Four of the greatest challenges we face at the turn of the last century are environmental degradation from waste pollution, energy crisis and the over exploitation of natural resources. While we consistently sought the path to industrialization, we over exploited our natural resources and generated so much waste that its disposal became harmful to our environment. In the UK, a growing concern is the 6.7 million tonnes of food wasted annually. Although ignorant at first, there is now a growing awareness of the importance of environmental responsibility and the need to address the problems of mounting waste. The energy crisis is another awakening call, in 2007 Britain due to a delayed release of a government white paper; our daily headlines screamed 'Britain grapples with a looming energy crisis' The statistics were frightening and claimed that in only eight years, demand for energy could outstrip supply by 23% at peak times (The Sunday Times, 20 May 2007).
News broke out of our move from being self-sufficient in oil and gas, as North Sea production declined. There was thus the need to find alternative green energy and environmental friendly ways to dispose our waste together with a fix for the damage already done. Surprisingly, while the scientist, engineer and politicians struggle to find answers, a couple of British farmers provided the solutions to the impending crisis. Their solution, quite simply low tech, low carbon and sustainable waste to energy scheme utilizing anaerobic digesters.
In February 2009, the UK Government outlined plans to make greater use of anaerobic digesters to generate electricity. Instigated at the National farmers Union conference in Birmingham, an ambitious plan was hatched to build a thousand farm-based anaerobic digesters by 2020 and in the months that followed, the debate regarding sustainable waste to energy was revived in the house of common, the environmental engineering industries, by sceptics and the nation at large. To the sceptics several 'green' initiatives have been proposed that simply do not add up either financially or more importantly, environmentally. To the environmental engineering professionals, simply too many knowledge gaps need to be filled about the technology of building and operating such huge numbers of plants over a short period of ten years. In the house of common, the debate ranged from its financial implication to its health implications and safety.
On 4th March 2009, the parliamentary under-secretary, Huw Irranca Davies addressed the house of common on the governments £10 million push to develop this technology and its wide spread applications and benefit to the UK economy. To the UK government, the time for debates were over, it was time to make drastic investments in anaerobic digestion.
So in my opinion irrespective of the sceptics' views, the fact remains, anaerobic digestion of organic waste streams can produce a hundred percent renewable electricity whilst the heat produced by the process can find its separate application. The treated residue can be returned to the land as fertiliser and the whole process reduces greenhouse gas emissions, assures food security, and aligns the UK in meeting its EU directives. If we process these wastes then we are truly making an environmental step forward, reducing landfill, producing power (that would otherwise be generated from fossil fuels) and moving away from a food chain towards a food cycle. For these reasons, it is a laudable initiative that should be encouraged regardless of the impracticalities in time frame.
1.1. Current Status of Anaerobic Digestion in the UK
As early as the late 90's a number of organisations manufactured and sold small scale domestic digesters running on kitchen waste, however digesters have been employed in sewage works for the treatment of sludge for more than a decade. To date in the UK, there are 11 anaerobic digestion facilities in existence, two permit applications in process and 51 proposed anaerobic digestion sites in the permit pre-application stage. (DEFRA, 2010). Almost all large anaerobic plants in the UK are designed to accept single source farm waste, simply because they were designed to solve the disposal of farm waste and located on these farms. Since digesters are designed based on the feedstock it received, most have been largely cattle waste, poultry or dairy waste and pig waste. In the bid to encourage more, the UK government announced on 1st February, the feed-in-tariffs for small scale electricity includes tariffs for anaerobic digesters of 11.5p per kw/h up to 500kW and 9p per kw/h between 500kW and 5MW. In my opinion, anaerobic digesters should be rewarded with at least twice this rate. Feed in Tariffs are an excellent solution provided they are biased towards those processes that genuinely make a positive environmental impact, such as a low cost, low technology farm based anaerobic solution that relies exclusively on waste.
However, there are a couple of success stories, Twinwoods plant, the first of its kind in the UK was conceived in 2003 with pig slurry from Bedfordia Farms as feedstock. The plant has a combined generating capacity of 1.2MW enough to power over 1000 homes. Another is the South Shropshire Bio digester, at Ludlow. The plant has been designed to process 5,000 tonnes per annum of household kitchen waste into biogas to produce renewable heat and electricity, and a bio fertiliser that is returned to local agricultural land.
1.2. Report methodology
There are two key aspects neglected and important in this early planning and conception stage of any digester implementation plan.
1. The technical knowledge and expertise involved in operating a digester. The microbiological and biochemistry of operating a digester indicate that the task is a herculean one.
2. A viable collection system. Though food waste is one of (if not) the largest fraction of the waste stream, it remains the case that it is barely touched by separate collection systems in England. In Wales, Scotland and Northern Ireland, food waste also remains a 'Cinderella material', barely captured, yet needing to be managed more sustainably.
Subsequently, in the course of this write up and in addition to tackling these issues above, the following shall be discussed in a bid to suggest an operating strategy for anaerobic digesters.
- Current practices of anaerobic digesters in the UK citing the microbiology and biochemistry of the anaerobic digestion process with particular emphasis on the current operations and control strategies adopted.
- A look at the types of organic wastes in the UK that are amenable to anaerobic digestion and how these affects operations and control
- Identifying the process control indicators affecting digester operations and hence providing a suitable operating strategic plan that can form part of a Company's Quality Assurance procedures and which can be modified to treat a range of different waste
2. ANAEROBIC DIGESTION
To understand the principles and working operations of anaerobic digestion, lets briefly consider how anaerobic organism carryout their metabolism. Anaerobic respiration is believed to be the oldest form of cellular respiration. It is still employed by many single celled organisms, particularly in anoxic environments as was the case 4 billion years ago. The process of anaerobic respiration begins with breaking down glucose (sugar molecules) that produces pyruvic acid. The pyruvic acid then undergoes a fermentation process to produce ATP, which cells employ for energy production.
Anaerobic degradation, the basic process in digesters has always been considered inferior to aerobic degradation in their kinetics and capacities. They are thought to be slow and inefficient, nonetheless in certain anoxic environment such as the cow's rumen, is a much faster process and requires less energy. These organisms are widely employed in the working of an anaerobic digester.
2.1. Process Microbiology for Anaerobic Digesters
The anaerobic degradation of complex organic matter is carried out by different groups of bacteria. There must be a coordinated interaction among these bacteria for success the process. These bacteria are;
1. Hydrolytic Bacteria: Complex polymeric substrates are broken down into soluble monomers. A number of extracellular hydrolytic enzymes that are capable of initiating the attack on these complex substrates may be produced within anaerobic digesters by hydrolytic genera such as Clostridium, Peptococcus, Vibrio, and Micrococcus. Fungi are also believed to play a relatively minor role within anaerobic digesters and have been shown to be capable of reproduction within operational digesters and to this extent are taking part in the digestion process by consuming nutrients for growth. Genera of fungi observed include Phycomycetes, Ascomycetes and fungi imperfecti.
2. Acidogenic Bacteria: Provide important substrates for acetogens and methanogens. They metabolize amino acids and sugars to the intermediary products, acetate, hydrogen and carbon dioxide. The acidogenic stage includes many different fermentative genera and species; among them are Clostridium, Bacteroides, Ruminococcus, Butyribacterium, Propionibacterium, Eubacterium, Streptococcus, Pseudomonas, Bacillus and Escherichia. The facultative members of this group also help protect the oxygensensitive methanogens by consuming traces of oxygen that may enter in the feed. Together, the hydrolytic and acidogenic bacteria are known as Fermentative Bacteria. Two common factors that affect this metabolism in a digester are the various types of substrates and the environmental conditions. Novaes (1986) suggest that this metabolism is thermodynamically favourable for the formation of acetate, CO2 and H2 at Low Hydrogen partial pressure. (D.D Mara & N. J Horan, 2003)
3. Acetogenic bacteria, these bacteria convert higher organic acids into acetate, H2 and CO2. As a key implications for digester operation and control, McCarty and Smith (1986) described how high rate anaerobic reactors could maintain low hydrogen partial pressure enough to efficiently oxidize intermediates (e.g. propionate and ethanol). This hydrogen produced during anaerobic oxidation of intermediates must be consumed rapidly by hydrogen consuming microbes for efficient oxidation of intermediates to take place. Therefore high rate anaerobic systems are considered to be highly efficient for maintaining lower hydrogen partial pressure. Homoacetogens, which are known to produce acetate, an important precursor to methane generation, could be either autotrophs or heterotrophs. An example is the Acetobacterium woodii usually found in sewage sludge. (Novea, 1986). An acetogenesis reaction is shown below:
4. Methanogenic Bacteria, these are regarded as rate limiting species in the methanogen metabolism and are the most complex of all microbial life forms utilised in a digester. They are responsible for the formation of methane from hydrogen and acetates. Previously classified as bacteria but more recently classified as archaea. Two important genera of the methanogens are Methanosacrcina and Methanosaeta. The Methanosarcina has been reported to account for the stability of anaerobic digestion; hence it is a common strategy to maintain their dominance during short solid retention time or high acetate concentration. The practical strategies of enhancing the growth of Methanosarcine during anaerobic digestion are two phase digestion or pre-treatment e.g. Ozonation. (S.K Khanal, 2008)
2.2. Process Biochemistry of Anaerobic Digesters
The widely accepted view for the biochemistry of anaerobic digestion is the six stage process enumerated below.
1. Hydrolysis of biopolymers: Break down of polymer into protein, lipids and Carbohydrates (C6H12O6) Proteins are broken down by an enzyme called protease that is secreted by fermentative bacteria. This enzyme separates proteins (polypeptides) into amino acids (peptides).
- Hydrolysis/Liquefaction reactions
- Lipids ? Fatty Acids
- Polysaccharides ? Monosaccharides
- Protein ? Amino Acids
- Nucleic Acids ? Purines & Pyrimidines
2. Fermentation of amino acids and sugars: Amino acids are broken down into higher fatty acids called Volatile Fatty acids.
3. Anaerobic oxidation of long chain fatty acids: Long chain fatty acids are converted into smaller acids.
4. Anaerobic oxidation of intermediary products (VFA's): Amino acids, sugars, intermediates (Valeric acid, Isovalerate, Propionate and Butyrate) are converted to acetate and a lesser proportion to CO2 and H2. Higher fatty acids (stearic, oleic), and alcohols such as ethanol are converted to acetate, CO2 and H2.
5. Conversion of acetate and hydrogen to methane: Methanogens combine the acetic acid made by acetogens with hydrogen gas, and carbon dioxide to produce methane gas, water, and carbon dioxide, according to the following equation:
2.3. Current Operations and Control Methods.
To understand the current operation strategies adopted in running digesters, a brief historical development of anaerobic biotechnology since the late 30's will shed more light on the technology so far. In 1930, Fair and More discovered the importance of seeding and pH control, and in 1950, Morgan and Torpey developed a digester mixing system, the high-rate anaerobic digestion. Shortly after, G. J Stander (a South African investigator) developed the Clarigester (high rate anaerobic process). Work in the united state yielded good progress when the anaerobic contact process similar to the aerobic activated sludge process was developed. From then on, reactors designs and technology improved and the anaerobic membrane bioreactor (AnMBR), the Up flow anaerobic sludge blanket reactors and the Expanded Bed reactor were developed in the late 70's. As waste management challenges mounted in the 80's, the anaerobic baffled reactor and the anaerobic sequential batch reactor were developed around 1983 and 1992 respectively. The latter, allowing a more controlled process carried out in a single vessel thus requiring low capital equipment investment and an efficient operation. Gradually, more waste types became amenable as feedstock in digesters. Because digesters differ in design and as such operations and control, it is also pertinent to briefly discuss the different digester configurations for a full grasp of their operations and control
2.3.1. Current Configuration
Anaerobic digesters can be designed and engineered to operate using a number of different process configurations:
Batch or Continuous: A batch system is the simplest form of digestion. Biomass is added to the reactor at the start of the process in a batch and is sealed for the duration of the process. Batch reactors suffer from odour issues that can be a severe problem when they are emptied. In continuous digestion processes organic matter is constantly or added in stages to the reactor. Here the end products are constantly or periodically removed, resulting in constant production of biogas. Examples of this form of anaerobic digestion include, continuous stirred-tank reactors (CSTRs), Up flow anaerobic sludge blanket (UASB) and Expanded granular sludge bed (EGSB)
Temperature, Mesophilic or Thermophilic: Two conventional operational temperature levels for anaerobic digesters, which are determined by the species of methanogens in the digesters Mesophilic which takes place optimally around 37-41°C or at ambient temperatures between 20-45°C where mesophiles are the primary microorganism present Thermophilic which takes place optimally around 50-52°C at elevated temperatures up to 70°C where thermophiles are the primary microorganisms present.
High rate or Low rate: Low rate reactors are unmixed. Temperature, SRT, and other environmental conditions are not regulated. Organic loading rate is low in the range of 1-2kg COD/m3.day. Not suitable for bioenergy production. High rate digesters are essentially a continuous stirred tank reactor (CSTR), operated under mesophilic or thermophilic conditions.
Complexity (Single stage or multistage): A single-stage digestion system is one in which all of the biological reactions occur within a single sealed reactor or holding tank. Utilising a single stage reduces construction costs, however facilitates less control of the reactions occurring within the system. Acidogenic bacteria, through the production of acids, reduce the pH of the tank. Methanogenic bacteria, as outlined earlier, operate in a strictly defined pH range. Therefore the biological reactions of the different species in a single stage reactor can be in direct competition with each other. In a two-stage or multi-stage digestion system different digestion vessels are optimised to bring maximum control over the bacterial communities living within the digesters. Acidogenic bacteria produce organic acids and more quickly grow and reproduce than methanogenic bacteria.
2.3.2. Digester Operations and control.
How then is an anaerobic digester currently operated? Anaerobic digesters have a defined process flow that consists of four distinct phases;
1) Pre-treatment: Some form of pre-treatment of the feedstock is usually essential to the successful operation of a digestion plant. This pre-treatment can range from some simple screening to remove items likely to cause blockages, right through to complex processes that remove contaminants, pasteurise and hydrolyse the feedstock. During pre-treatment wastes may be processed, separated, or mixed to ensure that they will decompose in the digester. A screen is good policy on all feedstock and some means of removing heavy items (stones, glass, grit, etc) would be recommended.
2) Seeding and Sludge Recirculation: This is dependent on the type of waste. For batch reactor, the method is known as seeding and involves mixing the waste with a certain proportion of sludge which has already been digested anaerobically.
3) Digestion: The third phase is digestion, where waste products are broken down by bacteria and biogas is produced. The digestion tank should be air tight and its size is determined by the organic loading rate, the retention time chosen and the daily production rate. With the advancements this technology, the environment and operations of the digester are monitored and control by computer automated systems. Control of mixers, temperature and PH control probes. Outlets for the removal of scum should be provided as scum constitute a huge problem in digester operations (P.J Meynell, 1982)
4) Gas Collection and Processing: Biogas is firstly stored and upgraded before find its application fuels. During upgrading, scrubbers, membranes, or other means are used to remove impurities and carbon dioxide from biogas.
5) Processing Digester waste: Digested waste has a high nutrient content and can be used as fertilizer so long as it is free of pathogens or toxics, or it can be composted to further enhance nutrient content. Removing the moisture content is an important post treatment.
2.4. Control Parameters
The major drawback to the advancement of digester technology was the instability and unpredictable nature of its operational processes. This unpredictable behaviour and instability can be attributed to the fact that the process is carried out by a consortium of different populations of bacteria whose activities must be balanced for stable optimum performance. Imbalance can lead to accumulation of intermediate fermentation products which result in depression of pH and inhibition of the process. Digester imbalance can be attributed to three causes: overload, toxic substances and sudden changes in the digester environment. Fluctuations in the loading rate are usually attributed to poor control of a wide range of digester parameters which shall be discussed subsequently.
1) Volumetric Organic Loading Rate: Anaerobic processes are characterised by high volumetric organic loading rates. It is the measure of the biological conversion capacity of the AD system. Excessive loading rates results in low biogas yield due to accumulation of inhibiting fatty acids substances in the digester slurry hence an important control parameter in continuous systems. (S.K Khanal, 2008)
2) Total Solids & Volatile Solids: These are important measurements which must be monitored for optimum stability of a digesters process. The total solid content is that proportion of the waste left after all the water has been driven off while volatile solids are those made of organic matter as opposed to total solids which also contain the inorganic materials. The reductions in total and volatile solids contents through digestion can be useful preliminary estimates of the efficiency of the digester at reducing organic matter and hence pollution hazard. The appearance of the effluent can be a good indication of digester function. The sludge should be black with a faint tarry, but not unpleasant smell and the solids should settle out fairly easily. If the sludge smells rancid and foul then something is wrong and would require investigation. (P.J Meynell, 1982)
3) Hydraulic & Solid Retention time: Hydraulic retention time (HRT) indicates the time the waste remains in the reactor in contact with the biomass. The time required to achieve a given degree of treatment depends on the rate of microbial metabolism. It is a good operating strategy for waste containing simple compounds such as sugar which are readily degradable, to require low HRT. More complex waste which are slowly degradable need longer HRT for their metabolism. Solid retention time (SRT) control the microbial mass in the bioreactor to achieve a given degree of waste stabilization. It is the measure of the biological system's capability to achieve specific effluent standards or to maintain a satisfactory biodegradable rate of pollutants. Maintaining a high SRT produces a more stable operation, better toxic or shock loading tolerance and a quick recovery from toxicity. The SRT also determines the permissible organic loading rate. . (S.K Khanal, 2008)
4) Start-Up Time: This is the initial commissioning period during which the process is brought to a point where normal performances of the biological treatment system can be achieved with continuous feedstock feeding. This parameter is important because anaerobic microorganisms particularly have a slow growth rate and are thus susceptible to changes in the environmental factors. Reducing the start-up time for digesters is therefore beneficiary to the stability of the process. This can be achieved by seeding as discussed early. The more seeds used the shorter the start-up time. (S.K Khanal, 2008)
5) Microbiology: This remains the most critical factors affecting the stability of a digester. The fragile nature of microorganisms especially methanogens to the changes in environmental conditions such as pH, temperature, nutrients/trace metals availability and toxicity determine the stability of the anaerobic process. If a system fails due to lack of proper environmental factors or biomass washout from the reactor, it may take several months for the system to return to a normal operating condition because of the extremely slow growth rate of methanogens.
Environmental Factors: The effect of environmental factors such as temperature, operating pH, Oxidation and reduction potentials, Nutrients, Toxicity and Inhibitors are as important to the stability of the digester process as the microbiological agents in the digester. Temperature in anaerobic digestion can occur under Mesophilic conditions (between 20-45 degrees centigrade) and Thermophilic conditions (between 50-65 degrees centigrade). The optimum temperature will however depend on the feedstock composition but sterilization of the waste is linked to the temperature. The higher it is the more effective it is in eliminating pathogens, viruses and seeds. The optimum pH values should range between 6.4 and 7.2 and the oxidation and reduction potential values should be maintained between 200 to 350Mv (Archer and Harris 1986). Nutrients and trace metals promote at an optimum rate the microbial cell growth in a digester. The presence of ammonia, heavy metals, halogenated compounds and cyanide inhibit anaerobic organisms and can increase toxicity in the digester (Parkin and Speece, 1982).
The relationship between the amount of carbon and nitrogen present in organic materials is represented by the C/N ratio. Optimum C/N ratios in anaerobic digesters are between 20 - 30. A high C/N ratio is an indication of rapid consumption of nitrogen by methanogens and results in lower gas production. On the other hand, a lower C/N ratio causes ammonia accumulation and pH values exceeding 8.5, which is toxic to methanogenic bacteria. Optimum C/N ratios of the digester materials can be achieved by mixing materials of high and low C/N ratios, such as organic solid waste mixed with sewage or animal manure.
It is therefore becoming more appropriate, based on the number of parameters which have effects on the stability and operations of a digester, to move away from the traditional manual evaluation of digester performance involving weekly measurements of these parameters mentioned. Digester imbalances often proceed to critical levels before results are obtained from these weekly evaluations. A more automated monitoring system can effectively provide a smooth operating strategy and control of the digester system.
2.5. Waste Fractions Suitable for Anaerobic Digestion
A wide range of organic waste streams in the UK are amenable to anaerobic digestion. The early digesters in the UK basically relied on feedstocks of a single stream because of the uniformity in the characteristics which gave a more predictable process and assured digester stability. However with the development in digester kinetic the understanding of the microbiological process, virtually all organic waste can be employed as feedstock in a digester.
Still the most controversial of suitable organic waste streams for a digester is the municipal solid waste streams. High-solids reactors have been designed to handle the processing of mixed MSW as well as biowaste. Mixed MSW or wastes of variable feedstock are all material set out as garbage particularly food waste and excluding recyclables, compostable or waste diverted from garbage by some other means.
A list of Biodegradable waste fractions acceptable as defined by the UK's Environmental agency and WRAP and listed in the EWC categories of waste types suitable for anaerobic digestion are listed below.
- Wastes from agriculture, horticulture, hunting, fishing and aquaculture primary production, food preparation and processing
- Wastes from preparation and processing of meat, fish and other foods of animal origin
- Wastes from fruit, vegetables, cereals, edible oils, Cocoa, tea and tobacco preparation and processing; conserve production
- Wastes from sugar processing
- Wastes from dairy products industry
- Wastes from baking and confectionary industry
- Wastes from production of alcoholic and non-alcoholic beverages (except tea and coffee)
- Wastes from wood processing and the production of paper, cardboard, pulp, panels and furniture
- Wastes from leather, fur and textile industry
- Wastes packaging; absorbents, wiping cloths, filter materials and protective clothing not otherwise specified
- Wastes from waste management facilities, off-site waste water treatment plants and the preparation of water intended for human consumption and water for industrial use
- Wastes from waste water treatment plants not otherwise specified Municipal wastes and similar commercial, industrial and institutional wastes including separately collected fractions
- Garden and park wastes (including cemetery waste)
The table below has been compiled to summarise the types of waste amenable to anaerobic digestion in the UK and lists important constituents of these wastes fractions that can inhibit biochemical or microbial activities and affect the control and stability of digestion process.
3. CONTROL PROCEDURE FOR QUALITY ASSURANCE
An operation and control procedure for digester operations is therefore required to ensure that end products from the digesters reach specified standard set by governing authorities. Therefore before outlining control procedures, it is essential that the criteria that define a quality output are known. The UK Environmental Agency and WRAP have set out Quality Protocol criteria for the production of quality outputs from anaerobic digestion of materials. These outputs include the whole digestate, the separated fibre fraction and the separated liquor. If these criteria are met, outputs from anaerobic digestion will normally be regarded as having been fully recovered therefore ceased to be waste and under the control of the waste management handling criteria.
3.1. Control Tests
No process can be operated without having adequate control and an indication of its progress. How then are controls and indicators defined in digester operation? Controls are short term and used for corrections. They are tests that can be run to confirm satisfactory operations or to indicate an action that would bring about change.
A. External control
External control tests are to help the operator control what is coming into the digester. An example, in normal operation, the operator should control the concentration of solids in the feed to avoid diluting the digester contents. To do this the operator takes a composite sample of the incoming feed and runs a total solid test. This test measures all solids and what percentage of the liquids is in solid form.
Other external control tests are quantity of sludge handled per day and tests which define the characteristics of the incoming sludge. This information tells
- Whether the existing grit removal system is operating as well as it should or whether new equipments are required.
- Whether toxic materials are present
- Whether the sludge is fresh or stale
- How much heat will be needed and if the digester operating temperature can be maintained
B. Internal Controls
Internal controls show what is happening inside the digester. Four tests are recommended for best control; Temperature, Volatile acids, Alkalinity and pH. Temperature directly affects the work of the methane bacteria as earlier explained thus variations in temperature should never exceed more than one degree per day. The best temperature for any given digester is based on;
- The highest gas production
- The ability to hold the volatile acids to alkalinity ratio between 0.1 and 0.25
- Maintaining the pH near neutral
The major internal control combines two lab tests, volatile acid and alkalinity. The alkalinity of a digester is important because it represent the ability of the digester to neutralise the acids. The results of these two tests are expressed in a ratio and expressed as a single number. The tests are run on sludge samples from the primary digester and flowing supernatant drawoff points. Ratio should range between 0-0.35 and an increase in this ratio is an indication of a malfunctioning digester. Volatile acid test methods include static acid or chromatographic method, straight distillation method and the titration or nonstandard method
The pH is one of the simplest test that can be run to indicate the progress of the digester and should be run frequently, however danger lies in depending too much on pH as a process control. Because of the alkalinity in the digester the pH changes very slowly.
The following general guidelines are given for best process control;
- Routine Volatile acid and alkalinity determinations during any startup process are a must in bringing a digester to a state of satisfactory digestion
- Measure the volatile acid/alkalinity ratio at least twice per week during normal operation and watch the trend
- Measure the volatile acid/alkalinity ratio at least daily when a digester is approaching trouble such as an increase solids load from waste discharges or a storm
- CO2 and pH tests may be substituted for volatile acid/alkalinity control in those cases where the loading is uniform and predictable and process upsets are infrequent. It is important, however to realise that failure are costly in terms of both money and time.
3.2. Planning and Design
Decisions about the equipment and technical design of the plant also need to be considered. The main requirements for good design for AD plants include, minimising mechanical and electrical equipment, effective insulation properties, and corrosion resistant materials, simple design and automatic operation, equipment fail safe devices throughout and environmental controls.
AD produces certain emissions and effluents, to air, ground and water, which need treatment to avoid damage to human health and the environment. As with any new development, there are specific issues arising from the construction of the power generating plant which are more related (for neighbours) to living near to a building site than to a power plant. These include construction noise and dust, impact on the access road, transport noise, light pollution, oil spillages, soil erosion, and Proper management should ensure that all these risks are controlled, and the best available technology should be used in all cases.
3.3. Feedstock Selection
The EA quality protocol set out acceptable feedstock from waste listed in appendix B and mentioned in section 2.5 above. For digesters of variable feedstock, pre-treatment, would be required. It is essential that toxic substances are minimised in feedstock, and certain materials should never be fed to digesters because they will arrest or kill the process. These include:
- Toxic materials that inhibit digestion (eg high ammonia levels, pesticide residues)
- Bioagents (aflatoxins, antibiotics)
- Disinfectants (e.g. creosol, phenol, arsenic).
- Long straw and non-biodegradable materials should be avoided as they can cause blockages in the system
A. Pre-treatment, Quantity and Quality of Feedstock
The main aim of managing the quantity and quality of feedstock for the digester is to maximise the quality and quantity of the outputs, and therefore the economic and environmental benefits from the feedstock. Different priorities for outputs will affect the quality criteria for the feedstock. For example:
- To maximise gas yields, the key factors will be organic matter content and the percentage of dry matter (5-12.5% maximum of feedstock should be dry/solid waste).
- Screening for foreign matter (e.g. bricks, sand, grit and long straw).
- Adding water, or taking water out. In general, it is not advisable to add water as the more water there is in the feedstock, the more energy is needed for the process; however, some water may need to be added to ensure the feedstock is the right consistency.
- Conditioning the waste (e.g. Shredding, chopping straw),
- Stirring the feedstock. The quality of the feedstock in terms of its gas yield will partly depend on its freshness. The fresher it is, the higher the gas yield will be and the less danger there is of it becoming acidic. An acidic feedstock may inhibit or even kill the bacteria in the digester. Ideally, the pH range in the digester should be 6.8-8
- It is good practice to test feedstock before processing, as well as the resulting products.
B. Storage and Transportation of Feedstock
Plants will require access to up to several hundred tonnes of feedstock at any time, so extensive storage facilities are likely to be needed to ensure continuity of supply over weekends and holidays when traffic movements may be controlled.
- The storage of farm slurry is covered by the Control of Pollution Regulations 1991, which requires the store to be big enough to hold at least four months slurry production.
- Planning requirements require feedstock to be stored in a totally enclosed space with tank covers to reduce the escape of odour.
- Health and safety regulations require that substances are controlled so that there is no exposure of hazardous substances to health.
3.4. Digester Operation and Control
It is possible to automate AD plants, but they will always need daily management and monitoring for quality, performance, health and safety. Anaerobic digestion is a robust process, but to ensure optimum performance the following aspects of operations and control should be considered;
- Choice of appropriate technology for the specific circumstances
- Achieving the desired dry solid content of feedstock: the more dry solids, the more gas per kg of feedstock that will be produced
- Ensuring efficient mixing and maintaining the correct temperature in the digester and ensuring sufficient heat transfer and capacity
- Regular process monitoring and regular maintenance of moving parts and heat transfer surfaces.
- Use the least starting volume available. In a multiple tank installation, start the digestion in one tank only. In a floating cover installation fill the tank only sufficiently to float the cover.
- Initially heat the added sewage or water to 35°C and maintain temperature within +- 1.7 °C throughout the period of the starting process.
- Seeding culture is recommended if available; it will hasten the start of the normal digestion. Recommended seed quantity is about 0.5 % (dry weight) of weight of liquid in the tank or about 15 % by volume.
- Raw feed should be no more than 10 percent of the anticipated ultimate daily load each day. When the gas produced is about 50 percent of that calculated to be available normally from the volatile solids added and the volatile acids do not rise sharply, increase the feed by 50 to 100 % of the initial daily feed until the ultimate design load is reached.
- Make volatile acids determinations daily before adding feed. Continue daily analyses until the design load is reached. If volatile acid concentrations show a continued increase, or if digestion does not start, reduce or interrupt feed.
- Measure and record gas production daily. Check the CO2 content of the gas. The volume of gas collected should be about 0.6 m3/kg added in raw feed per day. The CO2 ultimately should be between 30 and 35 % by volume.
- After one digester is operating properly at its design load, digested feed and overflow liquor then can be used to seed other tanks in a multitank system.
- For a new digester fill the tank with fresh raw wastewater or water and bring the temperature up to about 35 °C. Maintain this temperature +- 1.7 °C until digestion is established.
- When raw feedstock is added, also add hydrated lime. The raw feed probably would be less than the design load at first, but full loads are usually possible within 30 to 40 days.
- During the daily feeding of raw feedstock, use 4.5 to 6.8 kg of lime per 1,000 connected population, or more importantly, just enough to maintain the pH at 6.8 to 7.0. It is important that mixing be complete so that false low pH readings will not result in excess lime addition. Lime should be added in a slurry form.
- Recirculate the feed mass daily. The entire tank contents should be overturned from bottom to top.
- Keep a constant check on the pH at as many points in the tank as is possible so that it is reasonably certain that the 6.8 to 7.0 range is maintained throughout the feed mass.
- Digestion usually is established within 30 to 40 days. However, if too much lime was used many upsets can occur over a period of several months
- The gas collection system within the digester should be designed to facilitate exclusion of floating debris.
- Pipe and components within the digester should be securely anchored to prevent displacement from normal forces including loads from accumulated scum.
- Pipe should be designed for wet biogas. The pipe may need to be insulated to prevent frost build-up.
- Pipes should be constructed to enable all sections to be safely isolated and cleaned as part of routine maintenance.
- Transfer pipe can be buried or installed above ground and must include provisions for drainage of condensate.
- Equipment and components should be conveniently located and sheltered from the elements.
- The size of equipment and connecting pipe should be based on head loss, cost of energy, cost of components, and manufacturers' recommendations.
- Gas pipe installed within buildings should be of type approved for combustible gas.
Two general methods of start-up will be discussed and both have been used successfully. These are the Controlled Natural Process and the pH Control Process. The controlled natural process might be better suited for starting a digester in a plant where close laboratory control is difficult. The pH control method would appear to need closer laboratory control.
A. Controlled Natural Process
The following procedures are recommended either for starting a new digester or for restarting a sour one. However, if an existing sour digester is to be restarted, it is recommended that the contents be removed completely and that the inside bottom and walls be cleaned thoroughly.
B. PH Control Process
These procedures can be used either for starting a new digester or for restarting a sour one. It usually is not considered necessary to empty the sour sludge for this process in restarting a unit.
3.5. Gas Collection, Transfer and Control System.
The biogas collection, transfer, and control system shall be designed to convey captured gas from within the digester to gas utilization equipment or devices pipe and appurtenances shall meet the following:
A. Gas Control
B. Gas Utilization.
Gas utilization equipment should be designed and installed in accordance with standard engineering practice and the manufacturer's recommendations. As a minimum, the installation will include a flare to burn off collected gas and a means of maintaining the digester within acceptable operating temperature limits.
- The flare should be equipped with automatic ignition and have a minimum capacity equal to the anticipated maximum biogas production.
- Gas-fired boilers, fuel cells, turbines, and internal combustion engines, when a component of the system, should be designed for burning biogas directly, in a mix with other fuel, or should include equipment for removing H2S and other contaminants from the biogas.
3.6. Monitoring and Maintenance
Equipment needed to properly monitor the digester and gas production should be installed as part of the system. As a minimum the following equipment is required:
- Temperature sensors and readout device to measure internal temperature of Digester
- Temperature sensors and readout device to measure inflow and outflow temperature of digester heat exchanger
- Maintenance contracts for digesters and generators
- The potential for letting an operations contract (to run the digester and generator)
- Pressure systems, as defined by the Pressure
- Systems and Transportable Gas Containers
3.7. Start up and shut down
Starting up and shutting down the digester can be one of the most risky procedures in running an AD plant. Training must be provided for all operators to ensure they understand and comply with procedures. If the heat is turned off, a typical digester will lose at least 0.50C to 10C a day if loading of feedstock ceases. Once the temperature has dropped to 280C, the gas production will reduce significantly. To start the digester up again, the contents should be mixed continuously, so there is no mat on the top, and then slowly warmed up again.
This process can be used if the operator is in any doubt about contaminated feedstock, if feeding is stopped the digester will recover. As a matter of good practice, any developer of a scheme of any size will also need to take into account what will be done when the digester reaches the end of its useful life. It is likely that any planning consent will require decommissioning to be addressed, and advice should be taken from the Environment Agency and the Health and Safety Executive on any plans for decommissioning the digester and plant.
Over the course of this report, a brief discussion on the problems linked to our rapid industrialization has been discussed, specifically pointing out the reasons for a change in the way we deal with energy and waste. The UK and indeed the world's drive towards energy and environmental sustainability has further lead to huge technological advancements in the waste to energy schemes and anaerobic digestion has greatly benefitted from this. Therefore in the course of this report we have justified the UK's interests in the rapid utilization of this technology.
However, a thorough look at the topic at and the present situation with the technology has revealed that although the UK have been trailblazers in the technology, the failure to make decisive policy decisions has hampered wide spread application. This we have seen again in the current rulings on the feed-in-tariff.
Having applauded the governments initiatives to cause its wide spread application, neglected aspects of this earlier planning stages have been discussed with the aim of developing a strategy for digester implementation plan. In providing a strategy, a discussion on the anaerobic digestion technology, its development overtime, the microbial and chemistry of the process, amenable waste fractions, its operation and control sequence, reactor configurations and all aspects of the physical technology including authority views on a quality product were discussed.
Subsequently, this report then outlined the control procedures that providers of the technology, operators within the plants and engineers in the industries might need to critically consider to ensure the successful implementation and operation of an anaerobic digestion plant. In outlining a strategy however, this report has paid particular attention to operational and control aspects as against prescribing a specific strategy. It is my understanding that a strategy particularly customized for this scenario would require a vast amount of specific details and which might eventually change the aim of the report to appear more as technical paper.
A massive roll out of digesters in the UK will definitely achieve tremendous fits in terms of the economy, environmental, on the waste front, on the energy front and in achieving food security. However, it is also important that the operations and controls of these facilities are effectively mastered, or else the British landscape will be littered with well designed well built but poorly functioning digesters. Simultaneously, policies should be structured to support the growth of the technology. It is only by making it a viable venture and lucrative enough to attract private and public participation, will the true aims of a sustainable waste to energy scheme be fully achieved.
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