Aerobic co-composting of wwtp residual sludge with vegetal wastes

The most preferred method of stabilization of sewage sludge is composting. It is apparently a simple process aimed at stabilization of organic matter – ripening, destruction of pathogenic organisms, production of an environmental friendly material which can be sold or used as fertilizer. To reach the above aims the sludge has to be mixed with structural materials in suitable proportion to obtain a C:N ratio of about 25:1. In practice, the most often used structural materials contain cellulose (i.e. wooden chips, sawdust, bark, straw, leaf litter, other vegetal wastes). The present study presents several composting experiments performed using as raw materials dewatered anaerobic digested sludge from a municipal WWTP, two different vegetal wastes (wood chips, straws, vine shoots) in different proportions and water in order to ensure a 50 to 70% humidity. The experiments were performed using aerated heap method; the heap is arranged on a porous layer, which is periodically aerated by a blower. Composting process evolution was verified during the whole period of 3 months of experimentation by online measurement of temperature, periodic pH and humidity monitoring and by weekly microscopic observations. The quality of the obtained compost was good, similar to commercial products used for pot-flowers, with heavy metal concentrations below the limits imposed by 344/2004 Order.