The treatment of municipal solid waste air pollution control (MSW APC) residues with sodium silicate
Waste materials are hazardous if they display toxic, corrosive or other
characteristics which have the potential to cause danger to health or the
environment. New regulations to meet the requirements of the Landfill Directive
contain controls on hazardous waste including the requirement to pre-treat
hazardous waste prior to disposal and to ensure these wastes meet stringent
waste acceptance criteria (WAC) on leachability. Waste from the cleaning of
combustion gases produced from municipal solid waste (MSW) incineration is
classed as hazardous by virtue of their corrosive properties. The majority of these
air pollution control (APC) residues which contain dioxins, heavy metals and high
levels of chloride are currently sent to landfill. To meet the new controls, pretreatment
will be required to improve the handling properties and reduce the
release of chloride ions into the environment.
Ordinary Portland cement (OPC) is used to treat hazardous wastes. The solid form
produced with the addition of OPC is however susceptible to degradation from
aggressive leaching fluids and may release contaminants over time. Additives with
high silica content can be used to interact with free lime produced during OPC
hydration to improve the physical and chemical properties of the solid waste form.
The treatment of MSW APC residues with sodium silicate and cement produces a
solid waste form with a reduced structural integrity and a tendency to breakdown
under attack from aggressive fluids. Silica 'gels' are formed during initial setting
reactions which 'depolymerise' with fluid ingress to form new calcium rich silica
'gels' within cracks and voids of the solid waste form. Expansion due to water
absorption and continual 'gel' formation causes structural failure. The addition of
sodium silicate to sludges produced from a current treatment by mixing MSW APC
residues with other mixed hazardous waste improves strength development by
'encapsulating' the waste sludge inside a calcium/silica 'gel'. This stops
components of the sludge from interfering with normal OPC hydration.
The treatment of MSW APC residues with sodium silicate will not produce a solid
form to meet the new waste acceptance criteria. However, sodium silicate has the
potential to improve handling and structural integrity of the sludge produced from
the current treatment process.