Fig 2 indicates that the US-only,the CA-only and the US+CA have a higher daily biogas yield than the initial stage (during 2-5 d).the highest daily yield of 88.33 ml/(g vs d)occurred in the US+CA . in the following stage (from the 6th day to the end of the digestion),the US+CA maintained the biogas yield at a high level,whereas the CA-only experiment and the control re-mained the biogas yield at a iow -level, then the biogsa yield of the us-only got down and finally shwed on difference with that of the control.
....lt also howed that ca-only has higher intial metane content (nearly60%)than other experiments. but after the initial stage,the methane contents of all the groups achieved simlar values,nearly 70% ,indicated that both us and ca can promte the content of methane:us breaks down the oli molecules as well a the cone-obimum of microorganisms and snd sludge,making them easily dis-solved in the water ,releasing largr amount of cll components,leading to a high biogas yield and low methane content in the ini-tial stage.but if there is only ca,the oil molecules and the coeno-bium would not be broken down easily, proving that ca-only has a low biogas yield during the whole stage .besides,the pH value would decay by introducing ca.as shown in table4,in the us+ca, the acid could be easily used dy microorganisms since all of the substrate were dissolved and the contact area increased by us,thus the pH value would easily go back to the normal range (from 5.99 in the initial stage to 7.12 in the end stage).however,in the ca-only,the acid would accumulate because of the lack of contact