A carbon footprint analysis was performed comparing three biosolids treatment technologies: the Kruger BioCon ERS, a conventional natural gas fired dryer (with no post-incineration) and aerobic digestion for Class B biosolids (land application). The BioCon ERS is a biosolids convective air belt dryer and an integrated biosolids energy recovery system that utilizes heat generated by the combustion of biosolids to dry the biosolids. The rotary drum dryer consumes natural gas to generate the heat required to dry the biosolids. The aerobic digestion reduces a quantity of solids and allows for Class B land application.
Technology Design Data
The biosolids treatment technologies evaluations were based on the same inlet dry solids loading, biosolids transport distance offsite and expected life:
- Load-dry solids: 2,500 tons/year
- Transportation distance from treatment to disposal site: 25 miles (40 km)
- Expected Life: 20 years
Per MOP8 standards, the aerobic digester design airflow rate used was 20 SCFM/1000 ft3.
The carbon footprint of each process was analyzed from the standpoint of construction, equipment, operation and waste utilization. The carbon footprint attributed to construction includes the resources necessary to create the materials used, shipping and installation of the equipment and materials. Electricity, fuel heat, water, chemicals, and transport of waste were considered in the operational footprint.
System disposal was not considered.
Since the source of electricity varies throughout different regions of the United States, three different regions were selected to quantify the sensitivity of the electrical emission factor to the total carbon footprint. On the low end, the northwest US (EPA eGRID subregion NWPP) had an approximate emission factor of 0.41 kg equivalent CO₂ per kWh, while the Florida region had a higher end emission factor of 0.60 (EPA eGRID subregion FRCC), and the U.S. average is approximately 0.58. The three regions are depicted in graphs as "Low", "Avg." and "High".
Total Lifetime Equivalent CO2 Emissions with Varying Electrical Emission Factors
The Kruger BioCon ERS system provides the lowest carbon footprint of the three biosolids treatment processes evaluated. The BioCon ERS integrated heat supply by way of utilizing its dried biosolids provides significantly less carbon emissions over the rotary drum. The relatively low production of waste (ash) within the BioCon ERS process provides for low offsite transportation requirements, offering pronounced carbon benefits in comparison to aerobic digestion.