What is biogas, what are some approaches to digester optimization and how does it affect me?
If you work in or around the wastewater industry you have most likely heard of, or been involved in some sort of investigation on solids waste digesters. Their use in the industry has surpassed just being a trend and is getting closer to becoming an ubiquitous facet of most medium-sized to large-sized facilities.
I recently attended a 2-day conference on wastewater with my sole objective being to understand the general attitude toward the operation of digesters, what facilities are doing to optimize the production of biogas in digesters and what types of results can be expected in the quest to achieve energy independence through on site power generation. Well, starting with the basics and going into the specifics, here’s a rundown of what I’ve learned.
What is biogas?
Biogas is a gas that is formed by anaerobic microorganisms. These microbes feed off carbohydrates and fats, producing methane and carbon dioxides as metabolic waste products. This gas can be harnessed as a source of combustible fuel.
Biogas is considered to be a renewable fuel and we continue to see an increased emphasis and support on the use of renewable fuels from government agencies. Since wastewater facilities have the main ingredients to create this resource, namely digestible sludge, many of our organizations are investigating and implementing energy production strategies fueled by the biogas generated in the digesters.
What are some of the benefits of anaerobic digestion and biogas?
Key benefits of anaerobic digestion and biogas include the:
- Production of renewable power through combined heat and power cogeneration
- Disposal of problematic wastes
- Diversion of waste from landfill and an overall reduction of solid waste coming out of the plant.
- Production of a low-carbon fertilizer
- Avoidance of landfill gas escape and reduction in carbon emissions.
To break down the formula, we send our solid waste to an anaerobic digester where microorganisms consume a percentage of them and then produce a methane-rich gas. That methane is then used to run a power generation unit to produce power to run our plants. Sounds simple, right?
How do we make sure it works as planned?
One of the first things you need to do is optimize the amount of methane that you are producing in your digester. The relative percentages of methane and carbon dioxide in the biogas are influenced by a number of factors including:
- The ratio of carbohydrates, proteins and fats in the feedstock
- The dilution factor in the digester (carbon dioxide can be absorbed by water)
It has become a common practice to find local sources of high-strength waste (HSW) and add them into your digester to increase the amount of biogas produced. This is also referred to as co-digestion. There are multiple choices for co-digestion feedstock including restaurant and cafeteria food wastes, food processing wastes or byproducts; fats, oils and grease (FOG) from restaurant grease traps; energy crops; crop residues; and others.
The addition of HSW into a digester can produce 50% - 100% increases in biogas production. The actual increase will vary depending on the current state of your digester feed stream and the types of feedstock added.
One concern that I’ve heard many times is the local availability of these HSW, or at least the premium FOGs we are all looking for, within an easy to obtain distance from the plant. With the popularity of co-digestion increasing, it is prudent to check with your surrounding producers or haulers of FOG to make sure their waste stream isn’t already spoken for.
Another key to success is to not give up on energy reduction and conservation just because you are now producing your own power. One of the most helpful actions in the quest for energy independence is to reduce the amount of energy you consume to run your facility. This has become a pretty common emphasis for most of us, but we can also find a source of HSW in our efforts to reduce energy usage. However, implementing and enforcing grease traps and interceptor regulations and requirements will ultimately improve efficiencies at your facility. Keeping these FOGs out of your influent stream eases the biological load coming into the headworks of the plant. Efforts can then be put into getting the FOGs that are collected delivered straight to your facility where they can be added directly to your digester. So an emphasis on grease disposal enforcement can reduce the influent load, which will reduce the power consumption of the plant, and also supply you with a nice source of premium fats, oils, and greases to load into your digester.
Additionally, if you have producers of biological waste in the vicinity of your plant, it might be beneficial to directly pipe their HSW straight to your digester, again eliminating it from your influent stream and delivering it in a more manageable way than via the sewer. The added upfront cost should pay off down the road for both parties involved.
Finally, utilizing the most efficient means to generate the amount of power you need with the methane produced will go a long way in determining the success of your installation. Sizing the correct power generation unit with the methane available and load needed is critical. Whether you use engines or turbines, producing power also generates heat, and capturing that heat and efficiently using it in the digester process and throughout your facility is all part of piecing together an energy independent operation.
I’ve learned of facilities that could produce all the power they need if they could just produce enough biogas and of the facilities that are producing so much biogas that they are converting it into a type of compressed natural gas to fuel their automotive fleet at the plant. Most importantly, I’ve learned of facilities that claim to be net energy neutral today.
As digesters and the use of their byproducts become more prominent in our industry, we are increasingly seeing them as a source for renewable energy and as a revenue stream. I believe it will be a way of life for the wastewater industry for a long time. We are still constantly learning about ways to get the most out of our digesters and their systems but we seem to be at a place now where we shouldn’t be viewing biological influent as a waste, but more as a resource.