Disposal of old fridge freezer

Assuming you live in Brussels, Bruxelles-Propreté will do one collection of up to 3 cubic metres annuallly at no cost. See here;

https://www.bruxelles.be/dechets

Doesn't say anything about the age or type of fridge.

Collecte sur rendez-vous par la Région
L’agence Régionale Bruxelles-Propreté enlève vos encombrants à domicile sur rendez-vous. L’enlèvement à domicile est gratuit pour un maximum de 3 m³ par ménage, une fois par an.
0800 981 81 (gratuit)
https://www.arp-gan.be/fr/tri/7-encombrants-mnagers.html

The Flemish borough you live in doubtless has a similar program to the Brussels one: "Grofvuil" is the term for "encombrants" in Dutch.

I'm sure your best bet is to check out your commune website or your ophaalkalender if you get one each year.
My Flemish commune will pick something like that up so long as the two largest dimensions are less than 1.75 x 1 metre but they only collect every 3 months and you have to give 10 days notice. I assume that there is a charge but that's not clear from my ophaalkalender.
If you take it to the dump yourself, you then pay by weight.

When you buy anything electrical, the shop is obliged to take the old electrical item back. so when the new fridge is delivered, they take away the old one.