NOVELTY - A liquid stream (334) produced from separating formation fluid from subsurface in situ heat treatment is provided to a nanofiltration system (342) to produce a retentate that comprises clogging compositions and a permeate. The permeate is processed in the processing units downstream of a skid-mounted nanofiltration system to form a crude product. Liquid stream is provided to the nanofiltration system continuously for at least 10 hours without cleaning a feed side of a membrane of the nanofiltration system.    USE - For the production of crude products, hydrocarbons, hydrogen, and/or other products from various subsurface formations such as hydrocarbon-containing formations such as oil shale formations, coal formations, tar sands formations, and formations with heavy viscous oils. The produced hydrocarbons are utilized to form transportation fuel such as JP-8 jet fuel, gasoline, pipeline gas and products suitable for refinery use.    ADVANTAGE - The process economically produces crude products, hydrocarbons, hydrogen, and other products from hydrocarbon-containing formations. The heat source in production well allows for vapor phase removal of formation fluids from the formation. Providing heating at or through the production well may inhibit condensation and/or refluxing of production fluid when such production fluid is moving in the production well proximate the overburden. Heating increases heat input into the formation and increases production rate from the production well as compared to a production well without a heat source. It also inhibits condensation of high carbon number compounds (C6 and above) in the production well, and increases formation permeability at or proximate the production well. Pressure in the formation may be maintained high enough to promote production of formation fluid with an API gravity of greater than 20degrees. Maintaining high pressure in the formation also inhibits formation subsidence during in situ heat treatment. It also facilitates vapor phase production of fluids from the formation which allows for a reduction in size of collection conduits used to transport fluids produced from the formation. Maintaining increased pressure also reduces or eliminates the need to compress formation fluids at the surface to transport the fluids in collection conduits to treatment facilities. Large quantities of hydrocarbons of increased quality and of relatively low molecular weight are produced. The formation fluid produced has a minimal amount of compounds above a selected carbon number at most 25, 20, 12, or 8. Entrainment of high carbon number compounds and multi-ring hydrocarbon compounds in the vapor is also inhibited. These compounds may remain in a liquid phase in the formation for significant time periods sufficient enough for the compounds to pyrolyze and form lower carbon number compounds.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for the system of treating an in situ heat treatment fluid.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic representation of a system for treating the mixture produced from an in situ heat treatment process.    Liquid stream (334)    Filtration system (342)    Filtered liquid stream (344)    Retentate (346)    Waste stream (348)