Urban wastewater treatment

Number of countries in brackets. Northern: Iceland, Norway, Sweden, Finland; Western: Austria, Ireland, United Kingdom, Luxembourg, the Netherlands, Germany, Denmark; Southern: Greece, Spain. Primary wastewater treatment removes part of the suspended solids, but no ammonium; secondary (biological) treatment uses aerobic or anaerobic micro-organisms, retains 20 % to 30 % of the nutrients and removes around 75 % ammonium; tertiary treatment includes phosphorus retention and in some cases nitrogen removal.