A septic tank closure occurs for multiple reasons. Firstly, if you are demolishing a house. Secondly, when installing a new septic system. Thirdly, if a city sewer is within a certain distance of your property, and you are hooking up to it after previously having a septic system. In these instances, you will have to close your septic tank, because you will no longer use it.
It is important to decommission, or close, your old septic tank because of the hazards it poses when it is abandoned. If left unused, septic tanks over time cave in or collapse. Also, the possibility of methane gas becoming hazardous in the air for breathing becomes a threat. Not only that, but the old tank becomes an explosion hazard because of the methane gas. Generally, if a septic system caves in, it becomes a hazard for people, because they can accidentally fall into it if the ground above it caves in.
Abandoned septic systems are hazardous for humans, but they are also bad for the environment and animals. They have the possibility of leaking sewage over time. Along with the ground stability issues.