Nitrate removal from groundwater using a sequential aerobic granular sludge technology

Resumen

The dependence on groundwater for human consumption has increased worldwide over the last 50 years. Among the nutrient pollutants of concern, nitrate (NO3-) often reaches groundwater and causes significant degradation in groundwater quality. This dependence on groundwater has become even more important in the Mediterranean region due to increased desertification and global warming. In countries such as in Spain, 70% of the water resource demands of cities with less than 20000 inhabitants are supplied by groundwater. The mail goal of our experimental work was development a new biological technology based on aerobic granular systems. In this sense, a full-scale water treatment plant using aerobic granular sludge (AGS) was built to remove NO3- from nitrate-polluted groundwater intended for human consumption, in order to achieve a complete nitrate removal with low economic cost and minimizing the environmental impact. First of all (Chapter 1), four granular sequencing batch reactors (GSBRs) at lab-scale were inoculated with four denitrifying Pseudomonas strains carrying nosZ to study the process of granule formation, the operational conditions of the bioreactors, and the carbon concentration needed for nitrate removal. The selected Pseudomonas strains were P. stutzeri I1, P. fluorescens 376, P. denitrificans Z1, and P. fluorescens PSC26, previously reported as denitrifying microorganisms carrying the nosZ gene. Pseudomonas denitrificans Z1 produced fluffy, low-density granules, with a decantation speed below 10 m h1. However, P. fluorescens PSC26, P. stutzeri I1, and P. fluorescens 376 formed stable granules, with mean size from 7 to 15 mm, related to the strain and carbon concentration. P. stutzeri I1 and P. fluorescens 376 removed nitrates efficiently with a ratio in the range of 96%, depending on the source and concentration of organic matter. Therefore, the findings suggest that the inoculation of GSBR systems with denitrifying strains of Pseudomonas spp. containing the nosZ gene enables the formation of stable granules, the efficient removal of nitrate, and the transformation of nitrate into nitrogen gas, a result of considerable environmental interest to avoid the generation of nitrous oxide. In a second group of experiments (Chapter 2) a bioreactor was designed as a cylindrical sequential batch reactor (SBR), with a height of 3.52m and a diameter of 0.49m. The reactor was inoculated with 6L of mature granules previously formed at lab-scale in the Water Research Institute (University of Granada) with a total volume of 660L. In this research, a novel modification of aerobic granular sludge technology was developed for the treatment of nitrate-polluted groundwater, adding very low concentrations of a solution based on carbon and oligoelements in the groundwater to promote the growth of denitrifying microorganisms, avoiding expensive technologies to supply drinking water in small urban nuclei. The denitrification process was successfully reached at 0.15g C2H3NaO2 L1, meeting the Nitrate Directive of Europe for drinking water. The granular biomass was compact and dense with average values of mean size and settling velocity of 4.0mm and 40mh1, respectively. The prokaryotic and eukaryotic communities were studied by massive parallel sequencing techniques. The dominant prokaryotic phylotypes were related to influent composition, belonging to Comamonadaceae, Rhizobiales, Acinetobacter and Pseudomonas. The dominant eukaryotic phylotype was affiliated to Haematococcus microalgae. The diversity and evenness were high, regardless of influent composition. This study demonstrates support for the innovation of aerobic granular sludge technology application in terms of performance, operation, granular maturation and stability, as well as the role of denitrifying microorganisms to implement a low-cost, easy-to-use and maintain, environmental-friendly drinking water technology for rural populations. Finally, in a third group of experiments (Chapter 3) a full-scale water treatment plant using aerobic granular sludge (AGS) technology was built, under real conditions, to remove NO3- from nitrate-polluted groundwater intended for human consumption in village Torre Cardela (Granada) in the South of Spain. The impact of changes in the operational conditions of hydraulic retention time (HRT) and organic matter loading (OML) rate on NO3- removal and overall system performance were examined. Variations in the abundance of the denitrification genes and the diversity and composition of the prokaryotic and eukaryotic communities in the granule microbiome were studied. Regardless of the HRT, the AGS technology was successful in removing NO3- with removal rates greater than 50% with an optimal OML rate of 75 mg L-1. Regardless of the HRT and OML rate, the organic matter removal rate was greater than 90%. No significant changes in the abundance of denitrification genes were detected during the experimental period. However, the composition of prokaryotic and eukaryotic communities was affected by changes in the HRT and OML rate. Specific prokaryotic taxa were identified as responsive to changes in operational parameters and their relative abundances were strongly linked to the removal of NO3-, confirming that the microbes are critical to the NO3- removal process. The results obtained in this experimental work clearly demonstrate that the AGS technology can be successfully implemented to treat nitrate-polluted groundwater in rural villages to produce water of drinking quality.