Coconut, Rubber : waste characteristics, management, value addition

Coconuts are produced in 92 countries worldwide on about more than 10 million hectares. Indonesia, Philippines and India account for almost 75% of world coconut production with Indonesia being the world’s largest coconut producer. A coconut plantation is analogous to energy crop plantations, however coconut plantations are a source of wide variety of products, in addition to energy. The current world production of coconuts has the potential to produce electricity, heat, fiberboards, organic fertilizer, animal feeds, fuel additives for cleaner emissions, health drinks, etc.

The coconut fruit yields 40 % coconut husks containing 30 % fiber, with dust making up the rest. Coconut husk and shells are an attractive biomass fuel and are also a good source of charcoal. The major advantage of using coconut biomass as a fuel is that coconut is a permanent crop and available round the year so there is constant whole year supply.

Coconut husk has high amount of lignin and cellulose, and that is why it has a high calorific value of 18.62MJ/kg. The chemical composition of coconut husks consists of cellulose, lignin, pyroligneous acid, gas, charcoal, tar, tannin, and potassium. Because of high lignin and cellulose content, the materials contained in the casing of coconut dusts and coconut fibers are resistant to bacteria and fungi.

The predominant use of coconut husks is in direct combustion in order to make charcoal; otherwise husks are simply thrown away. Coconut husk can be transformed into a value-added fuel source which can replace wood and other traditional fuel sources. In terms of the availability and costs of coconut husks, they have good potential for use in power plants.

 Activated carbon manufactured from coconut shell is considered extremely effective for the removal of impurities in wastewater treatment processes.

Open burning of Coconut waste contributes significantly to CO₂ where as land filling leads to methane emissions.  Coconut shell is widely used for making charcoal. The traditional pit method of production has a charcoal yield of 25–30% of the dry weight of shells used. The charcoal produced by this method is of variable quality, and often contaminated with extraneous matter and soil. The smoke evolved from pit method is not only a nuisance but also a health hazard.

The coconut shell has a high calorific value of 20.8MJ/kg and can be used to produce steam, energy-rich gases, bio-oil, biochar etc. Coconut shell is more suitable for pyrolysis process as it contain lower ash content, high volatile matter content and available at a cheap cost. The higher fixed carbon content leads to the production to a high-quality solid residue which can be used as activated carbon in wastewater treatment.

Coconut shell powder is manufactured from matured coconut shells. The manufacture of coconut shell powder is not an organized industry in India. The product finds extensive use in plywood and laminated board industry as a phenolic extruder and as a filler in synthetic resin glues, mosquito coils and agarbathis. Coconut shell powder is preferred to other alternate materials available in the market such as bark powder, furfurol and peanut shell powder because of its uniformity in quality and chemical composition, better properties in respect of water absorption and resistance to fungal attack. The product is manufactured in sizes ranging from 80- 200 mesh. Keeping in view of the vast industrial uses, the demand for coconut shell powder appears to be promising

Coconut Shell Charcoal is an important product obtained from coconut shell. Shell charcoal is used widely as domestic and industrial fuel. It is also used by blacksmiths and goldsmiths and in laundries. Shell Charcoal is also used to produce activated carbon.

Activated carbon

Activated carbon is a non-graphite form of carbon which could be produced from any carbonaceous material such as coal, lignite, wood, paddy husk, coir pith, coconut shell, etc. Activated carbon manufactured from coconut shell is considered superior to those obtained from other sources mainly because of small macropores structure which renders it more effective for the adsorption of gas/vapour and for the removal of colour and odour of compounds. The activated carbon is extensively used in the refining and bleaching of vegetable oils and chemical solutions, water purification, recovery of solvents and other vapours, recovery of gold, in gas masks for protection against toxic gases, in filters for providing adequate protection against war gases/nuclear fall outs, etc.

The process of activation is carried out in two stages. Firstly the coconut shell is converted into shell charcoal by carbonization process which is usually carried out in mud-pits, brick kilns and metallic portable kilns. The coconut shell charcoal is activated by reaction with steam at a temperature of 900^oC -1100 ^oC under controlled atmosphere in a rotary kiln. The reaction between steam and charcoal takes place at the internal surface area, creating more sites for adsorption. The temperature factor, in the process of activation is very important. Below 900 ^oC the reaction becomes too slow and is very uneconomical. Above 1100 ^oC the reaction becomes diffusion controlled and therefore takes place on the outer surface of the charcoal resulting in loss of charcoal.

Coir Pith

Coir pith a waste product obtained during the extraction of coir fibre from husk is very light, highly compressible and highly hygroscopic. It is used as a soil conditioner, surface mulch/ rooting medium and desiccant. Composted coir pith is an excellent organic manure for indoor plants as well as for horticulture crops. Several firms are manufacturing composted coir pith in the country. Compressed coir pith in the form of briquettes for easy transportation is also manufactured in the country.

Rubber industries waste

Natural rubber (NR) processing sector is an industry which produces raw materials used for the manufacture of rubber industrial products (conveyor belts, rubber rollers, etc.), automotive products (fan belts, radiator hoses, etc.), latex products (rubber gloves, toys hygienic products, etc.) and many kinds of adhesives .The major users of natural rubber are tire and footwear industries.

The raw material used for natural rubber processing is latex mainly tapped from rubber tree (Hevea brasiliensis). Natural rubber processing sector consumes large volumes of water and energy and uses large amount of chemicals as well as other utilities. It also discharges massive amounts of wastes and effluents. The most common environmental issues are wastewater containing chemicals and smell, hazardous waste, noise, and thermal emission.

Natural rubber production processess

The raw material used for the production of natural rubber is “white milky fluid” called latex taken from the latex vessels of rubber trees, which can be categorized as field latex, scrap, soil lump, and bowl lump. Chemically, latex consists of rubber, resins, proteins, ash, sugar, and water. The rubber content in the latex comes from the trees is approximately 30 to 40%. Latex, which is a kind of biotic liquids, will be deteriorated if it is not preserved by ammonia or sodium sulfite which is called anticoagulant. Anticoagulants prevent latex from pre-coagulation. The kind of anticoagulant used is depended upon the production process. Sodium sulfite is preferred if crepe or sheet rubbers are to be made, but ammonia is more suitable for latex concentrate

In summary, the product of natural rubber can be broadly classified under two categories i.e. dry and liquid rubber. Dry rubber refers to the grades, which are marketed in the dry form such as rubber sheet, crepe rubber, and crumb rubber, whereas liquid rubber refers to the latex concentrate production in which the field latex is separated into latex concentrate containing about 60% dry rubber and skim latex with 4-6% of dry content. Skim latex is produced as a byproduct during the preparation of latex concentrate. It has a dry rubber content of only 3 to 7% and its dirt content is very low. Coagulation of skim latex can be either spontaneously or by acid treatment. It is important that the ammonia content is kept as low as possible. Further processing is the same as for smoked sheet.

Referring to the whole steps in natural rubber processing, it is obvious that both dry and wet processes are involved. Size reduction, digestion, washing, and drying are unit operations involved in these processing activities. The step of washing consumes large amount of water, so that wastewater generated from these processing operations mainly comes from this step. Brief descriptions of processing of each type of natural rubber are presented below.

Processing of rubber sheet

Rubber sheet could be categorized as Air Dried Sheet (ADS) and Ribbed Smoked Sheet (RSS). The main difference of ADS and RSS is on the method used for drying the sheet, in which ADS exploits air, whereas RSS uses smoke provided in a smokehouse with the temperature up to 60°C. The original type of smokehouses has been replaced by so-called “Subur” smokehouses. The principle of the design of these houses is to eliminate as much as possible manhandling of sheets. The smoking chambers are on ground level, so that trolleys can be loaded with sheets in the factory and then transported by rail into the smoke chambers. The smoking process in the “Subur” smokehouses is basically a continuous process. Rubber sheet processing is started from latex collection in the field. It is then diluted and screened before the addition of formic acid for coagulation process. The wet sheet is sheeted off to a thickness of about 3 mm and finally passes an embossed two roll mill. The sheets are dried whether by air or in a smokehouse for about one week at temperatures. The specific smell of the smoked sheets is caused by the wood and other organic materials such as coconut shells used to produce the smoke. The sheets produced are finally classified and packaged.

Processing of Crepe rubber

Crepe rubber is made from latex field coagulum. In the production of crepe rubber from latex, the raw material is prevented from coagulation by adding ammonia. After transported to the factory, latex is filtered through a screen to remove coagulated rubber, particles, or leaves. It is then transferred to mixing tank with stirring blade after determine dry rubber content (DRC), latex is diluted with water to reduce DRC to 20 – 22%. In the production of crepe rubber, there are three important steps. Diluted latex from mixing tank is transferred to stationary coagulation troughs through movable throughs. Acetic or formic acid solution (2%) is normally used to neutralize ammonia added in the field for coagulation prevention and to reduce pH to 5.0 – 5.2, near the iso-electric point of 4.3.

The second step is primary and secondary milling. After coagulation, water is added to coagulation troughs to float up the coagulum. In water, coagulum is easy to move to milling machine. After primary milling, slabs of coagulum is passed through pair of roller of which the final one is grooved so as imprint on each the rib to increase the surface area for drying. Each roller is equipped with water sprayers to wash away non rubber particles. Then coagulum is cut into small, then it is dried by hot air and pressed.

Processing of crumb rubber

This type of natural rubber product is relatively new, which in trade market it is known as “technical specification rubber”. There are some benefits of crumb rubber processing i.e. the process is faster, the product is more clean and uniform, and the appearance of product is more interesting. Raw materials used for making crumb rubber can be field latex or low quality lump. The steps included in crepe rubber processing using field latex are latex coagulation, milling, drying, bale pressing, and packing. Coagulation process uses 1% formic acid plus 0.36% melase.

Sodium bisulfate is usually added to the coagulation mixture to get brighter end-product. If the raw material used is lump, the step will be started by soaking and/or washing the lump, and then followed by hammer milling, crepe formation, milling, drying, bale pressing, and packing.

Processing of latex concentrate

Latex colleted from the field is pre-treated such as screen, wash and ammonia addition before processing. After processing, the field latex is centrifuged. Because the disperse phase (rubber) and the continuous phase (water mainly) differ in density, the concentrated latex (60%) rubber is separate and is collected from the center of centrifuge bowl, whereas skim, about 5% rubber, is taken from the outer edge of centrifuge bowl. The concentrate latex is bulked, ammoniated and then stored. The skim latex is deammoniated, coagulated with acid, creped and dried.

Environmental issues of natural rubber processing sector

The rapid growth of this industry generates large quantities of effluents coming from its processing operations which is really a big problem because of its wastewater contains high biological oxygen demand and ammonia.

High concentration of BOD, COD, & SS

Wastewater discharged from latex rubber processing usually contains high level of BOD, COD and SS. These characteristics vary from country to country due to difference in raw latex and applied technique in the process. The main source of the pollutants is the coagulation serum, field latex coagulation, and skim latex coagulation. These compounds are readily biodegradable and this will result in high oxygen consumption upon discharge of wastewater in receiving surface water.

Acidic effluent

The effluent from latex rubber processing industries is basically acidic in nature. Different extents of acid usage in the different factories attribute to pH variation of different effluent. Due to the use of acid in latex coagulation, the effluent discharged from latex rubber factories is acidic and re-dissolves the rubber protein. The effluent comprises mainly of carbonaceous organic materials, nitrogen and sulfate. The quantity of acid used for coagulation of the latex, specifically in skim latex after centrifugation operation, is generally found to be higher than the actual requirement.

High concentration of ammonia and nitrogen compounds

The high concentration of ammonia presents in the latex concentrate effluent posed another serious threat to the environment. Most of the concentrated latex factories discharge treated wastewater that contains high level of nitrogen & ammonia to a nearby river or canals leading to a water pollution problem. If high level of ammonia is discharged to water bodies, it could lead to death of some aquatic organisms living in the water. Land treatment system has been conducted to treat and utilize nitrogen in treated wastewater from the concentrated latex factory.

High level of sulfate

The effluent from latex concentrate factories contains high level of sulfate which originated from sulfuric acid used in the coagulation of skim latex. The high level of sulfate in this process can cause problem in the biological anaerobic treatment system as high levels of H₂S will be liberated to the environment and generates malodor problem. The free H₂S also inhibits the digestion process, which gives lower organic removal efficiency.

 High level of odor

The odor causing compound such as hydrogen sulfide, ammonia, amines, can be produced by many of wastewater treatment process. Most odor of organic nature arises from the anaerobic decomposition of compounds containing nitrogen and sulfur. The odor is detectable even at extremely low concentrations and makes water unpalatable for several hundred miles downstream from the rubber plants. The problems presents varies considerably depending on the plant site, the raw material used, and the number of intermediary product.

Waste disposal measures - waste treatment practices

Waste treatment practices include practices for wastewater treatment, air pollution control and solid management. Of all environmental issues generated from this industry, wastewater is the major problem with a wide range of effects on human health and environmental health. Solid management are not major problems in rubber industry .

Wastewater treatment practices

Wastewater collected from rubber processing factory contains a variety of substances as well as the commercially important constituent, in this case rubber hydrocarbon. It contains proteins, minerals, non-rubber hydrocarbons and carbohydrates. This wastewater has high concentration of ammonia_, BOD₅, COD, Nitrate, Phosphorus as well as total solids. Moreover, the wastewater from latex concentrate and skim crepe industry contains sulfate which comes from sulfuric acid in the skimming process and in some processes produce rather high content of zinc and cadmium. Wastewater treatment practices can be mentioned as pollution abatement.

Pollution abatement involves (a) in-plant control of waste and (b) end-process treatment of wastewater. Some in-plant control measures can be introduced to enable reduction in consumption of water, generation of pollutants and to increase the efficiency of the end-of-process wastewater treatment.

 In-plant control measures

In the crepe and crumb rubber units, in which field coagulum is processed, high required water quantity is generally used for soaking and also the soaking time allowed is not adequate. If the raw scrap rubber is properly soaked and primary dirt removal is done by scrap-washer, the quantity of water consumed in milling can be reduced. In the crumb units, wastewater from final milling can be collected separately from the effluent of the other milling section and can be used either for soaking the scrap rubber or for the first milling process. This is comparatively clean and the amount of reduction can be up to 25% of the total water consumption.

In centrifuge machine bowl, washing is done at the interval of 3-4 hours to remove the sludge. About 0.5% rubber is lost during this washing step. To reduce loss, washing step can be done at two stages. The first washing which is more concentrated may be segregated and collected in a separate tank and coagulated for recovery of the rubber lost during washing. This will result in reduction of pollution load in the effluent. The possibility of diverting this waste stream into the skim coagulation tank can also be considered.

The quantity of acid used for coagulation of the latex, especially skim latex kit after centrifugation stage is generally found to be higher than the actual requirement. The time needed in coagulation tank is also less. The incomplete coagulation results in the loss of rubber particles into the effluent along with the skim serum. The excess acid not only causes acidic effluent but also re-dissolves the rubber protein and causes delay in coagulation. Hence, it is suggested that proper acid concentration applied and sufficient coagulation time should be provided to obtain more or less clear liquid after complete coagulation. The skim latex if de-ammoniated before coagulation, acid requirement can be reduced and the ammonia concentration in effluent may also be reduced. In the latex process units the segregated first washing of the coagulum may be diverted to the skim coagulum tank where after skim coagulum recovery, the effluent may join the other wastewater streams.

 End of process treatment

Basically wastewater treatment can be divided into pretreatment, primary treatment, secondary treatment, and tertiary treatment.

The rubber trap used for arresting suspended matters should have holding capacity of at least 12 hours with proper baffles to induce continuous up and down flow pattern. If designed properly, this can reduce suspended solids by 40 to 60%. The equalization tank should have at least one day detention time. It is preferred to have two equalization tanks, each of them with one day detention time.

Primary treatment

For a latex processing unit, effluent from the equalization tank to be sent for neutralization and chemical treatment by alum and iron salt (about 200 mg/l). Combined wastewater of latex process units also needs neutralization by using of lime and settling of suspended solids by using of coagulants. The settler/clarifier should have adequate detention time for removal of suspended solids. The sludge may be taken to sludge drying beds for dewatering. The dewatering of sludge produced by primary clarifier is normally carried out on belt or vacuum filters which raises the sludge consistency from 20 to 40%.

Secondary treatment

Following the primary treatment, the effluent should be subjected to the biological treatment. If sufficient land area is not available, then the effluent after primary settling may be subjected to an extended aeration activated sludge type biological treatment process.

Before going for biological treatment, it must be ensured that:

(a) All the in-plant control measures are adopted

(b) Primary treatment e.g. rubber trap equalization neutralization and clarification steps are incorporated.

The above measures will reduce substantial quantity of pollutants particularly BOD and suspended solids. The primary treated effluent can be treated in a secondary/biological treatment unit. It is envisaged to render secondary treatment by adoption of extended aeration activated sludge process. The biological treated effluents should be settled in a secondary settling tank.

If there is no constraint of land, the biological treatment could be anaerobic followed by aerobic pond system with the proper dimensions, holding capacity and adequate detention time (10 to 15 days) for anaerobic pond followed by 5 to 10 days for aerobic ponding system. The type of soil and proximity to the wastewater and ground water table condition should be taken into consideration before going for these treatment systems. Protective lining is recommended to eliminate any risk.

In place of the anaerobic-aerobic system, an oxidation ditch of detention time of 2-3 days can also be considered as an alternative for treating the effluents of the crumb rubber unit.

Depending on the real conditions of countries and specific processes, some units of wastewater treatment are modified and adjusted to have better efficiency. For example, most of the latex concentrate factories in the South of Thailand discharge treated wastewater that contains high level of nitrogen to a nearby river or canals leading to a water pollution problem. Land treatment system is used to treat and utilize nitrogen in treated wastewater from the concentrated latex factory. The land treatment system resulted high removal efficiency for nitrogen.

In recent years, many studies were carried out to treat wastewater from this industry by biological methods such as ASP (activated sludge process) and use of oxygenic phototrophic bacteria for treating latex rubber sheet wastewater These studies aim at improving the efficient treatment of wastewater from this industry and contribute to partially reduce the emission of toxic gases into the environment.

Tertiary treatment

The remaining components after primary and secondary treatment are residual SS, residual BOD, odor and hydrocarbon. Tertiary treatment designed to remove these components are generally carbon adsorption, massive lime treatment and foam separation, mainly for treatment of Residual Refractory Organics.

Biological method incorporated with sulphate reduction system

Low cost operation, high removal efficiency and also producing the biogas as a useful energy sources are some advantages of anaerobic wastewater treatment system. However, this treatment results in the formation of H₂S due to consumption of sulphate instead of oxygen by sulphate reducing bacteria. H₂S is toxic and increases the smell of putrid eggs. The gas also causes a big problem in biogas producing systems. As a result, sulphide could inhibit the activity of methane producing bacteria due to its toxicity. It also revealed that the high amount of sulphide reduced the COD removal. Therefore sulphide elimination is an important stage for this kind of wastewater before a biogas production step.

Sulphate reduction reactor (SRR) has been used for treatment of sulphate rich rubber wastewater from concentrated latex and skim crape. The SRR is needed for reduction of sulphate concentration in wastewater before biogas production by UASB. However, the produced biogas does not have a good quality due to its high amount of H₂S. Therefore, the biogas was burnt to remove the very toxic and corrosive H₂S gas.Hence, converting the sulphide to sulphur by partial oxidation is needed. It is realized that levels of sulphide oxidation are dependent on oxygen concentration. Additionally, bacteria with ability of oxidizing reduced sulphur compounds can be used for removal of H2S from treated wastewater or gaseous systems. Thus, selection of a microbe that can grow at room temperatures and neutral pH with ability of oxidizing sulphide to sulphur in wastewater is important. The purple non-sulphur photosynthetic bacteria which were isolated from a concentrated latex effluent were cultured in a wastewater without any supplementation. After 40 h of cultivation, 34% of COD was decreased by Rubrivivax Gelatinosus and Thiobacillus sp.. The Thiobacillus sp. meanwhile is extensively used worldwide for removal of both organic and inorganic sulphur compounds in wastewater. Thiobacillus sp. can reduce inorganic sulphur compounds as an energy source and therefore is used for removing sulphide from wastewater. Four kinds of Thiobacillus sp. were isolated from domestic and rubber wastewaters in Thailand by Kantachote and Innuwat (2004). All isolates could grow in pH of 2.0 – 7.0 (optimum 6.5), temperature of 25 – 45°C (optimum 30 – 35°C) under both aerobic and anaerobic conditions. The results showed that the highest COD removal (54%) can be obtained by Thiobacillus sp. which cultivated in rubber sheet wastewater for 14 days. However, it does not show the good ability for BOD reduction and it declined by only 33%.

 

Biological method incorporated with precipitation

One of the parameter that can affect the efficiency of biological treatment processes is the presence of heavy metals such as zinc in wastewater. Adsorption, membrane separation and precipitation are some examples of effective technologies that have been used for removal of heavy metals from wastewater. Currently, simple and inexpensive method such as precipitation by hydroxide is the more common approach that are used in Malaysia. However, this method is not suitable for highly organic polluted rubber wastewater due to zinc-organic ligand complexes production. Therefore, reduction of organic matter that includes heavy metals from wastewater is required. It has been found that some kinds of microorganisms in anaerobic and aerobic processes can be used for this purpose. Microbial flocculation under aerobic conditions can be avoided by high amount of total dissolved solids (TDS) in rubber wastewater.

In order to meet the effluent standards, rubber factory have to use some tertiary treatment such as coagulation and filtration processes to remove the excess solids. This significantly increases their treatment and disposal costs. Therefore, reduction of the TDS to a level that does not inhibit or interfere with aerobic microbial aggregation is required. Another method for removal of heavy metal is sulphide precipitation. In this process, use neutral pH which is also suitable for microbial growth In addition to, the removal efficiency of sulphide precipitation is usually better than the hydroxide treatment under a low dissolved solid condition. Therefore, sulphide precipitation is a more promising option than the recent technology. In other hand, adjustment of optimal dosage in hydroxide precipitation system is much easier than sulphide method especially under frequent fluctuation of zinc concentration. In fact, high amount of sulphide can makes malodour and also excessive residual sulphide whilst inadequate dose of precipitant can results in an effluent with high amount of zinc. Therefore, study about the effect of important parameters on sulphide addition control system which is easy and cheap are needed. Chemical and biological processes without any pH adjustment were used for treatment of acidic latex wastewater with high amount of zinc.

Sulphide and hydroxide precipitation increased the total dissolved solids of treated effluent by 1.1 and 2.8 times, respectively. 92% of TDS was removed by anaerobic filter in more than 11.8 g COD L^-1 day^-1 of organic loading rate. For the activated sludge process, average removal efficiencies for COD and BOD were 96.6 and 99.4%. This combined system was verified to be an effective method for purification of rubber thread wastewater.

Another most cost effective system for zinc removal from the wastewater in Malaysia is using a mixture of 800 mg/l of sodium sulphide and 5 mg/l of polyelectrolyte LT27, respectively. The optimum settling time and flocculation time were 60 and 20 min. The best results can be obtained in a speed of 20 rpm in a 110 mm diameter reactor. The efficiency of an expanded bed biofilm reactor in the treatment of wastewaters contaminated with heavy metals has been investigated for rubber product manufacturing industry. Some advantages of biofilm systems are ability to retain relatively high biomass concentrations that results in shorter liquid retention times, better performance stability and higher volumetric removal rates. In the study, it has been found that the process could achieve 60 to 90% removal of Zinc. In addition, the efficiencies of an expanded bed biofilm reactor and a sequencing batch biofilm reactor for heavy metal adsorption were studied using Zn and Cu containing wastewaters. The results showed that heavy metal adsorption by these reactors are 50 – 95%.

Natural process

A constructed wetland is an artificial marsh or swamp that includes substrate, vegetation and biological organisms contained within a physical configuration. Suitability designed and operated wetlands have considerable potential for lowcost, efficient and self maintaining wastewater treatment systems. This system has demonstrated capability to remove nutrients, suspended solids, organic compounds, pathogens and metallic ions and to increase oxygen and pH levels in wastewater. In comparison to conventional systems, lagoons or land application flow, wetlands waste treatment systems require fewer amounts of capital and operating costs, minimal operator training and land area

In Thailand, experiment was conducted to treat wastewater using the pilot-scale constructed wetland (CW) from a rubber sheet factory. This system consisted of vertical flow constructed wetlands (VF) followed by subsurface flow constructed wetlands (SSF) with nut grass (Cyperus rotundus Linn.) plantation. The tested COD loadings in this experiment were 500, 750, 1000 and 1250 kg COD/ha.d. The results showed that the best removal efficiency of BOD₅, COD, SS and TKN were 99, 99, 93.6 and 97.8%, respectively, using VF followed by SSF with nut grass (C. rotundus Linn.) plantation at 750 kg COD/ha.d.