Environmental and Economic Performance of Sludge Composting Optimization Alternatives: A Case Study for Thermally Hydrolyzed Anaerobically Digested Sludge

Composting is one of the ways to return sewage sludge nutrients to the soil and thus keep them in the economic cycle. This well-known technique is still being developed in search of more advanced, optimal solutions. This study presents the results of an environmental and economic analysis of the sludge treatment processes used in a municipal wastewater treatment plant. The sludge (up to 4700 m3 per day) is subjected to thermal hydrolysis before anaerobic treatment. The energy produced is lower than consumed, mainly since 59% of the digested sludge is also dried. An even bigger problem is that the treated sludge does not meet the criteria for fertilizing products and can only be used for energy forests. Thus, three alternatives for composting thermally hydrolyzed anaerobically treated dewatered sludge with green waste from public areas were researched. The analysis revealed the environmental and economic benefits of such a decision, especially when using microbial inoculants in open composting and maintaining semi-anaerobic conditions. An increase in humic acids (by 63.4%) and total nitrogen (by 21.8%) concentrations, a minimization of NH3 emissions (by 26.6%), and the lowest cost price (53 EUR tonne−1 of sludge dry matter) are among the benefits.