Alberto Pivato

Effect of aerobic pre-treatment on hydrogen and methane production in a two-stage anaerobic digestion process using food waste with different compositions

Aerobic pre-treatment was applied prior to two-stage anaerobic digestion process. Three different food wastes samples, namely carbohydrate rich, protein rich and lipid rich, were prepared as substrates. Effect of aerobic pre-treatment on hydrogen and methane production was studied. Pre-aeration of substrates showed no positive impact on hydrogen production in the first stage. All three categories of pre-aerated food wastes produced less hydrogen compared to samples without pre-aeration. In the second stage, methane production increased for aerated protein rich and carbohydrate rich samples. In addition, the lag phase for carbohydrate rich substrate was shorter for aerated samples. Aerated protein rich substrate yielded the best results among substrates for methane production, with a cumulative production of approximately 351ml/gVS. With regard to non-aerated substrates, lipid rich was the best substrate for CH4 production (263ml/gVS). Pre-aerated P substrate was the best in terms of total energy generation which amounted to 9.64kJ/gVS. This study revealed aerobic pre-treatment to be a promising option for use in achieving enhanced substrate conversion efficiencies and CH4 production in a two-stage AD process, particularly when the substrate contains high amounts of proteins.

Food web modeling of a river ecosystem for risk assessment of down-the-drain chemicals: A case study with AQUATOX

Conventional approaches to estimating protective ecotoxicological thresholds of chemicals, i.e. predicted no-effect concentrations (PNEC), for an entire ecosystem are based on the use of assessment factors to extrapolate from single-species toxicity data derived in the laboratory to community-level effects on ecosystems. Aquatic food web models may be a useful tool to improve the ecological realism of chemical risk assessment because they enable a more insightful evaluation of the fate and effects of chemicals in dynamic trophic networks. A case study was developed in AQUATOX to simulate the effects of the anionic surfactant linear alkylbenzene sulfonate and the antimicrobial triclosan on a lowland riverine ecosystem. The model was built for a section of the River Thames (UK), for which detailed ecological surveys were available, allowing for a quantification of energy flows through the whole ecosystem. A control scenario was successfully calibrated for a simulation period of one year, and tested for stability over six years. Then, the model ecosystem was perturbed with varying inputs of the two chemicals. Simulations showed that both chemicals rapidly approach steady-state, with internal concentrations in line with the input bioconcentration factors throughout the year. At realistic environmental concentrations, both chemicals have insignificant effects on biomass trends. At hypothetical higher concentrations, direct and indirect effects of chemicals on the ecosystem dynamics emerged from the simulations. Indirect effects due to competition for food sources and predation can lead to responses in biomass density of the same magnitude as those caused by direct toxicity. Indirect effects can both exacerbate or compensate for direct toxicity. Uncertainties in key model assumptions are high as the validation of perturbed simulations remains extremely challenging. Nevertheless, the study is a step towards the development of realistic ecological scenarios and their potential use in prospective risk assessment of down-the-drain chemicals.

Landfill aeration for emission control before and during landfill mining.

The landfill of Modena, in northern Italy, is now crossed by the new high velocity railway line connecting Milan and Bologna. Waste was completely removed from a part of the landfill and a trench for the train line was built. With the aim of facilitating excavation and further disposal of the material extracted, suitable measures were defined. In order to prevent undesired emissions into the excavation area, the aerobic in situ stabilisation by means of the Airflow technology took place before and during the Landfill Mining. Specific project features involved the pneumatic leachate extraction from the aeration wells (to keep the leachate table low inside the landfill and increase the volume of waste available for air migration) and the controlled moisture addition into a limited zone, for a preliminary evaluation of the effects on process enhancement. Waste and leachate were periodically sampled in the landfill during the aeration before the excavation, for quality assessment over time; the evolution of biogas composition in the landfill body and in the extraction system for different plant set-ups during the project was monitored, with specific focus on uncontrolled migration into the excavation area. Waste biological stability significantly increased during the aeration (waste respiration index dropped to 33% of the initial value after six months). Leachate head decreased from 4 to 1.5m; leachate recirculation tests proved the beneficial effects of moisture addition on temperature control, without hampering waste aerobization. Proper management of the aeration plant enabled the minimization of uncontrolled biogas emissions into the excavation area.

Pre-treatment technologies for dark fermentative hydrogen production: Current advances and future directions

Hydrogen is regarded as a clean and non-carbon fuel and it has a higher energy content compared to carbon fuels. Dark fermentative hydrogen production from organic wastes is the most promising technology for commercialization among chemical and biological methods. Using mixed microflora is favored in terms of easier process control and substrate conversion efficiencies instead of pure cultures. However, mixed cultures should be first pre-treated in order to select sporulating hydrogen producing bacteria and suppress non-spore forming hydrogen consumers. Various inoculum pre-treatments have been used to enhance hydrogen production by dark fermentation including heat shock, acid or alkaline treatment, chemical inhibition, aeration, irradiation and inhibition by long chain fatty acids. Regarding substrate pre-treatment, that is performed with the aim of enhanced substrate biodegradability, thermal pre-treatment, pH adjustment using acid or base, microwave irradiation, sonication and biological treatment are the most commonly studied technologies. This article reviews the most investigated pre-treatment technologies applied for either inoculum or substrate prior to dark fermentation, the long-term effects of varying pre-treatment methods and the subsequently feasibility of each method for commercialization.

Lab-scale phytotreatment of old landfill leachate using different energy crops.

Old landfill leachate was treated in lab-scale phytotreatment units using three oleaginous species: sunflower (H), soybean (S) and rapeseed (R). The specific objectives of this study were to identify the effects of plant species combinations with two different soil textures on the reduction of COD, total N (nitrogen) and total P (phosphorous); to identify the correlation between biomass growth and removal efficiency; to assess the potential of oily seeds for the production of biodiesel. The experimental test was carried out using 20L volume pots installed in a greenhouse under different leachate percentages in the feeding and subsequent COD, N and P loads. Significant removal efficiencies were achieved: COD (ɳ>80%), total N (ɳ>70%) and total P (ɳ>95%). Better performances were displayed by the clayey soil. Plants irrigated with leachate, when compared to control units fed only with water and nutrient solution (Hoagland solution), developed a larger plant mass. Sunflower was the best performing species.

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.

Recirculation of reverse osmosis concentrate in lab-scale anaerobic and aerobic landfill simulation reactors

Leachate treatment is a major issue in the context of landfill management, particularly in view of the consistent changes manifested over time in the quality and quantity of leachate produced, linked to both waste and landfill characteristics, which renders the procedure technically difficult and expensive. Leachate recirculation may afford a series of potential advantages, including improvement of leachate quality, enhancement of gas production, acceleration of biochemical processes, control of moisture content, as well as nutrients and microbe migration within the landfill. Recirculation of the products of leachate treatment, such as reverse osmosis (RO) concentrate, is a less common practice, with widespread controversy relating to its suitability, potential impacts on landfill management and future gaseous and leachable emissions. Scientific literature provides the results of only a few full-scale applications of concentrate recirculation. In some cases, an increase of COD and ammonium nitrogen in leachate was observed, coupled with an increase of salinity; which, additionally, might negatively affect performance of the RO plant itself. In other cases, not only did leachate production not increase significantly but the characteristics of leachate extracted from the well closest to the re-injection point also remained unchanged. This paper presents the results of lab-scale tests conducted in landfill simulation reactors, in which the effects of injection of municipal solid waste (MSW) landfill leachate RO concentrate were evaluated. Six reactors were managed with different weekly concentrate inputs, under both anaerobic and aerobic conditions, with the aim of investigating the short and long-term effects of this practice on landfill emissions. Lab-scale tests resulted in a more reliable identification of compound accumulation and kinetic changes than full-scale applications, further enhancing the development of a mass balance in which gaseous emissions and waste characteristics were also taken into consideration. Results showed that RO concentrate recirculation did not produce consistent changes in COD emissions and methane production. Simultaneously, ammonium ion showed a consistent increase in leachate (more than 25%) in anaerobic reactors, free ammonia gaseous emissions doubled with concentrate injection, while chloride resulted accumulated inside the reactor.

The S.An.A.® concept: Semi-aerobic, Anaerobic, Aerated bioreactor landfill

Hybrid Bioreactor Landfills are designed to enhance and speed up biological processes, aiming at reducing the duration of post operational phase until landfill completion. S.An.A.® (Semi-aerobic, Anaerobic, Aerated) concept consists in a Hybrid Bioreactor featuring a first semi-aerobic phase to enhance the methane production occurring in the following anaerobic step and a forced aeration for the abatement of the residual emissions. At the end of the last step, semi-aerobic conditions are restored and flushing applied for leaching residual non-biodegradable compounds. Results of the application of S.An.A.® concept to a lab scale bioreactor system showed that pre-aeration was effective in controlling the concentration of VFA, increasing pH and stimulating methane production during anaerobic phase; in particular with intermittent airflow the methane potential was 50% higher respect to control reactors. Forced aeration reduced organic compounds and nitrogen concentration in leachate of an order of magnitude, better performing in low airflow reactors. S.An.A.® Hybrid bioreactors proved to be an efficient system both for increasing methane production and reaching landfill completion in shorter time, suggesting that with proper landfill management, the duration of post-closure care might be reduced by 25-35%.

Pre-treating anaerobic mixed microflora with waste frying oil: A novel method to inhibit hydrogen consumption

An innovative method was introduced to inhibit methanogenic H2 consumption during dark fermentative hydrogen production by anaerobic mixed cultures. Waste frying oil was used as an inhibitor for hydrogenotrophic methanogens. Simultaneous effect of waste frying oil concentrations (0-20 g/L) and initial pH (5.5, 6.5 and 7.5) on inhibition of methanogenic H2 consumption and enhancement of H2 accumulation were investigated using glucose as substrate. Enhanced hydrogen yields with decreased methane productions were observed with increasing the waste frying oil concentrations. On average, CH4 productions from glucose in the cultures received 10 g/L WFO were reduced by 88%. Increased WFO concentration up to 20 g/L led to negligible CH4 productions and in turn enhanced H2 yields. Hydrogen yields of 209.26, 195.35 and 185.60 mL/g glucoseadded were obtained for the cultures pre-treated with 20 g/L waste frying oil with initial pH of 5.5, 6.5 and 7.5 respectively. H2 production by pre-treated cultures was also studied using a synthetic food waste. Anaerobic mixed cultures were pre-treated with 10 g/L WFO and varying durations (0, 24 and 48 h). A H2 yield of 71.46 mL/g VS was obtained for cultures pre-treated with 10 g/L WFO for 48 h that was 475% higher than untreated control. This study suggests a novel and inexpensive approach for suppressing hydrogenotrophic methanogens during dark fermentative H2 production.

PLASMIX management: LCA of six possible scenarios

Only a small percentage of the separately collected plastic is recycled. The mechanical selection process of source segregated plastic materials generates considerable amounts of residues that are commonly named as Plasmix. By means of a life cycle assessment (LCA) modelling, the environmental performances of the main Plasmix management options (thermal treatment, energy recovery, and landfilling) were compared. Six treatment scenarios, with different pre-treatment alternatives, were evaluated. Landfilling after waste washing and Plasmix substitution of coke in a blast furnace represent the most favorable options, since the performances of thermal treatment and energy recovery are worsened by specific emissions of a variety of toxic compounds and heavy metals within plastic materials as additives.