Environmental Assessment of Sewage Sludge Management – Focusing on Sludge Treatment Reed Bed Systems

Sewage sludge is generated from the treatment of domestic wastewaters at wastewater treatment plants. Since the implementation of stricter requirements for wastewater treatment in the European Union in 2005, the amount of sludge produced has increased, creating the demand for more effective treatment and recycling.

In Denmark, the application of sludge on agricultural land is an often-used recycling strategy, as it returns nutrients and microelements to the soil, which can substitute for commercial fertilisers. Conventionally, sludge produced in Denmark is dewatered with mechanical devices; however, in the late 1980s, sludge treatment reed bed (STRB) systems were intro-duced in Denmark and in 2016, more than 100 STRB systems were operating in the country.

Sludge treatment in STRB systems is often considered more environmentally friendly compared to mechanical sludge treatment technologies, albeit only a few life cycle assess-ments (LCAs) comparing the environmental performances of sludge treatment technologies include STRB systems. Furthermore, as data on the STRB system technology suitable for LCA are scarce, the results of these LCAs are unreliable.

The project aimed at generating data on the STRB system technology that would be useable for LCA. Based on identified knowledge gaps, research focused on three areas; quantification of gas emissions directly related to treatment, establishment of substance flows through the technology and the fate of carbon and nitrogen-based compounds in treated sludge when applied to the land.

The overall goal of the project was to perform an LCA comparing the environmental performance of the STRB system technology with a conventional technology based on mechanical dewatering of sludge on a decanter centrifuge and subsequent storage. Geographically, the project focused on Denmark, and was carried out as a collaborative effort between the Technical University of Denmark (DTU) and the Danish environmental consultancy Orbicon A/S.

The outcome of the project was a dataset on the STRB system technology usable for LCA, and an LCA comparing the environmental profiles of the STRB system technology and a mechanical treatment technology, constituting a basis for decision-making in relation to choice of technology. A major part of the project involved performance of fieldwork and laboratory work.

Data were collected at three Danish, well-operated STRB systems; furthermore, data required to represent the mechanical treatment technology were collected alongside data on STRB sys-tems. Most of the data collection was undertaken at a wastewater treatment plant housing both technologies, thereby making it possible to make the two datasets as comparable as possible.

Fourteen environmental impact categories were included in the LCA, and the environmental loadings and saving provided by the sludge treatment technologies normalised to represent the treatment of 1000 kg wet weight of sludge. The life cycle inventory and the choices underlying the life cycle impact assessment were based on international acknowledged standards and recommendations.

An attributional LCA approach was chosen, and the loadings and savings for all impact categories were normalised to people equivalents (PE) (the annual loadings and savings provided by one average person). Three sludge treatment scenarios were defined: 1) mechanical treatment on centrifuge, followed by storage and finally land application, 2) treatment in an STRB system and finally land application (S-STRB), and 3) treatment in an STRB system, followed by post-treatment on a stockpile area (SPA) and finally application (S-SPA).

The project succeeded in generating data on STRB systems, which could form the basis for a LCA, and comparable data related to mechanical sludge treatment. The results of the LCA revealed that STRB systems performed comparable to or better than mechanical treatment. The two scenarios based on the STRB system technology (S-STRB and S-SPA) performed comparable which only minor differences.

According to toxic impact categories, which for both technologies were mainly impacted by metals contained by treated sludge applied on land, the three scenarios performed com-parable. Indeed, the substance flow analyses revealed that the metals held by sludge subjected to treatment for all scenarios were accumulated in the final sludge product.

For all scenarios, the net-loadings for the impact categories Human Toxicity – Non-Carcinogenic and Ecotoxicity corresponded to 2.010-2 PE, and for Human Toxicity – Carcinogenic to 5.0 10-4 PE. Emission rates of CO2, CH4 and N2O related to biological processes in sludge subjected to treatment in STRB systems were measured during all four seasons of the year.

The results revealed that seasonal variations were considerable, and should be taken into account when calculating annual, average emission rates. The emission rate of CO2 measured from external storage of mechanically treated sludge was much lower compared to those measured for STRB systems, reflecting a lower microbial activity in the mechanical dewatered sludge.

As the emission rates of the potent greenhouse gasses CH4 and N2O were larger for mechanical dewatered sludge, the net environmental loadings provided to the impact category Climate Change by this technology (S-CEN) and the STRB system technology (S-STRB and S-SPA) ended up being equally sized (9.010-4 PE), despite of higher biological activity in the STRB systems.

As a consequence of the lower microbial activity in mechanically treated sludge, the con-centration of carbon and nitrogen-based compounds in the final sludge product produced by this treatment technology was higher compared to the final sludge product produced by treatment in STRB systems. Hence, the loadings affecting impact categories related to eu-trophication and acidification were higher for the mechanical treatment technology, espe-cially in relation to the category Marine Eutrophication, the net-loadings to this category being 8.0 10-4 PE for mechanical treatment (S-CEN) and 3.0 10-4 PE for STRB systems (S-STRB and S-SPA).

The STRB system technology consumed fewer abiotic resources, due mainly to the fact that the mechanical treatment process requires an input of polymer coagulant, while a STRB system does not require this contribution. Furthermore, as mechanically treated sludge often have a stronger odour compared to sludge treated in STRB systems, the latter is often claimed by the local land application sites, while mechanically treated sludge often must be transported longer distances to land application sites willing to apply it.

Hence, the STRB system technology required a lower input of fuel for transportation. In the future, it would be relevant to use the obtained data on STRB systems to compare the technology with other sludge treatment technologies commonly used. Furthermore, it would be relevant to generate a comparable dataset on representing the performance of the technology in other climate zones, and to expand the data set with more data related to economics, making it possible to make more detailed economical assessments.