The effect of reducing the residence time of screened pig slurry in an on farm pilot scale oxidation ditch is described. The soluble organic fraction of the slurry was greatly reduced, either by mineralisation at long residence times, or, as residence times decreased, by conversion to suspended solids and organic nitrogen. Losses of inorganic nitrogen during treatment were high due to desorption and denitrification. Controlled loading of a laboratory system with the same waste showed that the amount of oxygen demand lost per day was constant, and that residence time determined whether mineralisation or solids production occured. A small increase in soluble COD took place during over-winter storage of aerobically treated waste, but it remained far more acceptable for land disposal than untreated material.