The role of salinity on the changes of the biomass characteristics and on the performance of an OMBR treating tannery wastewater
Graphical abstract
Introduction
Tannery industry belongs to the most polluting industrial sectors (Kandasamy et al., 2017; Maharaja et al., 2017). This industry generates huge volumes of potentially toxic wastewaters characterized by high organic matter and salt concentration. In order to transform raw skins into finished leather products, three groups of processes have to be carried out: beamhouse operations (for flesh, hair and epidermis removal from the skin), tanning steps (to stabilize and avoid putrefaction) and final processes, in which the leathers are finished, acquiring the desired aesthetic appeal and commercial value (Mosca et al., 2017). Typically, beamhouse operations generate alkaline effluents and tanning steps and final processes generate acidic effluents (Drioli and Cortese, 1980; Mendoza-Roca et al., 2010).
The most common treatment of the global effluents is based on a sulfide oxidation of the alkaline stream, a subsequent homogenization of desulfurized alkaline wastewater and acidic streams and a physic-chemical process of the global wastewater. The aim of the physic-chemical treatment is to reduce suspended solids and chemical oxygen demand (COD) content and to remove chromium by precipitation (George et al., 2015). However, the remaining high COD and salt concentrations require further treatments to meet the discharge limits. In this way, it is necessary to apply biological treatments to reduce the organic matter content. Membrane bioreactors (MBR) produce effluents with higher quality than conventional biological treatments (Wang et al., 2016) and are also employed for industrial wastewaters (Le-clech et al., 2006; Yang et al., 2006). MBR are characterized by high sludge retention time and low reactor volume. However, the main limitations of their application are the high energy consumption and the high maintenance costs due to the frequent membrane cleaning (Suganthi et al., 2013).
Recent research advances in wastewater treatment and reuse have promoted the development, as an emerging technology, of OMBR (Luo et al., 2016). OMBR uses FO membranes to separate treated water from a bioreactor mixed liquor. The separation process is produced because of the osmotic pressure difference between the biological reactor and the draw solution (DS) (Luján-Facundo et al., 2017; Wang et al., 2016). The use of the FO nonporous membranes offers important advances such as high rejection capacity for trace organic compounds (Xie et al., 2012), pathogens (Hoover et al., 2011) and ions (Hickenbottom et al., 2013), low membrane fouling and the energy consumption. However, the reverse salt flux (RSF) due to the Fick’s law and the accumulation of non-biodegradable substances in the reactor originate operating problems in the OMBR. Salinity build-up in the biological reactor may significantly reduce the membrane water flux, modifying the mixed liquor characteristics and enhancing scaling phenomena (Gu et al., 2015). Thus, the impact of solutes accumulation on sludge properties and on microbial community and its activity is one of the most important issues to be studied in OMBR. In this way, parameters such as microbial hydrolytic enzymatic activities (MHEA), intracellular and extracellular ATP, cellular viability and the study of the microbial population will give valuable information about the OMBR process (Nguyen and Chong, 2015).
The changes in the biomass characteristics during the OMBR operation have been evaluated by a limited number of authors. Wang et al. (2017b) studied the bacterial community analysis for the removal of silver nanoparticles from simulated wastewater using a combination of microfiltration with OMBR. They found that the main phylum in the sludge was Proteobacteria and the relative abundance of this phylum decreased as salt concentration increased. Luo et al. (2017) compared the microbial community structure of a conventional MBR with that found in an OMBR. They concluded that there were differences between each community analysis such as the presence of Planctomycetes only in the MBR and the increase in the relative abundance of Bacteroidetes during the OMBR operation time. Qiu and Ting, 2013 studied the effect of salt accumulation on microbial community during a OMBR operation. They published that almost all the dominant species in the sludge were replaced by salt-tolerant new species as a consequence of salinity build-up.
The main objective of this paper is to study how the characteristics of the biomass change with the operation time of an OMBR treating tannery wastewater using an actual industrial wastewater mainly consisting of ammonium sulphate as DS. Until now, the study of the performance of an OMBR process focusing on the biomass characteristics using two actual effluents (as feed and as draw solutions) cannot be found in the literature. In this work, the OMBR mixed liquor proposed characterization includes MHEA, ATP, bacterial viability and microbial population. The relation of these mixed liquor characteristics and accumulated COD and salts in the OMBR during its operation is assessed.
Section snippets
Wastewater and draw solution
An actual wastewater from a tannery industry was used as influent. The main characteristics of this wastewater were: COD between 1497 and 3468 mg L−1, pH in the range of 8.04–9.29 and conductivity varying from 10.8 to 19.41 mS cm−1. An actual wastewater from an absorption process for ammonia removal was employed as DS. This wastewater mainly consisted of ammonium sulphate (SO4−2 and NH4-N concentrations of 153 g L−1 and 19 g L−1, respectively). The pH and conductivity of this industrial
Mixed liquor conductivity and membrane water flux
Fig. 1 shows the mixed liquor conductivity changes during the OMBR operation. The numbers expressing percentages in the figure mean the percentage of tannery wastewater in the OMBR feed. As expected, there was salt accumulation in the bioreactor due both to the reverse salt flux during forward osmosis process and the high rejection capacity of the FO membrane (>97%) (Praveen and Loh, 2016). It can be observed that the mixed liquor conductivity increases progressively from 1.3 mS cm−1 to a final
Conclusions
In this work, the performance of an OMBR treating tannery wastewater has been evaluated. In this type of reactors salinity and non-biodegradable COD are accumulated due to the high rejection of the FO membranes. In addition to it, the reverse salt flux contributes to a faster conductivity increase in the reactor.
Process monitoring is difficult because COD removal efficiencies data avoid finding out the actual performance of the biological process. Thus, a series of biological indicators have
Acknowledgments
This study was supported by the Spanish Ministry of Economy and Competitiveness through the project RTC-2015-3582-5-AR.
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