Anaerobic treatment for C and S removal in "zero-discharge" paper mills: effects of process design on S removal efficiencies

Stringent environmental laws in Europe and Northern America lead to the dev elopment towards closure of the process water streams in pulp and paper mil ls. Application of a 'zero-discharge" process is already a feasible option for the board and packaging paper industry, provided in-line treatment is a pplied. Concomitant energy conservation inside the mill results in process water temperatures of 50-60 degreesC. Thermophilic anaerobic treatment comp lemented with appropriate post-treatment is considered as the most cost-eff ective solution to meet re-use criteria of the process water and to keep it s temperature. In the proposed closed-cycle, the anaerobic treatment step r emoves the largest fraction of the biodegradable COD and eliminates "S" as H2S from the process stream, without the use of additional chemicals. The a naerobic step is regarded as the only possible location to bleed "S" from t he process water cycle. In laboratory experiments, the effect of upward liq uid velocity (V Pw) and the specific gas loading rate (V,,.) on the S remov al capacity of thermophilic anaerobic bio-reactors was investigated. Acidif ying, sulphate reducing sludge bed reactors were fed with partly acidified synthetic paper mill wastewater and were operated at 55 degreesC and pH 6. The reactors were operated at organic loading rates up to 50 g COD,l(-1),da y(-1) at COD/SO42- atios of 10. The effect of V Pw was researched by compar ing the performance of a UASB reactor operated at 1.0 m.h-1 and an EGSEI re actor, operated at 6.8 m.h-1. The Vup had a strong effect on 27 the ferment ation patterns. In the UASB reactor, acidification yielded H-2, acetate and propionate, leading to an accumulation of reducing equivalents. These were partly disposed of by the production of n-butyrate and C n-valerate from p ropionate. In the EGSB reactor net acetate consumption was observed as well as high volumetric gas (CO2 and CH4) production rates. The higher gas prod uction rates in the EGSB reactor resulted in higher S-stripping efficiencie s. The effect of V,,,, was further researched by comparing 2 UASB reactors which were sparged with N-2 gas at a specific gas loading rate of 30 m(3).m (-2) day(-1). In contrast to the regular UASB reactors, the gas-supplied UA SB showed a more stable performance when the organic loading rates were inc reased. Also, the H2S stripping efficiency was 3-4 times higher in the gas- supplied UASB, reaching values of 67%. Higher values were not obtained owin g to the relatively poor sulphate reduction efficiencies.