How to improve efficiency of water usage..?

Using an Immersion Whirlpool Chiller, so a pump to recirculate the wort, and accelerate the thermal exchange. Consider a tap water between 24C to 28C (75 to 83F), and an environment temperature of about 26C to 30C (80 to 86F).

To chill about 20L (5 galons), I use only the tap water until I reach about 45C (113F) collecting all the hot water that exists from the chiller to reuse in cleaning.

From 45C I stop the wort recirculation, and I switch the chiller exit to a little repository of about 5L (1 gallon), so I start to recirculate (using an another pump) the same water that exits the chiller and so, I start to add ice to it. The cold water (smelted ice) near 0C (32F) is pushed inside the chiller by a pump.

I never tried to add additives (like alcohol) to reduce the ice melting point and have a water below 0C (32F). I think it isn't necessary, because we need to wait some time to settle the solids in the kettle, after turning off the wort recirculation.

I estimate about of 30L (8 gallons) of water collected (never measured) during the chilling. But, all is reused, so doesn't matter much for me if is 30 or 50L.

Cleaning: If you use PBW, one batch of PBW can be reused many many times. This may also be true of other cleaners, but I've only ever used PBW.

Sanitizing: As with PBW, one batch of StarSan can be reused many times. The key with StarSan is to make the batch with the cleanest water possible and then keep it in a sealed container.

Cooling: There are lots of ways to cut water usage during cooling. First, you can capture all the water you use to cool the beer and save it for later use as cleaning water. Second, you can build a recirculating cooler that will use far less water. In a recirculating cooler cold water is pumped through the immersion chiller, back into a bucket full of ice and then back into the immersion chiller.

As others have mentioned, you can also decrease the time it takes to chill your beer by using a whirlpool chiller. Faster chilling = less water.

First of all, don't be so hard on yourself. Think of all the efficiencies made by brewing and drinking your own stuff. The beer in the store was driven there in a big, fossil fuel burning truck, with a whole lot of water used in the process. Home brewed beer comes in either re-used glass bottles, or re-purposed soda kegs, never in cans that end up in a landfill if not recycled. So the act of homebrewing in and of itself is probably better for the environment to begin with.

Having said that, if you want to decrease your water usage even more, then you can do things like: getting a more efficient chiller setup (recirculating with a pump is a must), keeping sanitizing solutions like StarSan around longer by sealing them off from the air so you can use them the next day, or my own personal favorite, giving up the chiller entirely and doing No Chill Brewing.

You should be capturing hot water out of your chiller for cleaning so that water is dual purpose. Second I was really able to cut down on water usage with a more efficient chilling operation. For me I invested in a bigger immersion chiller (50ft with 1/2in tubing) and put a pump into operation. I recirc my wort while chilling creating a whirpool. This is nothing new but it works super well. My last session I went from 212F down to <75 in 8 minutes. Huge savings on water.
Other super efficient chill operations include plate chillers or counterflows. If you aren't moving your wort in the kettle with an IC it takes much longer than necessary and wastes water. Of course you need to invest some cash to get better water savings.