This review analyses the main challenges of the process of food waste composting and examines the crucial aspects related to the quality of the produced compost. Although recent advances have been made in crucial aspects of the process, such composting microbiology, improvements are needed in process monitoring. Therefore, specific problems related to food waste composting, such as the presence of impurities, are thoroughly analysed in this study. In addition, environmental impacts related to food waste composting, such as emissions of greenhouse gases and odours, are discussed. Finally, the use of food waste compost in soil bioremediation is discussed in detail.

Since society at large became aware of the use of nanomaterials in ever growing quantities in consumer products and their presence in the environment, critical interest in the impact of this emerging technology has grown. The main concern is whether the unknown risks of engineered nanoparticles (NPs), in particular their impact on health and environment, outweighs their established benefits for society. Therefore, a key issue in this field is to evaluate their potential toxicity. In this context we evaluated the effects on plants and microorganisms of model nanoparticles, in particular of a stable metal (Au, 10nm mean diameter), a well-known bactericide (Ag, 2 nm mean diameter) and the broadly used Fe(3)O(4) (7 nm mean diameter). The toxicity of these nanoparticles was assayed using standard toxicity tests. Specifically, germination (cucumber and lettuce), bioluminescent (Photobacterium phosphoreum) and anaerobic toxicity tests were performed. Germination tests were conducted at a NP dose of 62, 100 and 116 microg mL(-1) for Au, Ag, and Fe(3)O(4), respectively. The bioluminscent testing (P. phosphoreum) was conducted at a dose of 28, 45 and 52 microg mL(-1) for Au, Ag, and Fe(3)O(4), respectively. Finally, anaerobic tests were conducted at a NP dose of 10, 16 and 18 microg mL(-1) for Au, Ag, and Fe(3)O(4), respectively. In all cases low or zero toxicity was observed. However, some perturbation of the normal functions with respect to controls in germinating tests was observed, suggesting the necessity for further research in this field. At the same time, the effect of NP-solvents was sometimes more significant than that of the NPs themselves, a point that is of special interest for future nanotoxicological studies.

Ammonia emissions were quantified for the laboratory-scale composting of three typical organic wastes with medium nitrogen content: organic fraction of municipal solid wastes, raw sludge and anaerobically digested sludge; and the composting of two wastes with high nitrogen content: animal by-products from slaughterhouses and partially hydrolysed hair from the leather industry. All the wastes were mixed with the proper amount of bulking agent. Ammonia emitted in the composting of the five wastes investigated revealed a strong dependence on temperature, with a distinct pattern found in ammonia emissions for each waste in the thermophilic first stage of composting (exponential increase of ammonia emitted when increasing temperature) than that of the mesophilic final stage (linear increase of ammonia emissions when increasing temperature). As composting needs high temperatures to ensure the sanitisation of compost and ammonia emissions are one of the main environmental impacts associated to composting and responsible for obtaining compost with a low agronomical quality, it is proposed that sanitisation is conducted after the first stage in large-scale composting facilities by a proper temperature control. Ammonia emission pattern and correlation with process temperature are presented for the composting process of different organic wastes.

Slow degradation of sewage sludge is a disadvantage of anaerobic digestion leading to high sludge retention times in conventional digesters. Hydrolysis has been pointed as the rate-limiting step in this process. Thermophilic anaerobic digestion has been proved effectively to reduce the retention time needed for sludge stabilization. Sludge pretreatment has been also proposed as a strategy to accelerate the hydrolytic step. The effectiveness of high and low temperature thermal pretreatment, ultrasonic and microwave pretreatments in secondary sludge disintegration has been studied by means of the increment in filterable volatile solids to total volatile solids ratio (FVS/TVS) respect to untreated sludge. Increments in this parameter ranging from three-fold for microwave treatment to nine-fold for high temperature thermal treatment have been obtained. Biogas production under thermophilic (55 °C) conditions for treated and untreated secondary sludge has also been evaluated. In spite of the values of FVS/TVS ratio obtained for all the treatments studied, no differences in biogas production were observed when high temperature thermal treatment, ultrasonic and microwave treatments are compared with untreated sludge. A 50% increment in biogas production was observed for low temperature (70 °C) thermally treated sludge. This type of treatment has been pointed as a predigestion step enhancing biological activity of some thermophilic hydrolytic bacteria

The objective of this work was to investigate the effect of a low temperature pre-treatment (70 °C) on the efficiency of thermophilic anaerobic digestion of primary and secondary waste sludge. Firstly, effect of sludge pre-treatment time (9, 24, 48 and 72 h) was evaluated by the increase in volatile dissolved solids (VDSs), volatile fatty acids (VFAs) and biogas production in thermophilic batch tests. Secondly, semi-continuous process performance was studied in a lab-scale reactor (5 L) working at 55 °C and 10 days solid retention time. The 70 °C pre-treatment showed an initial solubilization effect (increasing VDS by almost 10 times after 9 h), followed by a progressive generation of VFA (from 0 to nearly 5 g L⁻¹ after 72 h). Biogas production increased up to 30% both in batch tests and in semi-continuous experiments. Our results suggest that a short period (9 h) low temperature pre-treatment should be enough to enhance methane production through thermophilic anaerobic digestion of sludge

Growth in production and use of nanoparticles (NPs) will result increased concentrations of these in industrial and urban wastewaters and, consequently, in wastewater-treatment facilities. The effect of this increase on the performance of the wastewater-treatment process has not been studied systematically and including all the microbial communities involved in wastewater treatment. The present work investigates, by using respiration tests and biogas-production analysis, the inhibitory effect of four different commonly used metal oxide (CeO(2) and TiO(2)) and zero-valent metal (Ag and Au) nanoparticles on the activity of the most important microbial communities present in a modern wastewater-treatment plant. Specifically, the actions of ordinary heterotrophic organisms, ammonia oxidizing bacteria, and thermophilic and mesophilic anaerobic bacteria were tested in the presence and absence of the nanoparticles. In general, CeO(2) nanoparticles caused the greatest inhibition in biogas production (nearly 100%) and a strong inhibitory action of other biomasses; Ag nanoparticles caused an intermediate inhibition in biogas production (within 33-50%) and a slight inhibition in the action of other biomasses, and Au and TiO(2) nanoparticles caused only slight or no inhibition for all tested biomasses.