The sponges that are commonly bought in shops are synthetic. These are made from polyester foam that is very durable, elastic, light and cheap.
Why is it not recyclable?
There is no machinery to recycle synthetic sponges.
Although some sponges are said to be biodegradable, they still contain antibacterial chemicals such as triclosan, to keep the bacteria at bay. These chemicals are not ideal for composting either.
Silicon dish cleaner
Wooden brush with compostable brittles