Study of the treatment of salty sludges

Summary

Industry and urban wastewater treatment processes produce large quantities of solid or liquid waste containing salts. Solid residues mainly include sludge from wastewater treatment plants, Residues from the Purification of Fumes from Household Waste Incinerators (REFIOM) and salt-laden filter cakes. This waste poses a problem in terms of recovery, as it cannot be accepted in the traditional channels of storage, land application, incineration or recycling in cement works. It is therefore placed in a class 1 landfill for hazardous waste, after undergoing a stabilisation process. With around 4,680,000 tonnes produced each year, sodium chloride is the salt that is most discharged into the natural environment. For this reason, the study focuses on waste containing this salt. Salts have no direct impact on the environment, but they do affect the stability of the environment. There are currently no national regulations on their discharge rates. Only large producers are subject to prefectoral decrees, and companies in the Rhine basin must comply with the Bonn Convention (1976). Most chlorine discharges are concentrated in north-eastern France, where most of the salt extraction and associated inorganic chemical industries are located. In addition, these regions are also those where the largest quantities of salts are used on roads. The main sources of chloride emissions are industrial effluents (3,275,600 tonnes per year), the use of salt on roads (between 400,000 and 1,000,000 tonnes of salt spread each year) and domestic wastewater (710,000 tonnes per year). These discharges have widely varying concentrations, which can reach 200 g/L. It has not been possible to quantify the quantity of salty solid residues produced. Around 500,000 tonnes of REFIOM are produced each year at a concentration of 18 g of chlorides per kilogram of dry matter. Salt residues can be desalinated by washing with water. In the case of sludge, some of the salt present should be eliminated by dehydration. Of the various existing techniques, electro-osmosis appears to be the most promising. Solids with a high dry content can be treated by electroremediation. Experimental trials are needed to validate these techniques. Industrially, only Resolest and EDF are developing large-scale techniques for treating salt residues. Desalination techniques for liquid effluent are more varied. Research into seawater desalination has led to the development of a large number of processes. These are mainly membrane or thermal processes. This report has made it possible to assess the quantities of salty waste as a function of salt concentrations and the origin of the waste. We have also identified and listed the various techniques currently used or developed for eliminating chlorides.