Nutrient solutions contain the main Elements (N, P, K, Ca etc.), trace elements (basically heavy metals) and chelating agents. Main elements will be consumed by microbes growing in the tank. They are also present in poop/the breakdown products of poop. The trace elements could be supplied by rust somewhere in your piping. Chelating agents are biodegradeable and are also used in washing agents to reduce water hardness.
In priciple draining it is okay. My main concern is that draining the solution provides too many nutrients and cause some problems, because I don't know that much about septic tanks.
if you overfertilize your garden the nitrates will wash into groundwater and rivers. Especially during autumn and winter plants can't use fertilizers. That's not a nice way to go about your business.