Treatment technologies can play a key role aiming to reduce the environmental burdens associated to agricultural organic waste management. Simultaneously, they can help to improve production efficiency and convert by-products into valuable resources while enhancing land functions linked to the role of soil in closing nutrient cycles. Moreover, choosing the most appropriate strategy amongst the different methods of valorization, recycling and/or reutilization of organic waste resources is key to reduce total N losses and mitigate both greenhouse gas (GHG) and ammonia (NH3) emissions. The basis to reach such decision is far from trivial due to the large and complex interactions between the different levels of organization (e. g. soil-plant-animal-waste), the biogeochemical cycles, (e.g. the C and N cycles are intimately coupled), the water cycle, energy flows, human decisions, technological possibilities and local conditions (i.e. soil and climate).
The main objective of this thesis was to develop and test a methodology that, striking a reasonable compromise between right complexity and utility, could be useful to evaluate GHG and NH3 mitigation strategies involving management of organic resources under different soil and climatic conditions. With this view and based on the most updated state-of-the-art scientific information, an integrated methodology that can be used across scales and production systems, and which coupled mathematical modelling and life cycle assessment has been developed. The specific steps to achieve this goal involved: (i) quantifying through a systematic review which strategies have the potential to reduce GHG emissions and N losses from solid waste management, (ii) based partly on this review develop and test a composting model (SIMSWASTE-CO) to identify potential mitigation strategies involving solid waste management, (iii) develop and test an anaerobic digestion model (SIMSWASTE-AD) to identify potential mitigation strategies involving liquid waste management, (iv) evaluate the environmental impacts of including agro-industrial by-products in the diet of dairy goats in Spain and compare this strategy against other potential competing uses of these by-products (e.g. Anaerobic digestion and composting) and (v) evaluate at the regional level the effect of different management strategies of organic resources on soil organic carbon accumulation in orchards and horticulture in the Spanish Mediterranean coastal area.
The systematic review showed that different management strategies of organic waste in solid form have a significant influence on GHGs and N emissions. Improving the structure of the solid waste heap was one of the most effective measures to mitigate GHGs, simultaneously reducing methane (CH4) and nitrous oxide (N2O) emissions without increasing substantially N losses through NH3 volatilization. Amongst composting methods, turned systems have potential for reducing GHGs emissions. Aerobic conditions enhanced in both composting methods guarantee the hygienization of the material by reaching high range temperatures (>60ºC), but they involve the risk of pollution swapping due to enhanced NH3 emissions. Covering or compaction help decrease N losses via NH3 volatilization, thus enhancing nutrient conservation. However, these practices did not show significant effects on reducing GHG emissions (CH4, N2O), and they may result in negative agronomic trade-offs. The use of specific additives can be a successful strategy for reducing gaseous losses during treatment of solid waste depending on the target substance, additive dosage and operational conditions of the process.
There is evidence to assume that N2O emissions from solid storage systems are at least in the same range as those from turned composting in passive windrow, thereby the respective EFs should be refined in the development of the future IPCC methodology.
The new models for composting (SIMSWASTE-CO) and anaerobic digestion (SIMSWASTE-AD) validated reasonably well, showing good agreement between modelled and measured values from existing experiments. Whereas SIMSWASTE-CO validation exercise indicated good model performance for the simulation of carbon dioxide (CO2) and NH3 emissions and was able to capture the effect of different management practices on GHG emissions, SIMSWASTE-AD validation exercise reflected a good model capability to predict biogas production and N mineralisation under a range of substrate mixtures and operational conditions. More measurements are still needed in order to fully validate N2O and CH4 emissions though.
The importance of climate, post-digestion emissions and its relationship with the anaerobic digestion (AD) performance and feedstock composition was identified as crucial factors in order to reduce the net GHG emissions of the whole process, but also to enhance digestate fertilizer potential.
Beyond the effects on the waste management stage, SIMSWASTE-CO and SIMSWASTE-AD aim to help account for potential effects of organic waste treatment on other stages and spatial scales by providing the C and nutrient flows. Its mass balance basis allows them to interact with other models, so they can be easily integrated in more holistic (e.g. LCA) and broader (e.g. regional studies) approaches. Such system scope is essential for stakeholders in order to help developing the most appropriate strategies involving the management of agricultural organic waste management.
As a way to show the capability of the methodological framework on different scales, the two models were coupled to (i) an LCA analysis and to (ii) soil C model within a regional assessment.
Firstly, the integrated approach coupling mathematical models and LCA was applied to investigate the GHG mitigation potential of including agro-industrial by-products in the diet of dairy goats. From an environmental point of view, the results indicated that the application of the selected by-products as dairy goat feed was the best management option in comparison to the baseline management strategies considered: composting and anaerobic digestion. In fact, the two dietary strategies tested offered promising overall reductions on the environmental burdens associated with goat milk production.
Finally, the integrated approach coupling mathematical models (e.g. SIMSWASTE) with a dynamic model of SOC turnover (RothC) allowed to investigate the potential of different management strategies of organic resources to enhance SOC stocks in orchards and horticultural system in the Spanish Mediterranean coast.
Sowing cover crops in orchards systems showed the largest SOC accumulation potential. Whilst this practice is currently not widely used in the area, it could also provide other additional benefits such as soil erosion protection and water holding capacity, which are of paramount importance to the Mediterranean conditions. Other combinations, for example including composting or anaerobic digestion, did not lead to large increases in soil C but, on overall (e.g. including the effect of producing biogas) reduced largely the GHG emissions.