Development of Technologies for Local Composting of Food Waste from Universities

Abstract

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.

Keywords: decentralized composting; dynamic composter; food waste; static composters; university.

Figure 1

Composting system at the School of Architecture: (a) First step Dynamic composter, Plana^® design, (b) Maturation static composters (2 Container KOMP 1050 units and 1 Container KOMP 340 unit).

Figure 2

Cumulative loads (BWUC + GrW) on the left and temperature evolution on the right in the Philology Faculty composters. Data corresponds to the second and third batches (loads) and second batch (temperature).

Figure 3

Composting process diagram at the Philology Faculty (Composter volume: 340 L).

Figure 4

Operation of the dynamic composter (DC) and the static maturity composter (SMC) in the School of Architecture: (A) Evolution of the load and discharge applied to the DC, (B) Temperature evolution in the DC, (C) Temperature evolution in the SMC. For SMC, only one unit is shown, which is the one that received the output material from DC between days 78 and 92 of its operation (Figures A,B). The behavior of other batches was similar.

Figure 4

Operation of the dynamic composter (DC) and the static maturity composter (SMC) in the School of Architecture: (A) Evolution of the load and discharge applied to the DC, (B) Temperature evolution in the DC, (C) Temperature evolution in the SMC. For SMC, only one unit is shown, which is the one that received the output material from DC between days 78 and 92 of its operation (Figures A,B). The behavior of other batches was similar.

Figure 5

Oxygen concentration in dynamic (DC) and static composters (SC receiving fresh BWUC, several SMC batches receiving the material from the DC).

Figure 6

Composting process diagram at the School of Architecture. The static maturity composter (SMC) in stage 2 could be replaced with a breathable bag (big-bag). The number of units in parallel in stage 2 ranged between 2 and 4.

Figure 7

Mean concentration (± S.D.) of heavy metals in the final compost from PF and SA. Concentrations are expressed as values relative to the legal and stricter Spanish limits for Compost Class A, and its comparison with the limits for Class A (relative value = 1) and Class B.