How do we do Gaṇeśa visarjana at home in an eco-friendly way?

do the uttara-pūjā and āratī, then immerse a clay idol in a bucket or tub of clean water at home and give the dissolved clay to your plants.

Visarjana is a loving farewell, not a discarding. Before it, the family does a final (uttara) pūjā and āratī, offers modaks or curd-rice for the journey, and asks forgiveness for any shortcomings. Many say “Gaṇapati Bappā moryā, pudhcyā varṣī lavkar yā” — come again soon next year.

Traditionally the idol went into a river, lake or sea. Today, with water pollution a real concern, home immersion is fully in keeping with the spirit of the rite: choose an idol of plain clay (śāḍū māṭī) with natural colours, immerse it in a clean bucket or tub with a prayer, let it dissolve over a day or two, and pour the water and clay into a garden or large pot. Some plant a seed in it.

Flowers and dūrvā can be composted. Many towns also run artificial immersion tanks, which are equally valid.

Choose clay (śāḍū) over plaster of Paris · Source: common family practice

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