I once read that it was because of osmotic pressure that it returns to the blood stream, by entering the venules. But why? If lymph originated as plasma how come that the solute concentration is higher in the venule? Doesn't plasma contain solutes such as salts, nutrients, oxygen, etc. ?
Technically 'lymph' is used to refer to the fluid found within the lymphatic system. If it's not in the lymphatic system, it is not lymph fluid. Thus, your question is really asking about interstitial fluid or the plasma that was filtered out of blood capillaries.
The answer to your question is based on the Starling equation. Normally fluid leaves a capillary due to a net pressure that favors the interstitium. This net pressure is based on the hydrostatic pressure within the capillary being greater than the interstitial pressure of the surrounding tissues, and the oncotic pressure of the capillary (that draws fluid in) being weaker than the hydrostatic pressure of the capillary (that pushes fluid out). At the venule end of this system, the capillary oncotic pressure is stronger than the capillary hydrostatic pressure, drawing fluid back into the circulatory system.
Remember that albumin is the most important component which establishes the oncotic pressure within a vessel, and that this protein is normally NOT released out of a vessel during filtration. Thus, it passes from the capillary into its corresponding venule directly.
Yes, plasma that enters the interstitium contains many of the same components as when it is in the blood, but the main difference is the protein content, losing the majority of proteins as it enters the interstitium.
Now, if you are asking how the majority of LYMPH fluid re-enters the blood stream, the answer is through the R lymphatic and L lymphatic (aka thoracic) ducts, at the location of the R subclavian or R internal jugular, and L subclavian veins, respectively. Once fluid enters lymphatic capillaries, it goes through a system of lymphatic vessels, trunks, and finally, the above mentioned ducts to rejoin blood.