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The Process within Food Waste Management, I am focusing on this study, is Anaerobic Digestion (AD). The natural process by which microbes break down organic molecules is known as anaerobic digestion (AD). The term “organic” in this context refers to products that are derived from plants or animals. There is no air in an anaerobic digesting process (or oxygen) (Meegoda et al., 2018). The abbreviation “AD” may refer to the anaerobic digestion process or the physical system where anaerobic digestion occurs, known as a digester.
The following items are commonly referred to as “Organic.” A digester can be used to break down these materials:
It doesn’t matter what type of feedstock is used in an anaerobic digestion system; the fundamental principles are the same. There may be some architectural changes between the systems, but the procedure is the same in both. Anaerobic digestion, in which microorganisms break down (eat) organic molecules without the presence of oxygen, generates biogas (or Oxygen). Gases other than methane and carbon dioxide (CO2) make up the majority of biogas (Hunter, Blanco and Borrion, 2021). Only methane remains after removing the carbon dioxide and other gases. Natural gas is made up mostly of methane, a hydrocarbon.
“Digestate” refers to the residue that remains following anaerobic digestion. It is common practise to separate digestate into a solid and a liquid form once the wet mixture has been thoroughly dried. Digestate is a nutrient-rich fertiliser that may be applied to crops.
Six Sigma is a methodology for improving strategic processes, introduction of new products, and service innovations that is based on the application of scientific and statistical approach to produce significant reductions in customer-determined deficiency rates. This methodology is described as being organised and methodical in its approach. The term “Six Sigma” refers to a technique or set of practises for managing projects, and its purpose is to remove faults and waste from processes that are already in place or that are in the process of being developed (Ahmed, Page and Olsen, 2019). Change is inherent to the Six Sigma methodology, and managing change successfully demands for continuous and open communication. Breaking old habits is necessary in order to successfully form new ones.
The purpose of Six Sigma is to reduce instances of waste and boost operational effectiveness, with the end goal of boosting quality and meet or exceeding the expectations. The Six Sigma approach is highly organised, and each participant is assigned a specific function to play in the process. Because it is a data-driven technique, Six Sigma necessitates the acquisition of precise data pertaining to the processes that are being reviewed. Placing outcomes on accounting records is an important part of Six Sigma (Patel and Desai, 2018). Waste management is necessary in food processing industry which will decrease the environmental impacts. The process management can be enhanced by the application of six sigma. It will optimize the supply process and reduce the transit waste of the collected food waste which will be converted to other functional products.
The Fundamentals of the Six Sigma Method
Six Sigma’s five stages are based on a process known as DMAIC among industry insiders. This acronym stands for defining, measuring, analysing, improving, and controlling. Processes may be improved by using DMAIC, a data-driven quality technique (Sm?tkowska and Mrugalska, 2018). The letters in the acronym symbolise each of the five phases of the process, as well as the tools needed to finish each part of the process.
The aim of this stage is to clearly articulate the business challenge, goal, prospective resources, project scope, and high-level project timeframe of the project that you are undertaking (Sm?tkowska and Mrugalska, 2018). The project charter paper is a good place to look for this information. To begin, we need to write down all of the information we presently have and get the facts straight, define goals, and build a team.
We are gauging how well our problem/goal has been defined. For the goal of establishing process baselines, this data collecting stage collects data. Measure phase baseline performance metrics will be compared to final project performance metrics to see whether any substantial improvements have been made. It is up to the team to select what and how to measure (Ponsiglione et al., 2021). A lot of time and attention is often put into determining whether or not a proposed measuring system is fit for purpose. The DMAIC approach relies heavily on the use of high-quality data.
Then, we need to find and eliminate the underlying source of the problem in this stage. Root cause analysis identifies a significant number of probable project problem sources (process inputs, X) (for example a fishbone diagram). Voting or other consensus methods are used to pick the top three or four possible root causes for further confirmation. Each root cause’s contribution to the project’s metrics, Y, is determined through the collection of relevant data. This procedure is carried out again and over again until “legitimate” root causes are found (Widodo and Soediantono, 2022). Complex analytic techniques are frequently employed in the Six Sigma process. Basic tools can still be used if they are appropriate. All or some of the “verified” root causes may be.
There are a number of things that need to be done in this stage, such as finding a solution to the problem, implementing it, and testing it. This depends on the circumstances. Fix and avoid process issues by identifying and eliminating the fundamental causes. Use tactics such as Six Thinking Hats and Random Word to brainstorm. A DOE (Design of Experiments) technique can be used for some tasks, but if obvious answers are available, focus on them (Ponsiglione et al., 2021). This stage can also be used to discover answers without really putting them into action.
Making the change ‘stick’ is a term used to describe this process, which aims to make the changes long-lasting. The DMAIC process of improvement concludes with the control step. Track progress; Change ways of working; Quantify and sign off on benefits; Close the project in a formal manner; and; make a case for releasing the resources are all part of it (Sm?tkowska and Mrugalska, 2018).
To break down complex polymers in organic material in an anaerobic digester, hydrolysis or other pre-treatments must be used, which is not possible without further hydrolysis or pre-treatment. As a result, the hydrolysis process is used to break down organic macromolecules into smaller pieces that acidogenic bacteria may use. Hydrolysis can be electrochemical, but in anaerobic digestion, it is mostly a biological process. Hydrolytic bacteria release enzymes that can break down carbohydrates, lipids, and proteins into sugars, long-chain fatty acids (LCFAs), and amino acids during hydrolysis (Li, Chen and Wu, 2019). Acidogenic bacteria can transport hydrolysis products through cell membranes after enzymatic cleavage. In addition, because of the complexity of the structures of some substrates (such lignin, cellulose, and hemicellulose), microorganisms may not be able to digest them, enzymes are frequently used to facilitate the hydrolysis of these carbohydrates.
Acidogenic bacteria can make intermediate volatile fatty acids (VFAs) and other compounds by absorbing hydrolysis products through their cell membranes. Most VFAs have a molecular weight between 75:15:10 and 40:40:20, with the exception of acetates and bigger organic acids like propionate and butyrate. This does not exclude out the presence of modest levels of ethanol and lactate, though (Chiappero et al., 2020). There have been reports that VFA concentrations can vary dramatically for digesters running at different pH levels, with seemingly conflicting results in different research. This suggests that the particular concentrations of acidogenesis intermediates may be affected by digester settings.
Acidogenesis produces acetate from the original substrate, making it a viable substrate for acetoclastic methanogenesis. However, methanogenic bacteria have yet to get access to additional, higher-concentration VFAs. To turn these higher VFAs into acetate, the process of acetogenesis takes place. Hydrogen is also created. In anaerobic digestion, hydrogen interspecies transfer is a fascinating syntrophic connection that occurs because of acetogenesis’s production of hydrogen gas (Pramanik et al., 2019). A partial pressure that is too high can harm acetogenic bacteria, even if they are hydrogen producers.
To complete anaerobic digestion, bacteria that are capable of methanogenesis ingest available intermediates and create methane as a by-product. It was shown that 99 percent of Methanococcus voltae and Methanococcus vannielli cells were destroyed in just ten hours when exposed to oxygen, showing the extreme sensitivity of methanogenic microbes to oxygen. Additionally, methanogenic bacteria can only grow on a few number of different types of substrates since they are oxygen-sensitive (Li, Chen and Wu, 2019). For the majority of methane production (about two-thirds), methanogenesis from acetate and hydrogenotrophic methanogenesis account for the remainder; nevertheless, methanogenesis from Methanol, Methylamines and Formate has also been seen.
Different configurations of anaerobic digesters may be developed and manufactured to run in batch or continuous mode, with mesophilic or thermophilic temperatures, high or low solids content, and single or multistage processes, as well as in batch or continuous mode. Despite the complexity of the design, continuous process may be more cost-effective than batch process since batch process requires more initial investment and a bigger volume of digesters to manage the same quantity of waste as a continuous process digester. Compared to mesophilic systems, thermophilic ones need more heat energy to function but take a lot less time, have a greater gas production capacity, and contain more methane. This trade-off must be carefully considered (Nguyen and Khanal, 2018). Solid content up to 15% may be handled with low. A higher solids content, commonly known as “dry digestion,” is regarded to exist above this point in the digestive process. Single-stage digestion utilises a single reactor to perform all four anaerobic digestion phases in a single step The methanogenesis and hydrolysis steps are separated in a multistage digestion process that uses two or more reactors.
Batch or continuous digestion can be used for anaerobic digestion. When using a batch reactor, all of the biomass is added at the beginning of the operation. Afterward, the reactor is shut down and sealed up for the period. When it comes to batch processing, anaerobic digestion begins with the addition of pre-processed material. In a typical case, biogas production will develop over time in a normally distributed manner. When digesting organic materials, operators can use this information to assess when they feel the process has finished, an open and emptying the batch reactor before the operation is finished might cause smell problems. Anaerobic digestion and in-vessel composting have been combined in a more sophisticated batch technique to reduce odours. Using recirculated, degasified percolate, inoculation is accomplished (Náthia-Neves et al., 2018). Anaerobic digestion of the biomass is followed by in-vessel composting in the reactor before it is opened. This type of digestion is often more cost-effective since it is simple and requires less equipment and design effort. Having more than one batch reactor in a biogas plant ensures a steady flow of biogas production.
This means that organic matter is continually added (continuous full mixed or continuous plug flow; first-in-first-out) or introduced in stages to the reactor (continual plug flow). In this process, the final products are continually or irregularly eliminated, resulting in continuous biogas generation. It is possible to employ a single digester or a series of digesters in succession (Nguyen and Khanal, 2018). For example, continuous stirred-tank reactors, anaerobic sludge blankets, expanded-granular-sludge beds, and internal circulation reactors can all be used in this manner of anaerobic digestion. There are varying degrees of intricacy to digestion systems. There is only one reactor or holding tank in a single-stage digesting system (one stage), where all the biological processes take place. A single stage might save money on building, but it gives you less control over system reactions. Bacteria that produce acids are responsible for depleting the tank’s acidity (Chen et al., 2020). Archaea that produce methane function within a narrow range of pH. A single-stage reactor allows for direct competition between the biological responses of various species. Anaerobic lagoons are also one-stage reaction systems. These lagoons are pond-like, earthen basins that are used to process and store manure. Rather, the pool’s naturally occurring anaerobic sludge serves as an anaerobic reaction container.
Two-stage digestion systems (multistage) have various digestion vessels that are optimised to regulate the bacterial populations in the digesters. Biocide-producing bacteria proliferate and multiply more rapidly than methanogenic archaea (Náthia-Neves et al., 2018). Methanogenic archaea need a constant temperature and pH to function properly.
Methane, carbon dioxide, and hydrogen sulphide make up the bulk of the biogas produced by bacteria digesting biodegradable material (the methanogenesis step of anaerobic digestion is carried out by archaea, who are on a separate branch of the evolutionary tree of life from bacteria). When producing biogas, water vapour is also present, with the volume of water vapour decreasing as the temperature of the biogas rises (Castellano-Hinojosa et al., 2018). The majority of the biogas is generated in the middle of digestion, when the bacterial population has increased, and decreases when the putrescible material is depleted. Inflatable gas bubbles or gas holder extractions are the most common methods for storing the gas from the digester.
A low-cost source of cooking and lighting energy, anaerobic digestion systems can be found in underdeveloped nations. A substantial, government-backed effort to convert tiny biogas plants for domestic cooking and lighting has taken place in both China and India. Anaerobic digestion projects in the poor countries can receive financial help from the UN Clean Development Mechanism provided they can demonstrate that their carbon emissions are decreased.
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