**5. Cannery wastewater treatment with anaerobic-anoxic-aerobic system**

Anoxic process is widely used in wastewater treatment. Anoxic means depletion or deficiency of oxygen. Anoxic process is a biological treatment process by which NO3-N is converted to molecular nitrogen gas in the absence of oxygen.

A system comprised of anaerobic-anoxic-aerobic reactors was tested to treat the wastewater from tuna cooker. This wastewater stream is characterized by high COD and N concentrations. The up-flow anaerobic sludge blanket reactor was used to achieve the anaerobic digestion in a two-step process. In the first, the COD concentration was varied and ORLs up to 4 g COD/(l.d) were achieved. In the second step, the 6 g COD/l and the HRT were varied between 0.5 and 0.8 day, and this step led to ORLs less than 15 g COD/(l.d). The denitrification process was carried out in an upflow anoxic filter, and the result of the project indicated that the efficiency of this process is dependent on the supplied carbon content. For optimum carbon content, the ratio between the COD and N equalled 4 and the denitrification percentage equalled 80%. Finally, the nitrification was reported to be fixed at 100% ammonia removal regardless of the amount of carbon in the range of 0.2–0.8 g TOC/l. The variation of the recycling ratios between the denitrification and nitrification reactors in the range of 1–2.5 was found to affect the efficiency of the COD and N-removal percentage, where 90 and 60% removal for COD and N was reported at recycling ratio between 2 and 2.5 [15].
