E. coli (Escherichia coli)

Escherichia coli (E. coli) is a rod-shaped Gram-negative bacterium that naturally inhabits the lower intestinal tract of warm-blooded animals. Most E. coli strains are harmless commensals; however, pathogenic strains (most notably O157:H7) cause severe gastrointestinal illness and occasional fatal complications. E. coli is the universally accepted indicator organism for faecal contamination — its presence in digestate, water, or food signals possible contamination with intestinal pathogens including Salmonella, Shigella, Campylobacter, Listeria, and various viruses and protozoa.

In digestate quality testing, E. coli is enumerated rather than identified by species. Standard methods report Most Probable Number (MPN) per 100 mL of liquid or per gram of solid using selective culture media (lauryl tryptose broth, EC broth, MUG-based fluorogenic media) or membrane filtration. The Fertiliser Control Order, 1985 amendment for FOM and PROM specifies E. coli below 1,000 MPN per gram of dry matter for fertiliser sold to farmers. The Indian Standard IS 17274 for compost from municipal solid waste and the upcoming CPCB standards for digestate set similar thresholds. Above these limits, the digestate cannot be marketed as organic fertiliser and must either be re-processed or disposed of as waste.

Achieving E. coli kill is one of the central digestate-quality design questions for biogas plants. Mesophilic digestion at 35–40°C achieves only partial pathogen reduction — typical 2–3 log reduction from feedstock to digestate, leaving the residue not always compliant. Thermophilic digestion at 50–55°C delivers 4–5 log reduction and reliably meets FCO standards. Where thermophilic digestion is not practical, post-digestion pasteurisation at 70°C for 1 hour (the EU Animal By-Products Regulation Category 2 standard, increasingly used in India) provides the necessary kill. Aerobic composting of solid fraction at 55–65°C for 14 days achieves similar results. Plants targeting fertiliser markets for food crops need at least one of these processes — testing alone without process control is insufficient because batch-to-batch variation in feedstock means E. coli levels can spike unexpectedly. The trade-offs are capex for hygienisation equipment versus market access — digestate that cannot demonstrate consistent E. coli compliance is restricted to fodder crops or industrial uses and earns 30–50% lower price.