10 Things to Consider When Buying Waste To Biogas

1. Plan for success.

   During the planning stage, identify and define clear project goals. To establish these goals, site-specific farm information should be collected, including ownership and managerial goals and projections, animal information (e.g., number, types, maturity, bedding type), type(s) of manure recovery, volume of manure, manure analytical information, past and current disposal practices, and operational costs. Working towards project parameters is also crucial in addressing and meeting goals. This includes, often in iterative planning stages, identifying available feedstock, defining the type of digestion system, conversion efficacy of that feedstock, economic or financial factors and limitations, and project risks associated with developing an AD/biogas system.

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2. Recruit and secure an experienced team.

   Seek out and work with an experienced and qualified team to help initiate and successfully implement the project. At project initiation, the “core” team should include:

   • an engineer/permitting specialist who knows the farm history and local regulations and
   • an engineer or specialist with seasoned experience implementing various AD/biogas systems.
   
   

   Verify the project references, experience, and historical success of the engineers and specialists. This “core” team should help identify an applicable system and ensure the development is feasible and planned to meet the owner’s goals and expectations. As the project progresses, the team may also expand to include technology vendor(s), equipment provider(s), a project developer, investors/bankers/lender(s), and/or operators to supplement the initial “core” team. The farm owner or operating personnel should also be included in the “core” team early on if the project is not being developed by the farm itself.

3. Develop a sustainable business model.

   A successful AD/biogas system requires a sustainable business model. The project should not only be cost-effective, but it must also meet financial goals. The economic factors include well-defined project costs, expenses, revenue or income, liabilities, among many others. Personal goals for the project’s liquidity and profitability potential define the financial factors. The business model could consider involving partners, utilizing third party investments, or other traditional ‘cooperative’ models.

4. Secure suitable feedstock supply.

   During the planning and engineering phase, identify all suitable feedstocks. The digester must be supplied with a consistent quality and type of feedstock (manure and co-substrates) to maintain a productive microbial community. This will result in consistent organic destruction and biogas production and minimize operational issues. It is of high value to ensure that feedstocks are free of toxic and inorganic contaminants that will “upset" the intended microbial and mechanical processes. Sand, gravel, and other inert material should be removed to the degree possible to minimize sediment accumulation in the digester. Feedstocks from outside sources should be routinely characterized to monitor consistency. Projects that focus on co-digestion feedstocks (i.e., feedstock supplementing manure) should include contractual agreements to specify material quantity and quality, testing frequency, revenue received, and duration to ensure the right type and amounts of materials and revenue are provided. Co-digestion feedstocks should be designed for flexibility as external supplies are likely to vary over time.

5. Use the most appropriate technology.

   The AD technology needs to be carefully evaluated to match the type and amount of feedstock that is expected to be processed. There is no single AD technology that can be used for all situations or feedstock.

   Among the many key factors that need to be considered include:

   • the type of manure and co-digestion feedstocks,
   • how the manure is collected,
   • conversion efficiency goals,
   • the climate where the digester is located,
   • bedding type and mass,
   • amount of allocated maintenance, and
   • other factors.
   
   

   AD technology selection should also consider management goals and needs and future plans of the farm.

6. Analyze options for biogas and digestate use.

   During the planning stage, considerations should include market availability, capital and operating costs, and potential revenue to determine how the biogas is best monetized, which can include on-site use or off-site sales.

   Potential uses include:

   • on-site use of thermal and/or electrical energy,
   • off-site sale of thermal and/or electrical energy,
   • off-site sale of compressed natural gas or liquified natural gas (CNG/LNG) typically used for transportation fuel or other applications,
   • on-site use of renewable natural gas (RNG),
   • off-site sale of RNG, and/or
   • bio-based material generation.
   
   

   The need for on-site digestate use as fertilizer or bedding should also be determined, and the market for digestate final products should be assessed, including fertilizer, salable compost, or other value-added digestate products. Proper management of digestate, whether recovered for its nutrient value or disposed of in an environmentally correct manner, is critical to the success of the project.

7. Develop off-take agreements.

   It is critical to execute off-take agreements or legal contracts with users of the AD/biogas products and byproducts (e.g., biogas, electricity, heat, RNG, digestate, fertilizers) early in the development stage. These agreements—including power purchase agreements (PPA), biogas/RNG sale agreements, or digestate sales agreements—define the price and detailed specifications for all materials that any third party will purchase.

8. Evaluate added benefits.

   Consider the added benefits of AD, which may be difficult to quantify, but could be critical reasons for implementing an AD/biogas project. These benefits may include odor control or reduction in greenhouse gas (GHG) emissions. Digesters often are installed to reduce odor problems, particularly on farms where there is public development encroachment.

9. Conduct community outreach.

   Community outreach and education is critical to obtain buy-in and approval from the community, including, but not limited to, regulatory approval and the community and neighborhood approval where the project is located.

10. Plan for operation and maintenance.

    Good operation and maintenance practices are key for effective operation of AD/biogas systems. This includes continuous monitoring and management to ensure the biological processes and mechanical equipment are working properly. Often, AD/biogas system operating expenses (OPEX) are underestimated. Analyzing other similar operating systems that have several years of operational history can assist in predicting OPEX accurately. It is also important to consider whether current staff or a third party will be used to perform these functions.

Part of the purpose of building the mobile food and apple grinder cart was to grind up kitchen scraps, garden leftovers, and even weeds for use in a biogas digester. I've been composting these things for years, but as I've read more about greenhouse gases and realized that methane is many times worse for the atmosphere than CO2, I began to think about capturing the methane that my household creates and doing something with it. I could just throw a tarp over the compost pile and light a match to the built-up gases every now and then, but a biogas digester would more efficiently convert the organic waste to methane, collect the methane, and provide a nutrient-rich compost liquid that I can use to water the garden.

Plus, I can use the methane to blow stuff up.

Part of the goal here too is to reuse materials I had cluttering up my garage and basement. Some I'd held on to with this project in mind, some I just happened to come across. Doing so probably wasn't cost-effective, given the number of plumbing adapters I had to buy to make X work with Y, but at least I cleared out some of the clutter. I won't get into too many specifics on dimensions for that reason - use your best judgment regarding materials if you plan on building your own digester.

I bought three different containers for this project: one with a removable lid, a 30-gallon drum, and a 50-gallon drum. All three are plastic (HDPE), and all three were sourced from a neighbor who specializes in second-hand barrels.

The removable lid container will be used for the digester itself, so it needs a feed pipe, a gas outlet, a drain, and an overflow provision. Both the 30- and 50-gallon drums will get their tops removed, and the smaller will fit inside the larger to trap the biogas.

I measured how far in from the edge of the lid to drill the hole to account for the gradual decrease in diameter toward the bottom of the barrel. I also eyeballed the depth so I could cut some 2-inch PVC to length - the bottom of the feed tube doesn't need to extend all the way down, just far enough to keep the end submerged and just short enough to allow whatever you dump down the tube to spread throughout the rest of the digester. I then fit a coupler to the top of the PVC and inserted a threaded adapter through the hole and into the coupler. PVC cement to keep it all together and some clear silicone to seal up the gaps (PVC cement does not work on HDPE, so it's not an option to simply glue the fill tube to the lid). A threaded male plug is easy enough to unscrew by hand when it comes time to fill.

I found a drum funnel that happened to have the same thread diameter and pitch as the PVC. Filling is much less messy with it, highly recommended.

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Also, I built a stuffer using the cut-out round from the hole saw (ground down a bit on the edges) and an old broomstick. Whatever goes down the feed tube should be ground up finely enough not to get stuck in the tube, but this will help encourage what isn't. If the stuffer doesn't get the job done, a big long section of metal pipe will.

To prevent damage from freezing over during the winter, I want to be able to drain it, so I adde another hole toward the bottom for a garden hose-style drain. I entertained the idea of heating it throughout the winter using the heating element from an old dishwasher, but the ultimate goal here is to produce energy, not consume it.

The overflow tube goes toward the top using 3/4-inch PVC pipe and fittings. To keep the digester sealed, I added a valve (for when the fluid level hasn't yet reached the overflow tube) and a J-bend (for when it has). This will just dump out to a 5-gallon bucket for now, but I may add an overflow tank later on.

Note the upturned entrance to the overflow tube inside the digester. This ensures that only fluid will enter the tube and that the system doesn't airlock. Also note the multiple adapters it took to get from 3/4-inch PVC to 1-1/2-inch sink drain piping.

I disassembled an old water softener system I got for free and ended up with a good amount of semi-rigid plastic tubing, fittings, and a valve that I figured would be perfect for the biogas outlet. Drilled another hole for the valve and threaded the adapter into it, then ran the line to another valve that will then connect to the collector.

Note the T fitting in the middle. That's acting as a placeholder for a sulfur scrubber that I intend to build later down the line. Biogas digesters produce plenty of methane, some CO2, and enough hydrogen sulfide to make the biogas stink. The sulfur isn't all that useful for us, so it'll be worth scrubbing out of the end product. Which my neighbors should appreciate.

As for the different-sized tubing, that's just a result of using what I had on hand. The larger tubing measures 3/8 inch and fits perfectly into compression fittings.

More 3/4-inch PVC. Starting with a 3/8-inch copper pipe and fitting salvaged from the used sink that formed the basis for the mobile grinder cart project, I then adapted that to a 90-degree elbow and then a pipe that extended to the bottom of the collector. Another 90-degree elbow takes it underneath the rim of the 30-gallon drum and toward the middle of the collector, then a third elbow directs the gas upward to bubble through the water.

Before I cemented it all together, I added a couple 1-inch T-fittings to the long pipe. As it fills with biogas, the 30-gallon drum will rise, but I wanted it to rise evenly and not wobble like a buoy, so I figured the biogas inlet pipe would also make for a good guide pole if I secured the drum to a couple T-fittings that would ride the inlet pipe up and down.

To secure the drum to the T-fittings without drilling into the drum (and thus creating potential gas leaks), I cut a couple sections of plastic-coated wire to about the circumference of the drum, looped the ends, and cinched the ends to the fittings via zip-ties routed through holes drilled in the T-fittings. With the wires as tight as possible, I then lowered the entire assembly into the 55-gallon drum. Another zip-tie through a couple holes drilled toward the top of the larger drum keeps the inlet pipe from flopping around.

Before moving the whole assembly into place, I made a nice little patio for it from old bricks. I then connected the line from the digester to the collector, opened the collector outlet valve, pushed as much trapped air as possible out from the collector, and then closed that valve before opening the two valves between the digester and collector.

As mentioned before, I intend to feed the digester with kitchen scraps and some weeds processed through the mobile grinder cart. Tap water typically contains enough chlorine to negate the bugs doing the digesting, so I'm feeding it with water from my basement dehumidifier. Eventually, I'll add some cow manure to really jumpstart the reaction.

Then, once I get the digester up to speed and the whole system purged of anything but biogas, the sky's the limit for what I can do with it. I'll likely run a small compressor to feed empty propane tanks for use in my barbecue, but I'm also considering buying a secondhand generator to convert to natural gas. Or, if anybody has experience building methane fuel cells, I'd love to hear from you!

(Spring ) So I neglected to drain the digester and collector over the winter and thus both froze solid. Oops. I only lost the zip tie holding the gas outlet pipe in place on the collector, but the expand-o ice block in the digester popped the lid off and broke the water outlet pipe. The silicone seal on the compost inlet and the water outlet didn't hold up well either, so I scraped those off and replaced them with plumber's putty.

I didn't collect much biogas in the fall - not enough time and not enough warmth to really get the reaction going - so I'll build up the digester again this spring and hopefully generate a good amount of gas this summer.

(Summer ) After fixing the winter's damage, I wasn't getting a sufficient amount of gas in the collector - like, none - so I started troubleshooting. It put off plenty of stank, so the digester was creating biogas; that meant I had leaks to address. Ultimately I identified two areas of improvement.

First, sealing - I replaced every point of entry/exit on the digester with a bulkhead fitting, and I added a water trap to the inlet so I wouldn't have to unscrew a cap every time I wanted to feed the digester, thus losing some built-up gas. I still get some minor leakage, but that's only when the digester is full to the brim with slurry.

Second, I took a commenter's advice and ditched the bubbler-style gas inlet to the collector. Now the gas simply feeds into the collector via what was once the gas outlet. There's a capped T-fitting in the gas line, so when it comes time to empty the collector, I can simply uncap that. I did find another use for the bubbler-style inlet: I ran the liquid overflow outlet from the digester via garden hose to the inlet. That way any gases that happen to escape via the overflow outlet (previous design let far too much gas escape) now go no farther thanks to the water trap formed by the droop of the garden hose. I had to raise the digester on some stout timbers I had laying around to allow gravity feed into the collector.

Note also that I took another commenter's advice and spray painted the digester black to take advantage of the direct afternoon sun on that side of the garage and to heat up the slurry within to aid in the digestion process.

With all the water now circulating through the collector tank, I decided to put an overflow outlet on it - with a bulkhead fitting and spigot, of course - and run the overflow liquid via garden hose to a big-ass PVC pipe that a previous owner of the house had used for channelling rainwater from the gutters away from the garage. Gutters are no more, so I extended the pipe to a water tank just below the pipe and just above my vegetable gardens, screened to prevent mosquitos but allow rainwater to collect in it. Another bulkhead fitting and spigot installed in the water tank will allow me to connect a drip irrigation system to the tank.

So far this spring, I've ground up most of the weeds from my gardens and flowerbeds (anything too stalky neither grinds up well nor breaks down well in the digester), mixed them with harvested rainwater or the water collected in my basement dehumidifier, and run them through the digester. Without any manure to jumpstart the process, I'm seeing far more rise to the collector than before. Next step will likely be to collect some manure from a nearby farm to really get the digester running.

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