Energetic Valorization of Wet Olive Mill Wastes through a Suitable Integrated Treatment: H2O2 with Lime and Anaerobic Digestion

Performance Evaluation of Pressurized Anaerobic Digestion (PDA) of Raw Compost Leachate

Anaerobic digestion (AD) represents an advantageous solution for the treatment and valorization of organic waste and wastewater. To be suitable for energy purposes, biogas generated in AD must be subjected to proper upgrading treatments aimed at the removal of carbon dioxide and other undesirable gases. Pressurized anaerobic digestion (PDA) has gained increasing interest in recent years, as it allows the generation of a high-quality biogas with a low CO2 content. However, high pressures can cause some negative impacts on the AD process, which could be accentuated by feedstock characteristics. Until now, few studies have focused on the application of PAD to the treatment of real waste. The present work investigated, for the first time, the performance of the pressurized anaerobic digestion of raw compost leachate. The study was conducted in a lab-scale pressurized CSTR reactor, working in semi-continuous mode. Operating pressures from the atmospheric value to 4 bar were tested at organic loading rate (OLR) values of 20 and 30 kgCOD/m3d. In response to the rise in operating pressure, for both OLR values tested, a decrease of CO2 content in biogas was observed, whereas the CH4 fraction increased to values around 75% at 4 bar. Despite this positive effect, the pressure growth caused a decline in COD removal from 88 to 62% in tests with OLR = 20 kgCOD/m3d. At OLR = 30 kgCOD/m3d, an overload condition was observed, which induced abatements of about 56%, regardless of the applied pressure. With both OLR values, biogas productions and specific methane yields decreased largely when the pressure was brought from atmospheric value to just 1 bar. The values went from 0.33 to 0.27 LCH4/gCODremoved at 20 kgCOD/m3d, and from 0.27 to 0.18 LCH4/gCODremoved at 30 kgCOD/m3d. Therefore, as the pressure increased, although there was an enhanced biogas quality, the overall amount of methane was lowered. The pressured conditions did not cause substantial modification in the characteristics of digestates.

Improvement of Biomethane Production from Organic Fraction of Municipal Solid Waste (OFMSW) through Alkaline Hydrogen Peroxide (AHP) Pretreatment

The organic fraction resulting from the separate collection of municipal solid waste (OFMSW) is an abundant residue exploitable for biofuel production. Anaerobic digestion (AD) is one of the most attractive technologies for the treatment of organic wastes thanks to the generation of biogas with a high methane content. However, because of its complex composition, the direct digestion of OFMSW can be less effective. To overcome these difficulties, many pretreatments are under development. In this work, the efficacy of alkaline hydrogen peroxide (AHP) oxidation was assessed for the first time as a pretreatment of OFMSW to enhance its anaerobic biodegradability. In this regard, many AHP batch tests were executed at pH 9 and by changing the peroxide dosages up to 1 gH2O2/gCOD, under room temperature and pressure conditions. Afterwards, biomethane potential tests (BMP) were conducted to evaluate the performance of anaerobic digestion both on raw and pretreated OFMSW. The pretreatment tests demonstrated that AHP induces only a weak reduction in the organic load, reaching a maximum COD removal of about 28%. On the other hand, notable productions of volatile fatty acids (VFA) were found. In fact, by applying a peroxide dose of just 0.025 gH2O2/gCOD, there was a doubling in VFA concentration, which increased by five times with the highest H2O2 amount. These results indicate that AHP mainly causes the conversion of complex organic substrates into easily degradable compounds. This conversion made it possible to achieve much better performance during the BMP tests conducted with the pretreated waste compared to that carried out on fresh OFMSW. Indeed, a low methane production of just 37.06 mLCH4/gTS was detected on raw OFMSW. The cumulated CH4 production in the pretreated samples increased in response to the increase in H2O2 dosage applied during AHP. Maximum specific productions of about 463.7 mLCH4/gTS and 0.31 LCH4/gCODremoved were calculated on mixtures subjected to AHP. On these samples, the satisfactory evolution of AD was confirmed by the process parameters calculated by modeling the cumulated CH4 curves through a new proposed formulation of the Gompertz equation.

Paraben Compounds—Part II: An Overview of Advanced Oxidation Processes for Their Degradation

Water scarcity represents a problem for billions of people and is expected to get worse in the future. To guarantee people’s water needs, the use of “first-hand water” or the reuse of wastewater must be done. Wastewater treatment and reuse are favorable for this purpose, since first-hand water is scarce and the economic needs for the exploration of this type of water are increasing. In wastewater treatment, it is important to remove contaminants of emerging concern, as well as pathogenic agents. Parabens are used in daily products as preservatives and are detected in different water sources. These compounds are related to different human health problems due to their endocrine-disrupting behavior, as well as several problems in animals. Thus, their removal from water streams is essential to achieve safe reusable water. Advanced Oxidation Processes (AOPs) are considered very promising technologies for wastewater treatment and can be used as alternatives or as complements of the conventional wastewater treatments that are inefficient in the removal of such contaminants. Different AOP technologies such as ozonation, catalytic ozonation, photocatalytic ozonation, Fenton’s, and photocatalysis, among others, have already been used for parabens abatement. This manuscript critically overviews several AOP technologies used in parabens abatement. These treatments were evaluated in terms of ecotoxicological assessment since the resulting by-products of parabens abatement can be more toxic than the parent compounds. The economic aspect was also analyzed to evaluate and compare the considered technologies.

Enhancement of Anaerobic Digestion of Waste-Activated Sludge by Conductive Materials under High Volatile Fatty Acids-to-Alkalinity Ratios

Anaerobic digestion (AD) represents a suitable option for the management of the waste-activated sludge (WAS) produced in municipal wastewater treatment plants. Nevertheless, due to its complex characteristics, WAS is often barely degradable under conventional anaerobic processes. The use of conductive materials during AD provides a promising route for enhancing WAS digestion, through the effects of direct inter-species electron transfer (DIET). The present paper aims to evaluate the effects of the addition of four different materials—granular activated carbon (GAC), granular iron, and aluminium and steel scrap powders—in semi-continuous lab-scale reactors under very high volatile fatty acids-to-alkalinity ratios. In particular, the use of metallic aluminium in WAS digestion was investigated for the first time and compared to the other materials. The AD of WAS without the addition of conductive materials was impossible, while the use of steel powder and zero-valent iron is shown not to improve the digestion process in a satisfactory way. On the contrary, both GAC and Al allow for effective WAS degradation. At stable conditions, methane yields of about 230 NmLCH4/gVS and 212 NmLCH4/gVS are recorded for GAC- and Al-amended reactors, respectively. These two materials are the most promising in sustaining WAS AD through DIET also in case of unbalanced volatile fatty acids-to-alkalinity ratios.

Advances in Struvite Precipitation Technologies for Nutrients Removal and Recovery from Aqueous Waste and Wastewater

The abatement of nutrient compounds from aqueous waste and wastewater is currently a priority issue. Indeed, the uncontrolled discharge of high levels of nutrients into water bodies causes serious deteriorations of environmental quality. On the other hand, the increasing request of nutrient compounds for agronomic utilizations makes it strictly necessary to identify technologies able to recover the nutrients from wastewater streams so as to avoid the consumption of natural resources. In this regard, the removal and recovery of nitrogen and phosphorus from aqueous waste and wastewater as struvite (MgNH4PO4·6H2O) represents an attractive approach. Indeed, through the struvite precipitation it is possible to effectively remove the ammonium and phosphate content of many types of wastewater and to produce a solid compound, with only a trace of impurities. This precipitate, due to its chemical characteristics, represents a valuable multi-nutrients slow release fertilizer for vegetables and plants growth. For these reasons, the struvite precipitation technology constantly progresses on several aspects of the process. This manuscript provides a comprehensive review on the recent developments in this technology for the removal and recovery of nutrients from aqueous waste and wastewater. The theoretical background, the parameters, and the operating conditions affecting the process evolution are initially presented. After that, the paper focuses on the reagents exploitable to promote the process performance, with particular regard to unconventional low-cost compounds. In addition, the development of reactors configurations, the main technologies implemented on field scale, as well as the recent works on the use of struvite in agronomic practices are presented.

Chemical Denitrification with Mg0 Particles in Column Systems

The removal of nitrate from aqueous environments through zero-valent metallic elements is an attractive technique that has gained increasing interest in recent years. In comparison to other metallic elements, zero-valent magnesium (ZVM) has numerous beneficial aspects. Nevertheless, the use of Mg0 particles for nitrate reduction in column systems has not been investigated yet. To overcome the lack of research, in the present study, a wide experimental activity was carried out to develop a chemical denitrification process through ZVM in batch column equipment. Several tests were executed to evaluate the effects of recirculation hydraulic velocity, pH, Mg0 amount, N-NO3− initial concentration and temperature on the process performance. The results show that the process efficiency is positively influenced by the recirculation velocity increase. In particular, the optimal condition was detected with a value of 1 m/min. The process pH was identified as the main operating parameter. At pH 3, abatements higher than 86.6% were reached for every initial nitrate concentration tested. In these conditions, nitrogen gas was detected as the main reaction product. The pH increase up to values of 5 and 7 caused a drastic denitrification decline with observed efficiencies below 26%. At pH 3, the ratio (RMN) between Mg0 and initial nitrate amount also plays a key role in the treatment performance. A characteristic value of about RMN = 0.333 gMg0/mgN-NO3− was found with which it is possible to reach the maximum reaction rate. Unexpectedly, the process was negatively affected by the increase in temperature from 20 to 40 °C. At 20 °C, the material showed satisfactory denitrification efficiencies in subsequent reuse cycles. With the optimal RMN ratio, removals up to 90% were detected by reusing the reactive material three times. By means of a kinetic analysis, a mathematical law able to describe the nitrate abatement curves was defined. Moreover, the relation between the observed kinetic constant and the operating parameters was recognized. Finally, the reaction pathways were proposed and the corrosion reaction products formed during the treatment were identified.

A novel energetically efficient combinative microwave pretreatment for achieving profitable hydrogen production from marine macro algae (Ulva reticulate)

This study aims to enhance the hydrogen (H₂) production from marine macro algae (Ulva Reticulate) by microwave combined with hydrogen peroxide (H₂O₂) under alkaline condition. Microwave (domestic type) (M) pretreatment of algal biomass at its optimal power (40%) resulted in 27.9% COD solubilization at 15 min time interval. When this optimal microwave power was combined with H₂O₂ (MH) an increment in COD solubilization was achieved at 24 mg H₂O₂/g macroalgae dosage. Under alkaline condition (pH 7-12), microwave and H₂O₂ combination (MHA) yielded better result than MH. At optimal alkaline condition (pH 10), MHA pretreatment shows a COD solubilization of 34%. Microwave in alkaline condition induces decomposition of H₂O₂ and more OH radical synthesis. This synergistically promotes solubilization. The MHA process considerably diminish time and specific energy required for biomass disintegration. Among the samples, highest H₂ yield of 87.5 mL H₂/g COD was observed for MHA.

Biogas Generation through Anaerobic Digestion of Compost Leachate in Semi-Continuous Completely Stirred Tank Reactors

The composting process of organic fraction of municipal solid waste, besides to the residual compost, generates a wastewater that is characterized by a high organic load. The application of anaerobic processes represents an advantageous solution for the treatment and valorization of this type of wastewater. Nevertheless, few works have been focused on the anaerobic digestion of compost leachate. To overcome this dearth, in the present paper an extensive experimental investigation was carried out to develop and analyse the anaerobic treatment of young leachate in completely stirred tank reactors (CSTR). Initially, it was defined a suitable leachate pretreatment to correct its acidic characteristics that is potentially able to inhibit methanogenic biomass activity. The pretreated leachate was fed to the digester over the start-up phase that was completed in about 40 days. During the operational period, the organic load rate (OLR) changed between 4.25 kgCOD/m3d and 38.5 kgCOD/m3d. The chemical oxygen demand (COD) abatement was higher than 90% for OLR values up to 14.5 kgCOD/m3d and around to 80% for applied loads equal to 24.5 kgCOD/m3d. At this OLR, it was reached the maximum daily biogas production of about 9.3 Lbiogas/(Lreactord). The CH4 fraction was between 70%–78% and the methane production yield in the range 0.34–0.38 LCH4/gCODremoved. The deterioration of biogas production started for OLR values that were over the threshold of 24.5 kgCOD/m3d when a volatile fatty acids (VFA) accumulation occurred and the pH dropped below 6.5. The maximum ratio between VFA and alkalinity (ALK) tolerable in the CSTR was identified to be 0.5 gCH3COOH/gCaCO3. Through an economic analysis, it was proven that the digestion of compost leachate could ensure significant economic profits. Furthermore, the produced digestate had characteristics that were compatible for agricultural applications.

Biofuel Production and Phosphorus Recovery through an Integrated Treatment of Agro-Industrial Waste

The present study aimed to develop an integrated treatment of agro-industrial waste for biofuel (biogas and syngas) production and for phosphorus recovery. In the first step, an anaerobic digestion (AD) process was carried out on two different mixtures of raw agro-industrial residues. Specifically, a mixture of asparagus and tomato wastes (mixture-1) and a mixture of potatoes and kiwifruit residues (mixture-2) were investigated. The results proved that the properties of mixtures notably affect the evolution of the digestion process. Indeed, despite the lower organic load, the maximum biogas yield, of about 0.44 L/gCODremoved, was obtained for mixture-1. For mixture-2, the digestion process was hindered by the accumulation of acidity due to the lack of alkalinity in respect to the amount of volatile fatty acids. In the second step, the digestates from AD were utilized for syngas production using supercritical water gasification (SCWG) at 450 °C and 250 bar. Both the digestates were rapidly converted into syngas, which was mainly composed of H2, CO2, CH4, and CO. The maximum values of global gasification efficiency, equal to 56.5 g/kgCOD, and gas yield, equal to 1.8 mol/kgTS, were detected for mixture-2. The last step of the integrated treatment aimed to recover the phosphorus content, in the form of MgKPO4ˑ6H2O, from the residual liquid fraction of SCWG. The experimental results proved that at pH = 10 and Mg/P = 1 it is possible to obtain almost complete phosphorus removal. Moreover, by using the scanning electronic microscopy, it was demonstrated that the produced precipitate was effectively composed of magnesium potassium phosphate crystals.