Discover: How we manage our Water!

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Across the bush, three-chamber septic tanks are widely used, often with overflow directed into a simple soak bed, for example, a perforated pipe inside stacked used tyres wrapped in cloth, then covered with a grid and soil. Near urban areas or public facilities, regular pumping is another standard practice. Both are effective solutions suited to their contexts. At our center, given our space, water balance goals, and available plant materials, we opted for a constructed reed bed (planted gravel filter bed). It naturally treats and absorbs the effluent while producing useful plants, with no pumping, no chemicals, and no waste leaving the site.”

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Children learn by seeing, not just hearing. Parents and local leaders see that bio-treatment works, even in harsh conditions.

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Rain Water Harvesting

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How to Size a Three‑Chamber Septic Tank and Gravel Filter Bed

Simple Guide for the Kalahari, Botswana

Important Notice – Please Read First

• This guide is provided for educational and informational purposes only. It reflects our real-world experience building a bio-water system in D’Kar, Botswana, but it is not a substitute for professional engineering advice or official building regulations.
• Before constructing any septic system, wastewater treatment facility, or rainwater harvesting structure, you must check with your local building authorities, council, or Ministry of Land Management, Water and Sanitation Services. Regulations vary by Country, district, and your site may have specific soil, groundwater, or setback requirements that this guide does not address.
• The formulas and dimensions shown here worked for our specific conditions in the Kalahari. Your situation may differ. Always consult a qualified technician or engineer before breaking ground.

This section shows you how to calculate the size of a three‑chamber septic tank, and a gravel filter bed (reed/wicking bed) that follows it, using simple formulas adapted for the Kalahari climate (hot, dry, high evaporation, low rain).

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Part 1 – Sizing the Three‑Chamber Septic Tank

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Step 2: How much water per person?

Reed bed sizing depends heavily on how much wastewater you send into the system, which varies widely by household habits, plumbing fixtures, and location. In many parts of the world, average household water use is around 100–150 liters per person per day, while in the United States indoor use averages about 227 liters per person per day, with toilets alone accounting for roughly 24–30% of that total.

In rural Botswana, per-person use can be much lower, studies in Ngamiland show around 20 liters per person per day, reflecting limited water access and low-flush or non-flush sanitation. For our Kalahari project, we are sending only toilet wastewater into the reed bed, while greywater from the kitchen and bathrooms already feeds separate plant beds that support on-site irrigation across the premises. This means our system handles blackwater only, and we design for 100 liters per person per day as a conservative, low-use figure appropriate for the Kalahari.

For other households, the actual load can vary significantly depending on toilet type and age: older toilets can use 19–26 liters per flush, standard modern toilets use around 6 liters per flush, and high-efficiency models use 4.5–4.8 liters per flush, so daily toilet water use per person can range from about 18 to over 100 liters depending on flush frequency and fixture efficiency.

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This ensures a minimum base capacity of 2,000 L (common in many standards for small dwellings) plus extra volume to handle your household’s daily flow comfortably. Example for our Kalahari household (4 people, 100 L/person/day):

This size is appropriate for a small household sending only toilet wastewater to the septic tank, with greywater from kitchens and bathrooms going to separate plant beds and / or for irrigation.

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Step 4: Divide the tank into 3 chambers

A 3-chamber septic tank gives wastewater several stages to settle and clarify, which produces much cleaner effluent for your reed bed. Inside the tank:

• Chamber 1 (primary settling): All wastewater enters here first. Heavy solids sink to form sludge, light fats and oils float as scum, and the middle layer (effluent) begins to clear.
• Chamber 2 (secondary settling): The effluent flows here for further settling, allowing finer particles to drop out and more anaerobic digestion to occur.
• Chamber 3 (final clarification): A final polishing stage where any remaining fine particles settle before the effluent leaves the tank toward the reed bed.

For a 3,000 L tank, a widely used best-practice split is 50% / 25% / 25%:

Chamber	Purpose	% of volume	Volume
Chamber 1	Primary settling	50%	1,500 L
Chamber 2	Secondary settling	25%	750 L
Chamber 3	Final clarification	25%	750 L

For a tank with a constant width and depth, these volumes can be easily achieved by using a length ratio of 2 : 1 : 1 for the three chambers. This 50/25/25 split is the most common guidance for 3-chamber septic tanks; some designs use other ratios (for example, 50/30/20 or a 2/3–1/3 two-chamber split), but they all follow the same principle: the first chamber is the largest, and the later chambers are smaller to provide progressive settling and clearer effluent.

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Part 2 – Sizing the Gravel Filter Bed (Reed/Wicking Bed)

The gravel bed treats the water from the septic tank and uses plants (e.g., Napier grass) to absorb it.

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Step 3: Calculate the bed area

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Step 4: Choose bed depth: 0.6–0.8 meters

For our Kalahari demonstration with and Napier grass we used 0.7 m to 0.8 approximate depth.

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Quick Summary for a 4‑Person Household in the Kalahari

Item	Size / Value
People	4
Water use per person	100 L/day
Daily wastewater flow	400 L/day
Septic tank volume	2,500–3,000 L
– Chamber 1	~1,500 L (50%)
– Chamber 2	~750 L (25%)
– Chamber 3	~750 L (25%)
Gravel bed area	~6 m² (e.g., 2 m × 3 m)
Gravel bed depth	0.7 m – 0.8 m
Gravel/sand media volume	~4.2 m³ – 4.8 m³

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Why This Works Well in the Kalahari

• High evaporation and strong plant uptake help remove water.
• With healthy Napier grass, the bed can absorb all treated water with no discharge.
• The system is simple, low‑cost, and easy to maintain.

System Maintenance:

The same steps can be used for schools, community centers, or lodges alike.

Come visit us and see the system at work!

Explore Our Infographics

Below you’ll find simple, easy-to-understand infographics that explain:

• How the three-chamber septic system works
• How wastewater becomes plant food
• How rainwater is collected and stored

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Download the Infographics:

Download all infographics as a ZIP file in English
Download all infographics as a ZIP file in Setswana
Download all Sketches as a ZIP
Did You Know: Rain Water Harvesting_PreK_ in English
Did You Know: Rain Water Harvesting_PreK_ in Setswana
Did You Know: Rain Water Harvesting_ in English
Did You Know: Rain Water Harvesting_ in Setswana
Did You Know: Reed Bed Filtration_ in English
Did You Know: Reed Bed Filtration_ in Setswana
Did You know: Water Cycle Overview_PreK_ in English
Did You Know: Water Cycle Overview_PreK_ in Setswana
Did you Know: 3 Chamber Septic Tank_ in English
Did you Know: 3 Chamber Septic Tank_ in Setswana
Did you know: System Maintenance_ in English
Did you know: System Maintenance_ in Setswana

Quick facts:

• Location: D’Kar, Botswana
• Partner organizations: Rheingauer Jugend für Afrika e.V. (Germany), Mosaico (local partner), Italian partner organization
• Buildings served: 5 (school, kitchen/dining, staff house, manager house, volunteer house)
• People directly involved: 20 staff members, 110 school children per year
• Indirect beneficiaries through knowledge sharing: 500 + D’Kar and Beyond
• Community reach: D’Kar and surrounding areas
• Supported SDGs: 4 (Quality Education), 6 (Clean Water & Sanitation), 7 (Clean Energy), 11 (Sustainable Communities), 12 (Responsible Consumption), 15 (Life on Land)

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Gallery:

“Thank you from Botswana!” – The local team in D’Kar