My (possibly inaccurate) summary is that they can be discharged if it would be impossible to live with a minimal standard of living while still paying off the loan.
More realistically you're going to get one of the other two options.
> Your loan may be partially discharged, and you will still be required to repay some portion of your loan.
> You may be required to repay your loan, but with different terms, such as a lower interest rate.
My (possibly inaccurate) summary is that they can be discharged if it would be impossible to live with a minimal standard of living while still paying off the loan.