Controlling a combined lagoon/reed bed system using the oxidation-reduction potential (ORP)

Lagoon systems achieve good and stable effluent data in regard to organic pollutants, but they charge the receiving waters with relatively high ammonium loads. Therefore an existing lagoon-plant was extended by a vertical flow reed-bed for the special purpose of nitrification. This paper presents the efficiency of the combination plant as well as the possibility to monitor and control the reed-bed operation by the oxidation-reduction potential (ORP). The results show that the combination plant achieved excellent purification results, the average efficiency degrees were 97% for COD, 77% for N_total and 94% for the TKN elimination. The ORP in the effluent of the reed bed showed a clear dependence in its characteristic course and its absolute values on the current nitrification performance, the oxygen supply and the hydraulic behaviour of the reed bed. Therefore the ORP is a very good indicator for the state of the reed bed, which ultimately results from the accumulation of a large number of different influencing parameters. As the preservation of aerobic conditions in the reed bed is the crucial prerequisite for a high nitrification performance and for the avoidance of clogging, the ORP thus offers the possibility of immediate operation control.