A method is provided for forming
zeolite membranes in internal surfaces of a plurality of conduits in a cylindrical porous
ceramic monolith, the conduits extending from one end of the
monolith to the other, said method including a step of: flowing a pre-treatment liquid including a
zeolite initiating agent into the conduits; causing at least part of a carrier liquid component of the treatment liquid to flow from the conduits into and through the body of the
monolith to the exterior; and causing
zeolite crystals to be deposited in the porous internal surfaces of the conduits as the carrier liquid component flows into the monolith. The substrates may be pre-conditioned for membrane formation by a method which comprises: (a) forming an
aqueous suspension of zeolite particles; and (b) passing the suspension alternately (i) through the tubular conduits and (ii) out through the walls of the tubular conduits so as to deposit a layer of zeolite particles on the inner surfaces of the tubular conduits; wherein the porous substrates are treated in chambers arranged e.g in annularly and the suspension is supplied to the chambers from a first common manifold via respective delivery tubes and is recovered via
recovery tubes leading to a second common manifold, the first and second manifolds and the supply and
recovery tubes being configured so that the
branch path to and from each chamber is substantially the same. After pre-conditioning, formation of membranes may be by depositing or crystallizing a zeolite membrane on the zeolite particles by gel
crystallization. A
membrane structure is also provided which comprises a tubular porous
ceramic monolith having tubular conduits each having an internal
diameter of (5) to (9) mm formed within the monolith with a zeolite membrane formed on the internal surface of each of the conduits, wherein either there are four conduits and the monolith is longer than 600 mm or there are five or more conduits. The invention also provides methods for removal of water from
organic liquids and methods for the purification of water using the above membrane structures e.g. to remove residual water from
ethanol or
butanol or to produce high purity water.