Sewage sludge is usually discarded at the bottom of settling ponds. Old sludges are of little value in terms of the useful organic content, as they have been biodegraded and may contain high levels of heavy metals. They are usually the anaerobically digested or aerated end products of waste water treatment. These sludges often have a useful nutrient content of phosphate and nitrates. New sludges still have a high organic content and generate sulfur gases in particular hydrogen sulfide.
U.S. Pat. Nos. 3,942,970 and 5,387,257 propose the use of sewage sludges in fertilizer.
However most governments set maximum acceptable levels for metal content in fertilizers and soil enhancers particularly for arsenic, cadmium, chromium, copper, mercury, molybdenum, nickel, lead, selenium and zinc because of the potential that these metals will enter the food chain.
U.S. Pat. No. 5,009,793 discloses the removal of heavy metals from sludges and contaminated soils.
U.S. Pat. No. 5,178,776 discloses the removal of metals from sludges by acidification followed by a heating step to sterilize the treated solids.
U.S. Pat. No. 5,051,191 treats non oxidized sewage sludge at a low solids content to remove heavy metals by acid solubilization and subsequent precipitation from the liquid phase to leave a decontaminated sludge that is neutralized and then used as a soil conditioner and fertilizer.
USA application 2002/0153307 improves on this process by operating at a higher acid pH to reduce the solubilization of fertilizing elements in the sludge.
A problem with prior art processes is that a waste stream is still produced. Also because the value of the decontaminated sludge is not high the cost of carrying out the process must be kept to a minimum. Some prior art processes use expensive chemicals and require heating of the sludge all of which adds to processing costs.
It is an object of this invention to provide a process that overcomes the problems of the prior art.