The concentration of PO4 3-, Mg 2+ , Ca 2+ , K + , VFA, NH4 + and HCO3 -are usually lower in influent/effluent than in the mixed sludge, therefore they are presented separately. Nitrogen and phosphorus species are always low in the effluent since they need to be removed to avoid eutrophication in the water bodies where the water is discharged. 70-80% of the influent nitrogen is ammonia , while nitrate or dissolved organic nitrogen (30) are the major nitrogen compounds in the effluent. Typical values for phosphorus in effluent in Europe are 1mg/L (91) and will depend on the local legislation. For example, countries bordering the Baltic Sea, designated as a sensitive area, have to cope with more stringent discharge limits for phosphorus (and nitrogen) to control eutrophication (28). The concentration of magnesium in the influent is greatly influenced by the presence of seawater (up to 350 mg/L according to while potassium is mainly influenced by the presence of industrial wastewater (up to 3000 mg/L according to 10). Similarly to calcium (4, 23), magnesium and potassium usually go untreated from the influent to the effluent. A small decrease in their concentration can be sometimes observed (4, 23), possibly due to their accumulation by Phosphate Accumulating Organisms (PAO's) as counter ion for the negatively charged polyphosphates . We expect this decrease to be more important for WWTPs using EBPR, but no full-scale experimental data were found to confirm it. Under aerobic conditions, NH4 + is gradually oxidized to NO2 -and NO3 -, consuming 7.14g of alkalinity per gram of N oxidized. In the later anoxic conditions, NO3 -is reduced to N2O and then release as gaseous N2, producing 3.57g of alkalinity per gram of N reduced . Alkalinity represents the internal pH buffer of a system and is mainly influenced by HCO3 -, NH4 + , PO4 3- and VFA concentrations in a WWTP . During the oxidation of the biodegradable organic matter in activated sludge systems, 1.375kg of CO2 is produced per kg of Biological Oxygen Demand (BOD) . The effect of this large CO2 release on the alkalinity does not appear to be important: desorption predominates in weakly alkaline solution (like wastewater), meaning that CO2 is emitted in the air and does not greatly influence the pH . As soon as anaerobic conditions are present, fermentation can occur and significant release of some compounds can be observed, mainly due to biological activity. Volatile Fatty Acids (VFA) concentration can strongly increase, especially during prefermentation, due to the decomposition of organic matter (23) that mainly takes place during the first two days of fermentation . The VFA produced, composed of 50-80% of acetate , explains why the pH of thickened sludge is usually lower than in influent/effluent .