The nitrogen concentrations of piggery wastewater and its activated sludge treatment effluent were surveyed in nine farms with various facilities. The biological oxygen demand and suspended solids in the wastewater were sufficiently removed by activated sludge treatment because their effluent concentrations averaged 59 mg·L^-1 and 45 mg·L^-1, respectively; however, nitrogen was not sufficiently removed because its effluent concentration averaged 430 mg·L^-1. When the nitrogen concentration was higher than the general permissible limit under Japanese law, the nitrogen composition of the effluent tended to be mainly ammonium when the pH was 8 or higher, and a mixture of ammonium, nitrite and/or nitrate when the pH was below 8. For the post-treatment of activated sludge from the latter-case effluents, we assumed the application of anammox or denitrification treatment using piggery wastewater as the electron donor and estimated nitrogen removal potential based on the simplified stoichiometric reaction. It was estimated that denitrification and anammox treatments could respectively remove 19 % and 40 % of the total nitrogen from the activated sludge effluent on average.