Disposal of used greasses is primarily dependent on your local and country environmental sensitivity laws, For instance in North America you need to have a minimum temperature in the combustion chamber and the fuel (hydrocarbon) has to be subject to these temperatures for a specific minimum retention time. Then there is sometimes the requirement to monitor the discharge stack to determine the level of particulate release. There are places in the US where they take the used grease and hot filter it and then mix it (~5%) with regular home heating fuel and sell it comercially to places like the coment kiln industry.
Of course all of this assumes that the grease has been checked for lead, arsnic, PCB's etc and is compliant with your legislated maximum levels.
So depending on your laws you might be able to recover a bit of the expense of the purchase of the lube by using the used grease as a fuel. As you know there is more than 90% oil in most NLGI grades. Remember that some of the solids like moly and graphite will not be incenerated during the combustion process. These solids are usually less than 6% and if you dilute the grease with 95% heating fuel then you are somewhere around the .03 % mark.
regards......