Why is Ganesh Visrjan done?
The ritual is done to signify the birth cycle of Lord Ganesha; just as he was created from clay/Earth, his symbolic statue is as well. The idol is immersed in water 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.
Since discarding or breaking would be disrespectful, 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 and takes away from the positive symbolism and impact of the ritual. As we pray to Lord Ganesha, we must remember to do so responsibly.