Walter Borzani

Anaerobic sequencing batch reactors for wastewater treatment: a developing technology

Abstract This paper describes and discusses the main problems related to anaerobic batch and fed-batch processes for wastewater treatment. A critical analysis of the literature evaluated the industrial application viability and proposed alternatives to improve operation and control of this system. Two approaches were presented in order to make this anaerobic discontinuous process feasible for industrial application: (1) optimization of the operating procedures in reactors containing self-immobilized sludge as granules, and (2) design of bioreactors with inert support media for biomass immobilization.

Feasibility of a stirred anaerobic sequencing batch reactor containing immobilized biomass for wastewater treatment

This paper proposes a new configuration of a stirred anaerobic bioreactor to improve the biomass retention in sequencing batch processes for treating low strength wastewater. In the proposed configuration, the biomass was immobilized on a polyurethane foam matrix, thus resulting in the elimination of or reduction in settling time. The reactor was operated at a temperature of 30°C and an agitation rate of 500 rpm supplied by means of a stirring bar at the bottom of the reactor. An 8-h cycle was used to treat 0.5 l of a synthetic substrate containing a chemical oxygen demand (COD) of 485 mg/l. The overall COD removal efficiency attained 86% in the first 3 h of the cycle, as shown by the concentration profile.

TREATMENT OF LOW-STRENGTH WASTEWATER USING IMMOBILIZED BIOMASS IN A SEQUENCING BATCH EXTERNAL LOOP REACTOR: INFLUENCE OF THE MEDIUM SUPERFICIAL VELOCITY ON THE STABILITY AND PERFORMANCE

An anaerobic sequencing batch bioreactor with external circulation of the liquid phase wherein the biomass was immobilized on a polyurethane foam matrix was analyzed, focussing on the influence of the liquid superficial velocity on the reactors stability and efficiency. Eight-hour cycles were carried out at 30oC treating glucose-based synthetic wastewater around 500 mgDQO/L. The performance of the reactor was assessed without circulation and with circulating liquid superficial velocity between 0.034 and 0.188 cm/s. The reactor attained operating stability and a high organic matter removal was achieved when liquid was circulated. A first order model was used to evaluate the influence of the liquid superficial velocity (vS), resulting in an increase in the apparent first order parameter when vS increased from 0.034 to 0.094 cm/s. The parameter value remained unchangeable when 0.188 cm/s was applied, indicating that beyond this value no improvement on liquid mass transfer was observed. Moreover, the necessary time to reach the final removal efficiency decreased when liquid circulation was applied, indicating that a 3-hour cycle could be enough.

Measurement of invertase activity of cells of Saccharomyces cerevisiae

The determination of the invertase activity of intact yeast cells presents a critical point, that is, the blockage of the enzyme action at a given moment. In this paper seven blockage methods were compared: the addition of 0.010 M sodium hydroxide solution, addition of 0.010 M sodium carbonate solution, addition of 0.010 M sodium carbonate solution followed by centrifugation (9750g; 10 min), immersion of the reacting mixture in a boiling water bath, immersion of the mixture in a -15 degrees C bath, filtration through a Millipore membrane, and addition of the first Somogyis reagent followed by immersion in a boiling water bath. Only the last two methods lead to a rapid and effective blockage of the invertase activity.

Variation of the ethanol yield during oscillatory concentrations changes in undisturbed continuous ethanol fermentation of sugar-cane blackstrap molasses.

During the oscillatory phase of an undisturbed continuous ethanol fermentation of sugar-cane blackstrap molasses, the relative ethanol yield oscillated between 70 and 92% of the theoretical value (0.511), while its actual value was 85.6%. The ethanol yield based on catabolic activity oscillated between 0.290 and 1.174 g/kcal, while its actual value was 0.686 g/kcal. The specific production rate of ethanol increased when the specific growth rate of the yeast cells increased; a linear equation correlates the above specific rates.

A weighing method to identify the microbial growth phases in solid-state fermentation tests.

A general equation correlating the variation rate of the fermenting medium weight (dry matter) and the cell biomass growth rate in solid-state fermentation tests is proposed with the main purpose of identifying the microbial growth phases.

Determination of the volume fraction of yeast cells in a sediment or in a suspension

SummaryA simple and precise method for the measurement of the volume fraction of cells in a sediment or in a suspension is described. Determinations carried out with yeast cells sediments obtained from completely fermented molasses mash lead to cells volume fraction values affected by a coefficient of variation equal to 1.9% and to a cell density equal to 1.115 g.ml−1.When the method was applied to cells suspensions the coefficient of variation of the cells volume fraction was 2.9% and the cell density was 1.120 g.ml−1.

Measurement of the gassing power of bakers' yeast: correlation between the dough volume and the incubation time

An empirical equation is proposed to correlate the dough volume and the incubation time during cylinder tests using thin flour dough carried out to evaluate the gassing power of compressed yeast. The above equation permitted to correlate the gassing power of the yeast and the proof time, as well as to calculate the specific rate of the dough volume variation at any time. It provided more information regarding the fermentation power of the yeast than the sole value of its gas-producing power. A physical interpretation of the proposed equation is presented.

Calculation of fermentation parameters from the results of a batch test taking account of the volume of biomass in the fermenting medium

The values of fermentation parameters calculated from the measured concentrations of substrates and/or products may be significantly affected by the volume of biomass in the fermenting medium. Corrections proposed in this paper should be evaluated and, depending on their magnitude, considered in order to obtain more representative results.

Product inhibition during the feeding phasein fed-batch ethanol fermentation of sugar-cane blackstrap molasses

SummaryThe following equations represent the influence of the ethanol concentration (E) on the specific growth rate of the yeast cells (μ) and on the specific production rate of ethanol (ν) during the reactor filling phase in fed-batch fermentation of sugar-cane blackstrap molasses: μ = μ0 - k · E and v = v0 · K/(K +E)