Improving the communication speed on a 10km cable (with high capacitance)
It seems your hardware is limited. If you can't detect more finite frequencies and you can't implement another method of modulation you might have reached your maximum in physical bits with your hardware. 

May I suggest a bit of coding theory? Implement LZW compression. The data rate will vary and it might not give you double the speed but will definitely increase the data rate.

Its already produced in c, try it out.

http://marknelson.us/attachments/1989/lzw-data-compression/lzw.c