Several countries, including the Philippines, have a solid waste problem. Waste is being dumped everywhere, and, even if the waste is dumped in a landfill, it wouldn’t be a surprise to discover these landfills do not comply with the environmental standards. The capacity of a population to produce waste is unlimited, but the land available is a finite resource. Several solutions, like upcycling plastic and fabric, are in place. However, upcycling is labor-intensive. One of the common and lucrative solutions is converting the waste into energy. As nations move towards a circular economy and seek to build smart green cities, a waste to energy scheme becomes even more relevant and necessary.
In order to see some of the processes involved with biogas projects, BOX visited one of the plants of Solutions Using Renewable Energy, Inc. (SURE). SURE works with biogas, solar energy, and hydroelectric energy. For this article, we will be focusing on their biogas projects.
Biogas is produced by the anaerobic digestion of any organic material such as cow waste. During anaerobic digestion, the organic solids in the feed are decomposed into carbon dioxide and methane. This biogas is then purified to isolate the methane from the other gases.
PROPOSING A WASTE TO ENERGY SYSTEM
One of the most important parts prior to installing the plant is proposing the system. For this particular project, Sunpower needed a system to convert the food waste into energy because the cost of disposing the food waste was expensive. While a food waste to energy process has a high upfront cost, clients can eventually get a return on their investment within a few years. To protect clients from a direct huge hit to their pockets, SURE offers a build-operate-transfer (BOT) scheme with no cash out for the client. This is a good model because it gives smaller firms an opportunity to become more energy-efficient.
For SURE to write a proposal, Sunpower had to gather data from their canteens. Because food waste is composed of different meals and residues, several approximations had to be made. For example, the total amount of vegetable and fruit waste was considered to be just fruit waste since it would be difficult to sift through the waste and separate the two.
If you’re a chemical engineering student, you were probably taught that you can determine the optimum inlet conditions and biogas yield of the system with the following steps. If you need to review these lessons, we recommend Chemical Reaction Engineering by Levenspiel and Principles of Chemical Reactor Analysis and Design by Uzi Mann.
- For the reactions of the following organic compounds below, get the heat capacity, activation energy, rate law, and rate constant data.
- Setup the dimensionless energy balance and extent equations.
- Determine the composition of biogas and the effect of composition of the feed, reactor volume, flow rate, and temperature by solving the ordinary differential equations (since it is a batch reactor). You can try using the Fourth Order Runge Kutta method.
However, a few things, which we’ll discuss below, will prevent you from using this solution.
Anaerobic Digestion (AD) is really complex
Methane is produced in a series of steps: hydrolysis, acidogenesis, acetogenesis, and methanation. Methane fermentation is also described in 2 stages due to the close link between the steps. The first stage is comprised of hydrolysis and acidogenesis, while the second stage is comprised of acetogenesis and methanogenesis. The grouping together of these steps has led to the design of a system with stage 1 and stage 2 in separate reactors for some systems.
During hydrolysis, with the aid of enzymes from the anaerobic bacteria, undissolved complex compounds react with water to form soluble monomers.[1,2] The length of this reaction depends on the compound being hydrolyzed. Carbohydrates hydrolyze within a few hours while the hydrolysis of proteins and lipids requires a few days.
During acidogenesis, the monomers are broken down into short-chain organic acids, alcohols, hydrogen, and carbon dioxide using anaerobic bacteria. During acidogenesis, a low hydrogen concentration is required. This is because acidogenesis produces hydrogen; therefore, in order to favor acidogenesis, hydrogen concentration must be low. (If this is confusing, read up on Le Chatelier’s Principle). The products of acidogenesis are CO2 and volatile fatty acids (VFA).
During acetogenesis, microorganisms reduce the carbon dioxide and hydrogen to acetate.[1,2] Because the reactions are endergonic (i.e., thermodynamically unfavorable) and the microorganisms also produce hydrogen, the acetogenic reactions shown in Table 1 will only proceed with low hydrogen concentration or partial pressure.
During the methanogenic phase, methane formation occurs. Methanogenesis has different yields of methane depending on the substrate (i.e., CO2 type, methyl type, and acetate type).
As you can see in Table 2, the methanogenic reactions consume hydrogen. Comparing the hydrogen-producing reactions of Table 1 and hydrogen-consuming reactions of Table 2, you can see that the acetogenic and methanogenic bacteria complement one another because acetogenic bacteria requires low hydrogen concentration while methanogenic bacteria require high hydrogen concentration.
A fast first stage increases acidity of the sludge. Because the sludge is more acidic, during the second stage, as seen in Table 2, methane formation will not be favored. As a result, carbon dioxide composition in the biogas increases.
As you’ve seen above, the reactions involved in anaerobic digestion are too complex. You will need so much data for all the compounds and reactions. You will need the reaction kinetics of each reaction, which is not always readily available. Also, several factors affect the reactions (pH, temperature, surface area of substrate, concentration of microorganisms). Per step, the amount of each substrate available in the food waste is also required. This leads us to the next obstacle in solving for biogas yield.
Limited Funds = Limited Given for the Typical Solution
Due to limited funds, only the the food waste data in kilograms (e.g., FOG from grease trap, vegetable and fruit waste, mixed food waste) was provided.
So, can this even be solved?
In order to calculate the biogas yield, you simply use biogas yield data you find from books and scientific journals. The data available is usually given in m3 biogas/kg VS. VS is volatile solids, and it is the amount of solids that sublimes at 550℃ is assumed to undergo anaerobic digestion. Unless the client is able to pay a lab to analyze the food waste data, the amount of VS must be assumed. The VS data is usually given as a percentage of total solids (TS) which is the amount of solids remaining after drying at 103-105℃. A good source of data is Biogas from Waste and Renewable Resources by Deublin and Steinhauser. Here’s an example of a calculation.
As you can see, it’s a simple calculation. But, the system is not being optimized. We are simply estimating the volume of biogas per day.
After installing the system, there is a commissioning phase. During this period, the system is loaded with the food waste for the agreed upon number of weeks. Then, the biogas production is monitored every hour during office hours. If everything turns out okay, then the system just has to be operated and maintained properly.
We don’t know the total cost of the project (from proposal to operation and maintenance), but, if you’re interested in your own biogas facility, you can contact SURE at email@example.com. For their municipal solid waste (MSW) to energy facilities, a positive return on investment (ROI) takes around 5 to 6 years. The savings from a MSW to energy facility would result from the tipping fees paid to a sanitary landfill. Under SURE’s business model, institutions can send their MSW to the facility for free.
As you can see the waste to energy processes are complex, so, unless you’re willing to shell out more cash, you will not be able to optimize the system. Nevertheless, even without optimizing the system, you can always get a suitable amount of biogas provided the system was designed properly. Thank you to the folks at SURE for having us!
 de Mes, T.Z.D., Stams, A.J.M., & Zeeman, G.. (2003). Methane production
by anaerobic digestion of wastewater and solid wastes. In Reith, J.H., Wijffels, R.H., & Barten, H. (Eds.), Biomethane and biohydrogen (pp. 58-94).
 Deublin, D. & Steinhauser, A. (2008). Biogas from waste and renewable resources: An introduction. Germany: WILEY-VCH Verlag GmbH & Co. KGaA.
 E Instruments International, LLC. (2016). Biomass to biogas – Anaerobic digestion. Retrieved from http://www.e-inst.com/biomass-to-biogas/
 U.S. Environmental Protection Agency. (2001). Method 1684: Total, fixed, and volatile solids in water, solids, and biosolids (Draft). Retrieved from https://www.epa.gov/sites/production/files/201510/documents/method_1684_draft_ 2001.pdf