How much food waste can you feed a biodigester?

We tend to think in buckets, 5 gallon (20 liter) paint buckets. That is something everybody in the world can relate to.

Figure about 1 bucket per cubic meter each day. That is, if you filled a bucket with food waste.  If you grind it you are adding water as you grind, but because it grinds up and occupies less space and you then add water you are still talking about a bucket (because generally when you grind with water you end up with a bucket having about 50/50 food particles to water).

So a 10 cubic meter system generally takes up to 10 buckets of food waste a day safely (without getting indigestion). That equals about a 55 gallon oil drum worth of food ( or a 200 liter blue barrel like you find all over the world). 

This isn't going to be exact, as what you feed will have an effect.  If it is mostly fats, which produce fatty acids, you may want to add less (going acid is what we worry about).  If it is mostly sugars or carbs it may also go acid.  If it is a balanced meal type of food waste then you are okay and may even exceed the 1 bucket per cubic meter of tank space rule.

Another rule has been (although this was developed for manures) 1:40, i.e. 1/40th of the volume of the tank.  But for a 1000 liter tank that still amounts to no more than 25 liters of waste material, so the general rule still holds.

Most biogas people agree with the aerobic compost folks that the ideal is to have feedstock that has the typical C:N ration of anywhere between 20:1 and 30:1.  25:1 is pretty darn good.  That means it would probably be good if you have some nitrogen rich foods (animal and/or plant proteins) and some carbon rich foods (leafy stuff, even brown leaves, some say even charcoal; coffee grounds should work well).

Don't make the mistake of putting in straight sugar or straight white flour or starch -- we've done that and rapidly made the tank go acid.

How do I recover from an acid event , a.k.a. "digester souring"?

Add sodium hydroxide or potassium hydroxide (lye), usually as crystals is better (be careful; they don't call it "caustic soda" for nothing -- wear gloves and eye protection, and don't do it with good clothes on as it will eat through them!).

I have also had success with sodium carbonate (washing powder).  In fact, I have had success feeding my digester 10 kg of sugar with 5 kg  of sodium carbonate mixed in. Three times.  This doesn't mean it will always work, but it did for me, thrice!

I've also buffered my pH and brought it back with sodium bicarbonate (baking soda). It took more of it than the sodium carbonate, and lots lots more than the sodium hydroxide.

We don't have exact figures yet... we are still tinkering and experimenting at the home scale!

Let us know what results you get!

Be patient with the bounce back.  I have seen it take a coupld of weeks and I have seen it take a couple of days (for that matter I've seen biodigesters bounce back to a neutral pH after getting to an acidic 4 just by sitting unfed for several weeks.  But it seems that the tanks that bounce back are the onces with the most internal surface area and lots of internal carbon, like straw and leaves and gravel mixed in -- ones that just had starter manure didn't recover -- perhaps the microbes had fewer places to hide and recover...)