Designing underground concrete tank

What are your ideas of a good design for a concrete underground tank? How can I filter the sediment after the first rains?

I think a concrete septic tank would be perfect. They're designed to be buried, so are engineered to withstand soil load over them. A 750 gallon tank (they're rectangular in shape, generally) costs about $500-$700. I know I would be hard pressed to buy either sacks of concrete, or have a few yards delivered, buy some remesh/rebar and lumber to construct forms to make one for that cost.

Yen - Giving your location might make responses more applicable to you. Not everyone lives in the US, and most of us probably don't really know what resources are available in all locations.

But if you can get a concrete septic tank, that might be your best bet.

We don't have anything ready made here so I will make it myself with wire and cement. Do I need to waterproof it from the inside?

As long as it is strong enough to keep from cracking, it will probably be waterproof enough. I would consider what any waterproofing coating would do to the water quality.

Glenn Herbert wrote:As long as it is strong enough to keep from cracking, it will probably be waterproof enough. I would consider what any waterproofing coating would do to the water quality.

Makes sense, thanks.

Also what do you think of the initial sediment? I was thinking I could seperate the tank into two. Like seperate 5% for initial flow and then the rest would be the overflow from there. And an easy access hatch to clean the 5% part of sediment once every couple of years. I'm in Cyprus.

I have built several masonry tanks, both above and below ground. They can and will leak unless waterproofed inside. This is the best product I have found, and it is rated for potable water- http://www.drylok.com/products/drylok-extreme-masonry-waterproofer.php I have slipformed tanks, used CMUs, and helped plaster a tank built with just wire and cement plaster, Doug

Doug Kalmer wrote:I have built several masonry tanks, both above and below ground. They can and will leak unless waterproofed inside. This is the best product I have found, and it is rated for potable water- http://www.drylok.com/products/drylok-extreme-masonry-waterproofer.php I have slipformed tanks, used CMUs, and helped plaster a tank built with just wire and cement plaster, Doug

Hello Doug

How you doing?

Drylok sounds amazing. Although we dont have it here in Cyprus. Also would you be concerned about pollution using a product like Drylok?

Yen Yus wrote:

Doug Kalmer wrote:I have built several masonry tanks, both above and below ground. They can and will leak unless waterproofed inside. This is the best product I have found, and it is rated for potable water- http://www.drylok.com/products/drylok-extreme-masonry-waterproofer.php I have slipformed tanks, used CMUs, and helped plaster a tank built with just wire and cement plaster, Doug

Hello Doug

How you doing?

Drylok sounds amazing. Although we dont have it here in Cyprus. Also would you be concerned about pollution using a product like Drylok?

No, as it is rated for potable water-meaning safe to drink from.

Hi Yen

We've just had a concrete underground tank built at our new home in the Philippines.  
See if you can read through this link, the advice given by the user SAVEH2O is really good regarding reducing sediment.
HomeOneAustraliaForums

Although I haven't updated the thread yet, I will post a few pics on here if I can work it out.
We built ours 6m x 6m x 3m deep, so it's about 100kL. I wish I could remember thickness and coatings that the builder used but unfortunately cant. It would be a local Filipino product anyway. We will be using it mostly for watering our orchard and gardens. I'll test it once I'm happy with the cleanliness of it. There's a bit of a stigma about drinking rainwater here in the Philippines, but I'm originally from the very dry West Australian wheatbelt so we grew up on tank water.

We have a pipe each coming from the house roof and the garage roof. Each has a sediment trap installed in the line, very similar to my drawings.

We have not tested our system yet, the concrete is still setting. Hopefully by next week we can start using it.

I highly recommend the book "water storage" by Art Ludwig.
It gives a broad overview of the options & methods, then focus on Ferrocement.
I have built 5 tanks of 35,000 to 65,000 liters from this book.
But all of mine have been above ground.

This is a drawing of the underwater tank I plan on building. Not sure of the design though.

There is an initial tank which acts as a sediment trap and when it fills the water then flows into second compartment. There is a hatch and drain for cleaning.

Apparently there is a cement which is non porous especially for water tanks.  

A quick note for your design,
Building a permanent wall between sections is not the best design I would say.  You would like to have as much as free area in an underground tank when you are cleaning it. I would build a simple brick wall with no cement. When the time comes It would be easy to put blocks aside and do the cleaning. Besides the voids among blocks/bricks will let water through, so it won't be load carrying. It might let some small sediments through, but it would be way easier to modify in the future.
The height of the tank is not given in the drawing, but I think it is safe to assume the wall is roughly 80% of the total height. There is no need for that; 50-60% should suffice.