A system for adding and mixing acid into manure present in a manure storage tank is disclosed. The system comprises a submersible pump (18) pumping manure through an inlet (17) positioned near the bottom of the manure storage tank (1) and through an ejector (5). The manure is preferably ejected above the liquid surface (19) in the manure storage tank (1). The ejector (5) geometry provides a reduction of the pressure in the manure flowing through an inner part (13) of the ejector (5). The reduced pressure sucks acid from a storage container, through a suction hose (6) and a void space (16) in the ejector (5) and into the manure flow through openings (12). The system may be a mobile unit. The present invention also relates to a method for injecting and mixing acid into manure present in a manure storage tank. The system and the method ensure a very uniform acidification of the manure using a safe system for reliable control the formation of foam during acid addition.
