Trying to put things into context here a bit, but if you're in the UK then I'm not sure a 'conventional' brew day is particularly un-environmentally friendly. Were have an abundance of water in this country and the water we use can be utilised for secondary purposes such as washing up kit, though the amount of water used on a typical brewery is minuscule - a few hundred litres sounds alot to us but really is not alot at all. Can always collect rainwater and use that to brew if you really wanted to minimise the use of mains water. Think this is more common in the US.
Electricity...well if you believe what we're being told almost half of our electricity is generated from renewable sources, and growing, these days and as a nation we don't actually use that much and our consumption is dropping (though that wont be sustained...and ignoring the fact we are a net importer of electricity and not all of it is generated via renewables). But if you want to be as efficient as possible...make sure your vessels are well insulated and the energy consumption will go to support the 'business case' for installing solar and batteries at your home if you haven't done so already. Arrange your vessels to take advantage of gravity wherever possible to avoid the use of pumps.
And co2 emissions from fermentation...well all that grain we use requires CO2 to grow..the more CO2 we produce as brewers the less fertilisers and water farmers need to use to grow it in the first place and can go to slightly in a minuscule way offset some of the CO2 emissions in grain production...again a minuscule contribution either in a positive or negative way...smaller than the accuracy we can measure these things, but if you really want to be more efficient in the use of CO2 then you can utilise it to purge kegs ready to receive beer, if you have a greenhouse, pipe it into the greenhouse to increase PPM of CO2 I there, which your crops will appreciate.
Cooling - that heat energy your expelling on your overnight chills is wasted...you could utilise that to heat up water for other purposes around the brewery and home...heat up more strike water if you're doing a double brewery, collect the heated water in an insulated vessel and use it for cleanup, or domestically in the home..washing up, I the washing machine etc.
Spent grain - lots of option here...compost, take round to your local farmer/small holding for animal feed, make other stuff out of it...bird/dog treats, bread
As for the chemicals we use, certainly unless you're using pretty hard core commercial stuff I don't believe that the typical chemicals we use are harmful to the environment at all. Most break down into benign compounds over time, but washing up kit immediately and not letting soil dry and go hard on surfaces will cerntaly make cleaning easy and less need to use any detergents or chemicals.
Yeast - get into harvesting your own yeast to reduce demand for fresh yeast that has to be manufactured, packaged and shipped.
I suspect the biggest impact we endure as brewers is not the brew day but the cost of running fridges, kegarators, glycol chillers. Cooling is pretty energy intensive, so any savings there can help reduce impact. I've got two fridges and a chiller....not all running constantly all the time, but still probably chew through a fair amount of electricity.
