There is an industrial glue for HDPE plastics available though it is very expensive in small quantities plus you must have the application gun and tips.
Here is a link the product's literature.
ClicK Here As welding you must keep the temperature within very close tolerances, too little result in poor bonding and too much result the plastic structure being burnt. Also as said before it needs to be a full penetration weld on clean materials. I have weld these tanks for my own use but I prefer to just replace the whenever possible on customers equipment; just not worth the liability.
