Q.The residue left after methane production from cattle dung is:

Cattle dung fed into a biogas plant undergoes anaerobic digestion by methanogenic bacteria, which break down the organic matter to produce methane-rich biogas. The process does not consume all the material—what remains is a nitrogen-rich slurry that has lost most of its pathogenic microbes and weed seeds during fermentation.

This spent slurry is an excellent organic fertiliser. It retains the mineral nutrients (nitrogen, phosphorus, potassium) originally present in the dung but is now in a more stable, odourless form that plants can readily use. Farmers routinely collect this residue from the outlet of the biogas digester and apply it directly to their fields, closing the nutrient loop and reducing dependence on chemical fertilisers. …

When cattle dung undergoes anaerobic digestion in a biogas plant, the process doesn't simply consume the dung and leave nothing behind. The microbial breakdown that produces methane is selective—it targets the organic carbon compounds that can be fermented, but leaves behind a substantial portion of the original material. This residue, called slurry or spent slurry, emerges from the biogas digester as a semi-liquid mixture.

The beauty of this residue lies in what remains after fermentation. The anaerobic bacteria have broken down complex organic molecules, but the mineral nutrients—nitrogen, phosphorus, potassium, and various micronutrients—stay largely intact. In fact, the digestion process often makes these nutrients more readily available to plants than they were in raw dung. The nitrogen, for instance, is partially converted into forms that plants can absorb more easily. …

A different angle: trace the nutrients through the system like a materials-flow diagram, instead of memorising "slurry = manure".

Cattle dung going into a biogas plant carries three things: fermentable organic carbon, and non-fermentable mineral nutrients (N, P, K). Anaerobic methanogens consume only the carbon fraction to make methane — the mineral nutrients have nowhere else to go, so they simply stay in the leftover slurry, now in a more plant-available, pathogen-reduced form. …