Muhammad what do you mean by using iron oxide nanoparticles to observe their effect? To compare the results of methane production you can do a batch experiments with different pretreatment methods and compare their methane yields. You can also compare your results with literature.In your case hydrolysis will be the limit stage, you also, run experiment to determine  the rate of production of VFAs.

My idea is to use both alkali pretreatment and Iron oxide nanoparticles.  First, the substrate (Wheat straw and rice straw will be pretreated with NaOH and then during the Anaerobic digestion of this pretreated substrate,  IO nanoparticles will be added to catalyze the biodegradation). In another batch, I only use alkali pretreatment and compare its result with the previous one. I think its clear now... You can say that I want to use a combine alkali pretreatment and IO nanoparticles (using alkali pretreatment and IO NPs in a same experiment) and observe their effect on biogas production and compare its effect with another replicate with only alkali pretreatment.

Yes,. You can do that. I think you should also focus on  pretreatment results before digestions. This  will give you more information about the availability of your cellulose before AD. Thanks 

Thanks...there are two more questions which I want to ask..

1. At which stage, Iron oxide nanoparticles be added in the anaerobic digester? At the start of the process of during the process? 

2. Can iron oxide nanoparticles alone be used for anaerobic digestion of lignocellulosic materials without any pretreatment? 

One  important thing is the economic implication of your methods vis-a-viz the biogas yield. These should be the guide post to help you make informed decision.