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Sydney Water Corporation: Environmental Life Cycle Assessment (LCA) of biosolids processing options

The Sydney Water Corporation (SWC) manages wastewater treatment plants, which produce large quantities of biosolids. As materials containing nutrients and embodied energy, as well as trace concentrations of metals and pesticides, there are trade-offs in the treatment and use of biosolids. These trade-offs include issues of nutrient recovery, energy recovery and safe disposal. Life Cycle Assessment (LCA) has been used as an analytical tool to examine potential impacts of biosolids treatment and management options. In undertaking the LCA, two key comparative assessments are made - a comparison of configurations (decentralised and centralised systems) and a comparison of technologies (thermal drying versus lime amendment), and the environmental merits or otherwise of the options.

Background

SWC supplies filtered drinking water, and treats and recycles wastewater (sewage), for more than 4 million residential customers and 73,000 businesses in Sydney, Illawarra and the Blue Mountains. In some areas, SWC also supplies drainage (storm water) services. SWC's area of operations, almost 13,000 square kilometres, extends from the Hawkesbury estuary in the north to Gerroa in the south, and from the Pacific Ocean westwards to Mount Victoria in the Blue Mountains.

Several components of SWC's legislative operating framework require the organisation to act in accordance with the principles of ecologically sustainable development, as defined in the Protection of the Environment Administration Act, 1991. In line with the Sydney Water Act, 1994, SWC is required to demonstrate a reduction in "the combined environmental impact of the per capita amount of energy and water used… and other materials discharged". LCA has emerged as a useful tool for assessing the potential environmental effects of major capital works, consistent with this stated desire for holistic assessment. SWC has decided to upgrade all biosolids treatment facilities to produce "A" grade biosolids (NSW EPA 1997), a standard that can be reached with several technologies. In early 1999, Sydney Water decided to apply LCA to the examination of particular proposed upgrades to its biosolids treatment infrastructure and potential "A" grade technologies. This presented an opportunity for SWC to examine the application of LCA performed to the ISO 14040 series of standards (ISO 1998, 2000a, 2000b). The LCA was performed with expert assistance from Centre for Water and Waste Technology (CWWT), School of Civil and Environmental Engineering, University of New South Wales (UNSW), Sydney. The LCA examined two key issues in the further development of SWC's biosolids business: the choice of system configurations (i.e. centralised versus decentralised systems), and the choice of treatment technologies (i.e. thermal drying versus lime amendment for a decentralised facility). More information about LCA.

LCA Goal and Scope Definition

The goal of the study was to assess the application of LCA to SWC's planning activities by a holistic environmental analysis of selected biosolids processing options for three major sewage treatment plants (STPs), which discharge treated effluent to the Pacific Ocean. The approach was prospective or change oriented, looking at the upgrades to plants required to meet future biosolids production rates. Therefore, the study has natural relevance to the ongoing assessment of upgrade options for the ocean plants.

The functional unit of the study was the treatment of the mass of biosolids expected to be captured at the three largest plants in Sydney. These STPs serve an estimated population of 2.8 million people and discharge treated effluent to the sea. This value was estimated as 178 dry tons per day for the year 2021, assuming full primary treatment.

In this study, two functionally equal system and technology configurations were compared with each other:

Thermal biosolids drying at a proposed Central Biosolids Treatment Facility (CBTF) was compared (Scenario Two) with treatment of solids according to excisting practices (Scenario One). In order to examine the consequences of building a possible central drying facility, calculations were performed for both a central system with drying and pelletisation (Scenario Two with drying), and for a central system producing the current ratio of lime stabilised and dewatered biosolids (Scenario Two without drying).

System Configuration - Status Quo (Scenario One)

System Configuration - Status Quo (Scenario One)


System Configuration - Centralised Drying (Scenario Two)

System Configuration - Centralised Drying (Scenario Two)

Alternative technologies for treatment of sludge were compared on a decentralised basis only. The environmental impacts of thermal drying and lime amendment at a typical wastewater treatment facility were assessed and compared.

Technology Comparison - Lime Stabilisation Versus Thermal Drying at a Wastewater Treatment Facility

Technology Comparison - Lime Stabilisation Versus Thermal Drying at a Wastewater Treatment Facility

All equipment was assumed to have an average operating lifespan of 20 years. As the materials used in construction of the plant and equipment (primarily concrete and steel) are recyclable and considered to reduce environmental impact in other product systems, the disposal of equipment was not considered in this LCA. Additionally, the material and energy flows associated with construction were found to be considerably smaller than those associated with operation of the plants, and that operational issues dominate the total environmental impact of the system relative to construction and disposal.

In the case of Malabar, there was a net production of electricity by the biosolids process under Scenario One, as a consequence of its generation in the cogeneration plant. However, despite the electricity exported to other parts of the STP, the biosolids process was a net consumer of energy due to the high-energy demand of the trucking operations. By contrast, in Scenario Two, this trucking occurs from the CBTF site, and the additional energy saved at Malabar due to the absence of dewatering operations makes the biosolids process a net producer of both electricity and energy.

Methodological choices and assumptions

In this study, several methodological choices and assumptions were made that could influence the results. The most relevant choices are:

System boundaries: The system starts at the outlet of the primary sedimentation tanks to delivery of biosolids at the farm site. The beneficial effect of biosolid nutrients on plant growth was excluded from this study due to the complexity of interrelated soil chemical processes affected by the nutrients.

Capital equipment: Non-recurrent (construction) impacts associated with process equipment were considered only for the upgrade of the facilities. Environmental impacts due to construction of excisting facilities were excluded.

Impact assessment model: In this pilot study, the three impact categories and environmental indicators of most relevance to SWC were selected - namely, primary energy consumption, global warming potential and human toxicity of air emissions.

Energy consumption: In regards to the primary energy consumption, all energy flows entering the defined system (consumed energy) and all energy flows leaving the defined system (produced energy) were considered. If energy was produced within one system and at the same time consumed, no energy flows occur across the system boundary.

Greenhouse effect: Carbon dioxide emissions resulting from the combustion of fossil fuels contribute to global warming and other climate change effects. However, carbon dioxide emissions from the microbial degradation of biosolids, or the combustion of biogas generated from the anaerobic digestion of human waste, merely complete a biological cycle, which begins with the assimilation of atmospheric carbon dioxide in crops and animals, follows through to consumption of these by humans and ends in final excretion of waste to a STP. These carbon emissions were not derived from the earth's fossil reserves but return carbon to the atmosphere from the biological cycle. Therefore, greenhouse gas emissions resulting from the combustion of gases generated by the anaerobic digestion of biosolids were excluded from the calculation of contributions to the enhanced greenhouse effect in this LCA. Fugitive methane emissions from the anaerobic digestion phase were assumed to be insignificant when compared to carbon dioxide emissions from the combustion of biogas.

Life-Cycle Inventory Data Collection

After validation of annual mass and energy balances, the data were related to unit processes and the functional unit (see the previous description of the functional unit and system boundaries). Finally, the data were aggregated for the life-cycle impact assessment (LCIA) phase (described in the next section). This procedure was performed several times. The iterative character of the process allowed SWC to determine which issues were most significant prior to finalisation of the calculations, which permitted efficient allocation of efforts to improve data quality when necessary. The data on these units were contained in several statutory and planning documents, supplemented when necessary with supplier data and literature values. Australian inventory data for the inclusion of background systems was obtained from the Centre for Design at Royal Melbourne Institute of Technology.

Life-Cycle Impact Assessment


Selection of Impact Categories and Environmental Indicators

The environmental indicator and impact categories chosen for this study were primary energy consumption, global warming potential and human toxicity potential. These were chosen on the basis that they were most relevant to the particular systems undergoing comparison. Importantly, they also represent three categories that have been the subject of considerable scientific debate and investigation.

Results and Discussion


Data quality and sensitivity analysis

Two types of sensitivity analysis were applied to this LCA. The first type consists of testing the assumptions about the system by varying system configuration and/or boundaries. The second type of sensitivity analysis involves varying input values for a particular system and calculating the corresponding variations in the output values obtained. Ideally, each data element in a life cycle inventory should be collected with an error margin reflecting the level of uncertainty associated with each datum. The environmental planning documents upon which most of the inventories for this LCA rely are accurate to within 25% of the estimated dollar cost value, so it was considered appropriate to apply a standard 25% error tolerance to the output of the assessments.

System configurations

The scenarios were compared on the bases of energy consumption, global warming potential and human toxicity potential.

Energy consumption

The net energy consumption of each of the options was of the same order of magnitude ranging from 17.9 to 23.0 terajoules per annum (TJ/a - 1 TJ is 1012 J). Scenario One draws 18.5 TJ/a compared with 23.0 TJ/a for Scenario Two with drying and 17.9 TJ/a without drying.

Energy Consumption Under Two Scenarios

Energy Consumption Under Two Scenarios

From an overall energy accounting perspective, the biogas produced and used within the plant does not appear in the above figure since it does not cross the system boundary. The volume of biogas produced from North Head biosolids was not sufficient for the drying of the biosolids from all three plants. Consequently, the additional energy drawn by Scenario Two with its drying system was predominantly the result of the natural gas (methane) required. With drying, Scenario Two uses 24% more energy than Scenario One (just less than the 25% error tolerance). Scenario Two without drying was not significantly more energy efficient than Scenario One (using only 3% less energy). This was the consequence of the avoidance of dewatering centrifuges at Bondi in this option.

Scenario Two with drying draws 28% more energy than Scenario Two without drying, so it can be seen that the drying technology, rather than the choice of a central facility, was more influential. The most significant contributor to the energy consumption of Scenario One was the fuel for the diesel trucks transporting the biosolids and lime. Trucking represents 36% of the total energy consumed by the biosolids handling processes at these three major STPs. This suggests that reducing trucking distances between the sewage treatment plants and biosolids users, and reducing biosolids moisture content, are relatively effective ways to improve the energy efficiency and environmental performance of biosolids handling.

Global warming potential

Scenario One makes a contribution to the enhanced greenhouse effect of 17.6 x106 kg carbon dioxide equivalents, compared with 18.3 x 106 kg carbon dioxide equivalents for Scenario Two with drying. This was an insignificant increase of 4% over Scenario One.

Global Warming Potential Under Two Scenarios

Global Warming Potential Under Two Scenarios


Human toxicity potential

In both scenarios, most of the potentially toxic substances emitted to the environment were airborne contaminants released as a result of the high-energy intensity of the processes. Therefore, the emphasis of the human toxicity potential assessment was on all substances released to air.

The combined human toxicity potential via airborne contaminants was lowest for Scenario Two without drying (4.14 x 104 kg (1,4 dichlorobenzene equivalent)/year) and insignificantly higher for the other two options (4.31 x 104 and 4.50 x 104 kg (1,4 dichlorobenzene equivalent)/year for Scenario One and Scenario Two with drying, respectively). The results were influenced by several factors:

Lime amendment versus thermal drying

The technology comparison of lime amendment versus thermal drying was made on the basis of the same key indicators used in the comparison of system scenarios.

Energy consumption

The following discussion considers the total annual energy demand for lime amendment and thermal drying. The values calculated for energy consumption of the lime and drying systems for North Head biosolids treatment were 133 and 42.1 TJ/a respectively. The energy consumed by trucking greater masses of biosolids generated by lime amendment, and the lime required for the process, result in the total energy consumption of the drying option being only 32% of the amount required for lime amendment. Because transportation of limestone to the plant represents only 8% of the trucked material by mass, the most significant issues were the distance the biosolids were trucked and their mass. The pelletised and the lime-amended biosolids were transported to the same sites in this comparative study, so the key difference was the moisture content of the biosolid product. Pelletised biosolids can be assumed to reach 92% solid material by mass, while lime-amended biosolids were typically only 34% solids.

Energy Consumption: Lime Amendment versus Thermal Drying

Energy Consumption: Lime Amendment versus Thermal Drying


Global warming potential

Excluding biogas combustion from the calculations, the contribution to the enhanced greenhouse effect of the drying option (8.75 x 106 kg CO2 equivalents) was 45% better than the lime-amendment option (1.59 x 107 kg CO2 equivalents). This was a significant difference, although relatively small compared to the difference in energy consumption. This variation was a consequence of the difference in the greenhouse intensity of fuel consumption by diesel motors and coal-fired power generators. It was significant that the fuel sources of the dryer were the digesters within the system boundary.

Global Warming Potential: Lime Amendment versus Thermal Drying

Global Warming Potential: Lime Amendment versus Thermal Drying


Human toxicity potential

Consistent with the overall calculation of the LCA human toxicity potential indicator under different configurations, the aggregated emissions of the lime amendment process exceeded the thermal drying process by 21%. Trucking operations made the most significant contribution to this impact category, primarily as a consequence of the greater human toxicity potential of benzene per mass emitted to the atmosphere. The emissions associated with drying were a consequence of the electrical consumption of the pumps involved in maintaining the mixed state of the digester and decanting it. Drying also involves electrical power (estimated at 240 kW) to rotate the drier drum and provide other ancillary services.

Human Toxicity Potential: Lime Amendment versus Thermal Drying

Human Toxicity Potential: Lime Amendment versus Thermal Drying


Conclusions

The environmental issues explored by this LCA represent only some of the factors that SWC must consider as it plans to upgrade biosolids facilities. In terms of regional and global issues, this LCA indicates that the system configuration choices (centralisation versus decentralisation) will not affect the environmental outcomes as much as the technology choices. It also suggests that the choice of biogas fuelled thermal drying would result in reduced environmental impacts. The life cycle resource consumption of the drying option was dominated by the operation of the system, rather than its construction. The resources were predominantly consumed by energy production for the plant.

A key issue in improving the environmental profile of biosolids handling was the avoidance of coal-sourced electrical energy. Selection of renewable energy sources such as biogas and cleaner energy sources such as natural gas, will improve the environmental performance of biosolids operations both by reducing the carbon-intensity of energy use, and the toxicity of emission byproducts.

LCA can provide unique contributions to strategic planning for SWC because it considers diverse environmental impacts related to wastewater treatment systems and processes. Environmental impacts were taken into account whether they occur on or off site. Hence, the holistic approach allows a comprehensive environmental assessment that prevents against 'problem shifting' - i.e. the transfer of environmental problems from one part of the technical system or the environment to another.

Contacts

Gregory Peters
Water Cycle Planner
Environment and Innovation
Sydney Water Corporation
115-123 Bathurst Street
Sydney NSW 2000
Tel: 02 9350 5604
Fax: 02 9350 5929
Email: gregory.peters@sydneywater.com.au
Website: http://www.sydneywater.com.au

Sven Lundie
Project Leader
Centre for Water and Waste Technology/CRC WMPC
School of Civil and Environmental Engineering
The University of New South Wales
Sydney NSW 2052
Tel: 02 9385 5097
Fax: 02 9385 8624
Email: S.Lundie@unsw.edu.au
Website: http://www.civeng.unsw.edu.au/cwwt/index.html

Case developed by the Centre of Excellence in Cleaner Production (Curtin University of Technology)
Last modified: June 2003