Specialist Group on ANAEROBIC DIGESTION Newsletter - IWA

Operation and modeling of anaerobic sequencing batch reactors (ASBR) in the
treatment of complex effluents
by Andrés Donoso-Bravo
Institution:
School of Biochemical Engineering, Pontificia Universidad Católica de Valparaíso
New PhD Contact:
Postdoctoral Researcher
Department of Chemical Engineering and Environmental Technology
University of Valladolid. Spain
Supervisor:
Dr. Rolando Chamy
Abstract:
Anaerobic digestion has become a consolidated and sustainable technology due to its
economical and environmental benefits. Nowadays, anaerobic digestion is applied in a
wide variety of organic waste and wastewater and presents comparable advantages over conventional aerobic
processes. Several anaerobic reactors have been developed, mainly UASB, EGSB and CSTR reactors, however,
new configurations are being developed and studied to increase the amount of waste treated by anaerobic
digestion. One of the most important features of the Anaerobic Sequencing Batch Reactors (ASBR) is its
operational flexibility as a result of its operational variables that may be manipulated such as the reaction time,
the feeding time or the type of feeding.
In this work, an Anaerobic Sequencing Batch Reactor (ASBR) has been operated for 8 months to treat low
strength sewage with high suspended organic matter content simulating domestic wastewater. Three phases of
operation with increasing Organic Loading Rates (OLR) have been carried out: 0.4 (phase I), 0.8 (phase II) and
1.2 kg COD/m 3·d (phase III). During the whole experimental period, adequate values of the stability parameters
(pH, total alkalinity) were obtained. During phases I and II, the removal efficiencies of organic matter
(expressed as total COD) and total suspended solids ranged between 50 and 60%. However, these values
decreased to 15-25% in phase III, due to the less HRT the methanogenic biomass was washed out of the reactor
Afterward, a two-phase anaerobic system using Anaerobic Sequencing Batch Reactors (ASBR) treating the
same simulated wastewater, to optimize the activity of both populations was implemented. Two reactors in
series were set, the first one was hydrolytic/acidogenic, while the second one was a methanogenic. Instantaneous
feeding was used in both reactors. This configuration will promote solid removal reaching higher efficiencies.
During the experiment, a 69% and 50% of removal efficiencies of total cod removal were obtained for a OLR of
0.63 and 1.22 kgCOD/m 3·d, respectively. The Solubilized Organic Fraction (SOF) was within a range of 0.3 -
0.6 gCOD solubilized /gpCOD removed and an average on the Acidified Organic Fraction (AOF) of 0.6 gCOD vfaproduced/gsCOD
fed , produced in the hydrolytic-acidogenic reactor. These are good results considering the low
COD influent and the high fraction level of solids content. The methanogenic reactor presented VFA removal
fraction (VFARF) value between 0.4 – 0.6 gCOD vfa-removed /gCOD vfa-fed for the OLR of 0.63 and 1.22. An overall
good behaviour of the system was achieved during the whole experimentation, showing that the two-phase
system was stable, robust and effective for simulated domestic wastewater by anaerobic digestion.
Then 2 anaerobic sequencing reactors were implemented assessing two operation modalities batch and fedbatch,
in the phenolic wastewater treatment using an unacclimated biomass was evaluated. It is important to note
that many industries do not have a continuous discharge of phenol in their effluents, though phenol may be
released occasionally because of a seasonal change or abnormality in the upstream process. In these cases, it is
necessary to acquire acclimated biomass or to utilize rapid biomass adaptation in order to treat the effluent. The
batch reactor showed a poor performance at 5 g/l of cod with a 10, 25 and 40% of the phenol cod, despite a
readily co-substrate was used in the treatment. A fast adaptation of the biomass was performed in the fed-batch
reactor using phenol as the only carbon source, reaching a 100% of phenol removal in 10 days. The best results
were obtained with a 400 mg/l (160 mg/l·d) after 40 days of operation reaching over 95% of phenol removal.
The phenol concentration impaired the biomass settleability in both reactors, causing an important washout of
biomass. A fed-batch modalities can be an alternative for the anaerobic treatment of phenolic wastewater.
Finally, a model with 3 reactions and 2 populations was developed, implemented and validated with the
experimental data previously obtained. A good correlation between the simulated variables and the experimental
data was obtained; hence this model might represent a potential tool to control and prediction purposes.
IWA <strong>Specialist</strong> <strong>Group</strong> on Anaerobic Digestion
16 June 2009 Newsletter