RidX contains millions (billions?) of microbial life designed to break down organic waste found in septic tanks. An email to the folks at RidX might provide you the answer you are looking for. With not knowing for sure, you could consider going ahead and using the the product as intended. if the microbial life is dead due to the freezing, no harm will happen to the septic tank, you will have just added organic matter to be broken down by the existing microflora.
