A method for purifying one or more target substances from a viscous source material, said process comprising: contacting the viscous source material with a diluent in an amount sufficient to reduce the viscosity of the viscous source material and form a continuous stream of diluted source material, wherein the diluent is contained in a separate vessel from the viscous source material; flowing the diluted source material into a recirculation loop of a first cross-flow filter apparatus; diafiltering the diluted source material with sufficient diafiltration buffer retained in a buffer reservoir so as to recover the desired yield of the target substance by passing said target substance into the first permeate fluid; flowing the first permeate fluid containing the target substance to an end product vessel; flowing out the first retentate solution from the recirculating liquid of the first cross-flow filter into a second cross-flow filter unit, wherein the flow rate of the first retentate solution is at the same flow rate as the diluted source material being fed into the recirculation loop of the first cross-flow filter apparatus; diafiltering the flow of retentate into the second cross-flow filter unit with sufficient diafiltration buffer so as to recover the desired yield of the target substance by passing said target substance into the second permeate fluid; flowing the second permeate fluid containing the target substance to the end product vessel; concentrating the first and second retentate fluid by flowing same to a third cross-flow filter apparatus communicatively connected with the second cross-flow filter unit, wherein the volume of the third retentate fluid is reduced to the approximate volume of the undiluted source material or less thereby forming a waste stream for further use; recirculating the third permeate fluid back to the diluent vessel for reuse; concentrating the first and second permeate fluid by flowing same to a fourth cross-flow filter apparatus communicatively connected to the end product vessel wherein target substance is concentrated and diafiltration buffer is removed in fourth permeate stream and recirculated for reuse. contacting the viscous source material with a diluent in an amount sufficient to reduce the viscosity of the viscous source material and form a continuous stream of diluted source material, wherein the diluent is contained in a separate vessel from the viscous source material; flowing the diluted source material into a recirculation loop of a first cross-flow filter apparatus; diafiltering the diluted source material with sufficient diafiltration buffer retained in a buffer reservoir so as to recover the desired yield of the target substance by passing said target substance into the first permeate fluid; flowing the first permeate fluid containing the target substance to an end product vessel; flowing out the first retentate solution from the recirculating liquid of the first cross-flow filter into a second cross-flow filter unit, wherein the flow rate of the first retentate solution is at the same flow rate as the diluted source material being fed into the recirculation loop of the first cross-flow filter apparatus; diafiltering the flow of retentate into the second cross-flow filter unit with sufficient diafiltration buffer so as to recover the desired yield of the target substance by passing said target substance into the second permeate fluid; flowing the second permeate fluid containing the target substance to the end product vessel; concentrating the first and second retentate fluid by flowing same to a third cross-flow filter apparatus communicatively connected with the second cross-flow filter unit, wherein the volume of the third retentate fluid is reduced to the approximate volume of the undiluted source material or less thereby forming a waste stream for further use; recirculating the third permeate fluid back to the diluent vessel for reuse; concentrating the first and second permeate fluid by flowing same to a fourth cross-flow filter apparatus communicatively connected to the end product vessel wherein target substance is concentrated and diafiltration buffer is removed in fourth permeate stream and recirculated for reuse.