Stockpiled municipal waste incinerator bottom ash is frequently consid
ered for utilization as a construction material. Two 360 kg lysimeter
experiments were conducted to study percolation washing of contaminant
s from stockpiled MSW bottom ash. One lysimeter was leached with a con
centrated sodium hydroxide solution, as a possible pre-treatment for i
mprovement of the bottom ash characteristics prior to utilization, whi
le the other was leached using distilled water. The lysimeter leachate
was analysed, and at the end of the 2-year leaching period, the botto
m ash from each lysimeter was subjected to several laboratory tests to
assess the effect of the treatments. The laboratory tests showed that
distilled water leachability of both treated ashes was an order of ma
gnitude lower than that of fresh ash, but long-term contaminant leacha
bility under acidic conditions had not changed. Although alkaline wash
ing clearly resulted in greater contaminant removal than did distilled
water washing, the chemical properties of the alkaline-leached bottom
ash were not significantly different from those of the water-leached
ash.