In waste streams, several waste components can be merged into one waste component - Merged output by virtual waste activity - Merging. Merged WCs can belong to the same waste stream or the different ones.
Open the Waste stream diagram window. Right-click on the suitable WC in the right panel of the Preview or modify waste stream window, in the context menu choose the Add merging item, type a name of merging activity and click OK.
Right-click on the merging activity (marked with Sigma sign) and in the context menu choose the Add waste component to merge item. In the displayed window choose WC to be merged and click the Link selected object button. The latter action should be repeated until all WC, which have to be merged, are included.