The term by-product is defined by Waste Framework Directive as a substance or object, resulting from a production process, the primary aim of which is not the production of that item, may be regarded as not being waste referred to in point (1) of Article 3 but as being a by-product only if the following conditions are met:
(a) further use of the substance or object is certain;
(b) the substance or object can be used directly without any further processing other than normal industrial practice;
(c) the substance or object is produced as an integral part of a production process; and
(d) further use is lawful, i.e. the substance or object fulfils all relevant product, environmental and health protection requirements for the specific use and will not lead to overall adverse environmental or human health impacts.

According to a Croatian Waste Act if a company produces by-product (all criteria are met) this has to be approved by the Ministry of Environmental Protection and put in a national Register of by-products. Otherwise, the by-product will be regarded as a waste. I think this poses unnecessary administrative burden for small and medium enterprises.
I would like to know what are the rules regarding by-products in other countries.