SEPTIC TANK WOES

I contacted Immex waste management (Port Elizabeth branch) around the 12th of November 2018 to suction out my septic tank. They came and spent about 15 minutes sucking my tank. At the time I thought it strange that they only spent 15-20 minutes to suck out my tank. My tank holds 6000 Liters so I thought it would take longer to empty the tank. They left telling me the tank was empty. about 5 weeks later I realized to my horror that the tank was full again. This is impossible as my tank lasts at least 3-4 months before it gets full. I then contacted another company to do the job and I witnessed the whole process. They spent at least 50 minutes to an hour at my property, and I could see from the way they worked that they were emptying my tank properly, as well as the fact that it took much longer to suction the tank compared to Immex. Immex also charges more to suction out a tank, so I will not be using them again as I feel **********. I do not care what they say, we use water very sparingly and also shower with buckets to save water going into the septic tank. We are also only 2 people on the property, so I know it is impossible to get a full tank in 5 weeks.