Three treatments composed of mixtures of raw sludge and co-substrate (commercial fats, protein, and cellulose) were carried out and compared to a control composed of raw sludge. Mixture conditioning was performed on the basis on air filled porosity (40%). The results obtained in the cocomposting processes reflected a higher biological activity and higher degradation percentages of dry and organic matter when compared with control.
Higher temperatures (60, 67 and 62ºC for fats, protein and cellulose, respectively) were also achieved in all co-composting experiments as compared to the control test (55ºC). Biological activity was measured using both Static and Dynamic Respiration Indices obtaining higher values in cocomposting experiments compared to the control test. Fats content reduction was higher (66%) at higher fats content in the initial mixture (10.6%). The addition of fats seems also to promote the degradation of cellulose and lignin. Co-composting experiments with fats and cellulose presented higher initial C/N ratio and lower nitrogen losses, 27.5 and 34.2% compared to 40% for raw sludge. It has been demonstrated that the addition of an adequate cosubstrate to raw sludge leads to a higher degradation percentages of the different biochemical fractions and higher nitrogen conservation.