Abstract
A new method of defibring wood chips and producing chemi-thermomechanical pulp with significant capital and energy savings has been developed. The pollution load of bleached CTMP effluent varied between 40-60 kg/t for BOD5 and 120-210 kg/t for COD. The process includes a sulphite stage and the effluent contains sulphur compounds which are toxic to methanogenic bacteria. Studies have been conducted on anaerobic fixed bed reactors with upflow or downflow feed modes. It is necessary to progressively acclimate bacteria to increasing concentrations of sulphur compounds. Permanent functioning of reactors is possible with COD/S ratios near 10-12 where propionate concentrations are increasing. Upflow reactors are more efficient than downflow reactors, with a maximum loading rate of 3.5 kg COD/m3.day corresponding to 800 mg/l as S. With hybrid sludge bed/fixed bed reactors it is possible to enhance anaerobic treatment efficiency. A COD loading rate of 4.7 kg/m3.day can be maintained with a residence time of 2 days and a pollution load removal efficiency of 45% for COD (55% with complementary oxidation) and 70% for BOD5. The economics of effluent treatment give an investment cost of about 20 US$/ADt for the activated sludge process and 12.5 US$/ADt for anaerobic fixed bed treatment.
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