Towards a comprehensive greenhouse gas emissions inventory for biosolids

Compiling life cycle inventories for wastewater-derived products

With the paradigm shift in wastewater management from pollutant removal to resource recovery, more wastewater-derived products are emerging from different recovery pathways. It is becoming increasingly important to understand the potential environmental impacts of these products through life cycle assessment (LCA). This study aims to compile life cycle inventories of wastewater-derived products from the perspective of the product end users (e.g., agricultural sector, packaging industry), and to explore the challenges of their compilation. Using inventories from wastewater resource recovery LCA literature, we compiled an attributional inventory (88 sets) and a consequential inventory (33 sets) of three categories of wastewater-derived products - phosphorus compounds, nitrogen compounds, and biopolymers. The two inventories differ by the choices of system boundary, how foreground systems are being modelled, and how co-products are being handled. We found that while there is a large body of literature related to wastewater resource recovery LCA, very few studies (29 out of 174 for the three categories of products) are suitable for end users to successfully compile inventories of derived products. The inventories were assessed by the technology readiness level assessment, the data quality assessment, and the cumulative energy demand indicator. The inventories can be used directly by end users or served as "screening" inventories for end users to prioritize data collection effort. The identified challenges of inventory compilation include diverse recovery settings, the absence of baseline scenarios, the multifunctional nature of wastewater treatment plants, the lack of inventory transparency and completeness, and low technology readiness level for some recovery pathways. While established or emerging approaches exist to address most of these challenges for end users, wastewater resource recovery LCA practitioners can enhance their assessments to be more end-user-oriented. This can be achieved by including baseline non-recovery scenarios, disclosing detailed life cycle inventory by system components, and assessing a wide variety of operating scenarios. Addressing some of these compilation challenges would enhance the comprehensiveness and quality of wastewater-derived product inventories.

Wastewater recovery for sustainable agricultural systems in the circular economy - A systematic literature review of Life Cycle Assessments

Water availability and quality are known to affect agricultural production and nutrition. The aim of this study was to elaborate a systematic literature review of the most sustainable ways of wastewater treatment towards achieving circular economy (CE) in agro-industry activities. From the SLR, the authors selected twenty-seven papers that they classified into the three research themes of recovery of wastewater into irrigation water, extraction of sludge for production of bio-based compounds, and recovery of nutrients for soil amendment, including recovering of feeds for aquaculture, and recovery of nutrient biosolids for soil amendment. Results underlined that the recovery of nutrients biosolids for soil amendment can generate a GWP gain up to - 37 kg CO2-eq. So, the review highlighted that wastewater recovery for multiple purposes can be truly effective for the environmental sustainability of agricultural systems, and that LCA is a valid tool to assess and improve that sustainability. Under this perspective, this SLR's findings can stimulate public administrations at national and local scales in their planning and funding activities towards implementing circular bioeconomy paths based upon wastewater recovery for a sustainable, resilient agriculture. Overall, the authors believe that their article was effective in overviewing the current wastewater recovery paths in the CE context, and in highlighting key methodological aspects and findings of the reviewed LCAs, to advance the specialised literature and knowledge, and to guide practitioners for future LCA applications in the field. Finally, through its main findings, the article effectively contributes to the whole research project which it is part of and which the authors are deeply involved in. That research is performed under the Progetto GRINS "Growing Resilient, Inclusive and Sustainable" thanks to a PNRR M4C2- Investment 1.3 - GRINS with the aim of "Building a dataset for the circular economy of the main Italian production systems".

Optimal pathways for upgrading China's wastewater treatment plants for achieving water quality standards at least economic and environmental cost

Wastewater treatment plants (WWTPs) in China must be upgraded to meet new discharge standards, but this incurs both economic and environmental costs and benefits. To select the optimal upgrade pathway, we developed ten upgrade paths based on two common decision-making scenarios for WWTP upgrade in developing countries. Using model simulation, life-cycle assessment, life-cycle cost, and multiple-attribute decision-making, we incorporated the full costs and benefits associated with the construction and operation into the decision-making process. We used a weighting scheme of attributes for the three regions and ranked the upgrade paths using the technique for order preference by similarity to an ideal solution (TOPSIS). The results showed that constructed wetlands and sand filtration were advantageous in terms of lower economic costs and environmental impacts, while the denitrification filter pathways required less land. Optimal pathways differed by region, highlighting the importance of a detailed and integrated assessment of the costs and benefits of WWTP upgrade options over the full life cycle. Our findings can inform decision-making on upgrading China's WWTPs to meet stringent discharge standards and protect inland water and coastal environments.

Relevance of sludge management practices and substance modeling in LCA for decision-making: A case study in Chile

Reducing the costs and environmental impacts of sludge management is currently one of the main challenges faced by the wastewater treatment sector. Anaerobic digestion followed by land application has been widely endorsed as a low-impact approach to sludge management, mainly due to the recovery of biogas and the valorization of digestate. However, the influence that the operational conditions of digestion and the management practices of land application can have over the environmental performance of this strategy has been scarcely studied. Furthermore, most of the previous studies dealing with the environmental assessment of this strategy use simplified methods for estimating emissions after land application of sludge, and the lack of systematic accounting of these environmental flows might significantly affect the validity and comparability of the results. Therefore, this work performed an assessment of the influence that 4 relevant practices can have over the environmental impacts of this approach in the context of south-central Chile, providing a mass-balanced inventory for nitrogen, phosphorus and heavy metals in soil based on the ad hoc implementation of models developed for agricultural Life Cycle Assessment (LCA). A total of 16 scenarios were defined and 10 impact categories were evaluated, with the results showing that the environmental impacts were greatly influenced by the variables under study. Overall, solids retention time and the inclusion of pre-treatment mainly influenced climate change, fossil resource depletion and terrestrial ecotoxicity potential, while sludge application rate influenced the eutrophication, water ecotoxicity and human toxicity categories. The type of crop in the receiving soil was a significant driver behind the differences observed in the human toxicity category, which showed the highest variation and relevance in the final weighted result. The results clearly highlight the relevance of using context specific data as well as of quantifying the fate of nutrients, metals and heavy metals during LCA of sludge management. Based on the results, some policy and decision-making recommendations are formulated to optimize the environmental performance of sludge digestion and land application.

Biosolids: The Trojan horse or the beautiful Helen for soil fertilization?

The simultaneous requirement to manage resources and wastes in more rational way has meant that many communities worldwide have begun to search for long-term alternative solutions. Reuse and recovery of biosolids is considered to be a constant solution of circular sustainability, as waste disposal without further reuse background like fertilizer is no longer an alternative to be promoted. There have been developed many treatment methods over the years for the stabilization and sanitization of biosolids. However, the literature concludes that none of them is fully integrated by meeting all the basic criteria. Each method has its Achilles heel, and the appropriateness of the method lies in what is the goal each time. There are conventional methods with positive reciprocity in terms of sustainability, reuse indicators and technological maturity, but have high risk of microorganisms' reappearance. New advanced sustainable technologies, such as cold plasma, need to be further studied to apply on a large scale. The reuse of biosolids as construction materials is also discussed in the context of circular economy. Biosolids reuse and management legislation frame need to be revised, as a directive adopted 30 years ago does not fully meet communities' current needs.

The application of life cycle assessment (LCA) to wastewater treatment: A best practice guide and critical review

Life cycle assessment (LCA) has been widely applied in the wastewater industry, but inconsistencies in assumptions and methods have made it difficult for researchers and practitioners to synthesize results from across studies. This paper presents a critical review of published LCAs related to municipal wastewater management with a focus on developing systematic guidance for researchers and practitioners to conduct LCA studies to inform planning, design, and optimization of wastewater management and infrastructure (wastewater treatment plants, WWTPs; collection and reuse systems; related treatment technologies and policies), and to support the development of new technologies to advance treatment objectives and the sustainability of wastewater management. The paper guides the reader step by step through LCA methodology to make informed decisions on i) the definition of the goal and scope, ii) the selection of the functional unit and system boundaries, iii) the selection of variables to include and their sources to obtain inventories, iv) the selection of impact assessment methods, and v) the selection of an effective approach for data interpretation and communication to decision-makers.

Life cycle environmental impacts of sewage sludge treatment methods for resource recovery considering ecotoxicity of heavy metals and pharmaceutical and personal care products

Sewage sludge handling is becoming a concern in Europe due to its increasing amount and the presence of contaminants, such as heavy metals and pharmaceutical and personal care products (PPCPs). Currently, over 70% of sludge in Europe is treated thermally by incineration or used as fertilizer in agriculture. New thermochemical methods are under development and are expected to be implemented in the near future. This paper considers the life cycle environmental impacts of the following five alternatives for sludge handling, taking into account the presence of heavy metals and PPCPs: i) agricultural application of anaerobically digested sludge; ii) agricultural application of composted sludge; iii) incineration; iv) pyrolysis; and v) wet air oxidation. The results suggest that anaerobic digestion with recovery of nutrients and electricity has the lowest environmental impacts in 11 out of 18 categories considered. For the mean to maximum resource recovery, composting is the worst alternative, followed by pyrolysis with lower recovery rates. Agricultural application of anaerobically digested sludge has the highest freshwater ecotoxicity due to heavy metals, unless their concentration is in the lowest range, as found in some European sewage sludge applied on land. Therefore, stricter control of heavy metals in the sludge is needed for this option to limit freshwater ecotoxicity to the levels comparable with the thermal processes. The results also indicate that PPCPs have a negligible contribution to freshwater ecotoxicity when compared to heavy metals in the anaerobically digested sludge. Since thermal processes are currently drawing attention due to their potential benefits, the findings of this work suggest that their adoption is environmentally beneficial only if high resource recovery rates can be achieved.

Benchmarking the sustainability of sludge handling systems in small wastewater treatment plants

A benchmarking strategy was developed to assess all aspects of sludge handling in small wastewater treatment plants and tested on a cross-section of Ontario facilities. Using operational data and on-site measurements, sustainability metrics that addressed energy consumption, chemical use, biosolids quality and disposition, and greenhouse gas (GHG) emissions were estimated. Electricity consumption for sludge handling ranged from 0.9 to 3.9 kW-hours per dry kilogram of raw sludge (kWh/dry kg) with thermo-alkali hydrolysis and auto-thermal thermophilic aerobic digestion (ATAD) processes consuming the least and most electricity for stabilization, respectively. Mechanical dewatering processes consumed between 2 and 5% of total sludge handling electricity, however, associated polymer dosages were higher than literature values in some cases. Disposition fuel requirements for plants with dewatering were up to 85% lower than facilities without dewatering. Biosolids contaminant (pathogen/metals) contents were observed to be substantially below Non-Agricultural Source Material (NASM) requirements. The copper content of the hauled biosolids exhibited the highest concentration relative to the NASM limit among all plants studied, ranging from 14 to 37% among facilities practicing land application of biosolids. Four biosolid products met Class A requirements for E. coli content, including one product generated via long-term storage. Carbon emissions ranged from -119 to 299 kg CO₂ equivalents per dry tonne of raw sludge (g CO₂ eq./kg). Six facilities that practiced land application exhibited net-negative GHG emissions; the carbon credits gained from fertilizer production avoidance outweighed emissions associated with sludge processing and transportation. The results provide evidence that this practice is sustainable from a GHG emissions standpoint. The benchmarking approach developed and information gathered is beneficial to plant owners and operators seeking to better understand how their utility is performing relative to peers, identify areas of need and further investigation, and improve the sustainability of their operations.

Efficient removal of siloxanes and volatile organic compounds from sewage biogas by an anoxic biotrickling filter supplemented with activated carbon

The removal of siloxanes (D4 and D5) and volatile organic contaminants (hexane, toluene and limonene) typically found in sewage biogas was investigated in a lab-scale biotrickling filter (BTF) packed with lava rock under anoxic conditions. Complete removal efficiencies for toluene and limonene were recorded at all empty bed residence time (EBRT) tested. The influence of EBRT was remarkable on the abatement of D5, whose removal decreased from 37% at 14.5 min to 16% at 4 min, while the removal of D4 and hexane remained below 16%. The packing material was supplemented with 20% of activated carbon aiming at increasing the mass transfer of the most hydrophobic pollutants. This strategy supported high removal efficiencies of 43 and 45% for hexane and D5 at the lowest EBRT. CO₂ and silica were identified as mineralization products along with the presence of metabolites in the trickling solution such as dimethylsilanediol, 2-carene and α-terpinene.

Environmental comparison of alternative treatments for sewage sludge: An Italian case study

A Life Cycle Assessment (LCA) was applied to compare different alternatives for sewage sludge treatment: such as land spreading, composting, incineration, landfill and wet oxidation. The LCA system boundaries include mechanical dewatering, the alternative treatment, transport, and final disposal/recovery of residues. Cases of recovered materials produced as outputs from the systems, were resolved by expanding the system boundaries to include avoided primary productions. The impact assessment was calculated using the CML-IA baseline method. Results showed that the incineration of sewage sludge with electricity production and solid residues recovery collects the lowest impact indicator values in the categories human toxicity, fresh water aquatic ecotoxicity, acidification and eutrophication, while it has the highest values for the categories global warming and ozone layer depletion. Land spreading has the lowest values for the categories abiotic depletion, fossil fuel depletion, global warming, ozone layer depletion and photochemical oxidation, while it collects the highest values for terrestrial ecotoxicity and eutrophication. Wet oxidation has just one of the best indicators (terrestrial ecotoxicity) and three of the worst ones (abiotic depletion, human toxicity and fresh water aquatic ecotoxicity). Composting process shows intermediate results. Landfill has the worst performances in global warming, photochemical oxidation and acidification. Results indicate that if the aim is to reduce the effect of the common practice of sludge land spreading on human and ecosystem toxicity, on acidification and on eutrophication, incineration with energy recovery would clearly improve the environmental performance of those indicators, but an increase in resource depletion and global warming is unavoidable. However, these conclusions are strictly linked to the effective recovery of solid residues from incineration, as the results are shown to be very sensitive with respect to this assumption. Similarly, the quality of the wet oxidation process residues plays an important role in defining the impact of this treatment.