Biogas is primarily composed of methane (CH4) and carbon dioxide (CO2). It is produced in an anaerobic digester (AD) using a biological process, degrading organic matter in the absence of oxidant (e.g. oxygen). Figure 1 below is a simplified PFD of biogas production with conventional digester wastewater management.
While some of the organic matter will be broken down into biogas, digestate slurry remains. It is a mixture of solids (leftover organic matter, biomass) and wastewater, in which inorganic salts are dissolved. Solids are separated and sent for disposal/used as fertilizer.
However, without further treatment the digester wastewater cannot usually be re-used directly or discharged into a public sewer. Typical issues include total suspended solids (TSS), total dissolved solids (TDS), ammonia, biological oxygen demand (BOD), and chemical oxygen demand (COD). These characteristics of the digester wastewater are impacted by the composition of the organic matter feed, for example: municipal organic waste, livestock waste, poultry waste, manure etc.