Foreword:

· Presented below are selected statements pertaining the digester shown here. We think you will find them useful.

· The HBS discussed here measures 3mx3mx3m or 27 m3. It is larger than a standard HBS. It was specially designed for use in a farm with 80 to 100 pigs and with a gravity feed system.

· The digester is a project of a Matt Lorig from Alaska, USA for an 80-100 pig farm he is putting up in Pandan, Antique Province, Philippines.

The Digester Cleaning the pigpens

Water & manure from pig pens Removing excess water

Excess water goes to duck pond Biogas pipe to house

I have a big problem. The tank is not holding gas at high enough pressures to make a burner work. In hindsight, I know I should have done the pressure test with a bicycle pump. I incorrectly assumed that it was working because it was holding some gas and the water level in the expansion tank would go down when I opened the gas valve. But when I did try to let the pressure build on its own or by feeding the tank, the pressure soon dropped. It is holding about 1 cm of water in my manometer. I had it up slightly over 8 cm after feeding the tank but couldn't get the burner to light or even feel gas coming out of the burners.

I'm pretty sure that it is making methane. I captured some gas in a plastic bag and then inserted an empty ball pen tube and blew the gas out onto a small flame. The result was like a blowtorch.

I used soapy water to look for leaks. I found several. I used an elastomeric sealant to seal some of the obvious imperfections and painted the outside with an elastomeric paint yesterday. I can see that I need at least another coat to have any hope of making it work this way.

Do you have any other suggestions short of draining the tank and going inside? 

The tank was designed for about 80 pigs. But I only have 10 pigs right now and they will be sold next week.  I bought 2 more piglets last week mainly so that I can keep the digester working.

The total size is about 27 m3. 3x3x3. The actual working volume (to the bottom of the mixing tank) is about  20 m3.

I will get some pictures to you later today. I can also send your plans back to you if you need a reference. It has been about 2 1/2 years since I started this whole project. Things get done as I get money to do them. But I hope to be up to full production by next year.

You shouldn't worry about the leaks. They may not be as bad as you think.  First because your digester (at 20 or 27 m3) is too big & your manure feedstock (from 10 pigs) is too little.

It is unfortunate there seem to be more than the usual leaks but sealing "obvious imperfections" externally is not a good idea. As you successfully seal the leaks, the sealant material will form large bubbles & burst with high pressures.

A well-fed digester should produce sufficient & usable quantities of biogas even with some leaks. There are also other ways to minimize leaks. I can make better suggestions when I see pictures.

I finally got my burner working today! I just had to drill out the injectors. I'm pretty sure I read that somewhere either in your materials or elsewhere. I don't know why it took me so long to figure it out.

We've been making good gas for awhile. My "breakthrough" was to use your simple burner design until I got the two ring burner working.

Fire, Fire, Fire . . . Driling out the burner injectors.

Drilling the injector holes bigger allows the use of conventional burners including regular 2-burner stoves with igniters. These have screwed injectors that can be removed.

The problem with these burners however is "hole plugging." As biogas is very corrosive, the little burner holes corrode & close. Un-plug  & re-drill as needed.

Two-ring burner Burner with manometer & condensate collector

I'm wondering if you have any system or advice for preventing flashback. I get nervous that the flame might travel all the way back to the tank and then BOOM. I know it's unlikely without oxygen in the system but still makes me nervous. I have some ideas but would like to know what you think.

Flashback is a constant threat. Because of low biogas pressures, however, it is not possible to put a check valve. The likelihood of "lower than ambient pressures" in the digester that will trigger a flashback is very unlikely. But, try to always maintain pressure in the digester as a "precaution" against flashback.

Since I have so much extra capacity, I want to experiment with different feed stocks.  I've been adding our leftover food scraps.  One thing we have in abundance are leaves -- specifically from Philippine mahogany trees. Have you had any experience digesting leaves?  From what I've read, it seems like it would be ok as long as I have some hog manure to provide extra nitrogen.

Adding food scraps and connecting the toilet bowl pipe to the digester are good ideas. Food scraps specially in large quantities, however, tend to affect the pH & require pH management. When food scraps & human feces are added to the digester, some care is needed when using digester effluents as more germs & undesirable organisms will be present compared to simply using pig manure.

It is possible to use leaves & other organic wastes in other digesters, however, the HBS is not designed for use with leaves. Leaves float & the HBS is not equipped to keep leaves from floating. Leaves are also best used when shredded. The extra cost of shredding may make the process un-economical. I suggest using leaves to make compost.

We're getting enough gas to cook our rice every night with a little gas left over. I have 5 pigs now about 70 kg each and two piglets about 15 kg each. And I also have some jealous neighbors. Biogas is popular here but only a few people have made it work. The local "experts" don't seem to be much help.

If you are getting left-over biogas now with only 5 pigs and 2 piglets on an oversized digester of 27 m3, you will have LOTS when your farm becomes fully operational with 80 pigs.

The options, however, to store biogas are not simple. They can be expensive & quite complicated. This is the reason not many are mentioned except for well-financed & large applications.

A few hours of biogas produced can be stored for use in small engines to run pumps, generators or other machines. We can discuss this further if you wish.

In the meantime, may I suggest sharing excess biogas with neighbors, using it for enterprising activities like a cottage industry & simply burn or flare it to keep biogas from escaping into the atmosphere as it is a potent/dangerous greenhouse gas.

Thank you for permission to post your pictures in my website. They will help many others. Till when are you in the Philippines. If you are passing Manila on way back to or from (Alaska?), let me know -- if I am free I will try meet you near NAIA.

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Posted: 18 July 2010