Integrating liquid fraction of pig manure in the composting process for nutrient recovery and water re-use

Abstract

Composting of solid fraction of swine manure is a usual practice in most farms in order to obtain a fertilizer of better quality. Due to the negative hydric balance of the composting process, watering the composting material is necessary, what may be carried out with liquid fraction of pig manure. In this way, substantial amounts of liquid fraction can be treated by composting, allowing the recovering the nutrients and reducing the volumes to be transported to the more distant crop fields or subjected to further treatment. Thus, the main objective of this research was to study the treatment of liquid fraction of pig manure by co-composting with solid fraction of pig manure and other solid biowaste generated in rural areas. The present research is part of a project to find an integral solution for pig manure consisting of nutrient recovery through compost production and water re-use after biological purification in constructed wetlands. In accordance with the European waste management hierarchy, sustainable and low cost cleaner technologies aiming at resource recovery must be developed as an alternative to conventional technologies applied to the treatment of pig manure. This paper presents the results of composting of liquid fraction of fresh manure, which is conceived at the same time as a pig wastewater pre-treatment, wastewater volume reduction and a nutrient recovery system. Two 30 m3 turned windrows were constituted with solid fraction of pig manure and Populus sp. wood chips as bulking material at volume ratios of 1:1 and 1:2 and watered intensely with liquid fraction whilst thermophilic temperatures were maintained. Subsequently, both windrows were divided and the new windrows each received the same quantity of a different organic waste (solid fraction of pig manure, sawdust and grape bagasse), being watered with liquid fraction for a further 30 days. Stabilised composts with a nitrogen content ranging from 1.8 to 2.0% and a carbon to nitrogen ratio from 14.0 to 18.8 were obtained. Water balances showed evaporation rates ranging from 14 to 76 L/t total solids·d and overall evaporation ratios from 1 to 2.7 m3/t total solids, referred to dry matter of solid waste. While the reduction of liquid fraction volume ranged from 58 to 88% (depending on the watering rate), mass reduction of pollutants reached approximately 90% of total Kjeldahl nitrogen, ammonium and suspended solids. In comparison with traditional composting processes of solid fraction, our results show that huge amounts of liquid fraction can be treated by co-composting with solid fraction and other solid wastes. Integrating the liquid fraction of pig manure in the composting process has improved the compost quality and has reduced the pollutant load in the remaining liquid fraction, which makes possible an advanced treatment in constructed wetlands in order to reach the necessary water quality to be recycled or even to discharge in natural water bodies. In this way, both composting and constructed wetland systems can offer an integral solution for the recovery of water and fertilizer elements contained in pig manure and diverse locally generated solid wastes. However, in spite of these benefits, more research focussing on nitrogen balances, ammonia volatilisation and greenhouse emissions will be of great interest.