How do you clean a water tank without using chemicals?

I'm in a community that's small enough to have their own well and water tank, I think it's total 70K gallons split in two, they were built in the 70s, very dependable.
But every once in a while they need maintenance and the people who take care of that empty the tank, do the fixing and then chlorine shock the water.
We don't put any chems in the water so nobody in the community likes that taste, and even someone is very allergic to it and starts getting sick each time they do that.
So, what is an alternative to chlorine shocking?
thanks y'all

I would love to see a picture of that tank.  That is huge.

There is a product called star san. Available at amazon and any wine  or brew supply house. When diluted,  its acidic and is used to sanitize wine bottles and equipment.  If I'm not mistaken,  it does not need to be completely emptied out of the bottle (or rinsed and dried) before adding wine to the bottle.

It couldnt be used like bleach added to water,  you would have to spray it on interior walls (is there access?) Let it settle to bottom and drain what you can.

I don't know if this will work, but worth looking into.

Does this water tank have access to it where a person can get inside? I think a hydrogen peroxide spray would work great, and after it does its job, chemically there's just water leftover.

Yes there is access inside. Oxygen peroxyde is an interesting idea, it is used for contact lenses after all.
Do you know of any use at such scale, also that is aproved by some governmental agency? To err on the side of caution.
I'll take a photo of the tank, they're about 15 feet tall.

I have no idea of anyone using peroxide on that sort of scale. It was just the first thing that popped in my mind about sanitizing without using or leaving any chemical residue.

the closest I found is this:
https://www.cleanwaterstore.com/blog/eliminate-well-water-odors-four-reasons-why-hydrogen-peroxide-well-water-treatment-is-best/

Water is one of my areas of expertise.
I have a few questions;
- are the tanks capped?
- how long have they been in use?
- where is the source of water for the tanks?
- do you climb over the side to maintain them?
- Do they have external drainage valves?
- Is there much sediment in the bottom when the tanks are cleaned?
- How many people use this system?
- Has there been a large increase in the number of people using the water?
generally you may find chlorinating in the manner you speak of is useless, continual application is normally carried out.
If the tank volume/ number of people using the water is low, the time to turn over the volume of water may be great enough that the plant is self sterilising.

The tanks are closed
One has been in used for 20 years and the other since 1972
The water comes from 400 feet underground, below sea level
They climb from the top which is normally closed
I'm not sure on the external drainage valve, the usual procedure consist in shutting off well>reservoir valve, once the reservoir is empty, scrub, then spray with chlorine on the wall and finally restore well>reservoir. Done one tank at a time, one season spread.
I don't know about the amount of sediment but each time the pumps fail or a leak or a tank cleaning, there is brown deposit coming from everyone's faucet, this was determined to be oxidized Manganese
About 70 houses are on this system, only 30 are inhabited all year around.
The increase is very small, maybe 1 person a year, based on the graph, consumption has been mostly flat for the past 10 years and we are well below the groundwater max allowed extraction, off course it being water, nobody knows anything about where it comes from but the prevalent theory is that sea water gets filtered through the rock.
I believe chlorination is what the maintenance company does as an old habit.

From; webpage
I learn that
Iron (Fe) and Manganese (Mn) are metals that occur naturally in soils, rocks and minerals. In the aquifer, groundwater comes in contact with these solid materials dissolving them, releasing their constituents, including Fe and Mn, to the water. At concentrations approaching 0.3 mg/L Fe and 0.05 mg/L Mn, the water’s usefulness may become seriously impacted, e.g., there may be a metallic taste to the water and staining of plumbing fixtures may become common. At these concentrations, however, the health risk of dissolved Fe and Mn in drinking water is insignificant.
Also, from the same document;
Treatment for dissolved iron and manganese takes advantage of the natural process of oxidation, through the use of aeration, i.e., injecting air into the water prior to the tap to precipitate iron and manganese from the water.
Chlorine is also an effective oxidizer and will cause iron and manganese to precipitate, plus it provides protection from microbial contaminants.
Usually a physical filter follows the treatment so that the particles will not exit through the tap. Additional treatment methods include greensand filters and water softeners.
Going back to your notes;
Closed tanks keep birds etc from washing their food in the water.
It sounds like sediment from the water is not an issue, here in Australia we use channel water for remote towns and suspended solids can be an issue.
I am guessing that small number of homes will allow a long time for the water to stand and clean itself which will happen.

Since the chlorine is used spasmodically it may be possible to just stop using it. I guess somebody along the line assumed tha the cleaned operation may have added 'bugs' to the water and so a single dose of chlorine was applied to deal with that.
I will think more about that issue.

Pipe work can be installed into the tank across the floor, and a washout from the base can be undertaken without emptying that tank.
If air was pumped into the tanks, in much the same way as in fish tanks, any issues caused by Iron or Manganese may be solved.

But air would then precipitate inside the tank, and physical filters are ground for bacterial growth etc... basically moving the problem elsewhere and this place is expensive to fix when broken.

I've tried Oxygen peroxyde on myself without any ill effect. I don't know the chemistry with PVC pipes and the rest of the delivery system.

The talk about filters is from the link to the Oregon papers, called WEbpage.
Filters are not always a source of bad bacteria, but I see I have not been clear in mixing the paper and my own notes.
Where I say 'Going back to your notes', I mention a bottom of tank cleaning system.
It consists of pipes laid across the floor, of a material people can live with, full of holes.
When the outside valve for that system is opened, water is drawn across the floor between the pipes and this movement causes the bottom layer of water and sediment to escape outside the tank.
I hope I am clearer now.