Salmonella (Salmonella spp.)

Salmonella is a genus of rod-shaped Gram-negative bacteria — including species like S. enterica with serovars Typhimurium, Enteritidis and Typhi — that cause human illness ranging from gastroenteritis (food poisoning) to typhoid fever. Animal manure, slaughterhouse waste and untreated sewage sludge routinely carry Salmonella at concentrations of 10³ to 10⁶ CFU per gram, making pathogen elimination a binding requirement before digestate from a CBG plant can be sold or land-applied to food crops.

In India the Fertilizer Control Order (FCO) 1985 Schedule VI, which governs Fermented Organic Manure quality for sale and use, specifies that Salmonella must be absent in 25 g of the finished product. The complementary E. coli limit is below 1,000 MPN per gram. These thresholds align with WHO and FAO recommendations and with European regulations on the same materials. Sale of digestate that exceeds these limits is not just unmarketable; it is a regulatory violation under FCO with penalties under the Essential Commodities Act.

Salmonella in digestate is killed by combinations of time, temperature and pH. Mesophilic AD at 35-40°C with HRT above 20 days achieves 1-3 log reduction (10× to 1,000× kill) — not enough by itself to consistently meet FCO. Thermophilic AD at 50-55°C with HRT above 15 days achieves 3-5 log reduction and usually meets FCO directly. Pasteurisation at 70°C for one hour, applied either pre-digestion or post-digestion, achieves 5-6 log reduction and guarantees compliance. Composting at 55-65°C for 5-7 days in well-managed windrows achieves similar kill. Plants on poultry manure or slaughterhouse co-substrates almost always need pasteurisation to be safe.

The trade-offs are capex and energy. A pre-digester pasteurisation skid for a 5 TPD CBG plant costs ₹40-90 lakh and consumes 1-2% of the plant's gas output for heating; post-digester pasteurisation costs slightly less because the digestate volume is smaller. Composting the dewatered cake is cheaper in capex but requires land, turning equipment and 5-7 days of curing. Skipping the kill step is a false economy: a single rejected lab certificate stops digestate sales for months and can trigger regulatory action that endangers the plant's overall licensing status. Treating pathogen kill as a non-negotiable design item from DPR stage is the disciplined approach.