Depending on the polarity and solubility of the alkali substance you are using you could dissolve the substance in a volatile solvent  to lower the viscosity (make it more liquidy) and extract the substance of interest afterwords. This can be done in many different ways depending on what solvent you use. Distillation or simply heating the mixture to a temperature above the boiling point of the volatile solvent but below that of the alkali would work. The issue then becomes the reason for filtering in the first place. If you are filtering to remove particulates than filtering the solvent prior to the aforementioned as well and covering the vessels at all times might be enough. If you are filtering to use this for cell culture than if becomes a bit more tricky. I am not sure this kind of work that I described can be done in a cell culture hood rather it should be done in a fume hood. 

With what kinf of material are you dealing with? Are you interested to recover the components present in the resulting soluble hydrolysate and/or the insoluble/particulate residue?

I need to extract the filtrate after alkali treatment of lignocellulosic biomass. Though filtration is the obvious way, owing to the pulpy nature of the hydrolysate, filtration through G4 sintered funnel has not been too satifying. Can I use glass wool first followed by filtration ??

Do you need to perform any experiments on the filtrate one such example being HPLC? 

Glass wool first followed by filtration is apparently worth to try. However I guess that centrigugation at high speed would be the best approach to separate the insoluble from the insoluble components. Otherwise neutralization followed by increase in the whole osmolarity could possibly reduce the swelling by promoting  expression of the solvent from the particulate and thus helping filtration and/or centrifugation. For instance mixing one volume of 3 M NaCl with two volume of the whole hydrolysate will result in a final 1M solution.

Dear Nicholas,

i need to assay for glucose, acetic acid, phenolics.

Michel Sir,

glass wool i have tried, centrifugation too i have tried but i am having difficulty with the balancing, since volume make up will change the concentration of the filtrate for further analysis. 

Doesn't matter provided you know the starting and the final total volume. For analytical purposes another possibility is to insert the alkali hydrolysate in a strong dialysis sack, suspend it in a known volume of distilled water and wait until low molecular weight species diffuse outside the dialysis sack. Then make your determinations on the diffusate.