Wet weather problems
Septic Tank Problems When it Rains
If your septic tank becomes full of water when it rains, this can lead to problems such as flooding or unpleasant odours
This guide explains the common reasons septic tanks overflow during extremely heavy rain events and some preventative measures that could possibly reduce the risk of it happening again.
What causes septic tank problems when it rains?
Comparing the above image to the one on the "Is my septic tank full" page, it shows how due to the high levels of rainfall experienced in Far North Queensland, heavy rainfall can totally saturate the ground surrounding a septic system, so increasing the height of the underground water table which restricts outflow from the tank and which also reduces or prevents the drain field/trenches capacity to absorb wastewater after it leaves the tank. As shown in the image above, the soil becoming totally waterlogged can lead to wastewater backing up through the system and, in some cases, surfacing above ground.
In addition, rainwater can carry grit, silt, and soil particles into the drainage network. Over time, these materials can settle inside the pipes, restricting flow and creating blockages that place additional strain on the septic tank. This combination of hydraulic overload and physical obstruction increases the risk of overflows, foul odours, and damage to the wider system. Unfortunately there are certain types of soil, such as those in low lying marsh or swamp type areas that tend to exacerbate this problem as they have particularly poor drainage capabilities.
What are the signs your septic system needs emptying after heavy rain?
Slow-draining toilets and sinks
If toilets are taking noticeably longer to flush, this can be an early indication that your septic tank is overfull or that pipework within the system is obstructed. As water levels rise, flow through the network slows, creating delays and reduced capacity.
Unusual noises
Gurgling or bubbling sounds coming from drains and pipework suggest trapped air caused by restricted wastewater flow. These noises often develop before visible issues occur and may indicate that the tank is reaching capacity or that blockages are beginning to form.
Sewage backing up
One of the signs of septic tank overload is untreated sewage re-entering toilets, sinks, or drains, fortunately this is not usually a common occurrence, unless the initial installation was poorly designed, and there would be early warning signs such as those above, that a problem was about to occur. This is due to the tank having no further capacity to hold or process wastewater, forcing it back up through the system, or due to a clogged drain field forcing waste back through the system and into your home drains.
Saturated drain field/trenches
A consistently wet or waterlogged area around your septic tank indicates that the drain field is no longer able to absorb wastewater properly. If you notice standing water or saturated soil around your drain field, these are signs that the soil has reached its limit, often
due to prolonged rain or excessive loading.
Septic tank alarm
Some septic treatment systems such as “Bio-Cycle”, “Aqua-Nova” and “Taylex” include alarms designed to alert operators when the levels in the tank have risen above the safe operating level. These warnings should never be ignored, as they indicate that action is needed to prevent overflow and normally indicate the failure of a discharge pump.
How to mitigate issues with your septic system during the wet season
Minimise household water usage: Unfortunately there is very little that can be done to combat flooding caused by high rainfall events which cause a rise in the underground water table, however, excessive water use stresses the already overloaded septic tank and drain field. Directing flow from activities producing “Grey Water” such as laundry or equipment washing onto garden areas helps prevent the system from being placed under unnecessary strain.
Divert storm water away from the system
Diverting storm water away from the top of the septic tank and drain field can help mitigate flooding of the drain field or "trenches" as they are quite often referred to. Sealing up the manhole and both inspection openings on a concrete tank rather than diverting the water is not an ideal solution for a number of reasons, the most obvious is that it creates difficulty accessing the inside of the tank for service reasons such as clearing blockages or having to empty the tank. (A bit like welding the sump plug to the sump in your car after an oil change). The second reason is that the lid of the tank is not always sealed to the top lip of the tank itself so ground/stormwater is going to find its way in anyway.
Dangers surrounding emptying a flooded septic tank
The main concern regarding emptying a full or flooded septic tank when the surrounding ground is waterlogged is the possibility that the tank, once empty, will float out of the ground.
When warning customers of this possibility I've often been met with the reply "It should be OK, it's a concrete tank". Unfortunately this will not prevent it "floating" in the right conditions as there are apparently still a large number of Ferro-Cement yachts, (which was a popular build option in the 1970's), floating around on the oceans and concrete swimming pools have also been know to float up out of the ground.
Should the tank float out of the ground it will have major ramifications, not only financially, but will also effectively shut down the whole septic system until it can be satisfactorily reinstalled. Unfortunately this does not merely involve refilling the tank with water hoping it will settle back into its original position as once it has risen off its base the surrounding soil washes in beneath it preventing it from doing so. What then needs to be done is the tank must be removed completely from the ground which will involve digging around the complete circumference of the tank so that it can be lifted out, the base which it will stand on be re-prepared and then the tank reinstalled at the correct height in relation to the fall or slope of the sewer pipes from the dwelling, and then any pipework which has been damaged when the tank floated will also need to be repaired.
Unfortunately this process can only be carried out once the water table has subsided, so, depending on the duration of the wet season, there's a possibility that the system may be inoperable for weeks or more before a plumber is able to carry out this work.
When the rain stops
Once the rain eventually stops it is still not advisable to empty the tank immediately due to the existing residual high water table as it is quite possible to experience the problem detailed above. A secondary but also very important concern is being able to get the septic truck close enough to the tank without it becoming bogged. The truck weighs 8.5 tonnes empty and at a minimum would weight 10.5 tonnes once loaded with the contents of just one septic tank so even if it's possible to drive your Land Cruiser on the grassed areas around your house To avoid damage to lawns or the truck getting bogged, it needs to stay on a firm hard surface, within a maximum of 30 metres from the tank, until the ground has dried out completely.
The images below show an incident which occurred in Millstream Ravenshoe during a particularly wet year a week or so after the rains had stopped. As can be seen by the shadows it was a bright sunny day and everything appeared to have dried out but unfortunately the water table was still at an elevated level.
If your septic tank becomes full of water when it rains, this can lead to problems such as flooding or unpleasant odours
This guide explains the common reasons septic tanks overflow during extremely heavy rain events and some preventative measures that could possibly reduce the risk of it happening again.
What causes septic tank problems when it rains?
Comparing the above image to the one on the "Is my septic tank full" page, it shows how due to the high levels of rainfall experienced in Far North Queensland, heavy rainfall can totally saturate the ground surrounding a septic system, so increasing the height of the underground water table which restricts outflow from the tank and which also reduces or prevents the drain field/trenches capacity to absorb wastewater after it leaves the tank. As shown in the image above, the soil becoming totally waterlogged can lead to wastewater backing up through the system and, in some cases, surfacing above ground.
In addition, rainwater can carry grit, silt, and soil particles into the drainage network. Over time, these materials can settle inside the pipes, restricting flow and creating blockages that place additional strain on the septic tank. This combination of hydraulic overload and physical obstruction increases the risk of overflows, foul odours, and damage to the wider system. Unfortunately there are certain types of soil, such as those in low lying marsh or swamp type areas that tend to exacerbate this problem as they have particularly poor drainage capabilities.
What are the signs your septic system needs emptying after heavy rain?
Slow-draining toilets and sinks
If toilets are taking noticeably longer to flush, this can be an early indication that your septic tank is overfull or that pipework within the system is obstructed. As water levels rise, flow through the network slows, creating delays and reduced capacity.
Unusual noises
Gurgling or bubbling sounds coming from drains and pipework suggest trapped air caused by restricted wastewater flow. These noises often develop before visible issues occur and may indicate that the tank is reaching capacity or that blockages are beginning to form.
Sewage backing up
One of the signs of septic tank overload is untreated sewage re-entering toilets, sinks, or drains, fortunately this is not usually a common occurrence, unless the initial installation was poorly designed, and there would be early warning signs such as those above, that a problem was about to occur. This is due to the tank having no further capacity to hold or process wastewater, forcing it back up through the system, or due to a clogged drain field forcing waste back through the system and into your home drains.
Saturated drain field/trenches
A consistently wet or waterlogged area around your septic tank indicates that the drain field is no longer able to absorb wastewater properly. If you notice standing water or saturated soil around your drain field, these are signs that the soil has reached its limit, often
due to prolonged rain or excessive loading.
Septic tank alarm
Some septic treatment systems such as “Bio-Cycle”, “Aqua-Nova” and “Taylex” include alarms designed to alert operators when the levels in the tank have risen above the safe operating level. These warnings should never be ignored, as they indicate that action is needed to prevent overflow and normally indicate the failure of a discharge pump.
How to mitigate issues with your septic system during the wet season
Minimise household water usage: Unfortunately there is very little that can be done to combat flooding caused by high rainfall events which cause a rise in the underground water table, however, excessive water use stresses the already overloaded septic tank and drain field. Directing flow from activities producing “Grey Water” such as laundry or equipment washing onto garden areas helps prevent the system from being placed under unnecessary strain.
Divert storm water away from the system
Diverting storm water away from the top of the septic tank and drain field can help mitigate flooding of the drain field or "trenches" as they are quite often referred to. Sealing up the manhole and both inspection openings on a concrete tank rather than diverting the water is not an ideal solution for a number of reasons, the most obvious is that it creates difficulty accessing the inside of the tank for service reasons such as clearing blockages or having to empty the tank. (A bit like welding the sump plug to the sump in your car after an oil change). The second reason is that the lid of the tank is not always sealed to the top lip of the tank itself so ground/stormwater is going to find its way in anyway.
Dangers surrounding emptying a flooded septic tank
The main concern regarding emptying a full or flooded septic tank when the surrounding ground is waterlogged is the possibility that the tank, once empty, will float out of the ground.
When warning customers of this possibility I've often been met with the reply "It should be OK, it's a concrete tank". Unfortunately this will not prevent it "floating" in the right conditions as there are apparently still a large number of Ferro-Cement yachts, (which was a popular build option in the 1970's), floating around on the oceans and concrete swimming pools have also been know to float up out of the ground.
Should the tank float out of the ground it will have major ramifications, not only financially, but will also effectively shut down the whole septic system until it can be satisfactorily reinstalled. Unfortunately this does not merely involve refilling the tank with water hoping it will settle back into its original position as once it has risen off its base the surrounding soil washes in beneath it preventing it from doing so. What then needs to be done is the tank must be removed completely from the ground which will involve digging around the complete circumference of the tank so that it can be lifted out, the base which it will stand on be re-prepared and then the tank reinstalled at the correct height in relation to the fall or slope of the sewer pipes from the dwelling, and then any pipework which has been damaged when the tank floated will also need to be repaired.
Unfortunately this process can only be carried out once the water table has subsided, so, depending on the duration of the wet season, there's a possibility that the system may be inoperable for weeks or more before a plumber is able to carry out this work.
When the rain stops
Once the rain eventually stops it is still not advisable to empty the tank immediately due to the existing residual high water table as it is quite possible to experience the problem detailed above. A secondary but also very important concern is being able to get the septic truck close enough to the tank without it becoming bogged. The truck weighs 8.5 tonnes empty and at a minimum would weight 10.5 tonnes once loaded with the contents of just one septic tank so even if it's possible to drive your Land Cruiser on the grassed areas around your house To avoid damage to lawns or the truck getting bogged, it needs to stay on a firm hard surface, within a maximum of 30 metres from the tank, until the ground has dried out completely.
The images below show an incident which occurred in Millstream Ravenshoe during a particularly wet year a week or so after the rains had stopped. As can be seen by the shadows it was a bright sunny day and everything appeared to have dried out but unfortunately the water table was still at an elevated level.
