Method for determining the effectiveness of removal of viruses in a purification process for producing an aqueous stream, wherein the viruses present in a water sample are used for investigating the effectiveness of the purification process, said method comprising the following steps: i) taking a water sample, at a point located upstream of the purification process; ii) taking a water sample, at a point located downstream of the purification process, wherein the water sample is taken from a water source, selected from the group of salt water, brackish water, groundwater, surface water, seawater, sewage treatment plant effluent, storm water, rainwater, and process water, or a combination thereof; iii) isolating genetic material from one or more viruses present in the aforementioned water samples, obtained according to steps i) and ii), wherein the concentration of said one or more viruses in said water sample i) is in a range from more than one million to 10 billion per litre of water; iv) carrying out a technique for multiplying RNA and/or DNA on the genetic material obtained according to step iii); v) based on the results obtained from step iv), establishing the number of gene copies of specific virus types in the water sample obtained according to step i); vi) based on the results obtained from step iv), establishing the number of gene copies of specific virus types in the water sample obtained according to step ii); vii) determining the effectiveness of removal of viruses in the aforementioned purification process based on comparing the values obtained in step v) and vi), wherein in step iii) the genetic material is DNA and/or RNA, and when genetic material of the RNA type is present in step iii), step iv) is preceded by a step of synthesis of cDNA from the RNA, wherein there is no dosage of markers in the water samples to be taken in step i) and/or step ii). i) taking a water sample, at a point located upstream of the purification process; ii) taking a water sample, at a point located downstream of the purification process, wherein the water sample is taken from a water source, selected from the group of salt water, brackish water, groundwater, surface water, seawater, sewage treatment plant effluent, storm water, rainwater, and process water, or a combination thereof; iii) isolating genetic material from one or more viruses present in the aforementioned water samples, obtained according to steps i) and ii), wherein the concentration of said one or more viruses in said water sample i) is in a range from more than one million to 10 billion per litre of water; iv) carrying out a technique for multiplying RNA and/or DNA on the genetic material obtained according to step iii); v) based on the results obtained from step iv), establishing the number of gene copies of specific virus types in the water sample obtained according to step i); vi) based on the results obtained from step iv), establishing the number of gene copies of specific virus types in the water sample obtained according to step ii); vii) determining the effectiveness of removal of viruses in the aforementioned purification process based on comparing the values obtained in step v) and vi), wherein in step iii) the genetic material is DNA and/or RNA, and when genetic material of the RNA type is present in step iii), step iv) is preceded by a step of synthesis of cDNA from the RNA,