José Alberto Domingues Rodrigues

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.

Influence of agitation rate on the performance of an anaerobic sequencing batch reactor containing granulated biomass treating low-strength wastewater

Abstract The present work reports on the influence of the mechanical agitation rates on the performance of a stirred anaerobic sequencing batch reactor with a six-vertical-blade disk turbine on the basis of chemical oxygen demand (COD). The reactor containing granulated biomass treating synthetic domestic wastewater was operated at 30 °C and an 8-h cycle was applied to treat approximately 2.0 l of the synthetic substrate with COD of nearly 500 mg/l. The studied agitation rates ranged from no agitation to 75 rev./min. The system attained non-filtered and filtered substrate removal efficiency of 80 and 88%, respectively, at the agitation rate of 50 rev./min, presenting a relatively good solid retention and no granule break-up. Moreover, the use of agitation mainly increased the efficiency throughout the cycle of the reactor and, consequently, enabled reduction of the total cycle time. An empirical equation and a first order kinetic model with a residual organic matter concentration were proposed to analyze the influence of agitation rates on the reactors performance.

Enhancement of the performance of an anaerobic sequencing batch reactor treating low-strength wastewater through implementation of a variable stirring rate program

This work focuses on enhancement of the performance of an anaerobic sequencing batch reactor with a six-vertical-blade-disk-turbine impeller, containing granulated biomass treating low-strength synthetic wastewater, through a study of the feasibility of implementing a variable stirring rate program. The reactor was operated at 30oC and a six-hour cycle was used to treat approximately 2.0 L of the synthetic substrate with a chemical oxygen demand (COD) of nearly 500 mg/L. Two different stirring rate program were implemented: a constant rate of 50 rpm and a variable rate consisting of 75 rpm for one hour, 50 rpm for four hours and 25 rpm for 0.5 hour. The last 0.5 hour of the cycle was used for the settling step. In both cases, a very short start-up period and unfiltered and filtered substrate removal efficiencies of 81% and 88%, respectively, were attained. However, use of the variable stirring rate enhanced efficiency of the reactor dynamics without impairing biomass morphology, thus resulting in a reduction in the total cycle time and a possible decrease in energy consumption. Additionally, a simplified model of the anaerobic metabolic activity, using apparent kinetic parameters, was proposed as a consecutive first-order kinetic model with substrate and total volatile acid residual concentrations in order to analyze how the variable stirring rate affects reactor performance.

Effect of feeding strategy on a stirred anaerobic sequencing fed-batch reactor containing immobilized biomass

The present work reports on the influence of feeding strategy on the stability and performance of a stirred anaerobic sequencing fed-batch reactor containing biomass immobilized on polyurethane foam. The reactor treated low-strength wastewater and was operated at 30 degrees C with an agitation rate of 200 rpm. A 180-min cycle was used to treat approximately 0.5 l of synthetic substrate with a chemical oxygen demand concentration of nearly 500 mg/l. The reactor was operated in batch mode with a 3-min feeding step and in constant rate fed-batch mode with feeding steps of 30, 60 and 180 min. During batch operation, the system attained stability and had a removal efficiency of 86% based on non-filtered substrate concentration. However, during fed-batch operation stability and efficiency were impaired and formation of suspended material was identified. Stability was achieved only for the 30-min feeding step. The poor performance and instability observed in the fed-batch experiments were credited to the formation of considerable quantities of extracellular polymers, which impeded contact between substrate and biomass with consequent negative effect on the mass transfer fluxes. The biopolymer formation was very likely a result of the fed-batch operational mode, in which part of the bioparticles were deprived of contact with the liquid medium for a relatively long period of time.

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.

External and internal mass transfer effects in an anaerobic fixed-bed reactor for wastewater treatment

Abstract An evaluation of mass transfer phenomena in horizontal-flow anaerobic immobilized biomass (HAIB) reactors is presented. The data refers to two similar HAIB reactors filled with polyurethane foam matrices for biomass immobilization: a pilot unit and a bench-scale reactor. The pilot unit of 237 l was used for the treatment of pre-screened domestic sewage while the bench-scale reactor of 2 l was fed with glucose-based synthetic substrate. Mass balances of substrate for single particles were applied to derive expressions correlating the substrate concentration in the bulk liquid to its concentration in any position of the bioparticle. These permitted the evaluation of the substrate concentration on the solid–liquid interface and the substrate concentration profiles inside the bioparticle. The data obtained were used to perform an analysis of the mass transfer phenomena involved in the organic matter conversion process. The results indicated that the solid-phase mass transfer was the limiting step of the overall substrate conversion rate in the pilot-scale HAIB reactor treating pre-screened sewage. The liquid-phase mass transfer in the bench scale reactor treating synthetic substrate mainly affected the overall conversion rate.

Fed-batch and batch operating mode analysis of a stirred anaerobic sequencing reactor with self-immobilized biomass treating low-strength wastewater

This work presents an analysis of a stirred anaerobic sequencing discontinuous reactor with different substrate feeding strategies resulting in batch, fed-batch/batch and fed-batch operating modes. The reactor, containing granulated biomass, was fed with approximately 2.0L of synthetic domestic wastewater with Chemical Oxygen Demand of nearly 500 mg/L per cycle and operated at 30 degrees C and 50 rpm. Three feeding strategies with a total cycle time of 6 h, including 30-min settling, were adopted: batch mode with a fill cycle of 6 min, a fed-batch/batch mode with fill cycles of 60, 120 and 240 min and fed-batch mode with a fill cycle of 320 min. The system attained average non-filtered and filtered substrate removal efficiency of 78 and 84%, respectively, for all operating conditions, presenting good stability, solid retention and no granule break-up. A first order kinetic model with a residual organic matter concentration was proposed to analyze the influence of the feeding strategy on the performance during a cycle and bicarbonate alkalinity and total volatile acids concentration profiles were also quantified in order to verify the transient stability behavior.

Effect of impeller type and mechanical agitation on the mass transfer and power consumption aspects of ASBR operation treating synthetic wastewater.

The effect of flow type and rotor speed was investigated in a round-bottom reactor with 5 L useful volume containing 2.0 L of granular biomass. The reactor treated 2.0 L of synthetic wastewater with a concentration of 800 mgCOD/L in 8-h cycles at 30 degrees C. Five impellers, commonly used in biological processes, have been employed to this end, namely: a turbine and a paddle impeller with six-vertical-flat-blades, a turbine and a paddle impeller with six-45 degrees -inclined-flat-blades and a three-blade-helix impeller. Results showed that altering impeller type and rotor speed did not significantly affect system stability and performance. Average organic matter removal efficiency was about 84% for filtered samples, total volatile acids concentration was below 20 mgHAc/L and bicarbonate alkalinity a little less than 400 mgCaCO3/L for most of the investigated conditions. However, analysis of the first-order kinetic model constants showed that alteration in rotor speed resulted in an increase in the values of the kinetic constants (for instance, from 0.57 h(-1) at 50 rpm to 0.84 h(-1) at 75 rpm when the paddle impeller with six-45 degrees -inclined-flat-blades was used) and that axial flow in mechanically stirred reactors is preferable over radial-flow when the vertical-flat-blade impeller is compared to the inclined-flat-blade impeller (for instance at 75 rpm, from 0.52 h(-1) with the six-flat-blade-paddle impeller to 0.84 h(-1) with the six-45 degrees -inclined-flat-blade-paddle impeller), demonstrating that there is a rotor speed and an impeller type that maximize solid-liquid mass transfer in the reaction medium. Furthermore, power consumption studies in this reduced reactor volume showed that no high power transfer is required to improve mass transfer (less than 0.6 kW/10(3)m3).

Influence of feed time and sulfate load on the organic and sulfate removal in an ASBR.

The removal of sulfate and organic matter was assessed in an ASBR, which treated wastewater containing 500 mg CODL(-1) (3 g CODL(-1)d(-1)) in 8h-cycles at 30 degrees C. The wastewater was enriched with sulfate at [COD/SO(4)(2-)] ratios of 1.34, 0.67 and 0.34 (8.8,4.5 and 2.2 gSO(4)(2-)L(-1)d(-1)). For each COD/[SO(4)(2-)] ratio fill times used were: 10 min (batch), 3 and 6h (fed-batch), achieving sulfate reduction of 30%, 72% and 72% (COD/[SO(4)(2-)] of 1.34); 25%, 58% and 55% (COD/[SO(4)(2-)] of 0.67) and 23%, 37% and 27% (COD/[SO(4)(2-)] of 0.34), respectively, and organic matter removal of 87%, 68% and 80% (COD/[SO(4)(2-)] of 1.34); 78%, 75% and 69% (COD/[SO(4)(2-)] of 0.67) and 85%, 84% and 83% (COD/[SO(4)(2-)] of 0.34), respectively. The results showed that fed-batch operation improved sulfate reduction, whereas organic matter removals were similar for batch and fed-batch operation. In addition, increase in sulfate loading in the fed-batch operation improved organic matter removal.