Assessing the potential of purple phototrophic bacteria for the simultaneous treatment of piggery wastewater and upgrading of biogas
Graphical abstract
Introduction
The annual production of pig meat in the European Union (EU) accounted for 24.1 million tons in 2017, which ranked the EU as the second largest pig producer in the world. In this context, a total of 150.1 million pig heads were produced in the EU in 2017 (Statista, 2018). Unfortunately, this key economic sector annually generates in the EU between 217 and 434 million m3 of piggery wastewater (PWW) containing high concentrations of organic matter, nitrogen and phosphorus (De Godos et al., 2009, García et al., 2017). The management of such high volumes of PWW represents nowadays an economic, environmental and technical challenge for the EU livestock industry. Anaerobic digestion and activated sludge processes are typically implemented on-site or in centralized facilities in order to fulfill with European wastewater discharge regulations (Andreoli and Von, 2007). In addition, alternative technologies based on the intensification of algal-bacterial symbiosis have been also tested both at lab and pilot scale in order to reduce the operating costs and enhance nutrient recovery during PWW treatment compared to conventional technologies (De Godos et al., 2009, García et al., 2018, García et al., 2017). Nevertheless, PWW treatment based on algal-bacterial symbiosis is limited by the high NH4+ concentrations of this type of wastewater and its poor performance at low temperatures, which requires the development of more resilient biotechnologies capable of cost-competitively recovering the carbon and nutrients from PWW.
In this context, purple phototrophic bacteria (PPB) have emerged as a promising technology platform for wastewater treatment based on their ability to assimilate a higher fraction of the carbon, nitrogen and phosphorous present in wastewater compared to their aerobic and anaerobic counterparts (Hiraishi et al., 1991, Khatipov et al., 1998, Takabatake et al., 2004). Compared to microalgae, PPB utilize infrared radiation (IR) as source of energy, which reduces the power required by photon emission and allows a deeper light penetration into the cultivation broth (thus reducing the footprint of the process) (Hülsen et al., 2014). In addition, the influence of temperature on the growth of PPB is low, which makes them ideal microorganisms to support wastewater treatment under multiple weather conditions. Literature studies have shown the promising potential of these microorganisms for municipal and PWW treatment. For instance, Kim et al. (2004) reported chemical oxygen demand (COD) and orthophosphate removals of 50% and 58%, respectively, under anaerobic conditions in a PPB photobioreactor. PPB have been also successfully applied for industrial wastewater treatment in membrane photobioreactors and sequencing batch stirred tank photobioreactors with COD removal efficiencies of 73–75% (Chitapornpan et al., 2012, Kaewsuk et al., 2010). The ability of PPB to simultaneously remove COD, nitrogen and phosphorus from domestic wastewater has been recently evaluated in photo-anaerobic batch tests and in a continuous membrane photobioreactor (Hülsen et al., 2016, Hülsen et al., 2014). A recent comparison between the use of PPB and microalgae for the recovery of carbon, nitrogen and phosphorous from pork, poultry, sugar, dairy and red meat wastewater was carried out by Hülsen et al. (2018), who confirmed that PPB are more efficient for organic and nutrient removal than microalgae.
On the other hand, biogas from the anaerobic digestion of wastewater or organic solid waste represents a renewable energy vector with potential to partially reduce the current world’s dependence on fossil fuels (Andriani et al., 2014, Muñoz et al., 2015). In the EU, the contribution of biogas to the energy sector has increased by a factor of 3 concomitantly with an increase in the number of biogas plants from 6227 in 2009 to 17,662 by the end of 2016 (European Biogas Association, 2017). Biogas upgrading to biomethane is required prior injection into gas grids or use as a vehicle fuel due to the large number and high concentrations of impurities: CO2 (15–60%), H2S (0.005–2%), O2 (0–1%), N2 (0–2%), CO (<0.6%), NH3 (<1%), volatile organic compounds (<0.6%) and siloxanes (0–02%) (Ryckebosch et al., 2011); while most international regulations require concentrations of CH4 ≥ 95%, CO2 ≤ 2–4%, O2 ≤ 1% and negligible amounts of H2S (Muñoz et al., 2015). Algal-bacterial systems have been consistently investigated as a low cost and environmentally sustainable technology to simultaneously remove CO2 and H2S from biogas. However, O2 stripping from the cultivation broth to the biomethane as a result of the oxygenic nature of algal photosynthesis represents the main limitation of algal-bacterial systems in biogas upgrading (Marín et al., 2018, Posadas et al., 2017, Posadas et al., 2015). In this sense, the versatile metabolism of PPB, capable of using H2S in biogas or the organic matter present in wastewater as electron donor to reduce CO2 from biogas without O2 generation, could eventually support a cost-effective biogas upgrading. Overall, there is a lack of comparative studies assessing the potential of PPB and algal-bacterial systems in order to determine the most cost-effective and environmentally friendly biotechnology for biogas upgrading.
This study aimed at evaluating, for the first time, the potential and limitations of using PPB for the simultaneous treatment of PWW and upgrading of biogas under IR in batch photobioreactors. The influence of PWW dilution and phosphorous concentration on PPB-based PWW treatment coupled to biogas upgrading were also investigated batchwise. The mechanisms and limiting factors underlying wastewater treatment and CO2/H2S removal by PPB were investigated. A comparative evaluation of PPB-based biogas upgrading vs. algae-based photobioreactors was finally conducted batchwise. The use of batch photobioreactors allowed to systematically test multiple environmental conditions. This work constitutes, to the best of our knowledge, the first proof of concept of the biogas upgrading using PPB under IR.
Section snippets
Cultivation media
Fresh centrifuged PWW was collected from a nearby farm at Segovia (Spain) and stored at 4 °C prior to use. The composition of the PWW was: total organic carbon (TOC) concentration of 10350 mg L−1, inorganic carbon (IC) concentration of 215 mg L−1, total nitrogen (TN) concentration of 2685 mg L−1, P-PO43− concentration of 15 mg L−1, total suspended solids (TSS) concentration of 5.9 g L−1. Prior to each test, PWW was centrifuged at 10000 rpm for 20 min in order to separate the soluble from the
Influence of piggery wastewater dilution on purple phototrophic bacteria-based piggery wastewater treatment coupled to biogas upgrading
TOC concentration in biotic and abiotic control tests conducted with MSM remained constant at 134 ± 16 mg L−1 and 69 ± 9 mg L−1, respectively (Fig. 1a). On the other hand, TOC concentration in undiluted PWW and non-irradiated biotic control tests remained constant at 10318 ± 957 mg L−1 and 3535 ± 236 mg L−1, respectively (Fig. 1a). A significant TOC removal from 3977 ± 336 mg L−1 to 1453 ± 134 mg L−1 (TOC-removal efficiencies (REs) of 63%) in 2 times diluted PWW tests, and from 1989 ± 12 mg L−1
Conclusions
PPB represent an innovative biological platform for the simultaneous treatment of PWW and upgrading of biogas under photo-anaerobic conditions. PWW with TN concentrations of 600 mg L−1 provided the best conditions for wastewater treatment and biogas upgrading. The presence of VFA in PWW supported CO2 fixation in the Calvin cycle, thus allowing biogas upgrading. The low phosphorous concentrations inherent to PWW did not significantly impact on wastewater treatment performance but slightly
Acknowledgements
This work was supported by the regional government of Castilla y León and the EU-FEDER programme (UIC 71 and CLU 2017-09). Daniel Puyol greatly thanks the Spanish Ministry of Science, Innovation and Universities for the Ramon y Cajal grant.
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