*4.3. Comparison to Other Studies*

Although this study reports on a specific case study, the results are relevant worldwide, especially those with significant livestock and agricultural productivities. In the literature, some studies have investigated the economics of implementing AD plants on small- to medium-sized farms in various countries and regions [68,90–92]. The overriding theme has been that the financial viability of a plant often needs to be assessed on a case-by-case bases as it is often highly dependent on local conditions, such as cost of energy, feedstock type and availability, and governmen<sup>t</sup> incentives. Therefore, careful consideration of such variables needs to be taken at the project planning stages.

SSAD has increasingly become a topic of interest for researchers, mainly driven by the growing emphasis to reduce the negative environmental impact of agriculture waste streams and increasing investment in renewable energy production. Research trends in the topic have included the optimisation of plant design and operations [93–95], feedstock pre-treatments [95–97], the impact of trace compounds [98–100], and biogas cleaning technologies, and its integration to a fford further energy generation [101–103]. Further research could expand the potential integration of these technologies with small-scale AD systems, and make its implementation more likely.
