Carbon footprint of sludge treatment reed beds

Sludge treatment reed beds (STRBs) performance has become a popular solution to dewater and stabilize surplus sludge from wastewater treatment systems because of the low investment, operational and maintenance costs of the systems. The environmental impact associated with this technology has, however, to be assessed in order to evaluate the appropriateness of the technology. The goal of this study is to determine the carbon footprint of STRBs. To this end, a Life Cycle Assessment was performed using methane and nitrous oxide emissions quantified in two full-scale STRBs located in Northern and Southern Europe (Denmark and Spain). Methane emissions ranged between 1000 and 3700 mg CH4/m2 d, while nitrous oxides ranged between 200 and 750 mg N2O/m2 d. Thus, greenhouse gas emissions correspond to 0.09 and 0.25 kg CO2eq/m2 d for the Danish and Spanish system, respectively. The treatment of 1 ton of sludge (wet weigh) in the Spanish system corresponds to 2.1 kg CO2eq, while in the Danish system it corresponds to 1.0 kg CO2eq. The contribution of greenhouse gas emissions in the carbon footprint is insignificant in comparison with raw materials and energy consumption. However, the results put forward the relevance of STRB design in the environmental impact of the technology. Thus, the excavation of basins or the use of recycled concrete are recommended to guarantee a low carbon footprint of this technology.