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.

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

Environmental impacts and gaseous emissions associated to home and industrial composting of the source-separated organic fraction of municipal solid waste have been evaluated using the environmental tool of life cycle assessment (LCA). Experimental data of both scenarios were experimentally collected. The functional unit used was one ton of organic waste. Ammonia, methane and nitrous oxide released from home composting (HC) were more than five times higher than those of industrial composting (IC) but the latter involved within 2 and 53 times more consumption or generation of transport, energy, water, infrastructures, waste and Volatile Organic Compounds (VOCs) emissions than HC. Therefore, results indicated that IC was more impacting than HC for four of the impact categories considered (abiotic depletion, ozone layer depletion, photochemical oxidation and cumulative energy demand) and less impacting for the other three (acidification, eutrophication and global warming). Production of composting bin and gaseous emissions are the main responsible for the HC impacts, whereas for IC the main contributions come from collection and transportation of organic waste, electricity consumption, dumped waste and VOCs emission. These results suggest that HC may be an interesting alternative or complement to IC in low density areas of population.

Odours (OU(E)) and volatile organic compounds (VOC) emission during biological process used to treat MSW were studied under standardized conditions in order to detect potential risk for workers and population. Results obtained indicated that odours and VOCs emitted depend on the biological stability of waste measured by the dynamic respiration index (DRI) and a very good correlation were found between these parameters (OU(E) vs. DRI, r=0.96, p<0.001, n=6; VOC vs. DRI, r=0.97, p<0.001, n=6). GC-MS study of the VOCs indicated the presence of a group of molecules that were degraded during the process. On the other hand, a second group of molecules, i.e. aromatic and halogenated compounds, and furan persisted in the waste sample, although molecule concentrations were always lower than Threshold Limit Value-Time Weighted Average (TLV-TWA).

There is actually a common consensus in using biological technologies for the treatment of organic wastes. For instance, composting is used for aerobic biological stabilisation of organic wastes. The amount of materials and the variety of wastes composted are increasing. However, composting is inherently a process generating gaseous emissions. Greenhouse gases (GHG) such as carbon dioxide, methane and nitrous oxide from compost are of special relevance for global warming. Although a part of these gases is inherent to the process, another important part can be abated by low-cost biological technologies such as biofiltration and its variations. This article reviews the emission of GHG from composting gases, from detection and measurement to minimisation and abatement. Special emphasis is given to the measurement of GHG to obtain reliable emission factors for the different composting technologies. These factors will help to compare different waste treatment options based on overall analysis tools such as life cycle assessment. A specific chapter is related to carbon and nitrogen dynamics in the composting matrix, and their consequences on the production of carbon and nitrogen gases. Finally, we present a review of the best available practices to minimise the GHG emissions from composting and the final treatment of composting off-gases.