Make use of sludge from wastewater and grass

How can the value of sludge from wastewater and roadside grass be increased? 

Primary sludge (directly from sewage water) and activated sludge (excess sludge and secondary sludge), which arise at wastewater treatment plants, are residual flows that are currently not used effectively. Most of it is fermented into biogas (but with a very low yield), dewatered and sent to incinerators. This current sludge processing (dewatering, transport and incineration) costs € 500 – 1000 per ton of dry matter (DS), which is a major cost item with a sludge flow of 350 kton (DS) per year. The removal of roadside grass (240 kton DS/year) is also a big expense for which authorities are looking similarly for cost reduction and a higher-grade use of their residual flow. Now, mowed roadside grass often remains along the road or is composted.

In this project, a laboratory-scale process has been developed in which low-grade sludge or juice from pressed roadside grass is converted into acetic acid (in concentrated solution) in a cost-efficient manner. The poor digestibility of activated sludge is notorious; therefore, the sludge was first pre-treated, and then subjected to biological anaerobic acidification. After a solid/liquid separation, the alkali and the acids were separated. The product is a mixture of highly concentrated organic acids with acetic acid. The acids can be used on the short term as a raw material for the production of green gas and as an auxiliary substance for denitrification in wastewater treatment plants. In the long term, the product can also be used as a raw material for caproic acid and PHA production, and as a silage agent.

Check the infographic for the innovative process. You can read the research in this (Dutch) report.