BIOTRANSFORMATION OF FRESH CASSAVA WASTEWATER TO BIO-FERTILIZER IN A LIQUID PHASE ANAEROBIC BIOREACTOR

ABSTRACT

The aim of this study was to convert fresh cassava wastewater into bio-fertilizer in a liquid phase anaerobic bio-reactor. Fresh cassava wastewater was collected from a garri mill company at Ogurugu road, in Nsukka Local Government Area of Enugu State, Nigeria. The fresh cassava wastewater was sieved using a mesh (0.154 mm aperture) to remove large solid particles and debris and the filtrate stored at 4◦C. Filtered fresh cassava wastewater (1 liter) was charged into a continuous stirred bio-digester tank. It underwent bio-digestion in an anaerobic condition using the indigenous facultative anaerobes at ambient temperature (25ºC – 27ºC) for 28 days. The physicochemical properties as well as microbial analysis were carried out for both the fresh cassava wastewater and that of the sludge (obtained after 28 days of bio-digestion). The result showed 0.57% increase in ash content (0.31% – 0.88%), 2.97% increase in crude protein (0.96% – 3.93%) and 0.12 % increase in phosphorous content (0.12% – 0.24%). Whereas, other properties decreased as follow: moisture content, 0.8% (84.40% – 83.60%), crude fat, 0.15% (0.55% – 0.40%), crude fiber, 0.50% (0.80% – 0.30%), carbohydrate, 2.09% (7.98% – 5.89), carbon content, 1.09% (4.31% – 3.22%), potassium, 00033% (0.00065% – 0.00032%), Hydrogen cyanide, 2.16% (2.70% – 0.54%), calcium 0.00060% (0.00053% – 0.00047%), magnesium 0.00050% (0.00080% – 0.00075%), chemical oxygen demand 64% (154.4% – 90.4%) and biochemical oxygen demand: 19.2% (46.4% – 27.7%). Microbial colony in the cassava wastewater was 11, with microbial bio-load 5.5 × 108 cfu/ml for 10-6 serial dilution per ml. The gram staining revealed that the digestate contains five gram-negative microorganisms, with two gram- positive microorganism. From the result of the biochemical identification, the presence of the following microorganisms were speculated: Proteus mirabelis, Vibro species, Bacillus subtillis, Staphylococcus species, Escherichia coli, Salmonella species, and Bacillus licheniformis. The result for GC-MS analysis of the fresh cassava wastewater revealed the presence of the following persistent organic compounds (POC); 2.58% of (Hexamethylenediacrylate cyclic sulphite, Cis- 1,2-cyclohexadiol and 2-cyclopentene-1-undecanoic acid), 6.20%  of (1,1,4,14-tetredecanediol, 1,2- dimethylcyclohexene and 1,6-dimethylcyclohexene), 2.59% of ( 1,2:4,5:9,10-tetraepoxydecane, 8- methoxy-1,6-octadiene and 5-methylenecyclopropyl-1-pentanol), 2.77% of (10-undecyn-1-ol, 1-decyne and 1,2:4,5:9,10-tetraepoxydecane) and 85.86% of (9,12-octadecadienoic acid, (z,z)-2,3- dihydroxypropylester, 2-butyl-5-hexyloctahydro-1H-indene and E,2,3,12-nanodecatriene). However, the POCs observed in the cassava wastewater after 28 days of bio-digestion were 32.28% of (2 molecules of propanenitrile and 1 molecule of 2-propyn-1-amine). The observed increase in nitrogen and phosphorus is evidence that fresh cassava wastewater could be transformed into bio-fertilizer. The resultant agro- product is good and efficient for soil fertilization and plant growth. The bio-fertilizer, however, haD less or no risk of environmental pollution.

CHAPTER 1

INTRODUCTION

The agricultural revolution and diversification initiative by the federal government of Nigeria are welcomed and acceptable development. The production and generation of solid and liquid waste due to massive cassava processing are issues for research and discussion. There is a perceived risk of environmental pollution, ranging from stinking smell, to its detrimental effects on soil microbes. However, the stinking odour at the garri mill factories are eloquent testimonies that the effluents are being acted upon by the inherent microorganisms such as bacteria and fungi. These indigenous microorganisms feed on the cassava wastewater; and in turn generate a high concentration of two basic mineral elements, such as; nitrogen (N) and phosphorous (P) which are essential in soil fertility for improved agricultural yield. Compared with other organic agricultural wastes, cassava wastewater has higher contents of the main essential elements (N and P) required by plants (Magalhães et al., 2016). This characteristic allows the biotransformation and utilization of this waste as  organic fertilizer considering the soil  chemical composition  (Duarte  et al.,  2012). Channeling all the wastewater from garri mill industry into an anaerobic bio-digester enables the facultative anaerobes inherent in the effluent to digest the substrate and fortuitously release bio- methane, carbon (iv) oxide and water. On completion of digestion, the resultant sludge serves as liquid bio-fertilizer, rich in P, N and high microbial biomas which play an indispensable role in soil fertility. Apart from production of liquid bio-fertilizer, bio-methane and sequestration of carbon (iv) oxide, this approach will reduce land, air and water pollution thereby enhancing sustainable environment.

This process is plausible with high performance, economically affordable in terms of capital expenditure and operating expenditure as we adopt Rapid Transformation of Organic Residues in

the cassava waste water (Onifade et al., 2015). Cassava-processing wastewater (effluent) is a feedstock that is abundant, readily available and cheap with a high concentration of starch and biodegradables (Guo et al., 2010). It is a potential substrate for the production of bio-fertilizer, bio-methane and bio-carbonic acid. This application has advantage of adding value to a clean energy source of a highly polluted wastewater (Cappelletti et al., 2011). The wastewater of starch production contains nitrogen and phosphorus, among other nutrients (Ribas et al., 2010). The composition of the residue is a key issue in the choice of substrate used in the production of bio- fertilizer as the costs of sources of carbon are high in implementing this technology for an industrial plant, representing around 40% of the total cost of bio-fertilizer production (Wang et al., 2008). Therefore, this study sought to produce bio-fertilizer from a fresh cassava wastewater in a continuous anaerobic fixed bio-digester, in which analysis were carried out on the fresh substrates and on the resultant sludge and the two results compared in terms of their total protein, total carbon, phosphorous, persistent organic compounds, microbial analysis, potassium (K), Calcium (Ca) and Magnesium (Mg), respectively.

1.01 Facultative anaerobic organism

Figure 1: Test tubes of thioglycollate broth; for identification of Aerobic and anaerobic bacteria. Source: https://upload.wikimedia.org/wikipedia/commons/thumb/9/90/Anaerobic.png/800px- Anaerobic.png, Retrieved, 28/07/2018.

1: Obligate aerobes need oxygen because they cannot ferment or respire anaerobically. They gather at the top of the tube where the oxygen concentration is highest.

2: Obligate anaerobes are poisoned by oxygen, so they gather at the bottom of the tube where the oxygen concentration is lowest.

3: Facultative anaerobes can grow with or without oxygen because they can metabolise energy aerobically or anaerobically. They gather mostly at the top because aerobic respiration generates more ATP than either fermentation or anaerobic respiration.

4: Microaerophiles need oxygen because they cannot ferment or respire anaerobically. However, they are poisoned by high concentrations of oxygen. They gather in the upper part of the test tube but not the very top.

5: Aerotolerant organisms do not require oxygen as they metabolise energy anaerobically. Unlike obligate anaerobes, they are not poisoned by oxygen. They can be found evenly spread throughout the test tube, see figure 1 for illustration.

A facultative anaerobe is an organism that generates energy in the form of ATP by aerobic respiration in the absence of oxygen, in which case, is capable of switching to fermentation or anaerobic respiration (Hogg, 2005).  Some examples of facultative anaerobic bacteria are Staphylococcus species, Streptococcus species. (Prescott, et al., 1996) Escherichia coli, Salmonella species, Listeria species. (Ryan and Ray, 2004) and Shewanella oneidensis (Ryan and Ray, 2004). Certain eukaryotes are also facultative anaerobes, including fungi such as Saccharomyces cerevisiae, (Singleton, 1999) and many aquatic invertebrates such as Nereid (worm) polychaetes (Carlile, et al., 2001).

1.02 Overview of Anaerobic Bio-digestion

Anaerobic bio-digestion  is  a bio  transformational  process  that  deals  with  the breakdown of biodegradable material  in  the  absence  of oxygen  (NNFCC,  2011).  The  process  is  used  for industrial or domestic purposes to manage waste or to produce fuels (Tabatabaei, 2010). Much of the fermentation used  industrially  to  produce  food  and  drink  products,  as  well  as  home fermentation, uses anaerobic digestion (Ugwu and Aoyagi, 2011).

Anaerobic digestion occurs naturally in some soils, in lakes and ocean basin sediments, where it is usually referred to as “anaerobic activity” (Tabatabaei, 2010). The digestion process begins with bacterial hydrolysis of     the     input     materials.     Insoluble organic     polymers,     such as carbohydrates,  are  broken  down  to  soluble  derivatives  that  become  available  for  other bacteria. Acidogenic       bacteria       convert       the sugars and amino       acids into       carbon dioxide, hydrogen, ammonia, and organic acids. These bacteria convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide, (Tabatabaei,

2010). Methanogens convert these products to methane and carbon dioxide (Tabatabaei, 2010). Archaea populations play an indispensable role in anaerobic wastewater treatments.  Anaerobic

digestion is used as part of the process to treat biodegradable waste and sewage sludge. As part of an integrated waste management system, anaerobic digestion reduces the emission of landfill gas into the atmosphere. Anaerobic bio-digestion process is widely used as a source of renewable energy  (Tabatabaei,  2010).  The  process  produces  a biogas,  consisting  of methane, carbon dioxide and traces of other ‘contaminant’ gases. (NNFCC, 2011). The nutrient-rich digestate also produced can be used as liquid bio-fertilizer.

With the re-use of waste as a resource and new technological approaches that have lowered capital costs, anaerobic digestion has in recent years received increased attention among governments in a number of countries, among these the United Kingdom, Germany and Denmark (Osakwe, 2012).

1.02.1 Anaerobic Digestion

Anaerobic (no oxygen) digestion is the process whereby naturally occurring bacteria, which can only live in places where there is no air, break down organic, biodegradable material over time and converts it to biogas and organic fertilizer. One way to create supplemental heat is to use a bio-digester. Bio-digesters use bacterial digestion, similar to the process of a human stomach, to digest organic material and produce methane rich biogas that can potentially be used as heating fuel (Hogg, 2005). In this case, we shall connect an electrical lamp of about 60 watts to alter the temperature of the bio-digester, a form of rapid biotransformation.

1.02.2 Process of bio-digestion

Many       microorganisms       affect       anaerobic       digestion,       including       acetic       acid forming bacteria (acetogens)  and  methane-forming archaea (methanogens).  These  organisms promote a number of chemical processes during the convertion of the biomas. Gaseous oxygen is excluded from the reactions by physical containment. Anaerobes utilize electron acceptors from sources other than oxygen gas (Hogg, 2005). These acceptors can be the organic material itself or

may be supplied by inorganic oxides from within the input material. When the oxygen source in an anaerobic system is derived from the organic material itself, the ‘intermediate’ end products are primarily alcohols, aldehydes, and organic acids, plus carbon dioxide. Populations of anaerobic microorganisms typically take a significant period of time to establish themselves to be fully effective (Eze and Onyilide, 2015). Therefore, common practice is to introduce anaerobic microorganisms from materials with existing populations, a process known as “seeding” the digesters, typically accomplished with the addition of sewage sludge or cattle slurry (Hogg, 2005).

1.02.3 Process stages

The four key stages of anaerobic digestion are;

Hydrolysis, Acidogenesis, Acetogenesis and Methanogenesis.   The   overall   process   can   be described by the chemical reaction, where organic material such as glucose is biochemically digested into carbon dioxide (CO2) and methane (CH4) by the anaerobic microorganisms, see equation 1.

C6H12O6 → 3CO2 + 3CH4 —————————————————————————Equation 1

Glucose is hydrolysed to CO2 and CH4.

1. Hydrolysis

Usually, biomass is made up of large organic polymers. For the bacteria in anaerobic digesters to access the energy potential of the material, these chains must first be broken down into their smaller constituent parts. These constituent parts, or monomers, such as sugars, are readily available to other bacteria. The process of breaking these chains and dissolving the smaller molecules into solution is called hydrolysis. Therefore, hydrolysis of these high-molecular-weight

polymeric components is the necessary first step in anaerobic digestion (Sleat and Mah, 2006). Through hydrolysis the complex organic molecules are broken down into simple sugars, amino acids, and fatty acids (Sleat and Mah, 2006).

Acetate and hydrogen produced in the first stages can be used directly by methanogens. Other molecules such as volatile fatty acids (VFAs) with a chain length greater than that of acetate must first be catabolized into compounds that can be directly used by methanogens (Boone and Mah,

2006).

2. Acidogenesis

The biological process of acidogenesis results in further breakdown of the remaining components by acidogenic (fermentative) bacteria. Here, VFAs are created, along with ammonia, carbon

dioxide, and hydrogen sulfide, as well as other by-products (Sleat and Mah, 2006). The process of acidogenesis is similar to the way milk sours.

3. Acetogenesis

The third stage of anaerobic digestion is acetogenesis. Here, the resultant simple molecules of the acidogenesis phase are further digested by acetogens to produce largely acetic acid, as well as carbon dioxide and hydrogen (Boone and Mah, 2006).

4. Methanogenesis

The terminal stage of anaerobic digestion is the biological process of methanogenesis. Here, methanogens use the intermediate products of the preceding stages and convert them into methane, carbon dioxide, and water. These components make up the majority of the biogas emitted from the system (Lee et al., 2003). The remaining, indigestible materials that the microbes cannot use and any dead bacterial remains constitute the digestate which serves as liquid bio-fertilizer (Nova Gas). (figure 2).

Figure 2: Processes of biotransformation in an anaerobic bio-digester

Source: (Peixoto et al., 2011).

1.02.4 Anaerobic Digester

An anaerobic digester is a large, air-tight container or tank that contains no oxygen. The tank is filled with organic material and maintained at an optimum temperature for anaerobic bacteria to digest the material. Depending on what is put into it, the contents can be wet or dry, see Fig. 2.

A Digester is an autonomous bacterial system that takes the influent feed and produce methane rich biogas and nitrogen/phosphorous rich fertilizer as effluent (PEIXOTO et al., 2011).

1.02.5 List of Materials for the design of the Bio digester

Figure 3: Diagrammatical design of a Bio-digester. Source: (Peixoto, et al., 2011).

a.   200 gallon barrel

b.   PVC tubes

c.   Rim tube 1 Jessica plastic rag (preferable transparent)

d.   Funnel pipe

S.   Accessory rim valve adapter

r.  Manometer or pressure gauge

O.  PVC c/thread male Adapters

J. PVC elbows

P. PVC Tee

m. Female plug c/thread.

V. Wrench of pasco metal

1.02.6 Organic materials

Organic materials are from that living organism. Wasted or spoiled food, plant clippings, cassava waste water, animal manure, meat trimmings, and sewage are common types of organic material used in anaerobic digestion. In contrast, inorganic material includes things like rocks, dirt,

plastic, metal and glass (Kunzeler et al., 2013).

1.02.7 Configuration

Anaerobic digesters can be designed and engineered to operate using a number of different configurations and can be categorized into;

1. Batch versus continuous process mode,

2. Mesophilic versus thermophilic temperature conditions,

3. High versus low portion of solids, and

4. Single stage versus multistage processes.

1. Batch or continuous

Anaerobic digestion can be performed as a batch process or a continuous process. In a batch system, biomass is added to the reactor at the start of the process. The reactor is then sealed for the duration of the process. In its simplest form batch processing needs inoculation with already processed material to start the anaerobic digestion. In a typical scenario, biogas production will be formed with a normal distribution pattern over time (Kim et al., 2012). Operators can use this fact to determine when they believe the process of digestion of the organic matter is completed (Aikantechnology.com, 2012). There can be severe odor issues if a batch reactor is opened and emptied before the process is well completed. A more advanced type of batch approach has limited the odor issues by integrating anaerobic digestion with in-vessel composting. In this approach inoculation takes place through the use of recirculated degasified percolate. After anaerobic

digestion is completed, the biomass is kept in the reactor and used for in-vessel composting before it is opened (Aikantechnology.com, 2012).  As the batch digestion is simple and requires less equipment and lower levels of design work, it is typically a cheaper form of digestion (Aikantechnology.com, 2012). Using more than one batch reactor at a plant can ensure constant production of biogas.

In continuous digestion processes, organic matter is constantly added (continuous complete mixed) or added in stages to the reactor (continuous plug flow; first in – first out). Here, the end products are constantly or periodically removed. A single or multiple digesters in sequence may be used. Examples of this form of anaerobic digestion include continuous stirred-tank reactors, up flow anaerobic  sludge  blankets, expanded  granular  sludge  beds and internal  circulation  reactors (Isabirye et al., 2007).

2. Temperature

The two conventional operational temperature levels for anaerobic digesters determine the species of methanogens in the digesters (Song, et al., 2004).

Mesophilic digestion takes  place  optimally  around  30  to  38 °C,  or  at  ambient  temperatures between 20 and 45 °C, where mesophiles are the primary microorganisms present.

Thermophilic digestion takes place optimally around 49 to 57 °C, or at elevated temperatures up to 70 °C, where thermophiles are the primary microorganisms present. A limit case has been reached in Bolivia, with anaerobic digestion in temperature working conditions of less than 10 °C. The anaerobic process is very slow, taking more than three times the normal mesophilic time process (Aikantechnology.com, 2012). Mesophilic species outnumber thermophiles, and they are also more tolerant to changes in environmental conditions than thermophiles. Mesophilic systems are, therefore, considered to be more stable than thermophilic digestion systems. In contrast, while

thermophilic digestion systems are considered less stable, their energy input is higher. The increased temperatures facilitate faster reaction rates. Operation at higher temperatures facilitates greater pathogen reduction of the digestate.

3. Solids content

In a typical scenario, three different operational parameters are associated with the solids content of the feedstock in the digesters:

High solids (dry-stackable substrate) High solids (wet-pumpable substrate) Low solids (wet-pumpable substrate)

High solids (dry) digesters are designed to process materials with a solids content between 25 and

40%. Unlike wet digesters that process pumpable slurries, high solids (dry-stackable substrate) digesters are designed to process solid substrates without the addition of water (Auer et al., 2017). The primary styles of dry digesters are continuous vertical plug flow and batch tunnel horizontal digesters. Wet digesters can be designed to operate in either a high-solids content, with a total suspended solids (TSS) concentration greater than ~20%, or a low-solids concentration less than

~15% (Auer et al., 2017).

High solids (wet) digesters process a thick slurry that requires more energy input to move and process the feedstock. The thickness of the material may also lead to associated problems with abrasion. High solids digesters will typically have a lower land requirement due to the lower volumes associated with the moisture. High solids digesters also require correction of conventional performance calculations (e.g. gas production, retention time, kinetics, etc.) originally based on very dilute sewage digestion concepts, since larger fractions of the feedstock mass are potentially convertible to biogas (Richards et al., 1991).

Low solids (wet) digesters can transport material through the system using standard pumps that require significantly lower energy input. Low solids digesters require a larger amount of land than high solids due to the increased volumes associated with the increased liquid-to-feedstock ratio of the digesters (Arotupin, 2007). There are benefits associated with operation in a liquid environment, as it enables more thorough circulation of materials and contact between the bacteria and their food. This enables the bacteria to more readily access the substances on which they are feeding, and increases the microbial biomas (Auer et al., 2017).

4. Residence time

The residence time in a digester varies with the amount and type of feed material, and with the configuration of the digestion system. In a typical two-stage mesophilic digestion, residence time varies between 15 and 40 days, while for a single-stage thermophilic digestion, residence times is normally faster and takes around 14 days (Auer et al., 2017). The plug-flow nature of some of these systems will mean the full degradation of the material may not have been realised in this time scale. In this event, digestate exiting the system will be darker in colour and will typically have more odour.

In the case of an up flow anaerobic sludge blanket (UASB) digestion, hydraulic residence times can be as short as 1 hour to 1 day, and solid retention times can be up to 90 days. In this manner, a UASB system is able to separate solids and hydraulic retention times with the use of a sludge blanket (Finstein, 2006). Continuous digesters have mechanical or hydraulic devices, depending on the level of solids in the material, to mix the contents, enabling the bacteria and the food to be in contact. They also allow excess material to be continuously extracted to maintain a reasonably constant volume within the digestion tanks.

1.03 Inhibition

The anaerobic digestion process can be inhibited by several compounds, affecting one or more of the bacterial groups responsible for the different organic matter degradation steps. The degree of the inhibition depends, among other factors, on the concentration of the inhibitor in the digester (Goddard, 2017). Potential inhibitors are ammonia, sulfide, light metal ions (Na, K, Mg, Ca, Al), heavy metals, some organics (chlorophenols, halogenated aliphatics, N-substituted aromatics, long chain fatty acids), etc (Chen, et al., 2008).

1.04 Feedstock

The most important idea when considering the application of anaerobic digestion systems is the feedstock to the process. Almost any organic material can be processed with anaerobic digestion (Chen, et al., 2008).  Feedstocks can include biodegradable waste materials, such as waste paper, grass clippings, leftover food, sewage, and animal waste.   Woody wastes are the exception, because they are largely unaffected by digestion, as most anaerobes are unable to degrade lignin (Chen, et al., 2008). Xylophalgeous anaerobes (lignin consumers) or using high temperature pretreatment, such as pyrolysis, can be used to break down the lignin. Anaerobic digesters can also be fed with specially grown energy crops, such as silage, for dedicated biogas production. In Germany and continental Europe, these facilities are referred to as “biogas” plants. A codigestion or cofermentation plant is typically an agricultural anaerobic digester that accepts two or more input materials for simultaneous digestion (Lemmer and Oeschsner, 2007).

The length of time required for anaerobic digestion depends on the chemical complexity of the material. Material rich in easily digestible sugars breaks down quickly whereas intact lignocellulosic material rich in cellulose and hemicellulose polymers can take much longer time

to  break  down. Anaerobic  microorganisms  are  generally  unable  to  break  down  lignin,  the recalcitrant aromatic component of biomass (Marcio et al., 2017).

Anaerobic digesters were originally designed for operation using sewage sludge and manures. Sewage and manure are not, however, the material with the most potential for anaerobic digestion, as the biodegradable material has already had much of the energy content taken out by the animals that produced it (Lemmer and Oeschsner, 2007). Therefore, many digesters operate with codigestion of two or more types of feedstock. For example, in a farm-based digester that uses dairy manure as the primary feedstock, the gas production may be significantly increased by adding a second feedstock, e.g., grass and corn (typical on-farm feedstock), or various organic byproducts, such as slaughterhouse waste, fats, oils and grease from restaurants, organic household waste, etc. (typical off-site feedstock) (Al-Turki, 2010).

Slurry-only systems generate far less energy than those using crops, such as maize and grass silage. Using a modest amount of crop material (30%), an anaerobic digestion plant can increase energy output tenfold for only three times the capital cost, relative to a slurry-only system (Lemmer and Oeschsner, 2007).

1.05 Moisture content

A second consideration related to the feedstock is moisture content. Drier, stackable substrates, such as food and yard waste, are suitable for digestion in tunnel-like chambers. Tunnel-style systems typically have near-zero wastewater discharge, and so has advantages where the discharge of digester liquids are a liability. The wetter the material, the more suitable it will be to hand with standard pumps instead of energy-intensive concrete pumps and physical means of movement. Also, the wetter the material, the more volume and area it takes up relative to the levels of gas produced. The moisture content of the target feedstock will also affect what type of system is

applied to its treatment (Lemmer and Oeschsner, 2007). To use a high-solids anaerobic digester for dilute feedstock, bulking agents, such as compost, should be applied to increase the solids content of the input material (Lemmer and Oeschsner, 2007). Another key consideration is the carbon:nitrogen ratio of the input material. This ratio is the balance of food a microbe requires to grow; the optimal C:N ratio is 20–30:1.  Excess N can lead to ammonia inhibition of digestion (Lemmer and Oeschsner, 2007).

1.06 Contamination

The level of contamination of the feedstock material is a key consideration. If the feedstock to the digesters has significant levels of physical contaminants, such as plastic, glass, or metals, then processing to remove the contaminants will be required for the material to be used. If it is not removed, then the digesters can be blocked and will not function efficiently. It is with this understanding that mechanical biological treatment plants are designed. The higher the level of pretreatment a feedstock requires, the more processing machinery will be required, and, hence, the project will have higher capital costs (NNFCC, 2010)

After sorting or screening to remove any physical contaminants from the feedstock, the material is often shredded, minced, and mechanically or hydraulically pumped to increase the surface area available to microbes in the digesters and, hence, increase the speed of digestion. The maceration of solids can be achieved by using a chopper pump to transfer the feedstock material into the airtight digester, where anaerobic treatment takes place.

1.07 Substrate composition

Substrate composition is a major factor in determining the methane yield and methane production rates from the digestion of biomass (Mara et al., 2017). Techniques to determine the compositional characteristics of the feedstock are available, while parameters such as solids, elemental, and

organic analyses are important for digester design and operation (Jerger and Tsao, 2006). Methane yield can be estimated from the elemental composition of substrate along with an estimate of its degradability (the fraction of the substrate that is converted to biogas in a reactor) (Rittmann,

2001). In order to predict biogas composition (the relative fractions of methane and carbon dioxide) it is necessary to estimate carbon dioxide partitioning between the aqueous and gas phases,   which   requires   additional   information   (reactor   temperature, pH,   and   substrate composition) and a chemical speciation model (Hill and Barth, 1977).

1.08 Applications

Schematic of an anaerobic digester as part of a sanitation system. It produces a digested slurry

(digestate) that can be used as a fertilizer, and biogas that can be used for energy (Tilley et al., 2014)

1.09 Waste and wastewater treatment

1.09.1 Wastewater

The final output from anaerobic digestion systems is water, which originates both from the moisture content of the original waste that was treated and water produced during the microbial reactions in the digestion systems. This water may be released from the dewatering of the digestate or may be implicitly separated from the digestate. The wastewater exiting the anaerobic digestion facility will have elevated levels of biochemical oxygen demand (BOD) and chemical oxygen demand (COD). These measures the reactivity of the effluent and indicate the ability to pollute the ecosystem. Some of these materials are termed ‘hard COD’, meaning they cannot be accessed by the anaerobic bacteria for conversion into biogas. If this effluent were put directly into watercourses,  it  would  negatively  affect  them  by  causing eutrophication.  As  such,  further treatment of the wastewater is often required. This treatment will typically be an oxidation stage

wherein air is passed through the water in a sequencing batch reactors or reverse osmosis (Adelaide

et al., 2017).

1.09.2 Cassava Wastewater.

Cassava is a very important food in most tropical developing countries, particularly in Nigeria (FAO, 2005). However, methods for processing cassava and degrading cassava wastewater are still very poor resulting in accumulation of high levels of organic materials and some toxic compounds (Bradbury, 2004; Enyenihi, et al., 2009). Some studies have shown that cassava processing generates solid and liquid residues that are hazardous in the environment (Jyothi, et al.,

2005; Cumbana, et al., 2007). However, the amount of cassava wastewater produced would depend on the processing methods and the scale of production. For instance, when cassava is used in homes for culinary purposes, the quantity of wastewater generated is very little and thus, will not cause any significant environmental hazards. However, when cassava is used industrially, including small flour factories, the so called “Casa de Farinha” and garri meal industries, generate a considerable amount of cassava wastes water since they are traditionally concentrated in a certain place. Cassava wastewater when discharged on the soil results in environmental pollution. Also, cassava contains cyanogenic glucosides (toxic substances), mainly linamarin (92-98%), which releases hydrogen cyanide after hydrolysis by an endogenous linamarase (Okafor and Ejiofor

1986; Nok and Ikediobi, 1990). Furthermore, the odour from this discharged wastewater is dangerous to human health (Ayenor 1985; Akinrele 1986; Ezeronye 2003). The cassava wastewater also inhibits the growth of vegetations and thus, reduces the entire fertility of the Soil. Anaerobic digestion is particularly suited organic material, and is commonly used for industrial effluent, wastewater and sewage sludge treatment. Anaerobic digestion, a simple process, can

greatly reduce the amount of organic matter which might otherwise be destined to be dumped at sea, dumped in landfills, or burnt in incinerators (Juniper, 2005).

Pressure  from  environmentally  related legislation on  liquid  and  solid waste disposal  methods in developed countries has increased the application of anaerobic digestion as a process for reducing waste volumes and generating useful by-products. It may either be used to process the source-separated fraction of municipal waste or alternatively combined with mechanical sorting systems, to process residual mixed municipal waste. These facilities are called mechanical biological treatment plants (Svoboda, 2003). If the putrescible waste processed in anaerobic digesters were disposed of in a landfill, it would break down naturally and often anaerobically. In this case, the gas will eventually escape into the atmosphere. As methane is about 20 times more potent as a greenhouse gas than carbon dioxide, this has significant negative environmental effects. In countries that collect household waste, the use of local anaerobic digestion facilities can help to reduce the amount of waste that requires transportation to centralized landfill sites or incineration facilities. This reduces the burden on transportation and carbon emissions from the collection vehicles. If localized anaerobic digestion facilities are embedded within an electrical distribution network, they can help reduce the electrical losses associated with transporting electricity over a national grid (Tilley et al., 2014).

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