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 bi ogas production or the biogas flow rate as the only on-line variable. Furth ermore, other off-line parameters, such as methane content in the biogas an d COD of influent are also necessary to calculate the COD balance in the re actor. Two different start-up strategies were applied: fed batch and contin uous operation. In the fed-batch operation the frequency of feeding is auto matically set depending on the efficiency in COD removal. The residual orga nic load fraction (the fraction of influent COD not degraded to methane), i s the key parameter enabling the controller the modification of feeding fre quency. In order to improve reliability of the system, a second parameter k representing the gas flow rate, was introduced. By using this strategy, st arting 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 wa s operated in continuous mode, the key parameter applied is again the resid ual 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 inc reases the feeding pump speed automatically, and when it attains the maximu m value, the feeding flow is decreased proportionally. In order to ensure s table operation, a further parameter, the waiting time between the moment a t 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 thi s purpose. By means of this strategy, starting from an influent OLR lower t han 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%.