On the 21^st of May 1991 the then European Economic Community (EEC) issued the 91/271/EEC Urban Wastewater Treatment Directive (UWWTD) that would set in motion a series of reforms to protect the environment from the adverse effects of effluent being discharged from Wastewater Treatment Plants (WWTPs) [1]. The directive made recommendations on the collection, treatment, and discharge of urban waste water. One of the key recommendations made in the directive is that WWTPs serving agglomerations greater than 2,000 PE^† discharging final effluent into freshwater and estuaries, and all other agglomerations greater than 10,000 PE are to employ secondary treatment^‡. This requirement presented local and national authorities with the challenge of firstly assessing the state of their respective sewage systems, before bringing standards to required levels.

In Ireland it is the responsibility of the Environmental Protection Agency (EPA) to enforce the measures outlined in 91/271/EEC. In 2013 the EPA initiated a research project to benchmark the energy and resource efficiency of WWTPs in Ireland. The project involved four different approaches: development of a benchmarking tool, process auditing and optimization, exergy analysis and environmental performance assessment. It is the findings from the latter of these four approaches that is the subject of this study.

The advancement of environmental awareness at both societal and policy-making level over the last two decades has brought with it the need for appropriate tools to analyse products and systems for sustainability and environmental impact. A range of tools have been developed for this purpose such as:

• Environmental Risk Assessment (ERA) [3];

• Environmental Impact Assessment (EIA);

• Ecological footprint [4];

• Exergy Analysis (EA) [5].

It should be noted that the application of exergy analysis to assess environmental impact of a product or system is a relatively new concept and is currently the subject of much debate between those that advocate its use in this context [6], and those who do not [7]. Life Cycle Assessment (LCA) is an analytical tool that allows for a holistic approach to assessing the environmental performance of a product or system from cradle to grave [8]. It has been widely accepted as a decision support tool for government bodies, environmental authorities, and areas of the private sector [9]. The use of LCA is particularly suited to WWTP analyses due to the nature of the relationship between a plant’s technosphere and the surrounding ecosphere. Indeed, there has already been a variety of LCA studies carried out on WWTPs, each with their own unique set of objectives, but with the common underlying theme of seeking to quantitatively and qualitatively assess environmental impact.

The application of LCA to a wastewater treatment plant or system was first reported in The Netherlands in 1997. The study carried out by Roeleveld et al. [10] examined the environmental sustainability of wastewater treatment. The study concluded that improvements in the environmental performance of wastewater treatment should focus on minimizing effluent discharge pollutants, and sludge production. Since then, there have been over forty studies published in peer reviewed journals [11]. Much of the earlier work that was carried out involved assessing the environmental impact of variations in system parameters such as:

• System configuration [12];

• System boundaries and scale [13];

• Structural changes [14];

• Competing technologies for large [15] and small scale plants [16].

Other LCA studies examined wastewater reclamation and reuse alternatives [17].

As the body of work in this area increased some common findings began to emerge. Firstly, the environmental loading associated with energy production for use in the treatment process was found to be one of the main contributors to the overall environmental profile of treatment plants and systems [16], with particular emphasis on global impact categories such as global warming and acidification. This is contrary to the findings in the earlier work by Roeleveld et al. [18] who concluded that electricity use had a negligible impact. However, the results of the study were normalised with Dutch normalization factors that output the electricity use as a percentage of the overall electricity use of The Netherlands. It is not a surprise that the environmental impact from energy use can vary between studies. The magnitude of impact is not only dependant on the amount of energy used, but also on the way the energy is generated. For example, the impact from electricity generation in Norway where over 90% is hydro-electric power will be much less than that of Italy where over 60% of electricity is generated from fossil fuels [19].

The second significant finding was the impact resulting from sludge disposal. Much like the impact from energy generation, it is the mode of sludge disposal that dictates both the magnitude and the type of impact incurred. Traditionally, methods of sludge disposal in Europe have been either by application to agricultural farmland, incineration, or by landfill. In relation to the latter, the EU Directive on the landfill of waste (1991/31/EEC) recommends a reduction in the quantities of sewage sludge going to landfills [20], and in some countries such as Sweden the practice has been banned completely since 2005 [21]. The LCA study carried out by Pasqualino et al. [22] that examined several sludge disposal options concluded that land fill was the worst scenario. This finding was echoed by Houillon and Jolliet [23] who also include agricultural application as one of the least desirable methods of sludge disposal from a global warming perspective, and claim that incineration in cement kilns is preferable. Conversely, Lundin et al. [24] expanded the impact assessment of sludge disposal options to include more impact categories than just global warming and found that incineration had environmental restrictions. The debate on sludge disposal is ongoing.

Quite often the metric used to assess the performance of a WWTP is the percentage reduction of influent pollutants such as:

• Biochemical Oxygen Demand (BOD);

• Chemical Oxygen Demand (COD);

• Total Suspended Solids (TSS);

• Total Nitrogen (TN);

• Total Phosphorus (TP).

While control of these parameters is necessary for compliance with the regulations, it affects only eutrophication and aquatic toxicity, and there are many other environmental factors involved that must be taken into account such as global warming resulting from energy production and toxicity resulting from sludge disposal. In more recent times there has been a paradigm shift from considering not only water quality and human health, but also energy and resource recovery during wastewater treatment [11]. A common finding throughout many of the published LCA studies of wastewater treatment plants and systems is that there are trade-offs, where reducing the load in one impact category increases the loading in another.

This paper seeks to further expand the knowledge of WWTP analysis by examining the environmental loadings of two WWTPs in Ireland and add to the body of work that has been carried out around the rest of the Europe. To the best of the author’s knowledge, this is the first study of this type on the island of Ireland.

This study is part of a larger project that aims to assess the energy and resource efficiency of WWTPs in Ireland. The main goal of the LCA component of the project is to quantify the environmental loading that results from reaching the effluent quality standards set out in 91/271/EEC. The specific goal of this paper is to assess the environmental costs or gains associated with variations in plant size and location. It has been reported in previous studies by Tillman et al. [12] and Lundin et al. [13] that there are economies of scale to be achieved in terms of environmental impact, however, this claim has never been investigated from an Irish perspective. The variation in plant location focuses mainly on the effects of differences in discharge limitations.

PLANT DESCRIPTIONS

Plant A is the largest and most sophisticated of the two plants under study. It has a design capacity of 186,000 PE and a dry weather flow of 35,000 m³/day. The plant is situated on the coast and releases its final effluent through a long sea outfall where the waters are deemed to be non-sensitive, thus avoiding the need for nutrient reduction. The SCADA system used at the plant is bespoke and state of the art. All aspects of process control take place at the hub of the SCADA system. Figure 1 shows the treated influent flow line for the plant.

Figure 1.

Plant A treated influent flow line

The sludge line is as shown in Figure 2. Primary and secondary sludge go through a series of dewatering and thickening processes, and are then mixed prior to being sent to anaerobic digesters. The sludge is brought to 22% solids concentration before being sent off-site to an external composting company.

Figure 2.

Plant A sludge line

Plant B has a design capacity of 50,000 PE serving a current agglomeration of 37,500 PE. The plant also accepts industrial sludge from the surrounding area as well as the sludge outputs from the smaller rural plants. The flow for the plant is shown below (Figure 3). The plant is equipped with anaerobic digesters and biogas storage facilities, but the plant operators were having difficulties running the digesters effectively and they were out of commission during the study period. The sludge outputs from the plant are sent to a sludge management company that stabilizes the sludge with lime before sending it for application to farmland.

Figure 3.

Plant B influent flow and sludge flow lines

When considering the environmental impact associated with waste water treatment the main focus is generally on eutrophication caused by the final effluent. However, the 91/271 directive has established a set of acceptable limits for pollutant concentrations in final effluent for BOD₅, COD and TSS, as well as nutrient limits of phosphorus and nitrogen. If it is understood and accepted by the scientific community that these limits represent a sustainable, non-destructive level of eutrophication, then the primary focus should no longer be on the quality of the final effluent, but more on the impact that results from achieving this effluent quality.

This study found that there were two main contributors to environmental loading outside of final effluent discharge: the energy that goes into treating the influent and the sludge disposal. The electricity grid mix in Ireland contributes heavily to global impact categories such as MAETP, GWP, AP and ADPf, whereas sludge disposal contributes mainly to the ecotoxicity categories. Anaerobic digestion creates a link between the energy that is consumed during the treatment process and the embedded energy in the sludge. Plant A employs anaerobic digestion and this serves a number of purposes:

• AD reclaims a significant amount of energy that can be fed back into the operation of the plant, reducing aerial emissions associated with electricity generation;

• AD reduces the volume of sludge leaving the plant which reduces transport emissions and fuel consumption. This can be significant when the sludge has to travel long distances to its final destination as is the case with plant A;

• AD stabilises sludge which reduces the emissions associated with lime production, and reduces resource depletion.

It is without doubt that anaerobic digestion is a key process in wastewater treatment as it reduces the output of two of the main contributors to the overall environmental impact. In terms of what happens after the sludge is digested, there needs to be a definitive solution on how best to dispose of the sludge. The literature is filled with conflicting opinions on the topic of sludge disposal. However, most studies would agree that land-filling and direct application to farmland are the least favoured options.

The purpose of the study was to assess the environmental performance of two WWTPs in Ireland. Life cycle assessment was the analytical tool used. The plants that were chosen for the study varied in scale and location. In relation to the latter, the main focus was on the differences in discharge limits. It has been shown that more stringent discharge limits will reduce eutrophic environmental loading, but the extra energy and materials that are consumed reaching the higher quality final of effluent can increase environmental loading in several other impact categories. Lower BOD limits typically result in an increase in aeration energy requirements and this in turn increases loading in impact categories that are dominated by contributions from energy production such as MAETP, ADPf, GWP and AP. Phosphorous reduction requirements increase ferric chloride consumption, and thus production, which increases environmental loading in a number of impact categories such as ODP, GWP and MAETP.

Investigations of the effect of variation in scale proved difficult because of differences in system configuration and the fact that one plant was successfully running its anaerobic digesters while the other was not. The plant with the functioning digesters was reclaiming some energy and reducing sludge outputs. This has the effect of reducing the loading in several impact categories. However, even if the energy reclaimed from anaerobic digestion were excluded, there would still be a significant difference in energy consumption between plants. What could be concluded in relation to scale was that there were significant economies of scale to be achieved in aeration energy consumed during secondary treatment.

It would be prudent to research further, and in greater detail, the exact sources of any energy and resource savings that could be made. There are other areas of plants operation that are also worthy of investigation such as plant management, flow monitoring and system configuration. Indeed, there is an abundance of research opportunities in this field. One further point of note in relation to this research is that because the UWWTD is pan-European legislation, the research could be expanded to compare energy and resource efficiency of water and wastewater treatment systems across the continent. Life cycle assessment is growing in popularity, and is seen by many as a suitable analytical tool for environmental profiling of existing plants, and as a decision support mechanism for future development of more environmentally sustainable wastewater treatment systems.