piece 除了表示“张”“块”之外，还可表示“碎块”“碎片”等。如：

There are some pieces of broken glass on the ground. 地上有些玻璃碎片。

You have a piece of lettuce stuck between your teeth. 你牙缝里塞了生菜末。

Her broken bicycle lay in pieces by the side of the road. 她的自行车倒在路边，已经散架了。

I watched her rip the letter to/into pieces and throw it away. 我看到她把信撕成碎片扔掉了。

The old bridge was blown to pieces [=blown apart] during the war. 那座老桥在战争中被炸毁了。

你的句中的 a piece of litter 其实就是指像废纸、烟头、塑料袋等。例句如：

I didn’t see a single piece of litter there.