Feasibility of Reusing Water Treatment Plant Sludge in Treating the Sludge from a Wastewater Treatment Plant

This study was conducted on the feasibility of improving processes of thickening, anaerobic digestion, and sludge dewatering at wastewater treatment plants on a laboratory pilot scale, aiming to reduce polyelectrolyte consumption by 50% and replace it with chemical sludge. Parameters of Sludge Volume Index (SVI), Specific Filtration Resistance (SRF), compressibility coefficient, filtration rate, and biogas production rate were investigated. It is shown that by applying 200 mg/g of water treatment plant sludge to wastewater sludge, the compressibility coefficient was decreased by 27%, the filtration rate was increased by 70%, and SVI and SRF were decreased by 64.9% and 43.3%, respectively. Therefore, it has facilitated the sludge sedimentation and release of its free water. On the other hand, adding the sludge from a water treatment plant led to the proper access of microorganisms to micronutrients and improvement of their performance resulting in an increase of 7.77% in the total biogas production. The iron present in the digestion process caused an increase in biogas production by increasing the activity of enzymes, reducing oxidation-reduction potential, and improving the direct interspecies electrons transfer. In conclusion, adding the optimal amount of water treatment plant sludge, along with the environmental management of a meaningful portion of water and wastewater residuals, may play an important role in significantly reducing polyelectrolyte consumption and improving biogas production.