Fe2+ might be accelerating the granulation process through binding between negatively charged groups on cell surfaces and linking extracellular polymers. But the specific methanogenic activity (SMA) of granules decreased with increasing Fe2+ concentration in the feed, possibly due to the presence of a large amount of minerals deposited within the granules. The significant decrease in the water content of the granules, and the possible toxicity of high-concentration Fe2+ accumulated inside the granules might also have contributed to the
decrease in the SMA