Biogas plants have been widely used to both reclaim bio-energy from agricultural waste and to treat waste; however, the efficiency of these biogas plants has yet to be determined. In this study, the performance of five full scale biogas plants treating chicken manure (CM), pig manure (PM), a mixture of chicken and pig manure (MM), dairy manure (DM), and maize straw (MS) were investigated. The results showed that CM had the highest total energy (16.4 KJ/g-TS) and the MM had the highest bio-available energy (10.2 g-COD/g-TS). The CM plant adopted a suitable hydraulic retention time (HRT) but the other plants used a much longer HRT than necessary. The methane production from CM, PM, MM, and DM was improved by 12%, 22%, 32% and 25% with the addition of trace metals, and this also resulted in an increment in the methanogenic activity for CM, PG, MM and MS. The pH stability of all the biogas plants was maintained at an acceptable level; nevertheless, the high pH and ammonium in the CM digester negatively affected the methanogenic activity. The results, therefore, conclusively indicated that the operation of the biogas plants could be more effective. (C) 2017 Elsevier Ltd. All rights reserved.