This could help...  

https://www.researchgate.net/publication/247177296_A_Preliminary_Study_on_Cactus_as_Coagulant_in_Water_Treatment 

Article A Preliminary Study on Cactus as Coagulant in Water Treatment

There is also investigations made on the Moringa Oleifera seed, showing the same Z potential as the aluminum:

sulfate https://www.researchgate.net/publication/262558778_Estudio_Comparativo_de_Sulfato_de_Aluminio_y_Semillas_de_Moringa_oleifera_para_la_Depuracion_de_Aguas_con_Baja_Turbiedad

http://repositorio.uis.edu.co/jspui/bitstream/123456789/7112/2/142202.pdf

I'm working on my thesis on the same field, so I hope it could help you.

Sincerely,

Juan Sebastián Mora A.

Biology and Environmental Engineering student

University of La Salle, Colombia

Article Estudio Comparativo de Sulfato de Aluminio y Semillas de Mor...

cactus can be used but higher dose causes film like soap solution and aslo disturbs alkalinity

Thank you to all my colleagues for these valuable information. In fact I hope to reduce alum dose by at least some 30% or even more to overcome its health effects ,so I hope we continue to exchange knowedge in the future. 

Sincerely Yours 

Dr.Nabeel Al-Saati 

If the cactus contain the coagulant property which is prereqisit to destabilize solid particles in contaminated/ turbid water then it will be more hopeful for the treatment of water. Contamination varies from the source to source in case of water. 

Thank you Mr.Hossain for the valuable notes .Please refer to our article announced here about cactus ( Opuntia spp. ). It is ready a coagulant, but it works as a polymer not by a chemical action like alum. Thank you again sir. 

Looks some thing interesting. Economical also.