Julie, even though there might not be any context for this particular word, the overall context could really be helpful. I have checked five different sources (Dictionaries for Anlagenbau, Umwelt, Chemie, Petrolchemie and Agriculture just to cover the bases) and all offer fermentation tank for Gaertank (Tanklager: is without a doubt tank farm). Gaeren almost always denote fermenting/fermentation commonly in alcohol. A glossary from an "Anlagenbauer" where I used to work in Germany always offers "decompose" for gaeren. I am not too sure, if this is accurate. With regard to agriculture, there is a term called Gaerfutter or Silage (also in English). This again is fermented feed for cows. I hope that this might have shed some additional light on your question.