PRODUCTION OF BIOMETHANE FROM FRESH CASSAVA WASTEWATER IN A LIQUID PHASE ANAEROBIC BIO- DIGESTER.

ABSTRACT

One of the greatest challenges facing the societies today is the increased emission of greenhouse gas from fossil fuel combustion, with consequent change in climatic conditions. Hence the need for cleaner and renewable energy source. This study was carried out to produce biomethane from fresh cassava wastewater in a liquid phase biodigester. Fresh cassava wastewater (FCWW) (1000 ml) was charged into 5 liter capacity fixed bed bioreactor and were subjected to anaerobic digestion for a period of 28 days at ambient temperature. The physicochemical parameters and the persistent organic compounds in the wastewater were determined before and after 28 days of biodigestion using standard methods. Isolation, characterization and determination of microbial population were carried-out at the end of biodigestion using plate count method. The main problem of biogas production from cassava waste effluents is the acid forming-bacteria that produces acids resulting in the decline in pH below 7 thus reducing the growth of methane forming bacteria in the biodigester. One of the methods to overcome this challenge as adopted in this work is the use of Ca(OH)2 to regulate pH by sequestrating CO2 which can be used in the production of economically important substances such as biocarbonic acid. After 28 days of biodigestion, the composition of flammable biogas were determined using a Bacharach combustible gas analyzer. The following composition of gases were obtained: CO2  = 12%, CO = 8%, NO = 3%, H2  = 3%, CH4  = 74% indicating that FCWW is a good substrate for biogas generation. The microbial isolation and characterization after 28 days of digestion indicated the presence of both Gram positive and Gram negative bacteria (Proteus, Vibrio, Bacillus, Staphylococcus, Escherichia and Salmonella) and a microbial population of 5.5× 108cfu. The fresh cassava wastewater had a pH of 5.70 which decreased to 3.01 after biodigestion. GC-MS analysis were conducted to determine the persistent organic compounds in the FCWW and the Sludge. The chromatogram for the FCWW showed 5 peaks with various organic compounds. The chromatograph for the Sludge showed 3 peaks and the presence of two organic compounds in each peak, indicating that anaerobic digestion is an effective means of bioremediation. The anaerobic digestion of cassava wastewater and other organic waste substrates in the production of biogas will contributes to proving the domestic energy need. This will also improve the quality of the environment by ridding the processing sites of pollution and reducing the emission of greenhouse gases.

CHAPTER ONE

INTRODUCTION

Biogas technology is also known as anaerobic digestion (AD) technology is the use of biological processes in the absence of oxygen for the breakdown of organic matter and the stabilization of these material, by conversion to biogas and nearly stable residue (digestate) (Ngumah et al., 2013).  Alessando Volta first discovered biogas in 1776, while Humphrey Davy was the first to pronounce the presence of combustible gas known as methane in the farm yard manure as early as 1800.

The anaerobic fermentation of organic materials has long been used to generate useful resources which have been harnessed for the use of mankind (Uri, 1992; US EPA, 2001). As early as the 18th century, anaerobic process of decomposing organic matter was known, and in the middle of the 19th century, it became clear that anaerobic bacteria are involved in the decomposition process, also called fermentation.

Anaerobic digestion provides some exciting possibilities to global concerns such as alternative energy production, handling human, animal, municipal and industrial wastes safely, controlling environmental pollution, and expanding food supplies (Uri, 1992; Ofoefule and Uzodinma,

2006).

As the demand for energy is increasing, the fossil based fuels become scarce and more expensive, and carbon dioxide emission levels become a greater concern. Biogas is a by- product of anaerobic fermentation and as a renewable energy source it has been recognized globally  as  a  means  of  solving  the  problem  of  rising  energy  prices,  waste  treatment

/management and creating sustainable development (Rao and Seenayya, 1994; Ofoefule and Uzodima, 2006). Biogas is a colorless, flammable gas produced via anaerobic digestion

(fermentation) of animal, plant, human, industrial and municipal waste to produce methane (50- 80%), Carbon dioxide (20-40%) and traces of other gases such as nitrogen, hydrogen, ammonia, hydrogen sulphide, water vapour etc. (Ofoefule and Ibeto, 2010).  However, the composition of these mixture depends on the source of biological waste and management of digestion process (Ofomatah, 2011). The natural generation of biogas is an important  part of biochemical reaction which takes place under anaerobic condition in the presence of highly pH sensitive microbiological catalyst that are mainly bacterial (Uzodinma and Ofoefule, 2009). Biogas production comprises of three biochemical process, which includes; hydrolysis, acidogenesis/acetogenesis, and methanogenesis (Nagamani and Ramasamy, 1999). Complex molecules (carbohydrate, protein, fats) are broken down into a broad spectrum of end products (.i.e. acetic acid, H2/CO2, monocarbon compounds and organic fatty acids larger than acetic) by fermentative bacterial (Uri, 1992; EPA, 2001) Fatty  acids  longer than  acetate are metabolized  to  acetate by  obligate hydrogen- producing acetogenic bacteria (Ntengwe et al, 2010).  Hydrogen and carbon dioxide can be converted to acetate by hydrogen oxidizing acetogen or methane by aceticlastic methanogens (methanogenesis), (Nagamani and Ramasamy, 1999). At pH 6.0-8.0 and ambient temperature between 280C-40oC in a bioreactor or digester under anaerobic condition. (Ntengwe et al, 2010). Ntengwe et al (2010), reported that the production of biogas from biomass is dependent on the amount of acid formed which depends on the types of substrate (feedstock) used. (The biogas production rate was found to be different for different substrate or raw material).

Various efforts to increase the biogas production has been proposed and developed. Some patent provides methods of increasing the production of biogas using bacterial inoculums such as U.S. Patent No. 20080124775, U.S. Patent No. 20070062866, and U.S. Patent No.7560026 (Budiyono and Kusworo, 2012). Furthermore, implementation of biogas technology from cassava starch effluent has been investigated by many researchers (Manilal et al., 1990;

Anunputtikul et al., 2004). However, biogas production rate from cassava starch effluent is still very low unlike biogas production using poultry dung, grasses, swine dung, bambara nut, cow dung, sawdust etc. Research on the production of biogas using cassava waste water has not been thoroughly investigated. The most important consideration in biogas production from cassava effluent are nitrogen source to support the growth of the methane bacteria, pH control during biogas production to keep methane bacteria alive and the management of the digesteion process (Kossmann et al, 2008). Therefore, biogas technology is currently dominated by the efforts to improve concentration and retention time in the bio-digester in order to increase the rate of biogas  production  (Viswanath  et  al.,  1992).  The research  project  is  expected to contribute toward enhancement of knowledge in biogas technology by finding new technology of enhancing biogas production.

1.1. Biogas

Biogas is a biological gas, an alternative and renewable energy source which originates from bacteria in the process of biodegradation (fermentation) of organic material (from plants, animals and sometimes human origins) under anaerobic (oxygen free) conditions (Sárvári et al., 2016).

1.1.2. Composition of Biogas

Biogas is composed of methane (CH4) and carbondioxide (CO2) (Cvetković et al., 2014). Depending on the source of the organic matter and the management of the anaerobic digestion process, such as pH, temperature, ionic strength or salinity, nutrients and inhibitory substrates, small amounts of other gases such as ammonia (NH3), hydrogen sulfide   (H2S) and water vapour (H2O) may be present (Ogejo et al., 2009). In general, biogas consists of 55-80% methane and 20-45% carbon dioxide (CO2), with other gases such as hydrogen sulfide (H2S)

0-3%, 0-1% hydrogen, nitrogen and ammonia (Uzodinma et al., 2011). It is also characterized based on its chemical and physical properties (Uri, 1992).

1.1.3 Physical Properties of Biogas

Depending on the composition, (biogas) a gas considerably lighter than air, is colourless, it produces twice as less energy by combustion with equal volume of natural gas. Biogas burns with an almost odourless blue flame with heat of combustion equivalent of 21.5MJ/M3 (Ossai,

2012).  Relative  density  of  biogas  compared  to  air  of  about  0.8  kg/m3.  Auto-ignition

temperature in the range of 6500oC – 7500oC compared to petrol 5000oC – 6000oC and, 8000oC

– 8500oC. Like any pure gas, the characteristic properties of biogas are dependent on pressure and temperature. They are also affected by the moisture content (Uri, 1992; Ogejo et al., 2009).

1.1.4. Purification of Biogas/ Biogas Scrubbing

Biogas scrubbing involves the separation of unwanted components such as hydrogen sulphide (H2S) and carbon (IV) oxide (CO2) that can combine with water vapour to form acids. These acids can cause corrosion of metal parts. Again, these gases do not support combustion.

Hydrogen sulphide and carbon (IV) oxide can be separated from biogas by passing the biogas through concentrated solution of sodium hydroxide (Dioha et al., 2003). Gas chromatography can also be used as a better method (Dioha et al., 2003) and also the use of calcium hydroxide (quicklime), as shown in the chemical equation below.

By using NaOH

2���� +  ��2   − − − − − − − −→  ��2 ��3  +  �2�

��2��3 + ������ ��2  − − − − − −→  2�����3(�) ���.

By using Ca(OH)2

��(��)2 + ��2   − − − − − −→  ����3 +  �2� ���

Ammonia is also separated or retained by charcoal; while hydrogen sulfide (H2S) can be removed by passing the biogas over iron fillings or iron (III) oxide mixed with wood shavings, as shown in the equation below.

��2�3 +  3�2�  − − − − − −→  ��2�3  +  3�2�

2��2�3  +  3�2   − − − − − −→  2��2�3  + 3�2

These purification gives a pure biogas (biomethane) which burn with a high production of heat. (Young, 1982; Ogejo, et al., 2009).

1.2.0 Methane, a Component of Biogas

Methane is a simple chemical molecule, with the chemical formula CH4. It is the principal component of biogas (natural gas), (Reay et al., 2011). Methane occurs naturally as a component of natural gas, it is odourless, lighter than air and highly flammable. Methane can form mixtures with air that are explosive at concentration 5-15%. Methane is not toxic, but can cause death due to asphyxiation by displacing oxygen in confined environments or spaces. The heating value of pure methane is 1,000 BTU per cubic foot (Ogejo et al., 2009). Additionally, methane is considered a powerful greenhouse gas that can remain in the atmosphere for up to

15 years, with a global warming potential (GWP) of 30. (This means that every kilogramme of methane emitted to the atmosphere has the equivalent forcing effect on the earth’s climate of

30 times that of carbon dioxide over a two-year period) (Ogejo et al., 2009). Other gases, such as CO2, H2S and NH3, in biogas are not useful because CO2 limits the combustion power of methane and lead to bad quality of flames. In addition, H2S and NH3 are toxic, corrosive and have an irritating smell (Uzodinma et al., 2011).

1.2.1 Digesters/ Culture plant for Biogas Production

Biogas plant is also called a bioreactor or a biodigester. It is an air tight container in which organic wastes and waste water are fermented by bacteria in the absence of oxygen to produce methane. It contains a system for gas collection and storage (Ukonu, 2011). Digesters are made of concrete, steel, brick or plastic. They look like silos, troughs, basins or ponds and may be placed under ground or on the surface. Metal digesters are made with iron (steel) to avoid poisoning of the bacteria during the digestion.

The modes of operation of the digestion include batch, semi-continuous and continuous operation.  In batch culture operation system, the biodigestor is loaded with the substrate or organic material and allowing it to digest. Once the digestion is complete, the effluent is removed and the process repeated. One technical shortcoming of batch system is the risk of blockage of the leaching process caused by clogging of the perforated floor. This problem is alleviated by mixing the feedstock with bulking material (e.g. wood chips) and by limiting the thickness of the fermenting wastes in order to limit compaction (Vandevivere et al., 2003). Although batch systems have not succeeded in taking a substantial market share, especially in more developed countries, the system is attractive to developing countries. The reason is that the process offers several advantages as it does not require fine shredding of waste, sophisticated mixing or agitation equipments, or expensive, high-pressure vessels, which consequently lower the investment costs (Vandevivere    et al., 2003; Koppar and Pullammanappallil, 2008).

For semi-continuous operation, the digester is fed on a more regular basis usually once or twice daily. The digested organic matter is also removed at the same interval.

In continuous operation, the organic material is fed constantly into the digester. The material moves mechanically or by the force of the new feed pushing out digested material. This kind

of operation is most suitable for large scale operations. There is steady availability of usable biogas (Ofomatah, 2011).

1.2.2. Types of Bio Digester

Digesters are made of concrete, steel, brick or plastic. Several different types of anaerobic bioreactors are used worldwide for municipal, industrial-food, and agricultural waste treatment (Ogejo et al., 2009).

The two commonly used biodigesters are: floating drum digester and fixed dome digester

Other types of bio digester include: Bag/balloon plant, Plug flow plants, Ferro cement plants and prototype bioreactor plant.

1.2.3. Floating Drum Digester

The floating drum digester consist of a cylindrical or dome shaped digester and a moving, floating gas holder or drum. The gas holder floats either in the fermenting slurry or in a separate water jacket. This type of digester is popularly known as the gobar gas plant. The gas is collected in the gas drum, which rises or moves down, according to the amount of gas stored. The gas drum is prevented from tilting by a guiding frame. The advantages are the simple and easily understood operation (Ukonu, 2011). The volume of stored gas is directly visible. The gas pressure is constant, determined by the weight of the gas holder. The construction is relatively easy, construction mistakes do not lead to major problems in operation and gas yield. The disadvantages are high material costs of the steel drum, the susceptibility of steel parts to corrosion. Because of this, floating drum plants have a shorter life span than fixed dome plant (Uri, 1992). The floating drum plants were mainly built in India

1.2.3 Fixed Dome Digester

This consists of an underground airtight pit constructed with bricks, stone or concrete with a dome shape cover. The fixed dome digester is a popular digester used in most places such as Nepal, India and China. In this type, the fermentation chamber and gas holder are combined as one unit (Iloeje, 1998). Fixed-dome plant is relatively low in its cost of construction because of the absence of moving parts and rusting steel parts (Ukonu, 2011). If well-constructed, fixed- dome plant have a long life span. The underground construction saves space and protects the reactor from temperature changes. The construction also provides opportunities for skilled local employment. The disadvantages of fixed-dome plant are mainly the frequent problems with gas-tightness of the brickwork gas holder (a small crack in the upper brickwork can cause heavy  losses  of  biogas);  fixed-dome  plant  are,  therefore,  recommended  only  where

construction can be supervised by experienced biogas technicians. The gas pressure fluctuates substantially depending on the volume of the stored gas (Omer and Fadalla, 2003).

1.3.0. Feed Stock Substrate (Cassava waste water)

Cassava (Manihot esculenta) also known as manioc is a woody, perennial shrub of the Euphorbiaceae (spurge family) which grows from 1-5 m (9ft) in height, has large tuberous roots with leaves deeply divided into 3–7 lobes. The shrub is often grown as an annual, and propagated from stem cuttings after tubers have been harvested. It is one of agriculture’s oldest crops originating from South America but today spread all over the worlds tropical and subtropical regions (FAO, 2013). Because of its tolerance against drought and for marginal soils it is commonly grown by poor farmers in developing countries, and today millions of small-scale farmers in more than 100 countries grow cassava (FAO, 2013) and it is the third most important source of calories in the tropics after rice and maize (FAO, 2008). Cassava is a truly versatile crop and in fact the whole plant can be used. The roots, which are the main product, is somewhat dark brown in colour and grows up to 2 feet long, can be processed in to a variety of food products and animal fodder, but they can also be used for industrial purposes. Such as for making noodles and cakes, for frying meat and fish, in textiles, pharmaceuticals, cardboard, monosodium glutamate (MSG), glucose, maltose and plywood; these are all just a few examples of how the roots can be used. The leaves, which contain up to 25 % protein, can be used as animal fodder and the stem as firewood (FAO, 2013). However, since the whole plant contains high levels of cyanide compounds including linamarin (cyanogenic glucoside) and hydrocyanic acid it is always processed in some way such as roasting, soaking, or fermentation before consumption to avoid intoxication (FAO, 2013).

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