Your toilet bowl fills to a predetermined water level based on the inherent design of that particular model. Flushing the toilet releases water from the fill tank into the bowl to force it to drain through a pipe in the floor to empty the toilet of the waste inside it along with water already standing in the bowl. The water level will get higher following the flush as the tank refills the bowl and the water supply line refills the tank. Several reasons explain why the tank or bowl level rises higher than usual.
Adjusted Ball Valve
The ball valve or float valve located inside the toilet's tank is responsible for turning the supply of water on and off to allow the toilet to fill to the desired level. The floating ball rises on a lever until the water level reaches a height that causes the attached lever to shut off the water supply to the toilet. Adjusting this valve causes the water level in the tank to get higher.
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Some models have an adjustment screw that allows for a higher water level, while others can be adjusted by simply bending the lever arm upward to make the float ball higher in the tank, therefore allowing more water to enter prior to shut off.
If you wish to save water each time you flush the toilet, trick the toilet into thinking it has more water in the tank than it actually does. This is done by water displacement. Some people use a brick, carefully placed in the tank away from the mechanical parts of the toilet to displace the water, making the level rise with a smaller amount of water. Other options include using a plastic bottle weighted with pebbles or sand and filled with water to accomplish the same task.
If your water is rising to a higher level in the bowl itself, it means there is something blocking the drain. A clog can be caused by excessive toilet paper, foreign objects that have been dropped in the bowl or sediment buildup in the pipes. Often a plunger will free the clog and allow the water to freely flow out and return the water to its proper level. A toilet snake may also be used to clear more serious clogs in the pipes. Failure to clear a clog will result in overflowing and a big mess in your bathroom.
Full Septic Tank
If your toilets are backing up with dirty, rising water levels you may also notice tub drains and sinks doing the same. If this is the case, then you may have a serious septic tank problem. This backup could mean that the septic tank is full and is unable to drain properly through the leach lines. Assuming there is no blockage in the main line of the home, the problem is likely in the tank itself. This will prevent wastewater from leaving the lines in the home. When this happens, the water has nowhere to go except back where it came from. Toilet levels will rise and may not subside until work has been done to fix the septic tank.