Identifying Pressures

Two common and widespread difficult waste streams are sewage sludge derived from municipal wastewater treatment plants (SS) and the organic fine component from the mechanical sorting of municipal solid waste (OFMSW). OF-MSW cannot be recycled and is extremely heterogeneous (containing food waste, plastics, metals, paper and glass) and after composting is used as a cover material at landfill sites. Although at present 80% of SS is reused in agriculture, the agricultural outlet for SS is under increasing scrutiny, mainly because of perceptions of contamination risk. There are emerging restrictions, mostly driven by private organisations and food quality agencies, leading to a reduction in the availability of agricultural outlets. This is because of a tendency to exclude SS from lands used for food production under quality assurance schemes (An Bord Bia and the Grain Assurance Scheme). Consequently, it is important to investigate alternative outlets for SS to reduce the risk associated with depending solely on land spreading as an outlet.

Informing Policy

Thermal treatment of SS is undertaken in many European Union jurisdictions; this process enables the destruction of organic pollutants at high temperatures through complete inertisation. Advantages of thermal treatment include sterilisation and volume reduction. Pyrolysis is a thermal conversion technology that applies high temperatures to depolymerise organic molecules, resulting in a combustible syngas and solid char, which can potentially be used as a soil conditioner or as a solid fuel energy carrier. Because pyrolysis is performed at a small scale, it is suitable for deployment in small cities and towns; this can avoid the long-distance transportation of waste.

Developing Solutions

The OF-PYR project investigated the potential of slow pyrolysis as a technology for the treatment of OF-MSW and SS as an alternative to other disposal options. Pyrolysis of OF-MSW enables the recovery of energy in the form of a high-energy gas. However, the composition of the feedstock and variable levels of volatile fractions (relative to fixed carbon, ash and inert contaminants) affect energy recovery and can result in low-energy outputs, the value of which is unlikely to remunerate the cost of technology deployment. Pyrolysis results in large proportions of contaminated char that is unsuitable for use other than disposal as landfill cover. It is recommended that the high-energy fraction of OF-MSW (e.g. higher plastic content leads to a higher gas yield) is separated from the high mineral matter fraction before energy recovery.

Pyrolysis of SS provides valorisation of energy in the form of a gas, which could be directly used in a boiler, or in a gas engine after dedicated conditioning, to produce heat and electricity. Syngas conditioning technology needs to be optimised to comply with engine manufacturers’ standards or, if used in boiler applications, to achieve an “end-of waste” standard. Pyrolysis of SS is unlikely to contribute towards improved nutrient recycling without development of additional nutrient recovery technology, as phosphorus is mineralised in biochar in forms that reduce plant availability and some heavy metal residues are concentrated at levels that exceed recommended thresholds, precluding sale as a soil conditioner or disposal via land spreading. SS char is a solid fuel whose combustion results in inert or non-hazardous ash, as per current regulations.