In wastewater treatment some process variables may be very difficult to measure directly because of the non-availability or excessive cost of sensors. An alternative is to use ‘soft sensors” that provide online estimates of these inacessible variables by calculations based on auxiliary measurable variables. Two such sensors are proposed based on extended Kalman filtering or neural networks that could enable this monitoring of nitrate ion, ammonium ion and carbonaceous matter concentrations during nitrification of wastewater.