Start-up of a full-scale SNAD-MBBR process for treating sludge digester liquor

A simultaneous partial nitrification, anammox and denitrification (SNAD) process was started up successfully to treat sludge digester liquor in a full-scale moving bed biofilm reactor (MBBR). This reactor treated up to 530 kg N/d with a nitrogen removal efficiency of 70%. During the start-up period, efficient and stable partial nitrification was attained by controlling the concentration of DO (0.3-0.5 mg/L), free ammonia (0.6-8.2 mg/L) and free nitrous acid (0.03-1.20 mg/L). The full- scale anammox reactor (832 m(3)) took about 240 days to start up successfully after inoculation with nitrifying sludge and a small amount of anammox sludge. The preformation of anammox biofilm in the pilot-scale MBBR played a vital role in the fast start-up of the full-scale anammox MBBR. An average nitrogen removal rate of 0.41 kg N/m(3)/d was achieved. In the SNAD reactor, high-throughput sequencing techniques revealed that the anammox bacteria in the biofilm accounted for a higher proportion than in the suspended sludge, indicating that the carriers effectively protected the anammox from washout. This study provides valuable experience for treatment of wastewater with low carbon-to-nitrogen ratio.