A new pretreatment process for sulphur removal was developed and tested. The process is an ANaerobic TRICkling (ANTRIC) filter, in which sulphate and sulphite are reduced to sulphide. The sulphide is stripped from the liquid by a gas recirculation system, in which the gas is washed from sulphide and recirculated to the ANTRIC filter to remove more sulphide etc. The goal with the new process was twofold: 1) removal of sufficient sulphur such that sulphide toxicity in a conventional anaerobic process could be avoided, and 2) recovery of as much sulphur as possible. After the ANTRIC filter the wastewater was treated in an ANAMET system, i. e. in an anaerobic contact process followed by an activated sludge process.

The tests were carried out using wastewater from a sodium based sulphite pulp mill. Theoretical calculations showed that, unless action was taken to limit sulphur, anaerobic treatment would fail due to toxic effects of hydrogen sulphide.

The goal of operating the ANTRIC filter such that sufficient sulphur was removed to make further anaerobic treatment possible was achieved. More than 85 % of the inorganic sulphur (sulphate and sulphite) was removed in the ANTRIC filter, making efficient biological treatment possible. The ANTRIC filter was shown to be a very stable process and the start-up time after a longer shut-down period was short.

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