We look at the Anaerobic Digester Design in South Africa. South Africa’s electricity is produced mainly from coal because it is the most abundant source of energy. It is the most widely used primary source of fuel and contributes to about 77% of the country’s primary energy needs. Coal contributes to greenhouse gas emissions to the atmosphere that leads to global warming.
Fossil fuels contribute to the increase in the concentration of carbon dioxide in the atmosphere, hence alternative energy sources (renewable energy) must be used in the place of fossil fuels. The commercial production of biogas and another alternative renewable energy source such as solar energy, wind energy, hydropower, geothermal will definitely give a drive for the development of the economy.
Energy derived from biogas is used in the form of fuel, heat, and electricity. Biogas is a renewable source of energy derived from biodegradable substrates such as agricultural wastes, animal wastes, domestic wastes, crops and industrial waste. It is produced by anaerobic digestion, which is a biochemical process in the absence of oxygen. The main product of biogas is methane and carbon dioxide.
Table of Contents
1 The biochemical process of anaerobic digestion
Anaerobic digestion is often considered to be a complex process, the digestion itself is based on a reduction process consisting of a number of biochemical reactions taking place under anoxic conditions. Methane formation in anaerobic digestion involves four different steps: hydrolysis, acidogenesis, acetogenesis, and methanogenesis.
Generally in an anaerobic digestion process, the rate-limiting step can be defined as the step that causes process failure under imposed kinetic stress. In other words, in a context of continuous culture, kinetic stress is defined as the imposition of a constantly reducing value of the solids retention time until it is lower than the limiting value; hence it will result in a washout of the microorganism.
Most researchers report that the rate-limiting step for the complex organic substrate is the hydrolysis step due to the formation of toxic byproducts (complex heterocyclic compounds) or non-desirable volatile fatty acids (VFA) formed during hydrolysis: whereas methanogenesis is the rate-limiting step for easy biodegradable substrates.
The anaerobic digestion process can be divided into two phases as illustrated in Figure 1. The microorganism carrying out the degradation reactions in each of these phases differ widely regarding physiology, nutritional needs, growth kinetics, and sensitivity to the environment. Very often, it is difficult to keep a delicate balance between these two groups: the acid-forming and the methane forming microorganisms, which lead to reactor instability and consequently low methane yield. The two main groups of microorganisms could be physically separated with the intention of making use of the difference in their growth kinetics. In order to accomplish phase separation, several techniques have been employed such as membrane separation, kinetic control, and pH control.
Figure 1: Phase separation of the anaerobic digestion system
1.1 Anaerobic Digester Design: Hydrolysis
This is the first step in the anaerobic digestion process, it involves the enzyme-mediated transformation of insoluble organic materials and higher molecular mass compounds such as lipids, polysaccharides, proteins, fats, nucleic acid etc. into soluble organic materials i.e. to compounds suitable for the use as the source of energy and cell carbon such as monosaccharides, amino acids and other simple organic compounds. This step is carried out by strict anaerobes such as bactericides, clostridia and facultative bacteria such as streptococci etc. This first stage is very important because large organic molecules are simply too large to be directly absorbed and used by microorganisms as a substrate/food source. To accomplish biodegradation, certain microorganisms secrete different types of enzymes, called extracellular enzymes, which “cut” the larger molecules up into smaller pieces that the microorganism can then take into the cell and use as a source of energy and nutrition. Some microorganisms secrete several different enzymes, which allow them to break down different types of organic materials. Other microorganisms are specialised. For example, they secrete enzymes that break down either sugar or protein. Microorganisms that break down different sugars are called saccharolytic, while those that break down proteins are called proteolytic. There are different enzymes for sugars, proteins, fats etc. The rate of decomposition during the hydrolysis stage depends greatly on the nature of the substrate. The transformation of cellulose and hemicellulose generally takes place more slowly than the decomposition of proteins.
The monomers produced in the hydrolytic phase are taken up by different facultative and obligatory anaerobic bacteria and are degraded further into short-chain organic acids such as butyric acids, propanoic acids, acetic acids, alcohols, hydrogen and carbon dioxide. The concentration of hydrogen formed as an intermediate product in this stage influences the type of final product produced during the fermentation process. For example, if the partial pressure of the hydrogen were too high, it would decrease the number of reduced compounds. In general, during this phase, simple sugars, fatty acids and amino acids are converted into organic acids and alcohols.
The products produced in the acidogenic phase are consumed as substrates for the other microorganisms, active in the third phase. In the third phase, also called the acidogenic phase anaerobic oxidation is performed. Products which cannot be directly converted to methane by methanogenic bacteria are converted into methanogenic substrates, volatile fatty acids and alcohols (VFA) are oxidized into methanogenic substrates like acetate, hydrogen and carbon dioxide, VFA with carbon chains longer than one unit are oxidized into acetate and hydrogen. It is important that the organisms which carry out the anaerobic oxidation reactions collaborate with the next group, the methane forming microorganisms; this collaboration depends on the partial pressure of the hydrogen present in the system. Under anaerobic oxidation, protons are used as the final electron acceptors which lead to the production of H2. However, these oxidation reactions can only occur if the partial pressure of hydrogen is low, which explains why the collaboration with the methanogens is very important since they will continuously consume the H2, to produce methane. Hence during this symbiotic relationship inter-species hydrogen transfer occurs.
1.4 Anaerobic Digester Design: Methanogenesis
In the methanogenic phase, the production of methane and carbon dioxide from intermediate products is carried out by methanogenic bacterial under strictly anaerobic conditions. Methanogenesis is a critical step in the entire anaerobic digestion process as it is the slowest biochemical reaction of the process. Methanogenesis is the final stage whereby methanogens bacteria converts hydrogen, acetic acid, and carbon dioxide to methane and carbon dioxide. Equation 1 shows a simplified generic anaerobic digestion.
C6H12O6 ® 3CO2 + 3 CH4 (1)
Figure 2 shows the whole biochemical process.
Figure 2: The key process stages of anaerobic digestion.
2 Anaerobic Digester Design: parameters affecting anaerobic digestion
The activity of biogas production depends on various parameters that include: temperature, partial pressure, pH, hydraulic retention time, C/N (Carbon to Nitrogen) ratio, pre-treatment of feedstock, a trace of metals (trace elements) and concentration of substrate.
As a guideline, the C/N ration for anaerobic digestion should be around 25-30:1. Should more Carbon be present the biochemical decomposition slows down?
3 Anaerobic Digester configuration
Batch or Continuous configuration
AD can be performed as a batch or a continuous process depending on the substrates being digested and the configuration of the digester. In a batch process, the substrate is added to the digester at the start of the process and sealed for the duration of the retention time (RT). After digestion, biogas is collected and the digester is partially emptied. They are not emptied completely to ensure inoculation of fresh substrate batch with bacteria from the previous batch.
In a continuous digestion process, organic matter is constantly added in stages to the digester on a daily basis. In this case, the end products are constantly removed resulting in constant biogas production. Single or multiple digesters in a sequence may be used.
The selection of biogas digester depends on the dry matter (DM) content of the digested substrate. There are two AD technologies systems: wet digestion which is liquid digestion; when the average DM content of the substrate is less than 15% and dry digestion which is solid digestion; when the DM content of the substrate is more than 15% (usually from 20 to 40%). Wet digestion is applied for substrates like manure and sewage sludge, while dry digestion is applied for solid municipal bio-waste, solid animal manure, high straw content, household waste, and green cuttings, grass from landscape maintenance or energy crops. Table 1 shows the characteristics of anaerobic digesters technologies while Table 2 shows the comparison of various digesters types.
Table 1: Main characteristics of Anaerobic Digester technologies
Table 2: Comparison of various Digester Types
4 Anaerobic Digester Design: Conditions affecting the choice of a biogas plant
Developing a biogas plant design is essentially the final stage of the planning process. However, it is mandatory for the designer to familiarize themselves with basic design considerations in advance. Ultimately, a successful plant design should be able to respond to quite a number of factors, and these include:
The design should respond to the prevailing climatic conditions of the location. Bearing in mind that biogas plants operate optimally at temperature ranges between 30°C to 40°C, in cooler regions, it is advisable for the designer to incorporate insulation and heating accessories to the design.
B. Substrate quality and quantity
The type and amount of substrate to be used on the plant will dictate the sizing of the digester as well as the inlet and outlet design.
C. Construction materials available
If the materials required for the plant set up can be sourced locally at affordable rates so as to maintain the plant set up costs within manageable ranges, then the design is preferred to that whose materials have to be imported.
D. Ground Conditions
Preliminary geotechnical investigations can guide the designer on the nature of the subsoil. In cases where the hardpan is a frequent occurrence, the design installation plan must be done in such a way that deep excavations are avoided because this would then increase the construction costs tremendously.
E. Skills and Labour
Biogas technology is sophisticated and hence requires high levels of specialized skilled labour. The labour factor cuts across from the planner to the constructor up to the user. However, gaps can be reduced through training of the involved parties at a cost.
Prior to the commissioning of the design, the planner must carefully study the prevailing standards already on the market in terms of product quality and pricing, especially for large scale projects.
5 Anaerobic Digester Design: Technology selection methods
Several Decisions Support (DS) tools have been developed to give unbiased results when it comes to making decisions on technology selection. These include Multi-Criteria Decision Analysis (MCDA) techniques, the use of grey statistics and Technology Identification, Evaluation, and Selection (TIES) methods among others. In principle, all technology selection methods are based on the steps as summarised below;
- Identification of the problem,
- Identification of stakeholders,
- Seeking the unbiased opinions of the stakeholders in the form of solutions to the identified problem. The identified solutions are treated as alternatives and the measures of importance towards solving the identified problem become the selection criteria,
- Modelling the obtained solutions so as to obtain impartial results through detailed analyses. At the modelling, the stage is when the decision-maker decides on which particular selection method to employ based on the nature of the problem at hand.
MCDA is an approach employed by decision-makers to make recommendations from a set of finite seemingly similar options basing on how well they score against a predefined set of criteria. MCDA techniques aim to achieve a decisive goal from a set of alternatives using pre-set selection factors herein referred to as the criteria.
The selection criteria are assigned weights by the decision-maker basing on their level of importance. Then using appropriate techniques, the alternatives are awarded scores depending on how well they perform with regard to particular criteria. Finally, ranks of alternatives are computed as an aggregate sum of products of the alternatives with corresponding criteria. From the ranking, a decision is then made.
Multi-criteria decision analysis (MCDA) techniques can be successfully applied to choose a biogas digester technology from a list of potential alternatives for an anaerobic digestion (AD) system based on:
- Cost of the digester
- Local availability of the digester
- Temperature regulation ability
- Substrate/feedstock suitability
- Ease of construction
- Presence of agitation accessory
Using MCDA to analyse the various biodigester models presents a successful option owing to the fact that all the critical attributes are directly measurable and non-subjective.
6 Anaerobic Digester Design: Biodigester sizing
A detailed feedstock analysis to determine the quantity and quality of the selected feedstock, which directly impacts the selected technology’s sizing needs to be completed. Using the MCDA technique, a suitable biogas model can be selected and from the substrate analysis, the appropriate size of the biogas digester can be determined.
The size of the digester, i.e. the digester volume Vd, is determined on the basis of the chosen retention time RT and the daily substrate input quantity Sd.
Vd = Sd x RT [m3 = m3/day x number of days] (2)
The retention time, in turn, is determined by the chosen/given digesting temperature. For an unheated biogas plant, the temperature prevailing in the digester can be assumed as 1-2 Kelvin above the soil temperature. The seasonal variation must be given due consideration, however, i.e. the digester must be sized for the least favourable season of the year. For a plant of simple design, the retention time should amount to at least 40 days. Practical experience shows that retention times of 60-80 days, or even 100 days or more, are no rarity when there is a shortage of substrate. On the other hand, extra-long retention times can increase the gas yield by as much as 40%.
The substrate input depends on how much water has to be added to the substrate in order to arrive at a desirable solids content (typically between 4 – 8% for wet digestion).
Substrate input (Sd) = biomass (B) + water (W) [m3/day] (3)
In most agricultural biogas plants, the mixing ratio for dung (cattle and / or pigs) and water (B:W) amounts to between 1:3 and 2:1. The ratio of B:W for the organic fraction of municipal solid waste (OFMSW) is in the region of 1:4.
7 Anaerobic Digester Design: Estimating the biogas production
The amount of biogas generated each day G [m3 gas/d], can only be estimated based on actual data recorded for the digester and substrate type and is calculated on the basis of the specific gas yield Gy of the substrate and the daily substrate input Sd.
The calculation can be based on:
The volatile solids content VS
G = VS × Gy(solids) [ m3/d = kg × m3/(d×kg) ] (4)
the weight of the moist mass B
G = B × Gy(moist mass) [ m3/d = kg × m3/(d×kg) ] (5)
The temperature dependency is given by:
Gy(T,RT) = mGy × f(T,RT) (6)
Gy(T,RT) = gas yield as a function of digester temperature and retention time
mGy = average specific gas yield, e.g. l/kg volatile solids content
f(T,RT) = multiplier for the gas yield as a function of digester temperature T and retention time RT
As a rule, it is advisable to calculate according to several different methods, since the available basic data are usually very imprecise, so that a higher degree of sizing certainty can be achieved by comparing and averaging the results.
8 Anaerobic Digester Design: Digester loading
The digester loading Ld is calculated from the daily total solids input TS/d or the daily volatile solids input VS/d and the digester volume Vd:
Ldt = TS/d ÷ Vd [kg / (m3 day)] (7)
Ldv = VS/d ÷ Vd [kg / (m3 day)] (8)
Then, the calculated parameters should be checked against data from comparable plants in the region or from pertinent literature.
9 Gasholder sizing
The size of the gasholder, i.e. the gasholder volume Vg, depends on the relative rates of gas generation and gas consumption. The gasholder must be designed to:
- cover the peak consumption rate gcmax (->Vg1) and
- hold the gas produced during the longest zero-consumption period tzmax(->Vg2)
Vg1 = gcmax × tcmax = vcmax Vg2 = Gh × tzmax (9)
gcmax = maximum hourly gas consumption [m3/h]
tcmax = time of maximum consumption [h]
vcmax = maximum gas consumption [m3]
Gh = hourly gas production [m3/h] = G ÷ 24 h/d
tzmax = maximum zero-consumption time [h]
The larger Vg -value (Vg1 or Vg2) determines the size of the gasholder. A safety margin of 10-20% should be added:
Vg = 1.15 (±0.5) × max(Vg1,Vg2) (10)
Practical experience shows that 40-60% of the daily gas production normally has to be stored.
The ratio Vd ÷ Vg (digester volume ÷ gasholder volume) is a major factor with regard to the basic design of the biogas plant and ranges typically from 3:1 to 10:1 depending on the feedstock.
10 Anaerobic Digesters in South Africa
South Africa has experienced very limited market penetration for bio-digesters and biogas. In Germany, one of the leading European countries utilizing biogas, around 1000 plants are built every year. In India, more than 12 million plants, ranging from small domestic units to large commercial plants, are in operation. Even Uganda has more than double the estimated 300 biogas plants that South Africa are operating at present.
In South Africa the waste management landscape is changing rapidly since land around the larger metropoles for landfill use is becoming very limited and expensive. A rapid departure is needed from our current approach of only throwing away instead of recycling materials where possible and utilize the organic fraction of municipal solid waste (OFMSW) for biogas generation. South Africa has also ratified the Kyoto protocol and committed to reducing greenhouse gas (GHG) emissions by at least 34% by 2020, and a further 45% by 2030. The proposed Carbon Tax due for roll-out will also grease the wheels to promote more green technologies like biogas generation.
The market for gas in South Africa has been increasing steadily since the introduction of the natural gas from Mozambique to industries and in certain areas of Johannesburg even to residential customers. With the new technologies available to feasibly “clean up” the biogas by removing the trace elements of hydrogen sulphide and reduce the carbon dioxide it is possible to successfully store and pressurize the bio-methane. This enables not only the use of the biogas for electricity generation as in the past but also for utilization anywhere from heating, cooking and transportation fuels in the form of CNG or even LNG (Liquified Natural Gas).
EPCM Consultants has already monetized the only “natural” bio-gas reserves in South Africa by converting it to compressed natural gas (CNG) to run Harmony mining’s busses. EPCM is also currently developing the next step to liquefy the gas to increase the energy density and enable more opportunities for usage. More recently EPCM completed a design to convert a customer’s anaerobic digester to utilize the biogas generated from organic waste material as compressed natural gas (CNG), instead of generating electricity. They will be using the CNG to supplement their transportation fuel requirements increasing the value generation from their anaerobic digester.
Biogas as an almost previously untapped resource in South Africa presents huge opportunities for utilizing untapped renewable resources (from manure to organic wastes) to expand the gas market in areas not having access to the Sasol gas pipeline infrastructure by providing a cost-competitive clean energy source.