A septic tank and soakaway system provides an on-site method of managing household wastewater where a public sewer connection is unavailable. The septic tank receives wastewater from the building, separates settleable solids and allows partial biological treatment. The liquid effluent then moves into a soakaway system, where the surrounding soil provides further treatment and disperses the water.
Choosing the correct capacity requires more than selecting a tank based on the number of bedrooms. The design must account for household occupancy, daily water consumption, wastewater generation, sludge storage, soil absorption and groundwater conditions.
An undersized septic tank may allow solids to escape into the soakaway, while an inadequate soakaway can become saturated and cause wastewater to return to the surface. Both components must therefore work together as one wastewater management system.
What Is the Difference Between a Septic Tank and a Soakaway?
Although people often discuss septic tanks and soakaways as one installation, they perform different functions.
A septic tank is a watertight underground chamber that receives household wastewater. Inside the tank, heavier solids settle at the bottom to form sludge, while lighter materials such as grease float to form scum. Anaerobic bacteria break down some of the organic matter, allowing the liquid effluent to leave through the outlet.
A soakaway receives the treated liquid effluent from the septic tank and allows it to disperse into suitable surrounding soil. Depending on the design, it may consist of a soakaway pit, infiltration trenches or a larger drainage field.
The septic tank provides primary treatment, while the soakaway provides further treatment through soil filtration and biological processes. A soakaway does not replace the septic tank, and untreated sewage should never discharge directly into one.
Factors That Determine Septic Tank Capacity
The septic tank must accommodate incoming wastewater while providing enough space for settling, treatment and the accumulation of sludge and scum.
Several factors influence the required capacity.
1. Number of Household Occupants
The number of people using the building directly affects the volume of wastewater entering the tank.
A house occupied by ten people will generally produce more wastewater than a similar house occupied by four people. The design should consider the expected long-term occupancy rather than only the number of people currently living there.
For residential properties, the number of bedrooms can help estimate potential occupancy, but the designer should also consider the intended use of the building.
2. Daily Water Consumption
Household water consumption affects the amount of wastewater entering the septic tank.
Water from toilets, bathrooms, kitchen sinks, showers and laundry facilities may contribute to the wastewater load, depending on the plumbing arrangement.
A building with several bathrooms, frequent laundry use and high water consumption may produce a greater wastewater volume than a household with more limited water use.
The designer should estimate daily wastewater generation using the expected occupancy and applicable local design criteria.
3. Retention Time
Retention time refers to how long wastewater remains inside the septic tank before leaving through the outlet.
Adequate retention allows heavier solids to settle and lighter materials to separate from the liquid. If wastewater passes through too quickly, suspended solids may escape into the soakaway.
The required retention period depends on the design standard, expected flow and tank configuration. The tank must also provide additional volume for accumulated sludge and scum.
4. Sludge and Scum Storage
Solids gradually accumulate inside a septic tank. The tank must have enough additional capacity to store this material between desludging operations.
If the tank has inadequate storage volume, accumulated sludge may reduce its effective liquid capacity and allow solids to escape through the outlet.
Regular inspection and desludging help maintain the system’s performance.
5. Number of Chambers
Many conventional household septic tanks use two chambers.
The first chamber receives wastewater and allows most settling and scum separation to occur. The second chamber provides additional settling before the effluent leaves the tank.
Baffles or properly designed inlet and outlet arrangements help control flow and reduce the movement of solids into the soakaway.
The number of chambers and their proportions should follow the applicable design requirements.
How to Estimate the Required Septic Tank Capacity
Septic tank sizing begins with an estimate of the daily wastewater flow.
A simplified design approach is:
Required tank capacity = wastewater retention volume + sludge and scum storage volume
The retention volume depends on the estimated daily wastewater flow and the required retention period. The sludge storage allowance depends on the expected accumulation rate and the planned desludging interval.
For example, consider a house designed for six occupants. If the estimated wastewater generation is 120 litres per person per day, the expected daily flow would be:
[6 \times 120 = 720 \text{ litres per day}]
This gives an estimated daily wastewater flow of 720 litres.
The designer must then determine the required liquid retention volume and add the appropriate sludge and scum storage allowance. The final tank capacity must also satisfy applicable minimum dimensions, construction and local regulatory requirements.
This example illustrates the calculation method. It is not a universal recommendation for a six-person household because actual water use, design standards and sludge storage requirements vary.
How to Choose the Right Soakaway Capacity
The soakaway requires a separate assessment. A septic tank may have sufficient capacity, but the entire system can still fail if the surrounding soil cannot absorb the discharged effluent.
The most important factors include soil permeability, daily effluent volume, available land area and groundwater conditions.
1. Soil Type
Different soils absorb water at different rates.
| Soil type | General drainage behaviour |
|---|---|
| Coarse sand | Usually allows relatively rapid infiltration |
| Fine sand | Often drains well, depending on its composition |
| Sandy loam | May provide suitable infiltration under favourable conditions |
| Clayey soil | Generally drains slowly and may become saturated |
| Compacted soil | Can restrict water movement |
| Rocky ground | May prevent suitable infiltration or allow inadequate treatment |
These descriptions provide general guidance, not a substitute for testing. Soil layers can vary considerably within the same property.
2. Soil Percolation Test
A percolation test measures how quickly water moves through the soil at the proposed soakaway location.
The test helps determine whether the ground can accept the expected effluent flow and informs the required infiltration area.
A typical assessment involves excavating a test hole at the proposed depth, saturating the surrounding soil and measuring the rate at which water levels fall under controlled conditions.
The test should follow an appropriate recognised procedure and should account for variations in soil conditions across the proposed drainage area.
Do not size a soakaway simply by looking at the soil surface or assuming that sandy-looking ground will drain adequately.
3. Daily Effluent Volume
The soakaway must accommodate the liquid effluent leaving the septic tank.
Its required infiltration area depends on the daily discharge volume and the soil’s tested absorption capacity.
A household with more occupants generally produces more effluent and requires a larger infiltration area. The designer should use the expected long-term occupancy rather than relying on a temporary household population.
4. Groundwater Level
The soakaway must maintain adequate separation from the highest seasonal groundwater level.
If the groundwater table rises close to the infiltration area, the soil may have insufficient capacity to receive and treat the effluent. This can cause wastewater to surface or contaminate groundwater.
The designer should assess groundwater conditions, including seasonal changes, before choosing the soakaway location and depth.
5. Available Land Area
A soakaway requires enough suitable ground to distribute wastewater without overloading the soil.
Small plots may not provide sufficient space for a conventional soakaway, particularly where buildings, boundary walls, boreholes and other underground services restrict the available area.
Where the plot cannot accommodate a safe infiltration system, an alternative wastewater treatment and disposal arrangement may be necessary.
Common Soakaway Arrangements
The type of soakaway depends on the site, soil conditions and expected wastewater flow.
Soakaway Pit
A soakaway pit is an underground excavation designed to receive septic tank effluent and allow it to infiltrate into the surrounding soil.
Depending on the design, its walls may use suitable permeable construction or another approved arrangement. The base and surrounding ground must provide adequate infiltration capacity.
A soakaway pit is not suitable for every soil condition. Clayey ground, high groundwater levels and unstable excavation conditions may make it inappropriate.
Soakaway Trenches
Soakaway trenches distribute effluent through perforated pipes laid within drainage aggregate.
The trenches spread wastewater across a wider soil area than a single discharge point. Their dimensions, spacing and length depend on the design flow and soil infiltration results.
The system should distribute effluent evenly to avoid overloading one section of the drainage area.
Infiltration Field
An infiltration field uses a network of drainage trenches or pipes to distribute effluent across a designated area.
It may be suitable where the property has enough land and the soil can support the required infiltration rate.
The layout must provide adequate separation between trenches and protect the system from vehicle loading, surface runoff and other activities that could damage or compact the soil.
Important Distances and Site Selection Considerations
The septic tank and soakaway should not be positioned without considering nearby buildings, water sources and property boundaries.
Before construction, assess:
- Distance from the house and building foundations.
- Separation from boreholes and wells.
- Distance from neighbouring properties.
- Highest seasonal groundwater level.
- Existing underground water and drainage pipes.
- Flood-prone sections of the property.
- Access for inspection and desludging vehicles.
- The location and direction of groundwater movement.
Required separation distances vary according to local regulations, soil conditions, groundwater vulnerability and the type of installation. A designer should confirm the applicable requirements rather than applying one distance to every property.
The septic tank should also remain accessible for inspection and desludging. Avoid placing permanent structures, heavy equipment or inaccessible landscaping over its access covers.
Common Septic Tank and Soakaway Design Mistakes
Poor planning can reduce the life and effectiveness of the entire wastewater system.
| Mistake | Possible consequence |
|---|---|
| Selecting a tank that is too small | Short retention time and frequent overloading |
| Ignoring future occupancy | Insufficient treatment capacity |
| Skipping soil percolation testing | Incorrect soakaway sizing |
| Building in unsuitable clayey soil | Slow infiltration and wastewater backup |
| Ignoring groundwater levels | Contamination and system failure |
| Connecting roof runoff to the septic tank | Hydraulic overloading |
| Poor tank construction | Leakage and possible groundwater pollution |
| Inadequate access covers | Difficult inspection and desludging |
| Discharging untreated sewage into a soakaway | Soil and groundwater contamination |
Roof gutters and stormwater drains should remain separate from the septic tank. Large volumes of rainwater can overwhelm the treatment system and reduce its ability to settle and treat household wastewater.
Maintenance Requirements
Proper maintenance helps prevent blockages, unpleasant odours and premature failure.
Inspect the septic tank periodically to monitor sludge and scum levels. Arrange desludging when the accumulated solids reach the recommended limits for the tank design.
Avoid flushing sanitary products, plastics, excessive grease and other materials that do not break down easily. Do not pour harmful chemicals or large quantities of disinfectants into the system because they may interfere with biological treatment.
Keep the soakaway area free from heavy vehicle traffic and construction activity. Watch for wet patches, sewage odours, unusually lush vegetation or wastewater appearing on the ground. These signs may indicate that the soakaway has become overloaded or blocked.
Never enter a septic tank to inspect or clean it. The gases inside can cause serious injury or death. Qualified personnel should handle tank cleaning and repairs.
Conclusion
Choosing the right septic tank and soakaway capacity requires a coordinated assessment of household occupancy, wastewater generation, retention time, sludge storage and soil infiltration.
The septic tank must provide sufficient treatment and storage volume, while the soakaway must have enough suitable soil area to receive and disperse the effluent safely.
For residential construction in Nigeria, do not rely solely on standard tank dimensions or informal rules based on the number of bedrooms. Conduct a proper site assessment, carry out soil percolation testing and confirm applicable local requirements before construction.
A correctly designed system protects the building environment, reduces wastewater problems and helps prevent contamination of nearby groundwater sources.