September 26, 2023

Basic methods for Jaque sequential batch anaerobic digester

By Douglas Renk

This unit is designed to hold shredded organic wastes. We used leaves, grass clippings and about 10-15 percent pig manure from the nearby farm.


Starting up a batch

Use the bags we made from fish netting to line the tanks. The bags are intended for easy containment and hoisting of the spent compost at the end of the 4 week cycle.. There were 4 bags stitched together when I was there. Some really nice netting vanished from the store room at fe y Alegria when I was there- sorry I don’t have more there. (looks like it would make some nice hammocks) I could maybe send Ana Maria with some more fabric from the US if the bags are beyond repair. The bags are not absolutely necessary- they were made (we intended) to assist in handling the material.

Julio and I built an A frame structure over the tanks and I sent some pulleys from Panama City to use to hoist the heavy bags out. I don’t know if the pulleys were ever installed to the heavy wood frame made from large post beams.

We loaded the material- leaves, grass, manure and sometimes sawdust or wood chips from the furniture shop in layers. Put a few cups of wood ash in the layers. You may be surprised how much material it takes to fill the 100 gallon tank. Tie off the top and fill the tank about ¾ full of water- no more as the water can expand when ”carbonated: in the first (acetogenid) stage. We discovered this to be a problem when we filled too full with water and the liquid was coming through the top gas valve when we opened it- (not good). Surprise!

The first batch is the most difficult because there is an acidic first stage that can destroy a bacteria culture if not corrected in time. IT will actually ferment or “pickle” If you have a pH meter it would be very useful. I can send some pH strips with Ana Maria. We left some pH test kits with the system last year with indicator drops for solutions.

With subsequent batches, you may recycle the leachate from the first stable batch to both inoculate the material with the culture you have established and to neutralize the pH. I will see if I can find a bilge type pump to send for this recycling. The rotary pump we used last year was very hard to turn. See pictures.

The best local source of material to buffer these acids (caused from what is known as the acetogenic bacteria) is wood ashes. We went to various folks who cook with fire and asked to keep them dry for us. Test the ph of the ashes ( a little in solution)- I think it is very high-depending on concentration.

Last year we added about two pounds of wood ashes at start-up. We allowed the grass to dry out before loading into the tank. It packs better and water in the material does not produce gas. There might be some baking soda left over from last year also- but he ashes work fine and are “free”. Check the pH once or twice a day for this first and most critical batch. If more ash is needed we found the only easy way to introduce the ash to the digester was to take the lid off and pour ir on top- followed by water recirculation from bottom to top.

To do a pH test all you need to do is draw a little water from a ball valve. Often we would find different pH values at he top and the bottom. We would try to recirculate the liquid from the bottom to the top to get the leachate to dissolve the ash.


Making a slurry of inoculum

Julio and I collected some organic samples from areas likely to contain microbes decomposing in the absence of air. We went to a ditch at the end of a muddy swale along he road near the elementary school. There we extracted a few shovelfuls of mud from below the water level. We dug to the bottom of the pig manure pile and also the bottom of the compost heap at fe y Alegria, taking a couple of scoops from each area and mixing them together in a five gallon bucket and covering with water. I think this was helpful for the quick results we obtained, but some sources I’ve read say that this inoculation step is not critical, that anaerobic bacterium is ubiquitous in organic wastes.

This process is only needed for the very initial batch due to the ability to introduce liquid from one tank to another by way of the upper and lower ball valves with hose connections. See pictures.


Sealing the lids

This proved to be the most difficult part of the project. If the tanks are not leak-tight, then no gas will be collected and oxygen will get into the tank.

We used a borrowed bicycle pump and a short piece of the clear gas line tubing as a coupler over the tire pump end and into the top gas valve to pump air under pressure to test for air escaping. After pumping, we would lightly pour water from the hose over the lid and look for tiny air bubbles.. This method worked. Here in the US we use a pressure gauge plumbed into one of the PVC ball valves and watch the gauge needle to see if it remains absolutely constant at 5-10 psi. I don’t think this system can handle pressure much any higher than that. When we used the tire pump, I think we went a little beyond 10-15 PSI-

The tanks were built to be water cisterns, not pressure tanks, but they still worked quite well with some creative sealing. The lids are about a little larger than 20 inches. We used strips of rubber from 20 inch bicycle inner tubes to use as large o-ring sealing gaskets. I sent several down from PC to Julio on my last shipment before I left for the US. I also sent some Perma-Tex and axle grease. I really hope this is all there still.

The pieces of inner tubes were cut about 2 inches wide and length-wise to get. I think, two o-rings out of each inner tube. There was a store in Jaque that did have some more inner tubes when we were there. The o-rings stretched nicely around the top of the tank. We pushed some plastic shopping bags under the edge of the rubber seal on the tank side to puff up the rubber to make a nice thick rubber cushion. I hope there is at least one of these still set up like that for you to see how we rigged it. The permatex was sent to make a good seal around the rubber gaskets at all of the valves. Use it thick there if you need it. You will see three valves on each tank: two plastic PVC ball valves to drain, fill and recirculate leachate liquid plus one metal ball valve for the gas flow. See picture.

The axle grease is used to seal the treads of the lid. Simply apply a thick layer of grease to the threads.


Heating the reactors

Our crude local technology method to heat was to use composting grass as a blanket around the tanks. It seemed to work well. This can be experimented with and refined if necessary. This material should be maintained to the height of the second valve. See picture. Ideal temperature is about 35-38C, the same as our body temperature. The grass keeps the tanks warm at night. See photos.


Gas Collection

We used a series of inner tubes. Ideally we wanted four tubes for each tank, but at $20.00 per tube, we were short a few. I sent more tubes to Jaque again on the last shipment. I think there would be a total of 8 large inner tubes. When we set up the tanks, we linked all of the gas tubes together with the small clear tubing.

As new tank batches were started, we kept one particular tube for a newly started batch separate until we knew for certain that the gas had produced the proper percentage of methane to hold a lit flame. To test for methane content, we simply tested by seeing if the gas would light. Once the gas in the tube would burn, then that tube can be linked in with the othe

r full tubes.


Data that is useful for students to record

Total weight of solids before and after

Amount of wood ash used to neutralize solution.

Cubic feet or liters of gas produced. May need to estimate volume of the tires.

Note the day after start-up on which the gas began to maintain a flame. It should hold a flame after three to four days, but don’t be surprised if it takes almost a week for the first batch. Gas that doesn’t burn may be high in CO2. Check pH, it should be neutral.

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