Seychelle's #35 US "in-line-filter would probably work with the Ortliebe. (Seychelle's "Pure Water Bag" and Katadyn's "Base Camp" gravity filter bags are similiar products, but don't claim the Ortliebe's multi-use.

http://store.seychelle.com/Pure_Water_Bag-Pure_Water_Bag_Advanced.html

A simple pre-filter for the Ortliebe is probably a good addition.