Palm oil mill effluent (POME) has been a major environmental concern in countries producing them. This effluent is a land and aquatic pollutant when discharged fresh and fermented due to the presence of moderate amount of organic load in it and its phytotoxic properties (Okwute and Isu, 2007). The uses of wastes such as POME in agriculture and for land reclamation are a common practice in regions with its abundant supply especially for irrigation, soil conditioning, amendment and conservation purposes (Navas et al., 1998; Pascual et al., 2007). Palm oil production requires addition of large quantity of water which is eventually discharged as waste effluent (Nwoko and Ogunyemi, 2010). POME is a mixture of water, oil and natural sediments (solid particles and fibres), large quantity of which is generated annually during crude palm oil production (Salihu et al., 2011b) and is amenable to microbial degradation (Nwoko and Ogunyemi, 2010; Nwoko et al., 2010). POME includes dissolved constituents such as high concentration of protein, carbohydrate, nitrogenous compounds, lipids and minerals, which may be converted into useful materials using microbial processes. Nevertheless, POME if not discharged properly and treated, may lead to considerable environmental problems (Singh et al., 2010). In nature, both nitrogen and phosphorus come from the soil and decaying plants and animals (Logan et al., 1997; Navas et al., 1998). Fertilizers, fresh and fermented sewage as well as domestic and wild animal wastes are common sources of plant nutrients.
The input of effluent materials with high organic matter content will help replenish the soil for sustainable agriculture. POME application to soil can result in increases of some beneficial soil chemical and physical characteristics such as increases in organic matter, carbon, major nutrients (such as nitrogen, potassium, calcium, and magnesium), water-holding capacity and porosity (Logan et al., 1997; Navas et al., 1998). However, it may bring about undesirable changes such as decrease in pH and increases in salinity (Kathiravale and Ripin, 2000). These effects mostly occur very slowly and take many years to be significant. Soil microbiological and biochemical properties have been considered early as sensitive indicators of soil changes and can be used to predict long – term trends in the quality of soil (Ros et al., 2003). Soil microbial properties are equally affected by environmental factors. (Dick and Tabatabai, 1992) reported that high rate of inorganic fertilizer application suppresses microbial respiration and dehydrogenase activity. Other factors such as increase in salinity or decrease in water availability may also reduce biological activity (Paredes et al., 2005). POME contains high organic load, substantial amounts of plant nutrients and represent a low cost source of plant nutrients when fermented (Onyia et al., 2001). It is generally believed that the toxic effect of POME is due to its possession of phenols and other organic acids which are responsible for phytotoxicity and antibacterial activity (Capasso et al., 1992; Piotrowska et al., 2006). However, the polyphenolic fraction is degraded with time and partially transforms into humic substances (Piotrowska et al., 2006). Little information is known on the impact of POME on the biochemical and microbial properties of soil. Studies show that effects of wastes applied to soil occurred mainly in the first weeks after amendment (Martens et al., 1992; Perucci, 1992; Binder et al., 2002). Indeed, investigating the effects POME has on soil properties would help farmers mostly in rural areas to improve food production through expanding their understanding of the importance of POME as well as the quantity to be added to soil during farming operation prior to planting. Also, knowledge on the remediating effect of POME on the soil will assist government in its drive to increase food production by helping farmers to improve soil fertility through adequate harnessing and processing of POME (Nwoko and Ogunyemi, 2010).
1.1 Scientific Classification of Oil Palm (Elaeis guineensis)
Elaeis guineensis is a member of the family Arecaceae (Reeves and Weihrauch, 1979). It is native to West and Southwest Africa and is vastly cultivated as a source of oil in Nigeria. It has a lifespan of over 200 years, while the economic life is about 20-25 years. The nursery period is 11-15 months and first harvest is done 32-38 months after planting (Reeves and Weihrauch, 1979). The yield is about 45-56% of fresh fruit bunch and the fleshy mesocarp of the fruit is used as oil source. The yield of oil from the kernel is about 40-50% (Rupani et al., 2010). The plant is classified as follows:
Specie Elaeis guineensis.
Source: Reeves and Weihrauch (1979).
While oil palm is recognized for its contribution to economic growth, the rapid development of palm products has also correspondingly led to environmental pollution.
1.2 The Palm Seeds
Palm seeds are reddish, about the size of a large plum and grow in large bunches. Each fruit is made up of an oily and fleshy outer layer (the pericarp) with a single seed (the palm kernel) that is also rich in oil. The seeds are used for propagating the plant and are eaten roasted or boiled. The pulp is pressed to produce palm oil while the kernel is used to produce palm kernel oil (Rupani et al., 2010).
Fig 1: Fresh fruit bunch (FFB). Fig 2: Palm seeds
One of the seeds was cracked open to show the pulp segment, the kernel shell and the kernel seed.
1.3 The Palm Oil Production
Palm oil is edible oil derived from the fleshy mesocarp of the fruit of oil palm. It is one of the most widely consumed plant oil across the world (Rupani et al., 2010). In general, the palm oil milling process can be categorised into dry and wet (standard) processes. The wet process of palm oil milling is the most common and typical way of extracting palms especially in Nigeria (Okwute and Isu, 2007). Despite the fact that the POME can cause environmental pollution, not much has been done to mitigate this effect. The technology applied in almost all palm oil mills is based on methods developed in the 1970s and 80s (Zaini et al., 2010). The major steps in the oil palm processing as reported by Zaini et al.(2010) are as follows:
Threshing: This is the removal of fruits from bunches. The fresh fruit bunches consists of the fruits that are attached onto the spikelet growing on a main stem. The fruit-laden spikelet are cut from the bunch stem using axe for manual threshing before separating the fruits from the spikelets (Zaini et al., 2010).
Sterilisation: Loose fruits are boiled in batches using high temperature wet-heat treatment. This is carried out in autoclave by steam application at temperature and pressure ranges of 120-140°C at 3-3.5 bar, for 75 minutes. Boiling prevents fatty acid formation and assists in fruit stripping as well as prepares the fruit fibre for the next processing step. Boiling breaks down oil-splitting enzyme and stops hydrolysis and auto-oxidation(Zaini et al., 2010).
Crushing process: In this step the palm fruits are passed through shredder and pressing machine to separate the oil from the fibre and seeds (Zaini et al., 2010).
Digestion of the Fruit: This process releases the palm oil in the fruit through cracks in the oil-bearing cells. The digester consists of a steam-heated cylindrical vessel with central rotating shaft that is filled with several beater arms. The fruit is pounded by the rotary beater arms at high temperature to reduce the oil viscosity. This destroys the exocarp fruits or the outer covering and completes the disruption of the oil cell already begun in the sterilisation process. The digester must be filled to ensure the maximum storage and the effect of the agitation (Zaini et al., 2010).
Extracting the Palm Oil: There are two distinct methods of extracting oil from the digested palm fruit: one system uses mechanical presses and is called the “dry” method. The other called the “wet” method uses hot water to leach out the oil (Zaini et al., 2010).
Kernel Recovery – The residue from the press consists of a mixture of fibres and palm nuts which are at this stage sorted. The sorted fibres are covered and allowed to be heated by itsinternal exothermic reactions for about two or three days. The fibres are then pressed in spindle press to recover second grade (technical) oil that is used normally in soap making. The nuts are usually dried and sold to other operators who process them into palm kernel oil (Zaini et al., 2010).
Refining: Refining converts the crude palm oil (CPO) into refined form. The CPO is processed to segregate fat and obtain refined palm oil (Zaini et al., 2010).
Oil Storage: The palm oil is stored in large steel tanks at 31 to 40°C to keep it in liquid form during bulk transport. The tank headspace is often flushed with CO2 to prevent oxidation. Higher temperatures are used during filling and draining of the tanks (Zaini et al., 2010).
Fig 3:Palm oil mill effluent dumpsite
a = Fermented POME
b = Fresh POME
Table 1: Summary of Unit Operations in Palm Oil Production (Rupani et al., 2010)
|Fruit fermentation||To loosen the fruit base from the spikelets and to allow ripening processes to abate|
|Bunch chopping||To facilitate manual removal of the fruit|
|Fruit sorting||To remove and sort the fruit from spikelets|
|Fruit boiling||To sterilise the fruit and stop enzymatic spoilage, coagulate protein and expose microscopic oil cells|
|Fruit digestion||To rupture the oil-bearing cells in order to allow oil flow during extraction while separating fibres from nuts|
|Mash pressing||To release the fluidal palm oil using applied pressure on ruptured cellular contents|
|Oil purification||To boil the mixture of oil and water in order to remove the water-soluble gums and resins in the oil as well as to dry decanted oil by further heating|
|Fibre-nut separation||To separate the oiled fibres from palm nuts.|
|Second pressing||To recover residual oil for use as soap stock|
|Nut drying||To sun-dry nuts for later cracking|
1.4 The Properties of POME
The two main wastes resulting from palm oil production in oil mills are the solid and liquid wastes (Kathiravale and Ripin, 1997). The solid waste typically consists of palm kernel shells (PKS), mesocarp fruit fibres (MFF) and empty fruit bunches (EFB). The liquid waste generated from the extraction of palm oil in wet process comes mainly from the oil room after the kernel recovery. This liquid waste combined with the wastes from the steriliser condensate and cooling water is called palm oil mill effluent (POME) (Zaini et al., 2010).
Palm oil production requires input of large quantity of water which is eventually discharged as waste effluent. POME is an effluent generated from palm oil milling activities which requires effective treatment before discharge into watercourses (Nwoko and Ogunyemi, 2010). POME generated from mill operation is thick, brownish, highlyconcentrated,colloidal and slurry with pH ranging from 4.0 to 4.5 and a temperature of between 80 and 90°C (Zaini et al., 2010; Alrawi et al., 2012). It contains mainly water (95-96%), suspended solids (2-4%) and oil and grease (0.6-0.7%) (Ahmad et al., 2003). Palm oil production process does not utilize any chemical and hence POME is considered as a non-toxic wastewater.
Table 2: Characteristics of POME and its respective standard discharge limit
|Parameters||Experimental values obtained by some previous researchers on POME.||Standard limit (mg/L)|
|Oil and grease (mg/L)||4,000||4,900-5,700||8,845-10,052||50|
|Biological oxygen demand (mg/L)||25000||23,000-26,000||62,500-69,215||100|
|Total solids (mg/L)||40,500||–||68,854-75,327||–|
|Total nitrogen (mg/L)||750||500-700||1,305-1,493||150|
|Total volatile solids (mg/L)||34,000||–||4,045-4,335||–|
Sources: Department of EnvironmentMalaysian, (1999); Ahmad et al., 2003; Najafpou et al., 2006; Choorit and Wisarnwan(2007).
1.4.1 Physicochemical Characterisation of POME
220.127.116.11 Dissolved Oxygen (DO)
The dissolved oxygen is a measure of the amount of gaseous oxygen dissolved in an aqueous solution. Analysis of DO is a key test in water pollution. The DO levels in POME depend on the physical, chemical and biochemical activities in POME. Adequate DO is necessary for good quality of water. Oxygen is an essential element to all forms of life. The DO concentrations ought not to exceed 110% otherwise; it may be harmful to aquatic life. As DO levels in water drop below 5.0 mg/L, aquatic life is put under stress; the lower the concentration of DO, the greater the stress. Death usually occurs at concentrations less than 2 mg/L. The World Health Organization (WHO) suggested the standard of DO greater than 5mg/L for river water monitoring (Sehar et al., 2011).
18.104.22.168 Biochemical Oxygen Demand (BOD)
The standard, five-day BOD (BOD) value is commonly used to determine the amount of organic pollution in water and wastewater. Determination of BOD involves measuring the oxygen demand of both the organic matter and organism in the POME. BOD is the amount of dissolved oxygen needed by aerobic biological organisms in a body of water to break down organic material present in a given water sample at certain temperature over a specific period of time (Okwute and Isu, 2007). It is an empirical test that determines the relative oxygen requirements of wastewater, effluents and polluted water. BOD tests measure the molecular oxygen utilised during a specified duration incubation for the biochemical degradation of organic materials (carbonaceous demand) and the oxygen used to oxidise inorganic material such as ferrous iron and sulfides (Sehar et al., 2011).
22.214.171.124 Chemical Oxygen demand (COD)
The chemical oxygen demand (COD) is used to measure the oxygen equivalent of the organic material in wastewater. It is a useful measure of water quality. In most cases applications of COD determine the amount of organic pollutants found in water (Sehar et al., 2011). The COD test is commonly used to indirectly measure the amount of organic compounds in water. The COD is the amount of oxygen required to chemically oxidize organic water contaminants to inorganic end products (Okwute and Isu, 2007). Most applications of COD determine the amount of organic pollutants found in surface water. COD is often measured as a rapid indicator of organic pollution in water (Sehar et al., 2011).
126.96.36.199 Total Dissolved Solids (TS)
The total solids represent all solids in a water sample. They include the total suspended solids, total dissolved solids, and volatile suspended solids. The range of 37900 – 45 000 mg/L has been reported by Wood et al. (1979); Wong et al. (2009) and MPOB (2004). TS is a measure of the amount of filterable solids in a water sample (Sehar et al., 2011).
188.8.131.52 Total Suspended Solids (SS)
These are amounts of filterable solids in a water sample. POME samples are filtered through a glass fibre filter. The filters are dried and weighed to determine the amount of total suspended solids in mg/L of sample. Suspended solid of POME were reported to be 18,000 mg/L by Ahmad et al. (2003) and MPOB (2004). The higher suspended solid was 25,800 mg/L and was recorded by Wu et al., 2007
184.108.40.206 Volatile Suspended Solid (TVS)
Volatile solids are those solids lost on ignition (heating at 550 0C). They are useful in application for treatment plant operator because they give a rough approximation of the amount of organic matter present in the solid fraction of wastewater, activated sludge and industrial wastes. Wood et al. (1979) and Wong et al. (2009) reported VSS range of 27300 mg/L to 30150 mg/L.
220.127.116.11 Oil and Grease (O and G)
Oil and grease have poor solubility in water. Thus, oil and grease content of industrial wastes are important consideration in handling and treatment of the material for disposal (Salihu et al., 2011a). The concentration of oil in effluents from different industrial sources can be as high as 40,000mg/L (Arcadio and Gregoria, 2003). Unlike free or floating oil spilled in the sea, lakes or rivers, most of the industrial wastewaters contain oil-in-water emulsions among other basic contaminants. Emulsified oil in wastewater can lead to severe problems in different treatment stages. Oil in wastewaters has to be removed in order to: (1) prevent interfaces in water treatment units (2) reduce fouling in process equipment (3) avoid problems in biological treatment stages and (4) comply with water discharge requirements (Arcadio and Gregoria, 2003).
1.5 Heavy Metals
The term heavy metal refers to any metallic chemical element that has a relatively high density and is toxic or poisonous at low concentrations (Kızılkaya et al., 2004). Examples of heavy metals include mercury (Hg), cadmium (Cd), arsenic (As), chromium (Cr), thallium (Tl), and lead (Pb) (Lebedeva et al., 1995). The use of metals by humans was and is still accompanied by increasing inputs of metals into soils through different types of wastes (Welp, 1999). The major sources of chromium include the metal finishing industry, petroleum refining, leather tanning, iron and steel industries, production of inorganic chemicals, textile manufacturing and pulp production. Because metals persist in soils and their leaching is a very slow process, they tend to accumulate in the soils (Irha et al., 2003). Chromium is one of the heavy metals and has oxidation states ranging from chromium (III) to chromium (VI).Chromium compounds are stable in the trivalent state and occur in nature in this state in ores such as ferrochromite, while chromium (VI) is usually produced from anthropogenic sources (Cervantes et al., 2001). Hexavalent chromium compounds have been used in a wide variety of commercial processes (Turick et al., 1996). Upon the reduction of chromium (VI) to chromium (III), the toxic effects are significantly decreased in humans, animals and plants because of a decrease in the solubility and bioavailability of chromium (III) (Turick et al., 1996). The reduction of the highly toxic and mobile Cr (VI) to the less toxic and less mobile Cr (III) is likely to be useful in the remediation of contaminated waters and soils. This problem has stimulated interest in microorganisms as alternatives to conventional methods due to their eco-friendly nature. Cr (III) is transformed to Cr (VI) mainly inside root cells but also in the aerial part of plant (Cervantes et al., 2001). Roots accumulate 10-100 times more Cr than shoots and other tissues. As a consequence, inhibition of growth, photosynthesis and respiration processes in plants and microorganisms are observed (Cervantes et al., 2001). Reduction of soluble Cr (VI) to insoluble Cr (III) occurs only within the surface layer of aggregates with higher available organic carbon and higher microbial respiration (Tokunaga et al., 2003). Thus, spatially resolved chemical and microbiological measurements are necessary within anaerobic soil aggregates to characterise and predict the fate of chromium contamination (Tokunaga et al., 2003). Heavy metals can enter a water supply by industrial and consumer waste or even from acidic rain, breaking down soils and releasing heavy metals into streams, lakes, rivers and groundwater (Zheng et al., 1999). The impacts of elevated heavy metal levels on the size and activity of natural soil microbial communities have been well documented. Field studies of metal -contaminated soils have shown that elevated metal loadings can result in decreased microbial community size (Jordan and LeChevalier, 1975; Brookes and McGrath, 1984; Chander and Brookes, 1991; Konopka et al., 1999) in organic matter mineralisation (Chander and Brookes, 1991) and leaf litter decomposition (Strojan, 1978).
1.5.1 Impact of Heavy Metals on the Soil Enzymes
By taking part and playing an important role in chemical changes of carbon, nitrogen, phosphorus and sulphur compounds, soil enzymes can serve as a tool for determining the biochemical soil properties (Dick and Tabatabai, 1992). For this purpose, activity of dehydrogenases is most commonly assayed as it is usually positively correlated with the volume of yields which in turn may indirectly indicate, however, that the activity of those enzymes is related to soil fertility (Dick, 1997). The activity of other soil enzymes such as catalase, lipase, urease or phosphatase, can also be helpful because they are as sensitive as dehydrogenases in indicating processes occurring in soil. Soil enzyme activities are considered to be sensitive to pollution and have been proposed as indicators of soil degradation (Trasar-Cepedaet al., 2007). Catalase (hydrogen peroxide oxidoreductase, EC