2–24 hours (homogenisation time 2–24 hours)

The 2-24 hour window refers to the recommended holding time for feedstock in a homogenisation or mixing tank before it enters the main anaerobic digester. The purpose of homogenisation is to convert a varying, lumpy, multi-stream feedstock arrival into a consistent, pre-blended, partly-hydrolysed feed that the digester receives at uniform composition, temperature and pH. It is the buffering step that protects digester biology from sudden composition shocks.

The lower bound of 2 hours covers minimum residence time for physical blending: mechanical mixers homogenise the slurry, pH and temperature equalise across the tank, large floating or settling fractions distribute, and any free oxygen entrained during loading is consumed by aerobic facultative bacteria. Below 2 hours the tank functions more as a load-spike absorber than a homogeniser, and the digester receives essentially raw feed. The upper bound of 24 hours allows partial pre-hydrolysis — slow-degrading components like cellulose start the breakdown process under facultative-anaerobic conditions, easing the load on the main digester's microbial community and improving overall digestion efficiency.

Beyond 24 hours the trade-off reverses. Once the tank turns fully anaerobic, methanogenic activity begins in the homogenisation tank itself — but with no gas-collection infrastructure, the methane escapes uncaptured (a climate-impact loss and a workplace-safety hazard) and acid intermediates accumulate, dropping the pH and creating a sour feed that stresses the main digester. Plants that hold for over 36 hours often see VFA spikes, foaming and reduced gas yield downstream.

Indian operating practice typically settles in the 4-8 hour range for CBG plants on agricultural and food-waste feedstock, with mechanical mixing at 30-60 rpm and continuous addition of fresh slurry through one inlet and continuous extraction to digester through another. Tank volume is sized for one full HRT cycle of the planned holding time — a 5 TPD plant with 6-hour holding processing 80-90 tonnes per day of wet slurry needs roughly 22-25 m³ of homogenisation tank volume. Heat is sometimes added at this stage to bring the slurry to digester temperature, reducing thermal shock at the main reactor.