Viabilidade técnica e econômica da digestão anaeróbia do lodo de excesso da Cetrel.

Abstract

CETREL - a Company of Environmental Protection is responsible for the treatment of the organic and inorganic liquid effluents dischargeds from the petrochemical complexai Camaçari-Bahia. The organic part is treated at the Central Treatment Plant by means ofan activated sludge, process which resultsinthe generation of 30 t/d of excess sludge. Presently aerobic digestion is applied for sludge stabilisation, before its final destination at land farms. Although the aerobic mechanism associated to the land farms has proved its efflciency, it has become unsuitable because of the high energy costs. In an attempt to reduce operational costs, an experimental investigation was carried out with the objective to establish the technical and economical feasibility of excess sludge stabilisation by anaerobic digestion. Pilot and bench scale units were used in the investigation. On the basis of experimental data, it was verified that the application of anaerobic digestion is technically feasible for CETREL sludge. The existing main aerobic digester can be converted into an anaerobic unit and operated at environmental temperature (25 to 35°C) with a minimum retention timeof lOdays. The removal efflciency of suspended solids varied from 30 to 40 per cent for retention times of 10 to lódays. The latter retention time was identified as the optimal one. The methane production was estimated at 82m3 (10 d) to 134m3 (16 d) per ton of applied volatile sludge. The results show that there is a generation potentialof 3,8 t/d of methane from which a electric power of 800kW can be produced. At the same time the power required in the aerobic reactor (800kW) is strongly reduced. The total energy savings of reducing the consumption and having internai power generation represent a total of 1600kW which at current cost of 0,07 US$/kWh result in an economy of approximately US$1.200.000 per annum. The required investment is estimated at US$2.000.000 so that the conversion of aerobic to anaerobic digestion is vexy attractive from the economical point of view having a pay-back time of less than 2 years. It was also established that anaerobic sludge, after aperiod of settling, can be processed more easily on drying beds than aerobic sludge.