Borregaard Industries Ltd, Norway, is a Ca-bisulphite based pulp manufacturer which has an interest in an anaerobic wastewater treatment process. At this pulp factory a combination of sulphite evaporator (SEC) and caustic extraction liquor (CEL) would eliminate the costs for neutralization chemicals in an anaerobic treatment process. The anaerobic process was also suitable because of the high concentration of organic compounds in CEL, the biogas production and the low sludge growth.

Today a full-scale plant treating SEC in combination with CEL is in operation with a capacity to treat 60 tons of COD/d. All CEL qualities are treated anaerobically even those containing peroxide. The highest guaranteed AOX concentration in the CEL to be treated is 25 mg AOX/I.

The experience from a 2-year pilot plant study has influenced the design of the full-scale plant.

An adaption of the sludge to the CEL has to be performed due the toxic compounds in the CEL.

The incoming wastewater to the full-scale plant has a composition of 50% COD from the SEC and 50% COD from the CEL. With an organic load of 40 tons COD/d, a SEC/CEL ratio of 51/49, the COD reduction is 65% and the gas yield 0.20 m3 CH4/kg COD added.

This full-scale plant is unique due to the treatment of different CEL qualities and to the high combination of CEL.

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