I am not a carpet cleaner myself but used to be Head gardener at a 5 star resort in Wales and the problem you described with the lawn was common for me.
I used to get builders, contract cleaners, in house cleaners thinking lawns where there for dumbing there waste I would often find patches of diluted emulsion that the builder had used water to clean there paint brushes etc. Cleaning waste often consisted water rich in all sorts of chemicals,dust piles and have even had sewer waste, so I have vast experience of this problem and I can tell you it takes alot to kill grass. I do not know what chemicals are in your no2 solution so I cannot comment on that but it sounds very much to me that the water was hot, INSTANT KILLER.
If the grass is not completly dead it will recover well this time of year. if it is completly dead you can pick up some grass seed cheap this is the best time to sow seed, it will germinate in about ten days.
Just thought I would post my comments from the other side of the fence

Martin Heames