Of all the concerns in mezcal production sustainability--cultivated agave, agaves sylvestres (wildcrafted), firewood, and vinazas, the vinazas rank second in my opinion. Let me rephrase--if we count Tequila as a mezcal--vinazas are the #1 problem, but if we are talking about Mezcal (capital M--certified product, excluding Tequila), it's fortunately less of a problem. Still, it's a problem that scales with the growth of production. And like most problems, it's a problem because people are either lazy or are not required to pay the entirety of the costs of their economic activities. (You know--like how beverage manufacturers moved off of returnable glass bottles and into single-use plastic ones that pollute every corner of the globe.)
Vinazas are the unwanted cuts of the distillation process--the puntas and the colas. In small quantities they are not much of a problem. In large amounts the stuff becomes a real disposal issue. The issue is not the substance itself, but the illegal dumping of it. This problem arises because of two features of the vinazas; 1) the pH is around 4 (acid); and, 2) the chemical oxygen demand is very high. This means that if the vinazas get dumped into an arroyo and run to a creek, the oxygen in the creek water gets consumed and life in the creek is badly hurt (in warmer climates, where water is also warm, the dissolved oxygen levels are already low). While vinazas are comprised of usable components--mostly alcohols and nutrients that can be used as fertilizers, the problem, it seems--as always--is that the cost of separating the useful components is higher than the value of the resulting products. The producers' costs get externalized onto the people and wildlife that depend on (or simply appreciate) healthy streams. Vinazas simply must be disposed of responsibly.
The history of how this disposal problem has been handled in the past is nauseating. Noted agave critic and journalist, Mike Morales, wrote a short book on this problem, Vinazas: The Corpse of the Spirit of Mexico (recently renamed and updated to: Vinazas: The Tequila Industry's Dirty Little Secret--which will set you back a mere $2.99 on Amazon, or $0 if you have Prime), that those concerned about the issue *must* read in order to get a good handle on the history and scope of the problem. I do not wish to repeat Morales' work, but do want to point out that he told me in an email that while he was researching and writing this volume, he was threatened by one of the major Tequila producers in an effort to prevent him from publishing. That's how serious of a problem this is and how badly the Tequila industry wishes to prevent its being known.
Fortunately, according to Morales, after the famous Patron shutdown in 2008, a few of the Tequila companies seem to be taking the problem seriously, installing bioreactors to clean up the vinazas before discharging the (then drinkable water) into nearby arroyos. What the state of the industry is at this point is not entirely clear, although things have certainly improved since then, with many companies now doing the right thing.
As I have tried to stress in previous posts, mezcal is already sustainable. It's rapid growth is the concern. The tiny producers Cuentacuentos deals with simply either use their vinazas to cure plastic tombas of the plastic smell they have when new (for storage of mezcal), or pour them over the used bagazo (spent agave fiber) to help compost the bagazo. That composted material then gets spread on fields where it helps keep weeds down and fertilizes the land (the alcohols evaporate, leaving nutrient behind just like any other compost).
It is the larger producers that face this vinazas issue--those brands that are either industrial or are larger artesanal producers--because it is the volume of vinazas that is the problem, not the substance itself. I know of a few projects that use both the bazago and vinazas to make building materials, or are investigating the installation of community bioreactors to treat the vinazas before dumping (the resulting clean) water. The basic idea is to separate solids and sugars from the liquid and then either use or evaporate the liquid (the alcohol is not that useful as a fuel, while water can be reused or left to evaporate). As is typical, the issue vanishes if the business model takes into account treatment and disposal of effluent as what it is--a cost of goods sold. Like buying certified lumber, there is no good reason to not pay to dispose of vinazas properly, whether that is through bioreactor or a simple solar evaporator.