Are Septic Tanks really that bad for the environment?

A few months ago we had to have our septic Tank replaced and whenever the crew came out they dug out around the tank and then they picked it up with their crane and then just dumped it out in the yard and used their tools and spread the sludge around the yard and then they said just wait a while and this will make your grass grow great and sure enough a month later the grass was so green and thick and nice, which makes me think if they just dumped that stuff out then how is a septic tank so horrible for the environment? These guys were professionals too, they dig out and put in tanks five days a week all year long all over this state and others, They had cranes and a ton of equipment and were done in just a couple hours, and the only damage done was the hole that they had to refill for the tank.