Was there a supposed to be a picture? Without seeing anything, it's probably a fungus. Brown patches that spread very quickly (within a few days) are almost always fungal diseases, although I suppose a massive invasion of bugs could be the culprit. Ants generally aren't an issue in the north. I see you are in WI. Any idea of what kind of grass you have? What's your watering schedule like? How often and how high do you mow the grass? Ever see cottony like growths in your grass?

I don't understand peoples reluctance to use what is at their disposal. Remember the advertising slogan of that chemical company....
"BETTER THINGS, FOR BETTER LIVING...THRU CHEMISTRY"

Many, what you call, chemicals, are organic in nature...
i.e. they are made from living things, such as plants or animals and do not constitute harmful aspects of everyday living.
You have a weed....you either live with it, or kill it.
There's two methods...one, you get down on all fours and pull it out..and make sure you get all of it...roots and all, otherwise, all you have done is made the weed stronger to come back better than it did before.
Some weeds, such as dandelion have roots that are a foot long. So the need to get something down there to get it all is paramount. Digging is often too tiring and the gardener most times just gives up...preferring to wait until it shows up again and pull off the flower.
Others, knowing it wont die of its own accord, wish to end its life once and for all...by spraying something that the weed cant fight....systemically takes a poison down to its roots and letting the grass push it out.
You can look on store shelves for the type of herbicide that is organic. Or by hand spraying the individual weed with one that is chemical in nature. Its your choice.

As for the ants, given the size of the ant hill I suspect they are dining on dying organic matter in your lawn. Whether there is a fungus in there I yield to spraying a fungicide and see what happens.
yellow patches can be a sign the soil is in a bad way. Not enough moisture is gaining down deep enough to encourage strong root growth. To that, a layering of topsoil/compost over the area and re-seeding can improve the quality of the grass. Following a regimen of over layering soil every spring, every fall, then overseeding at the same times, can result in good turf grass.
To find out how much soil to buy: Measure the area you wish to work, LENGTH TIMES WIDTH....IN FEET multiplied by the depth of soil IN INCHES...multiplied by 3, divide by 1000 results in the number of cubic yards of soil to buy.

A schedule of fertilizing every spring through fall will result in strong roots and good growth and can result in filling in the bare spots that might crop up.

Now about the ants:
This is a formulae that works and is not harmful to pets if you follow directions.

You buy at your drug store some boric acid powder. It comes in a package, costs about $5.00 or so..for about 4 ounces..give or take.
Into two cups of boiling water, you add one cup of granulated sugar, stirring to dissolve.
Then you add 2 tablespoons of boric acid, stir.

You then use a container, such as one that holds margarine to which you poke holes into at the side at the bottom to let the ant enter. Poke holes north, east, west, south.
Then, at the drug store you buy some cotton balls--they come in a package or bag. You dip a cotton ball into the boric acid solution and place into the small container. Put as many of the cotton balls into how many the container allows, put the cover on to keep out bees and such, then place it on the ant hill or where you see them travel.
Do as many containers as you wish...any solution left over can be refrigerated....mark it so it isn't used for other purposes.

What this does; the sugar is the come on--the boric acid the poison that is taken back to queenie. She dies, the colony dies and you get about fixing your lawn.
Its simple....and it works. Replace the contents about every 3 days or so.

GOREN - Third post in a row of BAD ADVICE!

..afraid of how many more there's going to be!

tszyman, it sounds like, what you are explaining is exactly what I have going on in my lawn. Curious as to what the outcome was? smithmbjm thank you!

I do not know what is bring in the ant, but I do know that you have to control the ants. If you don't they will take over your yard. We could not work the flower gardens with out getting ant bits. We struggled with them for the first couple of years we had the house. They got so bad that they were coming up the foundation and getting into
the house. These were not the ones attracted by sweets, they were just ants. I called them turnpike ants, as so many would go straight between two points in our yard that they would create a shallow trench some over 40' across the yard.

We liberally applied the insect killer early in the spring to all of the areas where we had ants last year. I filled the cracks between the house and the concrete drive with insect killer. We use a generic Amdro for the fire ants, and a general lawn insect poison for the rest.

It has significantly lowered the number of ants we are seeing in out yard.