Luca Alibardi

Development and permeability of a dynamic membrane for anaerobic wastewater treatment

Dynamic membranes (DMs) have recently been proposed as an alternative to microfiltration and ultrafiltration in membrane bioreactors (MBRs) in order to contain capital and management costs. This study aims to develop an anaerobic dynamic MBR for wastewater treatment by using a large pore-sized mesh. The study demonstrated that a DM can be developed by using a mesh of 200μm pore-size and applying low cross flow velocity. The bench-scale reactor achieved COD removal efficiency between 65% and 92% and proved to be able to remove approximately 99% of the mixed liquor suspended solids, maintaining a solids retention time well above 200d. A significant quantity of biogas was produced by the external dynamic membrane module and was released with the effluent stream. The flux-step experiment, designed to estimate the critical flux in ultrafiltration MBR, can also be used for monitoring the development and stability of DMs.

Effect of filtration flux on the development and operation of a dynamic membrane for anaerobic wastewater treatment.

Dynamic membrane represents a cost effective alternative to conventional membranes by employing fouling as a means of solid-liquid separation. This study evaluated the effects of initial flux on both development rate of dynamic membrane and bioreactor performance during two consecutive experiments. The dynamic membrane was developed over a 200xa0μmxa0mesh and the reactor was operated under anaerobic conditions. It was found that the effect of an initial higher applied flux on dynamic membrane development was more pronounced than mixed liquor suspended solid concentration inside the bioreactor. The development of the dynamic membrane was therefore positively associated with the applied flux. The rapid development of the dynamic membrane during the second experimental run at high initial fluxes and lower MLSS concentrations also affected the performance of the bioreactor in terms of more efficient COD removal and biogas production. A major shortcoming of applying higher initial applied flux was the formation of a denser and robust dynamic membrane layer that was resistant to applied hydraulic shear to control desired permeability and thus represented an obstacle in maintaining a long term operation with sustainable flux at lower transmembrane pressure (TMP).

Anaerobic digestion for the treatment of solid organic waste: what’s hot and what’s not

Anaerobic co-digestion has been widely investigated, but there is limited analysis of interaction between substrates. The objective of this work was to assess the role of carbohydrates, protein and lipids in co-digestion behaviour separately, and together. Two sets of batch tests were done, each set consisting of the mono-digestion of three substrates, and the co-digestion of seven mixtures. The first was done with pure substrates--cellulose, casein and olive oil--while in the second slaughterhouse waste--paunch, blood and fat--were used as carbohydrate, protein and lipid sources, respectively. Synergistic effects were mainly improvement of process kinetics without a significant change in biodegradability. Kinetics improvement was linked to the mitigation of inhibitory compounds, particularly fats dilution. The exception was co-digestion of paunch with lipids, which resulted in an improved final yield with model based analysis indicating the presence of paunch improved degradability of the fatty feed.

Biological carbon dioxide utilisation in food waste anaerobic digesters

Carbon dioxide (CO2) enrichment of anaerobic digesters (AD) was previously identified as a potential on-site carbon revalorisation strategy. This study addresses the lack of studies investigating this concept in up-scaled units and the need to understand the mechanisms of exogenous CO2 utilisation. Two pilot-scale ADs treating food waste were monitored for 225 days, with the test unit being periodically injected with CO2 using a bubble column. The test AD maintained a CH4 production rate of 0.56 ± 0.13 m(3) CH4·(kg VS(fed) d)(-1) and a CH4 concentration in biogas of 68% even when dissolved CO2 levels were increased by a 3 fold over the control unit. An additional uptake of 0.55 kg of exogenous CO2 was achieved in the test AD during the trial period. A 2.5 fold increase in hydrogen (H2) concentration was observed and attributed to CO2 dissolution and to an alteration of the acidogenesis and acetogenesis pathways. A hypothesis for conversion of exogenous CO2 has been proposed, which requires validation by microbial community analysis.

Pre-treatment of tannery sludge for sustainable landfilling

The wastewater produced during tanning activities are commonly conveyed to centralised industrial wastewater treatment plants. Sludge from physical-chemical treatments (i.e. primary sedimentation) and waste activated sludge from biological treatment units are called tannery sludge. Tannery sludge is a solid waste that needs to be carefully managed and its disposal represents one of the major problems in tannery industry. Conventional treatment and disposal of tannery sludge are based mainly on incineration and landfilling. The aim of this study was to evaluate the effects of a pre-treatment process composed of aerobic stabilisation, compaction and drying, for a sustainable landfilling of tannery sludge. The process produced a reduction of volume, mass and biodegradability of treated sludge. Results also demonstrated a reduced leachability of organic and inorganic compounds from treated sludge. The pre-treatment process could allow to extend landfill life time due to lower amounts of tannery sludge to be disposed off, minimise long terms landfill emissions and obtain a state of carbon sink for tannery sludge landfilling.

Impacts of coagulation-flocculation treatment on the size distribution and bioavailability of trace metals (Cu, Pb, Ni, Zn) in municipal wastewater

This study investigated the impact of coagulation-flocculation treatment on metal form and bioavailability in municipal wastewater. Real humus effluent samples were separated into particulate, colloidal and truly dissolved fractions before and after treatment with either ferric chloride (FeCl3) or the biopolymer Floculan. Results revealed that both reagents effectively (≥48%) eliminated Cu, Pb and Zn from the particulate fraction and removed Cu and Zn from the colloidal fraction in conjunction with colloidal organic carbon (COC). Although organics in the truly dissolved fraction were resistant to removal, Floculan reduced Cu in this fraction by 72% owing to the complexation of free Cu ions to phenol and amino groups along the polymeric chains, revealing an additional removal pathway. In fact, COC removed in the CF process by Floculan was replaced with truly dissolved compounds, input as a result of this reagents organic composition. Floculan, therefore, reduced the soluble concentration of Cu and Zn without changing the DOC concentration, thus reducing the bioavailability of these metals in treated effluent. FeCl3 did not reduce the bioavailability of target metals, thus did not deliver any environmental benefit. This work provides important information for the selection and development of high performance coagulants to improve metal removal.

Use of oleaginous plants in phytotreatment of grey water and yellow water from source separation of sewage

Efficient and economic reuse of waste is one of the pillars of modern environmental engineering. In the field of domestic sewage management, source separation of yellow (urine), brown (faecal matter) and grey waters aims to recover the organic substances concentrated in brown water, the nutrients (nitrogen and phosphorous) in the urine and to ensure an easier treatment and recycling of grey waters. With the objective of emphasizing the potential of recovery of resources from sewage management, a lab-scale research study was carried out at the University of Padova in order to evaluate the performances of oleaginous plants (suitable for biodiesel production) in the phytotreatment of source separated yellow and grey waters. The plant species used were Brassica napus (rapeseed), Glycine max (soybean) and Helianthus annuus (sunflower). Phytotreatment tests were carried out using 20L pots. Different testing runs were performed at an increasing nitrogen concentration in the feedstock. The results proved that oleaginous species can conveniently be used for the phytotreatment of grey and yellow waters from source separation of domestic sewage, displaying high removal efficiencies of nutrients and organic substances (nitrogen>80%; phosphorous >90%; COD nearly 90%). No inhibition was registered in the growth of plants irrigated with different mixtures of yellow and grey waters, where the characteristics of the two streams were reciprocally and beneficially integrated.

Performance and stability of sewage sludge digestion under CO2 enrichment: A pilot study

Carbon dioxide (CO2) injection in anaerobic digestion has recently been proposed as an interesting possibility to boost methane (CH4) recovery from sludge and organic waste by converting a greenhouse gas into a renewable resource. This research assessed the effects of exogenous CO2 injection on performance and process stability of single-phase continuous anaerobic digesters. Two pilot scale reactors treating sewage sludge were operated for 130days. One reactor was periodically injected with CO2 while the other acted as control. Two injection frequencies and injection devices were tested. The results indicated that CO2 enrichment allowed an increase in CH4 production of ca. 12%, with a CH4 production rate of 371±100L/(kgVSfed·d) and a CH4 concentration of ca. 60% when dissolved CO2 levels inside the test reactor were increased up to 1.9-fold. Results also indicated an improvement in process resilience to temporary overloads and no impacts on stability parameters.

Assessment of dynamic membrane filtration for biological treatment of old landfill leachate

This study investigated the behaviour of dynamic membrane (DM) filtration for the treatment of stabilised landfill leachate in a bench-scale pre-anoxic and aerobic submerged dynamic membrane bioreactor (DMBR). Four meshes with different openings (10, 52, 85 and 200u202fμm) were tested to support the development of DM. Differences were observed among the meshes in supporting the development of the cake layer constituting the DM. The treatment of landfill leachate had an impact on sludge characteristics resulting in deteriorated filtration performance of the DM. Effluent turbidity was often higher than 100 NTU for larger mesh pore size (85 and 200u202fμm). Low effluent turbidity was achieved with meshes with 10 and 52u202fμm (13u202f±u202f2 and 26u202f±u202f4 NTU, respectively) although at membrane fluxes lower than 10u202fL m- 2 h-1. The bioreactor exhibited a moderate organics removal of 50-60% and an ammonia oxidation between 80 and 90%. Incomplete nitrification was observed due to increased concentrations of free ammonia and free nitrous acid, with nitrite effluent concentrations up to 1062 mgNO2--N L-1. Due to the large presence of refractory organic matter in landfill leachate, denitrification was limited resulting in a total nitrogen removal of approximately 20%.

Anaerobic Digestion of Microalgal Residues to Enhance the Energetic Profit of Biocrude Production

Microalgae as source of energy have generated an enormous interest in the last decades. Microalgae seem to be the most feasible option to obtain renewable liquid fuels due to high growth rates, CO2 fixation capability and large accumulation of oil compared to other crop plants. The bottleneck of this technology is anyway represented by the costs of the process, both from the economic and energetic points of view. In order to reduce the energetic costs and to make microalgae cultivation more attractive, the possibility of exploiting the energetic content of microalgal biomass residues after oil extraction by means of anaerobic digestion to produce biogas was studied. Two microalgal species, Scenedesmus obliquus and Chlorella protothecoides, selected for their high oil contents and fast growth rates, were tested for biogas production, before and after the oil extraction. Oil extraction was carried out by Soxhlet method, using a mixture of methanol and chloroform as the solvent. Biochemical Methane Potential (BMP) tests were carried out to evaluate biogas production capacity from microalgae and degradability rates. Two different kinds of inocula were used to compare the specific hydrolytic capacities and to assess the most suitable one to maximize the biogas conversion of microalgae. The digestion tests were performed at controlled temperature of 37 °C, in batch reactors. Production of biogas and the proportion of CO2 and CH4 content were measured. The results are discussed in view of feasible industrial application.