Why is Ganesh Visrjan done?
The ritual is done to signify the birth cycle of Lord Ganesha who is Lord Shiva’s son; just as he was created from clay/Earth, his symbolic statue is also created using clay. The idol is immersed in water (generally river) so that Ganesha may return to his home after his ‘stay’ at the devotees’ home or temple where the Ganesha Chaturthi rituals are conducted. While it might seem like a good idea to skip the visarjan and keep hosting the God for fortune and prosperity, it is said that the power that suffuses the statue after 10 days of worship is all a human can bear. So it is not to be kept longer. However due to paucity of time, people can keep it for 3 days period or 1 day as well.
Since discarding or breaking the idol would be disrespectful, hence the statue is ritualistically immersed in the water so it may break down to the clay from whence it came. Some statues are prepared as such that its constituent materials actually benefit the ecosystem they enter.
However in recent years the immersion of Ganesha idols made of non-biodegradable materials and the use of poisonous substances in the paints has led to pollution of water bodies. It takes away from the positive symbolism and impact of the ritual. As we pray to Lord Ganesha, we must remember to do so responsibly. This is also in sync with PM Modi statement of Swach Bharat.