The start-up and optimisation of a 380 m3 UASB reactor (Up-flow Anaerobic Sludge Blanket) treating wastewater from a fish-canning factory was carried out. At the beginning of the operation the Organic Loading Rate (OLR) was 1 kg COD/m3·d. Then, the load was gradually increased in steps of 50% OLR until the final capacity of the system (4 kg COD/m3·d) was achieved.

Wastewater characteristics were highly dependent on the canned product (mussel, tuna, sardines, etc.). In spite of that, a stable operation working at a hydraulic retention time (HRT) of 2 days was maintained. Total Alkalinity (TA) always presented values higher than 3 g CaCO3/l, while the IA/TA ratio (Intermediate Alalinity/Total Alkalinity) was always maintained lower than 0.3.

In order to improve granulation conditions, upward velocities from 0.5 to 0.8 m/h were applied. The highest values caused the washout of non-granulated biomass from the reactor, optimum operation being achieved at an upward velocity of 0.7 m/h.

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