Development of bioaugmentation and sewage sludge composting technologies to remove emerging and priority pollutants while reducing their toxicity

Abstract

About 45 million dry tons of sewage sludge (SS) are produced globally every year and it is estimated to keep increasing, mainly due to the outstanding population growth and the industrial development during the last century. Sewage sludge is a mud-like residue from wastewater treatment which comprises a broad range of trace components, mostly derived from the composition into the influent released from various sources. Among these components, valuable organic matter and nutrients are found in great concentrations and therefore be useful for agricultural purposes. Hence, concerning SS management and disposal system, priority should be given to its valorisation and transformation into a valuable resource always considering potential threats to environmental and public health. Occurrence and presence of several contaminants like heavy metals, pathogens and other organic compounds into the SS are the main reasons for the hesitancy toward SS direct application as an organic amendment. These concerns lead to explore more diverse SS stabilization treatments being ‘composting’ the most used and reliable biological option for minimizing potential environmental impacts. Nonetheless, the threatening estimation of composting technologies is still leery due to the outstanding increment of priority pollutants concentration in the wastewater influent and the lack of information of emerging pollutants (EPs) persistency in the compost after the treatment. Emerging pollutants are synthetic or naturally occurring chemicals and characterized by a perceived, potential, or threat human health. They consists of, among others, pharmaceuticals including antibiotics, personal care products, pesticides, nanoparticles and endocrine disruptors and many of them are currently under investigation to discover exactly how harmful they are or could become. Due to their hydrophobic properties, the EPs are strongly sorbed on sludge and thus, hardly remove from the matrix by conventional stabilization treatments. Moreover, their presence and concentration neither in wastewater nor in composted SS is not often well monitored causing the urgency of an effective regulatory measures. Then, the European Commission has published a more restrictive policy of SS management (Directive 2008/98/EC) and is to update and announce a biennial ‘Watch List’ of substances for Union-wide monitoring. The study of such substances should generate high-quality data that give rise to novel, more effective, straightforward and environmental friendly strategies for SS management focus on its recycling and EPs removal. Within this PhD thesis, diverse bioaugmentation strategies in composting processes are proposed and discussed to remove EPs. Each step of the bioaugmentation design protocol are explained in four different chapters (Screening, characterization and field studies). The screening of EPs degrading microorganisms resulted in different fungal and bacterial isolates and mixed microbial consortia with pharmaceuticals degradation potential without microtoxic effects. The field study comprehend the design, optimization and assembly of a two-step composting technology at industrial scale coupled with bioaugmentation using indigenous and exogenous microorganisms as inoculants. The determination of physicochemical and biological modifications ensued from the inoculation were also explored. Polluted sites are considered to be a great source for the isolation of contaminants degrading microorganisms. Then, chapter 1 is focused on the first step of bioaugmentation regarding the screening of EPs degrading microorganisms that comprises a selective enrichment assay under selective pressure with pharmaceutical compounds (diclofenac, carbamazepine and Ketoprofen) and the further isolation and identification of pure cultures. These compounds were selected based on their persistence after composting processes. Among these microorganisms, fungi Cladosporium cladosporioides H1, Alternaria alternata H4 y Penicillium raistrickii H6 showed the best pharmaceutical degradation rates. Although efficacy of single strains have been observed, employing mixed consortia might benefits the overall degradation performance by broaden the targeted pollutants. This approach was closely studied in chapter 2 through the assembly of an artificial mixed microbial consortia using the BSocial webtool. The three previously mentioned fungi isolates, a proven degrading fungus (Penicillium oxalicum XD 3.1) and coexistence bacteria from the enrichment assay (Micrococcus yunnanensis K1, Oligella ureolytica T4 y Sphingobacterium jejuense T15) were used for the consortia establishment. Because of the social stability, wide range of degrading targets and the lower toxicity level of the artificial consortia consisting of P. raistrickii, P. oxalicum XD 3.1 y C. cladosporoides, A. alternata y M. yunnanensis, this consortium was selected as the most optimal one to use for EPs removal. Although the benefits promised by bioaugmentation technologies for contaminants elimination, its application under real conditions has barely been developed, indeed only 5 % of the studies were focused on organic pollutants degradation have reach the field scale in 30 years until 2020 and only few of them have reported the co-occurrence and simultaneous elimination of different classes of organic pollutants during sewage sludge composting. In chapter 3, details of optimization and installation of a two-steps bioaugmentation-composting system is presented. Two different inoculants were use separately to explore both kinds of bioaugmentation seeds: an exogenous microorganism (P. oxalicum XD 3.1) and a native consortium obtained from enrichment. Both bioaugmented treatments showed great reduction in pharmaceuticals containing and phyto and micro toxicity, as well as offered a better quality, more stable and sanitized mature compost. Biological contributions resulted from the inoculation over native microbial populations and physicochemical parameters were investigated in chapter 4. P. oxalicum XD 3.1 did not modified neither the diversity nor the heterogeneity of native populations regardless of its exogenous nature. Moreover, it promoted the increment of microbial diversity and the physicochemical parameters involved in emerging pollutants degradation.