Influência da concentração de oxigênio dissolvido sobre o desempenho de sistemas de lodo ativado tratando efluentes da indústria petroquímica.

Abstract

The energy requirement for aeration is normally the largest factor of operational costs in activated sludge systems. In order to treat the industrial effluent generated at the Petrochemical Complex at Camacari CETREL installed an aeration capacity of more than 5 MW. Since the operational costs are high, the possibility of reduction of energy consumption without adverse effects on the effluent quality or the sludge properties was evaluated. Among the parameters defining the effluent quality, the concentrations of organic matter, suspended solids and ammonium are the most important ones, as these have limits imposed by the environmental authority (CRA, Regional Environmental Council). The most important sludge properties are the metabolic capacity in the degradation of substrates (organic matter and ammonium) and the sludge settleability. One possibility to reduce costs is to reduce the concentration of dissolved oxygen in the mixed liquor. Aeration theory affirms that the energy required for aeration is proportional with the oxygen deficit, i.e. the difference between the saturation concentration of dissolved oxygen and its value at any time in the liquid phase. Thus, the lower the DO concentration, the lower the energy requirement for aeration. However, it is necessary to verify the effect of the DO reduction on the quality of the effluent and the sludge. In an experimental investigation three pilot scale systems were operated, while different DO concentrations were maintained in each system. The DO concentration was maintained at the desired levels (3,0, 1,5-2,5 e 0,5-1,5 mg//) with the aid of respirometers. The influence of the DO concentrations on the effluent quality was observed during the experimental investigation. The respirometers were also used to assess the biological quality of the sludges. The influence on settleability was evaluated from data of the sludge volume index (SVI) of the three sludges. The heterotrophic sludge activity can be estimated from the oxygen uptake rate (OUR) as follows: after determining the OUR in the absence of substrate (i.e. the endogenous respiration rate), substrate is added in abundance so that the sludge starts to consume oxygen at maximum rate, both for endogenous respiration (which remains unchanged) and for its exogenous demand to metabolize the extra cellular material. The proportion between the maximum (with abundant substrate) and the minimum OUR (without substrate) is indicative for the metabolic capacity of the sludge. The same method can also be used to evaluate the autotrophic (nitrifying) sludge activity: in this case an ammonium salt (for example NH4CI) is added in abundance and the OUR related to this addition is measured. The results of the investigation showed that down to a DO concentration of 1 mg// there was no significant influence of this parameter on the effluent quality, or on the biological or mechanical sludge properties, with 83% removal of COD, 99% removal of TSS and ammonia, and LVI at 34ml.g"1.