Digester Gas Treatment with Powerful VOC Pre-Cleaning

From 21st to 23rd May 2019, the sewage sludge days, an event held by the German Association for Water, Wastewater and Waste (DWA) in Würzburg, will discuss current political, legal and procedural developments in sewage sludge treatment and disposal.
One way in which sewage treatment plants are increasingly involved in this context is the production of biomethane or bio natural gas from sewage sludge digestion. For example, Hitachi Zosen Inova is currently building a gas treatment facility for the Hamburg sewage treatment plant at the Köhlbrandhöft site in the port of the Hanseatic city via its German subsidiary HZI BioMethan.

Energy Made in Switzerland
For five years now, this concept has also proven itself to the local wastewater authorities in Solothurn-Emme (ZASE). In its sewage treatment plant in Zuchwil, the largest in the Swiss canton of Solothurn, the association was able to reduce the amount of sewage sludge by approx. 40% through biodegradation.
For this purpose, ZASE 2014 had extended the sewage treatment plant by adding sludge digestion with sludge de-watering, in which bacteria decompose the organic substance of sewage sludge into biogas in an anaerobic process. This consists of around 65 % methane (CH4) and 35 % carbon dioxide (CO2). Zuchwil was the first plant in the canton to oppose the combustion of the digester gas and advocate its processing into bio natural gas. This is done in a gas treatment plant by means of membrane-based gas permeation. Carbon dioxide is separated from methane, which is refined to natural gas quality. The biomethane produced in this way will be fed into the supply network of Regio Energie Solothurn.

Material separation with hollow fibres
The separation of carbon dioxide from methane takes place in membrane modules with hollow fibres made of high-performance plastics. The biogenic gas is introduced into the hollow fibres under pressure. Their walls are impermeable to methane molecules and so high-purity methane is removed at the end of the modules. However, carbon dioxide and the water and nitrogen contained in the gas mixture permeate the membranes. In a three-stage interconnection process, biomethane is produced with a purity of over 97%.

High-power pre-cleaning
The fluctuating nature of the raw gas, which is typical of sewage gases, influences the service life and performance of the membranes. HZI BioMethan designed the gas treatment system with an efficient, comprehensive pre-treatment of the digester gas for their best possible preservation and to avoid the entry of particles into the gas network. First it is washed, desulphurised in an active carbon filter and subsequently pre-compressed.
A special feature of the plant is the subsequent adsorptive and regenerating raw gas purification. Unwanted gas components, such as volatile organic compounds (VOC), are adsorbed in filters by a molecular sieve and thus cannot affect the quality of the membrane. This maintains the high performance of the membranes. Two adsorbers work in shift function in a double filter system. If one filter is fully loaded, the gas flow is diverted to the second, and the previous operating adsorber goes into the desorption phase. This cyclical, fully automatic regeneration of the filter system permanently ensures reliable pre-cleaning with varying trace substances. This plant design is based on over 10 years’ experience in planning, construction and commissioning of gas treatment plants for biogenic gases of all kinds.

Visit us at the sewage sludge days
You will find us at the accompanying exhibition of the conference from 21st to 23rd May 2019 in the Congress Centre Würzburg.
Contact: Carsten Steentjes, carsten.steentjes@hz-inova.com, Tel.: +49 162 4261477