EVALUATING THE PERFORMANCE OF A MUNICIPAL SOLID WASTE MECHANICAL-BIOLOGICAL TREATMENT PLANT

Abstract

This study analyses the performance of the mechanical-biological treatment plant (TMB) of AMIU Puglia s.p.a., located in Bari and able to treat approximately 700 tons of undifferentiated waste per day, mainly produced by the city of Bari and neighbouring municipalities. The AMIU MBT plant performed an aerobic biostabilization of the input waste and a consequent screening process. An MBT treatment reduces the volume and the amount of putrescible fraction of the waste to be landfilled. In addition, it reduces the moisture and the amount of low energy fractions in order to have a waste suitable for energy recovery in waste-to-energy (WTE) plants. The input mixed waste is converted into a Combustible Dry Fraction (CDF) and a Stabilized Disposable Waste (SDW). The first is mechanically refined for the production of Solid Recovered Fuels (SRFs), delivered to dedicated WTE plants; the latter is disposed in landfill. The MBT generates two further output streams: metal waste, selected through a magnetic separation and then sent for new raw material recovery, and process water destined for in-plant or off-plant treatment prior to disposal. Generally, there are two main categories of MBT plants: single stream MBT or separated streams MBT. In single stream MBT, the municipal residual waste is subject to a bio-drying process and then to mechanical refining. In separated streams, the input is first preliminarily divided into a wet and a dry fraction by sieving and then biodegraded through biostabilization. The baseline period for evaluating the performance of the MBT facility under investigation was 2015-2019. The following topics have been addressed: (i) composition (or product fraction) of the inlet waste and of the two main output streams (CDF and SDW); (ii) mass balance; (iii) monitoring of the potential Dinamic Respirometric Index (DRI_p) for CDF and SDW streams. The obtained results showed that the input waste was mainly composed of plastic and paper waste; anyway, a more or less significant presence of organic fraction was evident. The CDF stream was composed of dry waste - suitable for the WTE treatment - by over 90%. Instead, the SDW stream achieved about 50% of organic waste, which had a dimension of less than 20 mm. Mass balance analysis was carried out through STAN software, a freeware in line with “Austrian standard ÖNorm S 2096”. On average, the principal output streams of the plant were CDF and SDW, with about 35% and 42% of the input waste, respectively. These fractions were followed by mass loss (about 20%) instead metals and process water achieved lower values than the ones of other streams (both less than 1%). Mass balance showed that input waste strongly reduces its amount over time. As a consequence, CDF and SDW streams reduce their quantity. If a “single stream” configuration was adopted, the CDF and SDW streams differ from each other with the amount of SDW greater than CDF. Instaed, if a “separated stream” configuration was performed, CDF and SDW amounts are equal. The monitoring of DRI_p showed stable and decreasing values for CDF. The DRI_p values of SDW was more variable than the CDF ones and they depended by the configuration performed by the plant (single or separated stream). Therefore, SDW had to be monitored more frequently than CDF, especially while the plant was operating with a separated stream process. As a conclusion, the AMIU MBT plant can be considered an example of best practice for similar areas of Southern Italy and the technologies adopted, well consolidated, allow to achieve excellent performance.