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Anaerobic digesters are sized on the greater of two criteria: hydraulic retention time, 15-25 days for mesophilic digestion, and volumetric volatile solids loading, generally 1.6-3.2 kg VS/m3/day. Thin sludge is governed by retention time; well-thickened sludge is governed by loading.
Retention time basis
Volume = feed volume x retention time
Loading basis
Volume = volatile solids load / maximum volumetric loading
Digester gas is flammable and asphyxiating, and digesters are confined spaces. Design and operation must comply with gas safety and confined space regulations.
It reduces feed volume for the same solids, so the retention time criterion needs less volume. Digesters fed thin sludge waste capacity heating and holding water.
Volatile fatty acid accumulation and ammonia inhibition. Overloading drops pH and stalls methanogenesis, which can take weeks to recover.