[zucchen]

high food to biomass ratio = few bacteria and lots of carbon food: the bacteria eat the food, breath in O2, breath out CO2, but heck this tank is not aerated. The meter drops to zero dissolved oxygen ANOXIC CONDITIONS and the bacteria are still growing on the carbon. Add a side stream containing nitrate (oxidized ammonia from the aerated tank above)and the bacteria that are 'passing out' for lack of oxygen grab it from a couple of passing NO3 molecules and the two Nitrogens fly off as a pair back to the atmosphere from whence they originated. In these anoxic conditions, strict anaerobic bacteria die, (methanogens,the oldest bacteria on the planet - archea bacteria - that learned to live before there was any oxygen in the atmosphere might as individuals live for a while but the population fails to grow and is washed out). These bacteria find any form of free oxygen fatally toxic given a few seconds or minutes. There are other bacteria (facultative types) that can switch between living in aerobic and anoxic conditions and some will operate in fully anaerobic conditions by having two metabolic pathways available - useful in other ways to the engineer but that is another story. Truely anaerobic conditions are required for methanogenic bacteria and this means no nitrate and no dissolved oxygen for happy methanogens.