Would you share why you want to increase bicarbonate concentration?  injecting CO2 into the water will increase the dissolved [CO2], and the speciation will depend primarily on pH

Actually I'm trying to simulate wastewater for further research, one major component of wastewater is bicarbonate. I don't want to use sodium bicarbonate to avoid adding Sodium to the solution.

Municipal wastewater typically contain several hundreds mg/L of sodium. Can you explain why you need to avoid sodium so we can suggest a solution.

If you just need to avoid sodium use KHCO3 or K2CO3. There are dusins more carbonate salts. 

i think too much sodium in irrigation water might be unsuitable for crops. check these links...... 

http://citrusagents.ifas.ufl.edu/events/GrowersInstitute2014/PDF/Understanding%20the%20Potential%20Problem%20with%20High%20Bicarbonates-%20Morgan.pdf

http://www.lenntech.com/applications/irrigation/bicarbonate/bicarbonate-hazard-of-irrigation-water.htm

Many thanks paul, Henrik and Navdeep . Valuable suggestions and recommendations. 

In fact I need to stick to specific concentration of Sodium in the simulated wastewater to keep certain ratios in my research.  K2CO3 might provide unwanted excess amount of Potassium. I need to keep the essential elements in certain ratios particularly K, Ca, N and Na.

Lithium carbonate or magnesium carbonate are the closest matches. I think you will find the easiest to dose is magnesium oxide (MgO) which dissolves well and convert to hydrogencarbonate in water in contact with air.

Many thanks professor Henrik

I agree withProfessorr Henrik Rasmus Andersen about magnesium oxide (MgO) as the best option as per your requirement