteristics.

DISCLOSURE OF THE INVENTION

The invention disclosed herein addresses the problems currently faced with the use of conventional evaporation and drying systems for significantly reducing the moisture content of the liquid effluent discharged from palm oil mills to facilitate achieving zero discharge. Reduction in the moisture content of either untreated or partially treated palm oil mill effluent (POME) using an evaporation system, followed by mixing with one or more bulking materials and drying makes it possible to convert POME to a solid biomass with significantly reduced moisture content that is suitable for use either as a biofuel or as a biofertilizer.

It is an objective of the present invention to provide a simple, environmentally- friendly and cost-effective method for treating palm oil mill effluent (POME).

It is a further objective of the present invention to provide a simple, environmentally-friendly and cost-effective method that is suitable for adoption by commercial palm oil mills to convert POME from a product having very little economic value and requiring substantial investment for its treatment to comply with regulations on discharge standards to a product that can be used either as a biofuel or as a biofertilizer, thus enhancing its economic value.

It is a further objective of the present invention to provide a simple, environmentally-friendly and cost-effective method for treating the discharge from palm oil mills in a manner that obviates the need for treating the discharge biologically using tertiary treatment systems to comply with the more stringent regulations in environmentally sensitive areas.

The above objectives are achieved in the present invention by providing a method for producing a solid biomass from liquid effluent discharged from palm oil mills comprising the steps of:

(a) evaporating the liquid effluent to reduce its moisture content to produce a concentrated sludge and a condensate containing evaporated components as by-product;

(b) mixing the concentrated sludge with one or more bulking materials to produce a mixed biomass; (c) drying the mixed biomass to reduce its moisture content to an extent sufficient to produce the solid biomass.

The liquid effluent in the present invention may be either untreated (raw) or partially treated POME. If partially treated POME, it may be either the liquor from anaerobic digestion of POME or the liquor from aerobic digestion of POME.

The liquid effluent may be pre-treated prior to evaporation to minimize fouling of the evaporation means, producing a clarified sludge for evaporating and sludge solids. The separation of the suspended solids is advantageously carried out using a mechanical separation means, such as decanting centrifuge, a belt press, a filter press or a multi-disk screw press, or combination thereof. The liquid effluent may be chemically dosed to assist in the flocculation of solids to improve the separation of solids by the mechanical separation means. The clarified sludge discharged from the mechanical separation means will consist mainly of water, dissolved solids and fine fibrous suspended solids.

A stripping means may also be used prior to evaporation for the removal of soluble volatile components in the feed to the evaporation means that are responsible for foaming during evaporation. This produces a stripped sludge for evaporating and volatile components. This is normally achieved using steam as the stripping medium. Steam stripping allows for the removal of the heavier soluble organics that cannot be removed by air stripping.

The use of a multiple-effect evaporator system makes possible the removal of moisture from the liquid effluent using a fraction of the energy required by a drier to remove an equal amount of moisture. The evaporation may be carried out using various of types of evaporators. The falling film evaporation method and rising film evaporation method can be used when the viscosity of the product being evaporated is sufficiently low. The most suitable evaporators for highly viscous products are forced circulation evaporators and scraped surface evaporators or evaporators using a combination of these two evaporation methods. The concentrated sludge discharged from the evaporation means is mixed with one or more bulking materials to increase its porosity to facilitate drying. A fluffy material generated by the palm oil milling process that has low bulk density should ideally be used as the bulking material. The bulking material used should have a significantly lower moisture content than the concentrated sludge discharged from the evaporation means. The use of the bulking material facilitates air movement during drying due to the increase in voids ratio. The voids ratio is an important parameter since it controls both air and moisture movement during drying. Ideal bulking materials available for this purpose in a palm oil mill are empty fruit bunches and palm fruit fibre, or a combination thereof. Empty fruit bunches may be shredded and/or pressed before it is used as bulking material. A portion of the solid biomass may be recycled back for mixing if the amount of bulking material generated by the palm oil milling process is insufficient. The amount of water removed by evaporation must ensure that the moisture content of the mixed biomass is less than 65% to ensure sufficient porosity in the mixed biomass to facilitate drying.

The mixed biomass is subsequendy dried to produce the solid biomass. The amount of moisture removed by drying is kept to the minimum needed to facilitate its intended usage. To facilitate the usage of the solid biomass as a bio fertilizer, its moisture content should be in the range of 10% to 50%. At this moisture content, the solid biomass can be recycled back to oil palm plantations for use as mulch. To facilitate the usage of the solid biomass as a biofuel, its moisture content should preferably be less than 40%. At this moisture content, its net calorific value will be sufficiently high to permit its use as boiler fuel. Hence, only about 50% to 65% of the water in the mixed biomass needs to be removed by drying, thus minimizing the energy consumption and the drying time needed, making drying a relatively cost-effective and energy-efficient operation.

Drying of the mixed biomass can be achieved using an external heating source, such as in convection dryers, where the drying medium directly contacts the material to be dried and carries away the evaporated moisture. Some cost-effective convection dryers that can be used for this purpose are rotary dryers and conveyor dryers or combination thereof. Drying using convection dryers can be achieved using either hot air or the flue gas generated by boilers used in palm oil mills as the drying medium. The drying time will be short (about an hour or less) because of the external heat supplied.

In a rotary dryer, a slightly inclined rotating metal cylinder is fitted internally with flights to cause the material being dried to cascade through a stream of hot air as it moves through the dryer. The design of the rotary dryer can be customized to suit processing needs, including the flight design and pattern, percent fill, retention time and size. In a conveyor dryer, the material being dried is evenly spread onto a slowly moving conveyor. The conveyor moves through a drying chamber that is heated using hot air. The use of a multi-deck conveyor system facilitates the use of longer drying times, intermittent turning of the material being dried and significant savings in the floor space required.

The hot air that flows through either of the abovementioned dryers can flow in the same direction (co-current) or in the opposite direction (counter-current) to the material flow. Since the product being dried is not thermo-sensitive, the temperature of the air used for drying can be raised to achieve higher evaporation rates. With parallel (co current) flow, only high moisture content material comes into contact with the hot gases and, as a result, higher evaporation rates can be achieved than when using counter-current flow.

Drying of the mixed biomass can also be achieved using internal heat generated by biodrying. Biodrying is the process by which biodegradable waste is rapidly heated through the initial stages of composting to remove moisture from the waste and hence reduce its overall weight. Heat to facilitate the biodrying process is generated by the aerobic degradation of the biodegradable waste by living microorganisms. The drying rate is further augmented by aeration. Mixing the concentrated sludge with one or more bulking materials to produce a porous medium also facilitate biodrying. The improved porosity facilitates oxygen transport to the aerobic microorganisms to the facilitate biodrying process and moisture movement. The moisture content of the feed to the biodrying process should preferably be less than 65%. If the moisture level is above 65%, anaerobic conditions will be more likely because water rather than air fills the pore space limiting available oxygen. The time needed for biodrying will be significantly longer than drying using externally supplied heat. The biodrying process can normally be completed in less than one month. Shredding of empty fruit bunches will convert it to a more homogenous mass and accelerate the biodrying process. Composting accelerators may be added at the start of the biodrying process to shorten the biodrying time. The main advantage over normal drying is that no external heat source is needed to facilitate drying.

Biodrying can be achieved using either a windrowing system or an in-vessel system, or combination thereof. The windrowing system is more suitable for the large quantities of waste generated by palm oil mills. In-vessel or reactor biodrying systems can be used as an alternative to the windrowing system to provide a more environmentally-acceptable solution that can be operated under all weather conditions. In-vessel bio-drying occurs within a contained vessel, enabling the operator to maintain closer control over the process. Compared to windrowing system, in-vessel bio-drying generally requires less land area and less manpower to operate the plant. These systems tend to be more costly then windrowing systems because of the increased mechanization. The power consumption and the operating and maintenance costs of such plants are also likely to be higher.

The condensate discharged from the evaporation means may be treated biologically, chemically (for example using biocides), physically (for example using microfilters, ultrafilters, nanofilters, reverse osmosis or granular activated carbon filters), or a combination thereof, and recycled for use in the palm oil mill. The condensate that is not recycled back to the palm oil mill can be treated biologically using a small effluent treatment plant, such as a sequencing batch reactor (SBR) or membrane bioreactor (MBR), before it is discharged.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates schematically the preferred embodiment of the present invention. In describing the preferred embodiment of the present invention, which is illustrated in Figure 1, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the specific terms so selected and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.

MODE FOR CARRYING OUT THE INVENTION

In the present invention, the liquid effluent may be either untreated (raw) or partially treated POME. The partially treated POME may either be the liquor from anaerobic digestion of POME or the liquor from aerobic digestion of POME. Referring to Figure 1, the liquid effluent 200 is optionally pre-treated 201 to remove suspended solids prior to evaporation. The use of a mechanical separation means for pre-treatment 201 generates two phases. One phase is sludge solids 202 having moisture content less than 80%. Another phase is clarified sludge 203 containing reduced solids content. The separation is advantageously carried out using a decanting centrifuge, a belt press, a filter press or a multi-disk screw press. The liquid effluent 200 may be chemically dosed to assist in the flocculation of solids to improve the separation of solids by the mechanical separation means. The clarified sludge 203 discharged from the mechanical separation means will consist mainly of water, dissolved solids and fine fibrous suspended solids.

The liquid effluent 200 or clarified sludge 203 may be optionally treated using stripping means 204 prior to evaporation for the removal of soluble volatile components in the feed to the evaporation means to minimize foaming during evaporation. This is normally achieved using steam as the stripping medium. The use of steam stripping means 204 generates two phases. One is volatile components 205 and the other is stripped sludge 205 with significantly reduced concentration of the volatile components responsible for foaming during evaporation. Steam stripping 204 is usually carried using either continuous contact or staged contact distillation columns, with the liquid effluent 200 introduced at the top of the column and steam introduced at the bottom. In the stripping column, volatile organic components are transferred from the liquid phase to the vapour phase as the liquid phase travels down the column and the vapour phase travels up the column. The steam stripping column utilizes trays or packing internals to facilitate contact between the liquid effluent and the steam. The vapour that comes off the top of the stripping column may be condensed. The higher temperature used for steam stripping compared to air stripping allows for the removal of the heavier, more-soluble organics that are not strippable with air. The liquid effluent 200, clarified sludge 203 or stripped sludge 206 is processed using evaporating means 207 to remove the bulk of the water and other volatile components that it contains in an energy-efficient manner to form concentrated sludge 209 and condensate 208 containing the water and volatile components removed. Evaporation 207 may be carried out by using a falling film evaporator, a rising film evaporator, a forced circulation evaporator, a scraped surface evaporator or an evaporator using a combination of these evaporation methods. The use of the falling film and rising film evaporation methods should be limited to the first few stages of a multiple-effect evaporator system when the viscosity of the product being evaporated is still sufficiently low. Scraped surface evaporation or forced circulation evaporation methods will be used to minimize fouling of the evaporator as the concentration of solids increases. The scraped surface evaporation method is advantageously used as the final evaporation stage of a multiple-effect evaporation system to achieve a very high concentration of solids. Concentrated sludge 209 is then mixed with one or more bulking materials 211, such as empty fruit bunches or palm fruit fibre, to form mixed biomass 212. Empty fruit bunches may be shredded and pressed before it is used as bulking material. The sludge solids 202 discharged from pre- treatment means 201 and/or boiler ash may also be mixed. The amount of water removed from the feed liquor 200 by evaporation 207 must ensure that the moisture content of the mixed biomass 212 is less than 65 percent to ensure sufficient porosity _' in the mixed biomass to facilitate drying. Mixed biomass 212 is dried using a drying means 213 to generate solid biomass 214. The amount of moisture removed by drying is kept to the minimum needed to facilitate its intended usage. To facilitate the usage of the solid biomass as a bio fertilizer, its moisture content should be less than 50 percent. At this moisture content, the solid biomass can be recycled back to oil palm plantations for use as mulch. To facilitate the usage of the solid biomass as a biofuel, its moisture content should preferably be less than 40 percent. At this moisture content, its nett calorific value will be sufficiently high to permit its use as boiler fuel. Hence, only about 50 to 65 percent of the water in the mixed biomass needs to be removed by drying, thus minimizing the energy consumption and the drying time needed, making drying a relatively cost-effective and energy-efficient operation.

Drying of the mixed biomass 212 may be achieved using convection dryers where the drying medium directly contacts the material to be dried and carries away the evaporated moisture. Some cost-effective convection dryers that can be used for this purpose are rotary dryers and conveyor dryers. Drying using convection dryers can be achieved using either hot air or the flue gas generated by boilers used in palm oil mills as the drying medium. The drying time will be short (about an hour or less) because of the external heat supplied.

Drying 213 of the mixed biomass 212 may also be achieved by using a biodrying means. Biodrying, as the name implies, is a drying technique that relies on the heat generated by the metabolic activities during composting as well as forced aeration to reduce the moisture content of wet biomass. Biodrying is advantageously carried out using either the windrowing technique or a version of the in-vessel biodrying technique in a fully-roofed and weather-proof building to reduce the moisture content from about 65% to below 50 %. The amount of water removed by evaporation is controlled to ensure that the moisture content of the feed to the biodrying means is below 65% after mixing with the concentrated sludge discharged from the evaporation system. Composting accelerators may be added at the start of the biodrying process. The mixture in the biodrying means is turned periodically to homogenize and aerate it, and a portion of the discharge from the biodrying means may be recycled to increase the percentage of bulking material in the feed to the biodrying means.

Condensate 208 may be treated biologically, chemically, physically, or a combination thereof, and recycled for use in the palm oil mill, or otherwise it is treated biologically using a small effluent treatment plant before it is discharged.

The mode or embodiment of the invention described herein is only meant to facilitate understanding of the invention and should not be construed as limiting the invention to that mode or embodiment only. Those skilled in the art will appreciate that the mode or embodiment of the invention described herein is susceptible to variations and modifications other than that specifically described. It is to be understood that the invention includes all such variations and modifications which fall within the scope of the inventive concept thereof.

INDUSTRIAL APPLICABILITY

The present invention finds ready industrial applicability in the palm oil industry as it is a method for converting the liquid effluent discharged from palm oil mills from a product having very little economic value and requiring substantial investment for its treatment to comply with regulations on discharge standards to a product that can be used either as a biofuel or as a biofertilizer. The invention provides a simple method for addressing the problems faced with the use of evaporation and drying systems to achieve zero discharge of liquid effluent from palm oil mills.