Optimization of piggery wastewater treatment with purple phototrophic bacteria

Resumo

The increase in human population in the world entails future challenges for the sustainable development of mankind. This demographic expansion will represent global changes generated by anthropogenic activity on planet Earth, involving negative impacts on the biosphere, atmosphere, cryosphere and hydrosphere. In particular, the hydrosphere will be affected by the increased contamination of surface water and groundwater, and an increase in their eutrophication due to inadequate management of wastewaters. Thus, the generation of new biotechnologies to reduce pollution and improve the sustainable development of humanity will be a challenge for the next few decades. In this sense, efficient wastewater treatment in cities, industry and agriculture requires the development of innovative solutions to reduce the pollution generated by the intense anthropogenic activity. Thus, the need for meeting the increasing demand for animal protein has led to an increase in intensive livestock farming, with the subsequent increase in the generation of wastewaters containing high organic matter, nitrogen and phosphorus concentrations. In particular, piggery wastewaters (PWW) are characterized by their high concentrations of organic matter and nitrogen in the form of ammonium, and by their odour nuisance. Traditionally, PWW has been treated by disposal in open anaerobic lakes, with the subsequent production of high concentrations of greenhouse gases such as CO2, CH4, NH3 and H2S to the atmosphere due to their open configuration. Another technology used for the treatment of this type of wastewaters is anaerobic digestion in enclosed bioreactors, which is capable of generating methane (CH4) as a byproduct, a gas with high economic and energy value. However, this process is only capable of removing the carbon present in PWW, generating an effluent with a high nitrogen and phosphorus content that is not assimilated in the process. Lastly, activated sludge systems entail an effective removal of carbon, nitrogen and phosphorous with a high energy demand and a destruction of the nutrients present in PWW. Nowadays, the use of photosynthetic microorganisms for PWW treatment at low operating costs and with a recovery of carbon, nitrogen and phosphorous has been proposed. These microorganisms are capable of absorbing solar radiation through the photosynthesis process to obtain energy, which is used for their growth and nutrients assimilation from different wastewaters. Purple phototrophic bacteria (PPB) represent the photosynthetic microorganisms with the most versatile metabolism in nature. PPB can grow phototrophically and chemotrophically, absorbing energy from solar radiation or from the degradation of organic compounds, respectively. In addition, PPB are heterotrophic microorganisms capable of degrading different organic compounds and also able to grow using an autotrophic metabolism, fixing carbon dioxide (CO2) and nitrogen (N2) from the atmosphere. PPB can grow both under anaerobic and aerobic conditions, through photoheterotrophy or chemoautotrophy, respectively. On the other hand, microalgae represent the most studied photosynthetic microorganisms in recent years, due to their high growth rate, capacity to fix CO2 and to the high valorization potential of their biomass. Both PPB and microalgae have species with extraordinary metabolic capacities, capable of growing at low or extremely high temperatures, under different pH ranges (acidic and alkaline), high salinity and in the presence or absence of oxygen, which supports their extraordinary metabolism for the treatment of multiple wastewaters. In this context, Chapter 1 evaluated the influence of PWW load, air dosing, CO2/NaHCO3- supplementation and pH control on PWW treatment by mixed cultures of PPB in batch photobioreactors. PPB were able to grow under infrared irradiation in 1:5, 1:10 and 1:15 PWW dilutions, undiluted PWW inhibiting PPB growth. Under high PWW dilutions, PPB growth and carbon and nitrogen removal efficiencies were favoured, due to a higher penetration of the radiation into the cultivation broth, which enhanced photosynthesis and decreased the toxic effects caused by the high ammonium concentrations present in the PWW. PPB performed an efficient PWW treatment under anaerobic and aerobic conditions. However, under aerobic conditions an increased nutrients loss by stripping was recorded compared to the tests conducted under anaerobic conditions, where the main removal mechanism was nitrogen assimilation in the form of PPB biomass. CO2 supplementation resulted in an efficient nutrient removal by PPB, while PPB metabolisms was not affected by lack of trace elements during PWW treatment. Finally, pH control was a key parameter for an efficient nutrient removal by PPB during PWW treatment, improving PPB growth, carbon and nitrogen removals by pH control via addition of both HCl and CO2. PPB were able to perform and efficient nutrient assimilation from PWW, CO2 supplementation and its beneficial effects on pH control being key to PPB growth and efficient PWW treatment. In chapter 2, the influence of the type of radiation (photosynthetically active radiation (PAR), near-infrared radiation (NIR) and PAR filtered using a UV-VIS filter), temperature (13 °C and 30 °C), metabolism (photoheterotrophic and chemoheterotrophic), type of inoculum (mixed cultures and strain Rhodopseudomonas palustris) and wastewater load (1:5 and 1:10 dilutions) during PWW treatment by PPB in batch photobioreactors was evaluated. PPB were able to grow under the different radiation sources tested, performing an efficient PWW treatment. The use of UV-VIS filtered PAR supported both a high bacteriochlorophyll content in PPB and the highest total organic carbon removal (TOC-RE = 74%). Interestingly, PPB exhibited TOC-REs and total nitrogen removal efficiencies (TN-REs) at low temperature (71% and 45%, respectively) similar to those recorded at a temperature of 30 °C (73% and 37%, respectively), without exhibiting a decrease in PPB biomass production. PPB were not able to grow in darkness chemoheterotrophically during PWW treatment. On the other hand, mixed cultures of PPB achieved a higher nutrient assimilation rate than R. palustris, exhibiting a total assimilation of the volatile fatty acids present in 10-fold diluted PWW. PPB were able to grow and perform an efficient nutrient removal, regardless of the type of radiation and temperatures, exhibiting a high removal efficiency using mixed cultures of PPB during PWW treatment. In chapter 3, the long-term performance (450 days) of open and closed photobioreactors operated in continuous mode during PWW treatment at a hydraulic retention time of 7 days was evaluated. The influence of 1:4 and 1:8 PWW dilutions, ratio of illuminated area/volume of the photobioreactor, biomass settling and recirculation, and infrared radiation intensities on the removal of carbon and nitrogen from PWW by PPB were evaluated. The increase in PWW dilution from 1:4 to 1:8 did not entail higher total organic carbon removal efficiency (TOC-RE) in the open photobioreactor (87% versus 89%), but a significant increase in the TOC-RE in the closed photobioreactor (from 73% to 80%). The increase in the illuminated area/volume of the photobioreactors increased total nitrogen removal efficiencies (TN-RE) up to 99% and 49% in the open and closed photobioreactor, respectively, with a concomitant increase in the temperature of both systems. However, temperature control did not mediate a significant enhancement in PWW treatment. Biomass settling and recirculation resulted in higher final TOC-RE of 80% and TN-RE of 90% in the closed photobioreactor. The increase in infrared radiation from 100 to 300 W m-2 fostered PPB growth. High water evaporation losses were recorded in the open photobioreactor (deteriorating effluent quality), where CO2 and NH3 volatilization were identified as the main mechanism of carbon and nitrogen removal. Both open and closed photobioreactor were efficient in carbon removal from PWW, enhancing nitrogen removal in the open photobioreactor and exhibiting lower evaporation rates and higher PPB growth in the closed photobioreactor. Finally, the potential of an innovative configuration composed of an anaerobic PPB photobioreactor (PPB-PBR) sequentially coupled to an aerobic microalgae-bacteria photobioreactor (MB-PBR) was assessed during the PWW treatment under continuous operation in Chapter 4. The effects of hydraulic retention time (HRT) and intensities of near-infrared radiation (NIR) in the PPB-PBR were evaluated using mass balances and a complete characterization of the bacterial and microalgal communities in the photobioreactors. Maximum removal efficiencies of total dissolved organic carbon (TOC-RE) and total dissolved nitrogen (TN-RE) of 91% and 82%, respectively, were recorded at an HRT of 12.2 days. The decrease in HRT to 6.2 days reduced TOC-RE and TN-RE in both photobioreactors. However, the increase in NIR in the PPB-PBR enhanced TOC-RE, contributing to a global carbon recovery of 67% via assimilation in the form of PPB biomass. PPB-PBR was highly efficient in carbon assimilation, while MB-PBR supported high nitrogen and total suspended solids removals (63% and 76%, respectively). The culture broth of PPB-PBR was dominated by the bacteria Rhodopseudomonas sp., which represented up to 54% of the total bacterial population, supported by the high HRT and increased NIR. The environmental and operational conditions set in the sequential MB-PBR favoured the dominance of Mychonastes homosphaera. This research demonstrated, for the first time, the high efficiency of the sequential PPB and microalgae systems for the treatment of PWW. The results obtained in this thesis demonstrate that an efficient treatment of piggery wastewater using photosynthetic microorganisms (purple phototrophic bacteria and microalgae) is feasible.