Disposal of sewage from a septic tank is a crucial step in the treatment process to ensure the safe and efficient removal of wastewater. There are various methods available for the disposal of sewage, but one of them is not commonly used. Let's analyze each option to understand why option C, Vacuum filter, is not a method of disposal of sewage from a septic tank.

a) Upflow anaerobic filter:
- An upflow anaerobic filter is a wastewater treatment system that uses anaerobic bacteria to break down organic matter in sewage.
- In this method, sewage flows upwards through a multi-layered filter media that promotes the growth of anaerobic bacteria.
- The bacteria in the filter media break down organic waste, producing biogas and treated effluent that can be safely discharged or reused.

b) Soil absorption system:
- A soil absorption system, also known as a drain field or leach field, is a common method of sewage disposal from septic tanks.
- In this method, the treated effluent from the septic tank is distributed through a network of perforated pipes in a specially designed area of soil.
- The soil acts as a natural filter, allowing the effluent to percolate and undergo further treatment as it passes through the soil layers.
- The treated effluent is eventually absorbed into the groundwater or evaporates, while the remaining solids are retained in the soil.

c) Vacuum filter:
- Vacuum filtration is a process used in wastewater treatment plants to separate solids from liquid by applying suction or vacuum pressure.
- However, it is not typically used as a method of disposal of sewage from a septic tank.
- Vacuum filtration is more commonly employed in industrial processes or advanced wastewater treatment systems, where the objective is to remove fine particles or sludge from the liquid stream.

d) Biological filters:
- Biological filters, also known as trickling filters, are another method used for the disposal of sewage from septic tanks.
- In this method, the effluent from the septic tank is distributed over a bed of filter media, such as rocks or plastic media.
- Microorganisms attached to the filter media break down and consume organic matter in the effluent, further treating it before it is discharged or reused.

In conclusion, the method of disposal of sewage from a septic tank that is not commonly used is the Vacuum filter (option C). While vacuum filtration is a valid wastewater treatment process, it is not typically employed for the direct disposal of sewage from septic tanks.