Substitute for fresh lemons - I need to make 70 liters of homemade lemonade

You can very likely buy frozen lemon juice or juice concentrate, even if you can't buy fresh lemons. (This is different from frozen lemonade concentrate.)

This is what we do often when we're making large batch lemonade; while it's not quite as good as fresh lemon juice, it's usually close enough that it's not noticeable.

For example:

It's kept in the freezer section. That jar is roughly seven lemons' worth of juice; each one makes something like 2-3 liters of lemonade, depending on your strength requirements. I usually put a few sliced fresh lemons in with the pitcher after that.

Lemon juice contains somewhere between 4-6% citric acid by weight; let's call that an average of 5%. It does contain tartaric acid, but in trace amounts only, so this would not be an effective substitution. Lemon juice also contains about 2% sugar by weight. So to produce 1 liter of a rough lemon juice equivalent, you would need to combine:

• 50 g citric acid
• 20 g glucose (you'll probably have to use table sugar instead)
• 930 mL water (equivalent to 930 g)

This is a possibly effective, but honestly pretty poor substitute. Though citric acid is the major flavoring element, lemon juice contains many other vitamins/minerals, and lemons have a huge range of volatile flavor compounds that contribute to their aroma. If you squeeze fresh lemons, many of these compounds ultimately make their way into your lemonade and give it a distinctive "fresh lemon" quality. (You may be able to compensate for some of these by adding in a highly flavorful lemon extract.)

By the time we've approximated lemon juice with this fairly basic substitute, we've lost those distinctively "lemon" flavors, and may as well use a powdered, concentrated, or commercially produced lemonade instead.