The Use Of Organic & Inorganic Additives During AD Process

In order to increase the performance of bioreactors, i.e. process stability and biogas production, organic and inorganic additives used during AD process. Although, results of experimental researches revealed that some additives can cause inhibitory effect, mainly as a result of high ion concentration in the bioreactor medium. Various organic and inorganic additives have been used to increase biogas and methane production from AD processes. Organic additives include green biomass, microbial cultures, and enzymes increase the biogas production by stimulating the microbial activity. Inorganic additives classified in three groups, i.e. macro-nutrients, micro-nutrients, and carbonous materials. macro-nutrients (i.e. P, N, S) are fed to bioreactor in form of mineral salts to keep the stability of bioreactors. Micro- nutrients, i.e. Fe, Ni, Co, etc., are added to the reactor in form of mineral salts, bulk (oxide/zero valent), and nanoparticles (oxide/zero valent).

Micro-nutrients are essential constituents of cofactors and enzymes in numerous biological reactions involved in AD and their addition to anaerobic bioreactors has been shown to stimulate the performance of biogas production process. Carbonous materials, as conductive non-biological materials, are used in form of both bulk and different carbon nanostructures to boost biomethane yield and rate via accelerating interspecies electron transfer (DIET) between syntrophic microorganisms. Supplementation of inorganic additives in form of nanomaterials caused enhancement toward AD performance because it offers unique physio-chemical properties, i.e. high surface area, high reactivity, high specificity, self-assembly, promoted mobility in AD media, and dispersibility. Furthermore, the stimulating effects of nanomaterials may be attributed to the cellular uptake of NPs inside the microorganisms and integrating with the metabolic intermediates and key enzyme activity involved in AD process. All the Nanomaterials used in AD processes were categorized in four groups, i.e. zero valent metallic NPs, metal oxide NPs, carbonous nanomaterials, and multi- compound NPs.

Results clearly indicate that using nanomaterials could be a useful strategy to improve the performance of AD process. Using multi-functional nanomaterials with different components to get the benefits of all the components is a promising method to improve AD process. The perspective could be design, develop and fabrication of multi-functional nanomaterial with ability to catalyze all the probable bottle neck steps in AD process, so that it could be useful for AD of versatile feedstocks. In the future, the following stages should be considered to achieve a novel multi-functional nanomaterials:

Deep understanding of the effects of different nanomaterials on each step of AD process by using model substrates.

Fabrication of multi-functional nanomaterials, i.e. deposition of several metal/metal-oxide NPs on support materials such as carbonous nano structures, zeolites, TiO2, SiO2, etc.

Tuning the activity of multi-functional nanomaterial via optimization of composition ratios.

Assessment of fabricated nanomaterial through application in presence of different types of real substrates.