Specialist Group on ANAEROBIC DIGESTION Newsletter - IWA

Sulfate reducing biological processes in biofilms for metal precipitation
by Marisol Gallegos García (mgg@ipicyt.edu.mx)
Institution:
Instituto Potosino de Investigación Científica y Tecnológica.
División de Ciencias Ambientales.
Camino a la Presa San José 2055. Lomas 4ª Sección.
San Luis Potosí, SLP. México. 78216
Supervisors:
Dr. Elías Razo-Flores
Dra. Mª. Lourdes Berenice Celis-García
Abstract:
The environment contamination by heavy metals is an important matter due to its negative impact over the
public health, the environment and the economy. Sulfate reduction has become and alternative for the treatment
of wastewaters that contain heavy metals. The general objective of this work was to study the application of
biological sulfate reduction processes in biofilm reactors for metal precipitation and recovery of the metal
sulfides in one step. First of all, the competence between methanogens and sulfate reducers was studied in an
UASB reactor that was operated for over 240 days, for the development of sulfate reducing biomass from a
methanogenic granular sludge. The chemical oxygen demand (COD) removed by sulfate reduction was 20% and
the total sulfide concentration reached in the reactor was 310 mg/L. The specific sulfate reducing and
methanogenic activities of the granular sludge pointed out to the development of sulfate reducing bacteria
(SRB). The presence of SRB that oxidize incompletely the substrate to acetate, allowed the permanence of
methanogens that were not limited by substrate, and in consequence were not completely displaced by the SRB
at the end of reactor operation. Afterwards, a down-flow fluidized bed reactor (DFFB) was proposed for the
precipitation and recovery of metals. The DFFB reactor was operated continuously during 320 days and
precipitation efficiencies over 99.0% of Fe, Zn and Cd were obtained. The recovered metal sulfides amounted to
90% of the theoretical mass expected to be precipitated; pyrite (FeS), sphaleryte (ZnS) and greenockite (CdS)
predominated in the precipitate. The metal loading rate did not affect the reactor performance even at pH values
as low as 5.0. The COD removal efficiency was close to 54%, which suggested an incomplete substrate
oxidation by the immobilized SRB, this was confirmed by the high concentration of acetate in the effluent and
the absence of alkalinity production. Finally, the development of a sulfate reducing biofilm able to oxidize
completely the substrate was studied. To achieve this, a selection pressure by substrate was used; three different
assays were carried out in the DFFB reactor that was fed with a mixture of acetate/lactate with the following
ratios on a COD basis: 50/50, 80/20 and 90/10 during 21 days. The best biofilm performance in terms of COD
removal efficiency (71%) and sulfide production rate (2.9 mg H 2 S/L-h) was obtained with the biofilm developed
in the experiment with the 90/10 acetate/lactate ratio. With this biofilm, the reactor was operated continuously
110 days. When the pH in the fed was decreased to 4.0, any inhibitory effect was observed over the SRB, as the
reactor performance showed which achieved COD and sulfate removal efficiencies of 80 and 42%, respectively.
The alkalinity produced by the complete oxidation of the substrate neutralized the influent pH, and maintained a
pH of 7.0 within the reactor. This work is the first to report the precipitation and recovery of metals as metal
sulfides in one step and the development, in a short period of time, of a SRB biofilm able to oxidize completely
the substrate.
Link to Thesis: http://www.ipicyt.edu.mx/storage-sipicyt/materialbiblioteca/GallegosGarc%edaMarisol.pdf
List of publications:
Gallegos-García, M., et al. (2009). Precipitation and recovery of metal sulfides from metal containing acidic
wastewater in a sulfidogenic down-flow fluidized bed reactor. Biotechnol Bioeng. 102, 91-99.
doi.wiley.com/10.1002/bit.22049
Gallegos-García, M., et al. (2008). Competencia por sustrato durante el desarrollo de Biomasa sulfato reductora
a partir de un lodo metanogénico en un reactor UASB. Sometido, a la Revista Internacional de
Contaminación Ambiental.
Gallegos-García, M., et al. (In preparation). Formation of a sulfate-reducing biofilm of complete oxidation of the
substrate for the treatment of a synthetic effluent with sulfate.
IWA <strong>Specialist</strong> <strong>Group</strong> on Anaerobic Digestion
18 June 2009 Newsletter