Did you use hot water to wash them?
Probably yes. Last time I did the math making them out of plastic saved energy vs heating water to wash glass (it was a close call, it depended on if you used a dishwasher vs sink washing).
So it might seem environmental friendly, but if you run the numbers it's not necessarily so.
Did your calculations include things like the extraction and distillation of the petroleum?
Beyond the environmental impact, there's also just the stupid, wasted effort that goes into pointlessly recreating the same thing over and over when you don't have to. In this case, the bottle goes back on the return trip from delivering new milk, which had to happen anyway. The milk ends up being the only thing delivered, on net. It's just strikingly elegant in its lack of wasted effort and material. It's not clear if it's a net win, though, given the delivery part, unless there are others on the same street signed up for the same day.
If anyone's in the Bay Area and is interested in delivery sort of like this, which also tries to set up regular delivery routes to minimize the marginal impact of a customer, I recommend checking out Farmstead (https://www.farmsteadapp.com/). They provide a similar milk delivery service by picking up Straus milk bottles and refunding the deposit. They also solved my biggest issue with most of the delivery services by picking up their reusable bags and having minimal packaging inside, but COVID put a stop to the bag pickup, at least temporarily.
Power markets are very liquid, so I tend to favour calculating your personal footprint based on the average co2 output of the grid, not the particular energy production method you've chosen. This makes it clear that even if you're paying for wind power, it's still better not to waste electricity.
The issue isn't getting bottles hot enough to kill germs. It's doing so in a way that doesn't aerosolize the bacteria and preventing all the other ways cross contaimination can occur.
And not only can they be reused, broken glass bottles can still be recycled.
And in the real world it's virtually never recycled, instead its crushed and used as landfill cover (they cover the trash each night to keep down vermin).
shudders
Not too hard to get that heat by burning fuel, but through wind power? Not sure how it'll be done.
Not dismissing it. I just really don't know.
The above is why aluminum is smelted in low energy cost areas. the above is why my company runs the foundry only for the night shift - the power company gives us a discount for using power at night when they need to keep the base load power plants running even though nobody is using power, which more than makes up for the extra pay we give to for working the night shift. (Now that wind is >50% of the electric mix in my state I wonder if the shifts are changing)
In fact most high temperature is generated from electric not fuel. If you burn something to get high temperatures the process is inefficient, turning the fuel into electricity and then the electricity into heat is more efficient (look up carnot efficiency)
In India, they do use them often for delivering milk and it's very sturdy.
If they choose to use steel or glass, and it doesn't get cleaned properly, and it ends up causing damage, then they might be liable for damages. Their liability insurance will deny coverage for damages because they knowingly used a method that had a higher probability of harm than using new plastic.
I'm sure there might be some way to ensure cleanliness of steel and glass, but it's probably cost prohibitive versus just using plastic, so it might cause the cost of the product to be too high to compete in the market.
It's probably a combination of all of these things, but the fact that no glass alternatives exist in the market in a similar price range as plastic options indicates there's an overall cost advantage for plastic.
Through there are limits for white glass this is much less the case for brown and green glass.