Titolo della tesi: Novel Platform for Sewage Sludge Valorisation: Integrating pre-treatments with Anaerobic Bioleaching and Digestion
Wastewater treatment plants (WWTPs) have become crucial elements for the bioeconomy - mainly through energy and matter recovery as an element of sustainable development. Moreover, wastewater treatment generates a significant amount of sewage sludge as by-product. After dewatering and stabilization processes (i.e., anaerobic digestion) sludge can be further used in agriculture and other reuse, can be incinerated, or reach landfills. The land application route is considered the most virtuous option for sewage sludge destination, taking into consideration a series of criteria such as practicability, affordability, sustainability, and acceptability. Sewage sludge comprises both organic carbon and nutrients, but could contain toxic metals, organic micropollutants and pathogens, too. In order to minimize environmental and health consequences, the sludge needs proper treatment and processing. Sludge utilization in agriculture is currently regulated by the limits of heavy metals specified in the old European Directive 86/278, namely cadmium, copper, mercury, lead and zinc, which results unsuitable to prevent risks on vegetation, animals, and humans. Sev¬eral EU Member States introduced limit values for other ele¬ments. For example, Italy recently introduced the limit for arsenic, selenium, beryllium and for hexavalent chromium. Therefore, for sludge land application, a decontamination treatment could be necessary. On the other hand, most metals considered toxic to ecosystem organisms and regulated in biosolids that are land applied have significant potential economic value. Thus, recovering metals could be an economic and environmental opportunity.
Based on these considerations, a strategical approach on a cascade biorefinery platform to enhance hydrolysis and bioleaching of metals from municipal sludge, together with the production of VFAs, EPS and methane is here presented and investigated. The waste activated sludge (WAS), after appropriate pre-treatment, is subjected to a short fermentation phase in order to favour metals and organics solubilization, and acidification. Thus, the effluent stream, containing soluble metals, soluble EPS and volatile fatty acids, could be valorised (i) to produce methane with high yields due to the presence of bioavailable VFAs and trace metals, or (ii) to produce a decontaminated digestate by removing toxic elements (i.e., As, Se, Be) from the supernatant; (iii) to recover VFAs (to be commercialized or transformed into medium chain fatty acids, with high economic value), EPS and metals.
The feedstock used for this work, was a real waste activated sludge sampled along the entire study from one of the four WWTPs serving the city of Rome presented a total metal concentration that ranged over 4 orders of magnitude from ~107 μg kg-1 dry wt for Al, Fe and P, elements with high economic potential to 103 μg kg-1 dry wt for Cd, Co, and Sb. Relevant concentration of As and Se, potentially toxic metalloids, in the investigated WAS could be ascribed to the input from natural backgrounds sources typically of Central Italy territory, affecting also the biosolids quality to be disposed of.
Sludge pre-treatments as ultrasound and thermal hydrolysis promoted floc disintegration and organics solubilisation together with metals release into water phase. In particular for As, Co, Ga, Ni, Se, V and Zn a statistically positive correlation between disintegration degree and soluble metal concentration highlighted that a fairly significant component of these elements was organically bound. Besides, anaerobic bioleaching was tested in batch mode using different inocula, incubation duration and food/inoculum (F/I) ratio. The biological hydrolysis at high F/I, acting as pre-treatment, contributed to the leaching of As, Ga, Se and Zn, increasing their bioavailability, in particular with an acidified inoculum, rich in fermentative bacteria.
Based on these results, scaling up from batch to anaerobic reactors operated at semi-continuous conditions was performed applying two cascade mesophilic reactors to evaluate the impact of pre-treatment (ultrasound, and ultrasound and HCl) integration, high organic load (and short HRT) on pH, metals/metalloids bioleaching, EPS (extracellular polymeric substances) release and VFAs production, together with methane production and organics removal. The study proved that the addition of HCl to sonicated sludge promoted a significant solubilisation and acidification of the system during the feeding days, that however was removed and transformed into methane during the weekend days. Crucial strategy was the inhibition of methanogens by thermal pre-treatment of the start-up inoculum that, that assured pH stability, highest acidification, and disintegration degree, together with elements (As, Co and Se) solubilization.
In addition, bioavailable soluble micro-elements of the WAS proved to be crucial in anaerobic co-digestion of organic biowaste to maintain an active, healthy biomass assuring satisfactory process performance and stability.
For all the different systems operated during the thesis work, the assessment of the microbial community composition and dynamics provided valuable insight into the complex anaerobic stages. Soluble organics concentrations and profiles together with the linked bacterial and methanogenic community structures were discussed to gain understanding into the main peculiarities of the hydrolysis and acidification phase.
The recovery of the extracted components (i.e., EPS, As) from the supernatant after the acidogenic phase was investigated. Soluble extracellular polymeric substance (EPS) released during process e was extracted, quantified, and characterised. The extracted EPS were used to produce filtration membranes for industrial purpose. The membranes, built on polycarbonate and nylon supports, presented layers mainly composed by proteins uniformly distributed along the entire thickness. Satisfactory results were obtained in terms of promising filtration characteristics and particles retention efficiency observed in this study for the sEPS layers deriving from WAS disintegration could be of interest for possible future application of these layers as dynamic membranes.
Besides, an in-silico study was performed by using the software Medusa/Hydra to evaluate the optimal operating conditions (pH, pe) for As precipitation using FeS.
Finally, the results obtained at lab-scale were transferred to an industrial plant of 100’000 pe and discussed in terms of mass and energy balance.
The study was almost entirely conducted at the laboratories of IRSA-CNR (Water Research Institute – National Research Council), while EPS extraction and related analysis were carried out at Wetsus Centre (Netherlands).