There's also a huge opportunity here to look into the water and energy use from the weed industry. As a consumer, I would totally pay a bit more to know that my pot was coming from a sustainable source (recycled rain water, solar or wind powered farm, etc.)
Just to iron out unit problems: it looks like King and Snohomish, at least, are composting most of their waste. Composting by just the largest company is 3,000 to 4,000 pounds/week = 156,000 to 208,000 pounds/year, compared to 210,000 pounds produced/year.
That's it. As a eco-conscious consumer, I will boycott WA weed until the industry manages to compost its waste. ... ... Just kidding! But seriously, get it sorted, please.