Automatic start-up of UASB reactors

A control system to automate the start-up of anaerobic wastewater treatment reactors is presented. The system controls the feedflow rate, using the biogas production or the biogas flow rate as the only on-line variable. Furthermore, other off-line parameters, such as methane content in the biogas and COD of influent are also necessary to calculate the COD balance in the reactor. Two different start-up strategies were applied: fed batch and continuous operation. In the fed-batch operation the frequency of feeding is automatically set depending on the efficiency in COD removal. The residual organic load fraction (the fraction of influent COD not degraded to methane), is the key parameter enabling the controller the modification of feeding frequency. In order to improve reliability of the system, a second parameter k representing the gas flow rate, was introduced. By using this strategy, starting from an organic loading rate (OLR) lower than 0.5 kg COD/(m(3).day), a load higher than 8 kg COD/(m(3).day) was achieved in only 33 days, with an efficiency, in terms of COD removal, higher than 90%. When the system was operated in continuous mode, the key parameter applied is again the residual organic loading rate fraction, expressed as the percentage of COD that remained undegraded in methane. Two extreme values-"minimum" and "maximum"-have to be defined. Once the system reaches the minimum, the controller increases the feeding pump speed automatically, and when it attains the maximum value, the feeding flow is decreased proportionally. In order to ensure stable operation, a further parameter, the waiting time between the moment at which the threshold value is reached and the modification of the feeding flow rate, was introduced. 24 h has proven to be an excellent value for this purpose. By means of this strategy, starting from an influent OLR lower than 0.5 kg COD/(m(3).day), an OLR of 9-12 kg COD/(m(3).day) was achieved in 40 days, with COD removal efficiency higher than 95%.