Los posos de café van al compostador.

Because posos is normally used in the plural when talking about leftover coffee grounds or sediment.

• el poso = the sediment, residue, or dregs in a more general or singular sense
• los posos = the grounds/dregs left behind

In this sentence, los posos de café is the natural way to say coffee grounds.

Not exactly.

• café molido = ground coffee before brewing
• posos de café = used coffee grounds left after making coffee

So in this sentence, posos de café refers to the waste you throw away or compost after making coffee.

De links the noun posos to what they are from: coffee.

So:

• posos de café = grounds of coffee / coffee grounds

This is a very common Spanish structure:

• taza de té = cup of tea
• botella de agua = bottle of water
• granos de arroz = grains of rice

Van is the third person plural of ir (to go).

The subject is los posos, which is plural, so the verb must also be plural:

• el poso va
• los posos van

In recycling, sorting, or household instructions, Spanish often uses ir in the sense of go in / belong in:

• El vidrio va al contenedor verde.
• El papel va al contenedor azul.

So here, van al compostador means something like go in the composter or belong in the compost bin.

Because al is the contraction of a + el.

• a el compostador → al compostador

This happens whenever a is followed by el:

• Voy al mercado.
• El papel va al reciclaje.

So van al compostador literally means go to the composter, but in context it means go into the compost bin/composter.

Spanish uses articles more often than English does.

Here, los posos de café sounds natural and complete in Spanish, especially in instructions or general statements. English often leaves the article out, but Spanish usually does not.

Compare:

• Los perros son inteligentes. = Dogs are intelligent.
• Los posos de café van al compostador. = Coffee grounds go in the composter.

So the los is normal Spanish grammar, not extra emphasis.

Compostador means composter or compost bin/container.

In Spain, compostador is a perfectly natural word, especially for the physical container where organic waste is composted. You may also hear:

• compostera in some places
• contenedor de compost in certain contexts

But for Spain-focused Spanish, compostador is a good, natural choice.

In this kind of sentence, it usually means go into or belong in.

Spanish uses ir a in sorting instructions more broadly than English uses go to literally. So:

• van al compostador = they go in the composter / they should be put in the composter

It is not saying the coffee grounds move there by themselves; it is giving a classification or disposal instruction.

Yes, but the meaning shifts slightly.

• los posos de café = coffee grounds in general
• los posos del café = the grounds from the coffee, a more specific coffee already mentioned or understood

So if you are speaking generally, de café is more natural. If you mean the grounds from a particular cup or pot of coffee, del café can work.

Yes, absolutely.

This kind of simple present statement is very common in instructions, labels, and signs:

• Las botellas van al contenedor verde.
• El cartón va al azul.
• Los restos orgánicos van al compostador.

So Los posos de café van al compostador sounds like a normal instruction telling you where to put coffee grounds.

It could, but van al compostador is shorter and very natural for instructions.

Compare:

• Los posos de café van al compostador. = Coffee grounds go in the composter.
• Los posos de café se tiran al compostador. = Coffee grounds are thrown into the composter.
• Hay que poner los posos de café en el compostador. = You have to put the coffee grounds in the composter.

The version with van is especially common in categorising where things belong. It sounds clean and direct, which is why it is popular on signs and labels.