The amount of biowaste generated by university canteens (BWUC) in the faculties of the University of A Coruña (UDC) varies between 6 and 100 kg/day. In addition, the gardening services of the campus generate even higher amounts of garden waste (GrW), including pruning, which, once crushed, serves as bulking material for composting the biowaste from the canteens. Decentralized composting has been chosen with the aim of producing high quality organic fertilizers for university urban gardens while reducing the environmental burdens of both waste management and agricultural practice. Small static home composters of 340 L (SHC) for smaller amounts of generation (up to 20 kg BWUC/day) were used, while, for faculties of higher generation (up to 40 kg BWUC/day on average), the first composting stage was carried out in a closed and dynamic composter (DC). The dynamic composter was designed and built specifically for this project and its features were improved and optimized throughout the study. The pilot project was carried out in two centers of the UDC, which are known as the Philology Faculty (PF) and the School of Architecture (SA). All the organic waste generated by the canteens of these two colleges from January 2011 to July 2011 (approximately 3000 kg) was treated. Composting in SHC included a thermophilic phase that extended one month beyond the loading period for which thermophilic temperatures were also recorded. The use of the DC as the first stage in combination with static composters (SC) for the maturation stage reduced the overall thermophilic phase to 6–8 weeks. The complete maturation (Rottegrade class IV-V) was achieved after about four months in SHC and after two months when using the combined DC-SC system, if the right conditions of moisture were maintained. The chemical quality of the compost produced was compatible with Class A of Spanish legislation (equivalent to organic farmer quality) and the C/N ratio ranged from 9 to 15 depending on the relation BWUC:GrW.

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The aim of this research was to study nitrogen losses during the treatment of the liquid fraction (LF) of pig manure by co-composting and to establish the best conditions for compost production with higher nitrogen and low heavy metal contents. Windrows were constituted with the solid fraction (SF) of pig manure, different organic waste (SF of pig manure, sawdust and grape bagasse) as co-substrate and Populus spp. wood chips as bulking material and watered intensely with the LF. Results show that nitrogen losses ranged from 30% to 66% of initial nitrogen and were mainly governed by substrate to bulking mass ratio and liquid fraction to substrate (LF/S) ratio, and only secondarily by operational parameters. Nitrogen losses decreased from 55-65% at low LF/S ratios (1.7-1.9 m/t total solids (TS)) to 30-39% at high LF/S ratios (4.4-4.7 m/t TS). Therefore, integrating the LF in the composting process at high LF/S ratios favoured nitrogen recovery and conservation. Nitrogen in the fine fraction (ranging from 27% to 48% of initial nitrogen) was governed by operational parameters, namely pH and temperature. Final compost showed low content in most heavy metals, but Zn was higher than the limits for compost use in agriculture. Zn content in the obtained compost varied from 1863 to 3269 mg/kg dm, depending on several factors. The options for obtaining better quality composts from the LF of pig manure are selecting co-substrates with low heavy metal content and using them instead of the SF of pig manure.