I found the solution.
1) Go to /controllers/front/CategoryController.php
In line 232 you can read:
foreach ($this->cat_products as &$product)
if (isset($product['id_product_attribute']) && $product['id_product_attribute'] && isset($product['product_attribute_minimal_quantity']))
$product['minimal_quantity'] = $product['product_attribute_minimal_quantity'];
You must change by:
foreach($this->cat_products as &$cat_product) $cat_product['quantity_discount'] = SpecificPrice::getQuantityDiscounts((int)$cat_product['id_product'], (int)Shop::getCurrentShop(), (int)self::$cookie->id_currency, $id_country, $id_group);
2) Go to product-list.tpl
Now you can use $product.quantity_discount
With <pre>{$product.quantity_discount|print_r}</pre> you can get the array with the volume discounts and use them.