WooMa Doo                                                 za razvoj životne sredine proizvodnju                 Doline

Basics of organic waste processing

Pyrolysis of wastewater sludge and composted organic fines from municipal solid waste: laboratory reactor characterisation and product distribution


Sludge from municipal wastewater treatment plants and organic fines from mechanical sorting of municipal solid waste (MSW) are two common widespread waste streams that are becoming increasingly difficult to utilise. Changing perceptions of risk in food production has limited the appeal of sludge use on agricultural land, and outlets via landfilling are diminishing rapidly. These factors have led to interest in thermal conversion technologies whose aim is to recover energy and nutrients from waste while reducing health and environmental risks associated with material re-use. Pyrolysis yields three output products: solid char, liquid oils and gas. Their relative distribution depends on process parameters which can be somewhat optimised depending on the end use of product. The potential of pyrolysis for the conversion of wastewater sludge (SS) and organic fines of MSW (OF) to a combustion gas and a carbon-rich char has been investigated. Pyrolysis of SS and OF was done using a laboratory fixed-bed reactor. Herein, the physical characterisation of the reactor is described, and results on pyrolysis yields are presented. Feedstock and chars have been characterised using standard laboratory methods, and the composition of pyrolysis gases was analysed using micro gas chromatography. Product distribution (char/liquid/gas) from the pyrolysis of sewage sludge and composted MSW fines at 700°C for 10 min were 45/26/29 and 53/14/33%, respectively. The combustible fractions of pyrolysis gases range from 36 to 54% for SS feedstock and 62 to 72% from OF. The corresponding lower heating value range of sampled gases were 11.8–19.1 and 18.2–21.0 MJ m−3, respectively.


There is growing interest in the use of thermal conversion technologies for waste management (Syed-Hassan et al. 2017; Kumar and Samadder 2017). These generally aim to valorise waste streams while reducing risks associated with re-use of waste materials. Pyrolysis, the thermal conversion of a substance, is of interest in waste management because it can reduce health and environmental risks from problematic wastes (Lindberg et al. 2015; Trinh et al. 2013) while providing an avenue for the recovery of energy and nutrients (Buah et al. 2007; Song et al. 2014).

Two common and widespread waste streams are sludge from municipal wastewater treatment plants (SS) and the organic fine (OF) component from the mechanical sorting of municipal solid waste (MSW). Wastewater sludge is the organic by-product of municipal wastewater treatment. It consists of the solids, which are removed from wastewater during the treatment process. Treatment methods can be mechanical, biological or chemical (Haller 1995). Sludge from wastewater treatment plants is commonly applied to agricultural land as a fertiliser. The re-use of sludge is the most encouraged outlet, according to current EU waste policy objectives which also permit optional methods that provide the best overall environmental outcome.

Organic fines of MSW are an extremely heterogeneous material containing food waste, plastics, metals, paper and glass (Buah et al. 2007). After the screening of MSW, the fine material is routinely stabilised through controlled aerobic composting after which it is used as a cover material at landfill sites (RPS 2014).

The utilisation of these two waste streams is undergoing changes. Firstly, societal perceptions of risk and quality assurance schemes in food production have lessened the appeal of spreading treated sewage sludge on agricultural land. Secondly, in the Republic of Ireland and elsewhere, landfill sites are closing down. Therefore, the outlets for sewage sludge and organic fines are rapidly diminishing, and new solutions are sought for their safe handling and utilisation (Kim and Parker 2008).)


Pyrolysis decomposes organic materials into other products under inert atmosphere. Wood charcoal, which is produced from the pyrolysis of wood, is a familiar example. Char, however, is only one of the products of pyrolysis. The process also yields liquids (oils and tars) and gases (syngas). The distribution of pyrolysis products depends heavily on several process parameters whose influence follows a general trend for all organic feedstock (Wang et al. 2011). Harmful emissions from waste pyrolysis (Han et al. 2017) and undesirable product characteristics (Leng et al. 2015), however, are also parameter-dependent (Yu et al. 2016; Yuan et al. 2011; Zhang et al. 2017). Therefore, process parameters need to be optimised for a particular application to achieve the best overall benefits (Buah et al. 2007).

Temperature, residence time and heating rate are the main process parameters, but particle size of the feedstock and residence time of vapour-phase products are also important as these influence the contact between chars and gases, the extent of which affects char formation and the decomposition (cracking) of long-chain hydrocarbon gases (Mok and Antal Jr 1983ab). Long vapour-phase residence times encourage char-forming reactions. It follows that process flow conditions, for example batch versus continuous processes and reactor configurations themselves, strongly influence char-vapour interactions.


Physical characterisation of a pyrolysis reactor gives information on how process parameters influence heat transfer. This is essential knowledge in interpreting experimental results and in identifying differences between laboratory and pilot-scale processes and reactor design. Heat transfer conditions can be described accurately using the observable feedstock heating rate, the heat transfer coefficient and the Biot number.

The product distribution from pyrolysis of wastewater sludge and organic fines from municipal solid waste has been determined experimentally according to the principle of mass conservation. Pyrolysis char and gas yields were characterised and show potential for use as fuels in energy recovery from these two abundant feedstocks.