Highlights•Faeces are dehydrated during their ageing, reducing in weight by 72% after 16 weeks.•Ageing reduces water activity in faeces to 0.67 after 16 weeks of storage.•Ageing mostly impacts moisture related properties, not energy and nutrient content.•Ageing removes mainly interstitial bound water without causing any biodegradation.•Ageing can improve the faeces drying performance and mechanical handling.