The gasification of biomass, a raw organic feedstock available in several typologies in nature, represents a process through which it is possible to produce energy with a remarkable reduction of GHG and atmospheric pollution. This process also allows to reduce the costs related to the management and disposal of certain biomass that is continuously produced such as sewage sludge, the residual, semi-solid material that is produced as a by-product during sewage treatment of industrial or municipal wastewater. This biomass undergoes several mechanical, biological and chemical treatments in order to separate and clarify the water stream before it is returned to the environmental water cycle (sea, rivers…).
Sewage sludge management and disposal represents a relevant financial and environmental cost for operators and society, considering the increasing volume as population grows and land spreading limitations. These costs are mostly related to logistics, collection, treatment and disposal; furthermore, legislation on this matter is progressively limiting the ‘business as usual’ disposal consisting of land spreading in agriculture, as sludge contains not only organic matter and nutrients, but often also a certain amount of highly polluting inorganic substances, for example metals and hydrocarbons, which do not make them compatible with their use for agricultural activities.
SEWAGE SLUDGE GASIFICATION
An alternative and innovative approach to traditional land spreading is to use dried sludge as fuel for thermochemical processes, such as gasification, which can greatly reduce the volume of the separated waste by-product inside the main treatment plant. SyngaSmart technology is designed to process not only woody biomass but also other organic residues, including sewage sludge. In fact, through a proper pre-treatment process including drying and briquetting, sewage sludge can be introduced into SyngaSmart gasification unit in order to convert the organic compounds into a useful flammable gas (therefore energy), with a variable heating value depending on some feedstock parameters such as inert content, ashes, carbon content, L.H.V. and so forth.
This approach offers the double benefit of both volume reduction and energy generation. In fact, through a suitable gasification process, the whole dried sludge mass can be sterilized and reduced to 5/10% of the original volume, leaving as by-product a small percentage of char that can be further processed to recover elements such as phosphorous or metals, or disposed. This approach also allows to concentrate pollutants in a small solid fraction.
On top of that, there is the additional advantage of energy availability: in fact, the resulting syngas can be used, for example, in sludge drying, thus creating a circular economy and reduced-impact upgrade of the whole wastewater treatment plant.
SEWAGE SLUDGE GASIFICATION BENEFITS
Sewage sludge gasification is an innovative and profitable alternative to business as usual management, particularly in small-medium size plants (from 6.000 to 20.000 tons / year) where transportation and disposal represent a relevant cost item. The energy that can be recovered from sludge gasification offers additional financial benefits, especially in those wastewater plants where the is no biogas recovery.
On site sludge processing allows also to reduce the fermentation occurring to biological matter left in an open environment, therefore reducing the release of GHG such as carbon dioxide and methane. These gases can be better used in syngas, and SyngaSmart gasification also includes the carbon storage benefit (char) offered by the carbon-negative process.
In conclusion: sewage sludge valorization with SyngaSmart technology offers both financial and environmental benefits, and represents a perfect example of bio-circular economy application.