The fermentation process for the sludge valorization
University of Seville is in charge of the Fermentation Unit included in the Water2Return system. The purpose of this Fermentation Unit is to valorize the activated sludge produced during the biological treatment of the wastewater produced in the activity of the slaughtering and meat processing. This valorization is aimed to transform this by-product obtained into products with biostimulant capacity and multiplied biochemical methane potential.
Every day between 2 and 3 m3 of this activated sludge are generated from the treatment of around 50 m3 of wastewater.
The process involved within the fermentation unit is divided in three parts:
The first part is intended to concentrate sludge. Due to the fact that next step of the process, the pretreatment of sludge, is the bottle neck of the process, it is needed to reduce the volume of sludge to be treated. For this purpose it is used a rotary sieve that concentrate sludge from 3 % w/v (DM) to 6 % w/v (DM).
The second part is the pretreatment system. Due to the fact that agronomic products are expected to be obtained from this process, it is necessary to eliminate all potencial pathogenic microorganisms that integrate sludge. This is achieved by a high temperature and pressure pretreatment followed by a sudden decompression that favors the break of cellular structures. Thus, besides sanitizing sludge, it also makes sludge a more suitable product to be used as fermentative substrate to increase the plant growth promoting rhizobacteria (PGPR). In this sense, the pretreatment increase solubility and bioavailability of sludge.
The pretreatment system is based on a sterilizer to sanitize sludge. It is an equipment divided in two parts. In the top there is a mixer based on a system of inverted funnels that allows mixing sludge with counter current steam that quickly makes sludge to reach 120 °C and high pressure. In the bottom side there is a deposit where the sludge is maintained for 8 min at such temperature and pressure operational conditions. Thus, it ensures the complete sanitization of sludge and correct hydrolysis of sludge.
Finally, pretreated sludge, which is ready to be used as fermentation substrate, is sent to the fermenter. Daily 1.8 m3 of sludge are treated, and converted into 0.9 m3 of fermentation substrate. This volume of substrate is daily added to a fermentation that is already taking place in a semicontinous regime. The fermenative cycle lasts 6 days, after which, 4.5 m3 of fermented sludge are obtained.
During this fermentation time, the PGPR microorganism, through the secretion of degrading enzymes and other components of agronomic interest, will further increase the solubility and bioavailability of the sludge, giving rise to a product of interest in agronomy as a biostimulant, and whose energy capacity through anaerobic digestion is multiplied.
Currently the fermentation unit is fully assembled and has been tested with water. Some minor adjustments are missing to get started with the sludge produced at the Water2REturn plant.