Septic Tanks

Post dinner reading… naa, its sanitary all the way, no gross stuff.
Description to drawing #2 below, but hard to read.

I currently live in Chad, Africa and need to build a large septic tank for a large, high usage guest house.  I’m thinking a three tank system, each tank getting progressively larger. First tank is for solids settling out, second tank is the biodegrading tank and third is the light effluents (gray water at that point).  There will not be a leach field; there is no room for a leach field. If it occasionally needs to be pumped, it will. My soil is a mix of sand and clay. No rock in this area. Labor is very available and very economical. But, I want the construction to be safe. I vaguely remember reading the “How To” about this somewhere, a long, long time ago. Quick internet research, didn’t show anything. The largest tank I’m building is a 10 foot or 3 meter inner diameter round tank, 20 foot or 6 meters deep, made with concrete block. I’ll be making the concrete blocks, 6 in w x 8 in tall x 16 in long. 15cm x 20 cm x 40 cm. 

What I remember is that one needs to build a concrete collar the diameter of the tank and approximate width and height of the blocks. The collar has to be very strong, lots of steel re-bar welded together. I’m thinking 4 circumference rings of 3/8 in or 2 cm re-bar held together with box channels rings every 12 inch or 30 cm around the circumference. It should have re-bar protruding out the top for going thru the blocks cemented on top of it. Once this has cured, 30 days, start building the block wall on top of the collar. Build up to a yard or meter high, then the tricky part starts. One starts digging the soil out from under the ring and inside the tank area. Eventually the weight of the ring and blocks will lower the assembly into the hole and one adds another row or 6 of blocks. Then dig out under and inside the tank, letting the wall slowly lower under its own weight. This helps keep the diggers safe from wall collapses and allows good masonry work for block installation. The bottom half of the tank is to have lots of gaps out the sides for the water to seep out. Either leaving out some blocks or leave out the grouting on the sides of every other block.

OK, give me your comments. Will this work? I am going about this right. Is this logical?



Description attached to drawing #1 below, but hard to read

3 meter inner diameter

6 meters deep

Top of the tank is 40 cm above ground level

Apron around tank at ground level, 40 cm wide

 x 10 cm deep.

Cap on top of tank, 15 cm thick, lots of rebar,

with a 50cm x 50cm manhole, service access.

A hinged, lockable steel door. Keep kids out.

Tank made with 15 x 20 x 40cm cement blocks

From the bottom up to 3 meters leave gaps for

water to exit.

Circumference = d x pi;  3m x 3.14 = 9.42 meters

Block is 15 cm long + 2 cm of mortar = 17cm

942cm / 17cm = 55 blocks per row

Block is 20cm high + 2cm of mortar = 22 cm

600cm deep / 22 cm = 27 rows deep

55 blocks per row x 27 rows high = 1485 + 10%

for breakage = 1650 blocks

50 kg bag of cement makes 60 blocks

1650 / 60 = 28 bags of cement for blocks

75 bags for mortar

10 bags for the lid

3 bags for the apron

+ 10% = 125 bags of cement @ 50 kg per bag

is 6250 kgs of cement, then add appropriate

quantities of sand and water at

1 part cement, 3 parts sand to a paste.

After everything else is complete add septic

gravel to bottom third of tank and throw a party.

Author: jimjudywycliffejourney

Jim and Judy have been with Wycliffe since 1984. They have served in aviation maintenance/management, motorcycle, and 4X4 training, recruitment, and facilities maintenance in multiple locations in the US and Africa. They currently work with America's Area, North Region, Scripture Access services team. They are a mobile USA team using the internet and face to face encounters with churches, ministries, Native American communities, Diaspora communities, refugee aid organizations, and individuals, introducing them to Through this website, those who did not grow up speaking English can access the Bible and other Scripture resources in the language they understand best. Would you consider becoming a part of their Wycliffe ministry partnership team? You can join their prayer or financial team by clicking on

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