Importance of process design on carbon footprint from drinking water treatment by enhanced coagulation-filtration

Drinking water suppliers must meet increasing demand, stricter quality regulations, and additional challenges due to climate change. Better knowledge of the impacts of pollutants on human health has led to more stringent drinking water quality standards. Improved drinking water quality, treatment, and capacity often result in higher costs, increased resource consumption, and a greater environmental impact (Venkatesh & Brattebø 2012; Ribera et al. 2014; Molinos-Senante et al. 2022). Climate change is a growing challenge for the water sector. More extreme weather, drought, floods, longer growing seasons, increased temperatures, and intense precipitation affect raw water quality. Decreased raw water quality in turn results in reduced production capacity and increases the resources needed for water treatment (Eikebrokk et al. 2004; Delpla et al. 2009; Ritson et al. 2014; Moona et al. 2018; Herrador et al. 2021). Thus, climate change is linked to increased environmental emissions, and the water sector needs to minimize its own environmental emissions.

Enhanced coagulation-filtration processes, combined with pH, calcium (Ca), and alkalinity adjustment for corrosion control, UV disinfection, and/or chlorine disinfection, are widely used for natural organic matter (NOM) removal in drinking water treatment. Despite the development of novel technologies for NOM removal, coagulation-filtration is still one of the most common technologies. This process has proved to be efficient at removing NOM and it is relatively inexpensive.

Enhanced coagulation, using elevated coagulant doses and strict pH control, targets NOM removal, whereas conventional coagulation targets primarily turbidity removal. Options for enhanced coagulation processes include:

• Direct filtration, where a flocculation basin is utilized prior to filtration.

• Contact filtration, where coagulated raw water is led directly to the filters.

• Conventional filtration, where coagulation, flocculation, and sedimentation take place prior to filtration.

Direct and contact filtration (CF) processes are widely used for raw waters with low turbidity and low-to-moderate NOM levels. Their footprint is much smaller than that of conventional filtration, which requires large sedimentation basins. Therefore, conventional filtration often has a footprint several times larger than that associated with direct or CF plants.

The environmental impact on the life cycle of a water treatment plant includes construction and operation, as well as demolition and disposal of the plant at the end of its lifetime. Several authors have estimated that the operation phase accounts for 80% or more of the total environmental emissions, while construction accounts for 4–15% of the emissions during the plant's lifetime (Muñoz & Fernández-Alba 2008; Bonton et al. 2012; Choe et al. 2013; Igos et al. 2014; Amini et al. 2015; Roth et al. 2022). Igos et al. (2014) conclude that the overall environmental impact of a water treatment plant is driven by the consumption of fossil resources mainly related to energy production. When taking into consideration that energy consumption is linked to all value chains at a water treatment plant such as constructing the plant, pumping, operating treatment processes, manufacturing chemicals, and transporting materials to the site, there is great potential in reducing environmental emissions by choosing low-emission energy whenever possible (Biswas & Yek 2016; Zib et al. 2021; Cruz-Perez et al. 2022). Hofs et al. (2022) have investigated a future scenario for a new DWTP taking into account the EU goals for lower emissions for future energy production for plants with a lifetime of 50 years. The results suggested that when emissions from energy production are minimized, the environmental emissions from the construction phase can exceed 50% of the total emissions.

Manufacturing coagulants and pH-adjusting chemicals also have a large environmental impact on coagulation-filtration processes (Jones et al. 2018). Most environmental emissions are derived during the operation phase, and therefore strategic process design including water treatment chemicals should be a priority effort in addition to optimization of operation performance for safety, environmental emissions, and cost efficiency.

This study aims to assess how treatment process design, including the water treatment chemicals, affects the carbon footprint. Both the direct filtration (DF) and contact filtration (CF) processes compared in this study use the same basic technology, enhanced coagulation-filtration. The objectives of this study are to (1) compare carbon footprints (CO₂-eq) originating from the construction phase, (2) calculate carbon footprints (CO₂-eq) originating from resource consumption during the operational phase, and (3) identify key opportunities to reduce carbon footprint and costs.

The DF-Al process is set up as coagulation with an aluminium coagulant (Al), flocculation, and filtration followed by pH, Ca, and alkalinity adjustment with limewater for corrosion control. The CF-Fe process combines coagulation with an iron coagulant (Fe), filtration, and pH, Ca, and alkalinity adjustment in one filtration step using alkalizing limestone filter media as the bottom filter layer. This makes the process footprint smaller even though the filter bed is deeper than in conventional filters. Raw waters in Norway are mostly soft (low alkalinity and pH) surface water with low turbidity and low-to-moderate NOM levels, making enhanced coagulation-filtration the most applied drinking water treatment process (Eikebrokk 1999). DF and CF processes supply over 40% of the produced water in Norway with over 170 drinking water treatment plants (DWTPs). Both DF-Al and CF-Fe are widely used in Norway (Norwegian Institute of Public Health 2022).

The DF process includes a flocculation basin before the dual media filtration (2-M) stage. The dual media filters apply expanded clay (Filtralite) or anthracite and quartz sand as filter media. Al coagulation requires pH control to ensure optimal coagulation and an additional pH and alkalinity adjustment step for treated water for corrosion control (increase pH, Ca, and alkalinity). Limewater and CO₂ are used for the final pH and alkalinity adjustment in a clean water basin.

The CF process with integrated corrosion control is more compact, where NOM removal and corrosion control are performed in one filtration step. This process applies Fe-based coagulation and three media (3-M) filters using expanded clay (Filtralite) or anthracite, quartz sand, and limestone as the bottom filter layer.

In this study, both process alternatives are scaled to treat the same raw water quality and to meet the same finished water quality at the local conditions in Bergen, Norway (Table 1). Plant engineering design is retrieved from existing DWTPs and scaled to the same production capacity. To simplify very detailed and case-specific environmental emission estimations, the volume of concrete for the main civil structure is used as a proxy indicator of resource intensity during the construction phase (Foley et al. 2010). Because both enhanced coagulation-filtration processes compared in this study have similar building and treatment components, it is assumed that a fair comparison can be made between the two process design alternatives.

Typically, suspended solids (SS) in Norwegian raw waters are very low, often below 1 mg SS/l. For this study, the raw water is the same quality for both process alternatives, and therefore SS_RW is set to zero. The applied coagulant doses for the DF-Al (2.4 mg Al/l) and CF-Fe (4.0 mg Fe/l) process alternative both give a sludge production rate of 10 mg SS/l. Thus, the wastes generated in both processes are very similar and waste generation rates are therefore excluded from subsequent carbon footprint assessments.

The construction of a water treatment facility requires resources such as capital, land area, building materials, treatment components, transport of materials, and energy/power supply. The carbon footprint and financial costs are unique to each facility, depending on raw water quality, selected process design, location, climate, terrain excavation, access for building and treatment materials, production, transport of materials to and from the site, etc. The carbon footprint and investment costs for the construction phase are related to resource intensity, which in turn increases with increasing facility size.

In this study, the process footprints and volumes needed were calculated based on the engineering design of the two plants. These values are used to estimate the concrete volumes required for the main civil structures (Table 2).

As shown in Table 2, the process footprint for the DF-Al was estimated to be 70% larger than for the CF-Fe. The space required for flocculation (DF-Al) is responsible for 40% of the DF-Al process footprint. A larger process footprint requires a larger building and more land, thus increasing both the costs and resource intensity for the construction. The volume of concrete for the main civil structure for the DF-Al is 40% larger than that required for the CF-Fe. The volume of concrete for the main civil structure can be used as a multiplier to estimate the required quantities of other building materials. Calculations based on these volumes show that the process footprint is 70% higher and the emissions are 40% higher for the for DF-Al process alternative.

Calcium oxide (CaO) is a water treatment chemical that is commonly used despite the very large CO₂ emissions. This chemical can be either slaked at the plant or purchased as liquid calcium hydroxide slurry, commonly known as limewater. Calcium oxide production starts with quarrying and crushing limestone and transporting it to a manufacturing site. Limestone is burnt in a lime kiln where thermal decomposition takes place. Thermal decomposition breaks limestone down into calcium oxide and carbon dioxide. Production of calcium oxide produces CO₂ in two ways: energy used for heating the lime kiln for thermal decomposition and the CO₂ released from the chemical decomposition of limestone. Thermal decomposition is a very energy-demanding process, and CO₂ emissions for calcium oxide production are typically 950–1,200 kg CO₂-eq per ton product (EuLA 2014; European Environmental Agency 2019).

The production of crushed limestone is a much less intensive process. After quarrying limestone, the mineral is crushed to the desired grain size (1–3 mm) to be utilized as an alkalizing filter media. The CO₂ emissions for crushed limestone are low, only around 3 kg CO₂-eq per ton product (Kittipongvises 2017; Franzefoss Minerals 2021).

Corrosion control with crushed limestone, limewater, or slaked lime requires the addition of CO₂. When using crushed limestone, one mole of CO₂ is needed to release one mole of Ca²⁺ and two moles of bicarbonate, whereas, with limewater, two moles of CO₂ are needed to achieve the same effect. Stoichiometrically, limewater binds two times more carbon dioxide per mole than crushed limestone. For raw waters with a low free CO₂ concentration, the CO₂ addition required is twice as high for limewater than for crushed limestone. On the other hand, raw waters with a high concentration of free CO₂ do not always require the addition of CO₂ when using limestone-based products.

Preparation of limewater at the DF-Al water treatment plants requires additional investments and process area. This includes powder storage and several saturators for preparing the lime solution and removing impurities. Limewater can contain up to 20% solid impurities, which must be removed before use to ensure proper quality of the limewater. Impurities are usually discharged to the sewer through the bottom drainage of the saturation unit. Limewater requires mixing and settles easily. Careful design and operation as well as regular maintenance are needed both in the preparation of limewater and to avoid clogging of the limewater pumps. Hydrated lime and burnt lime, especially in powder form, require special attention from operators and designers of the lime preparation unit as these chemicals can cause skin irritation, skin burns, eye damage, and respiratory irritation.

At the CF-Fe water treatment plants, crushed limestone is added to the filters before backwashing. Limestone is stored and added to the filters either in a silo with a water ejector system or by lifting it into the filters in 1,000 kg bags with an overhead crane built above the filters. Limestone is not hazardous to work with and requires very little operation and attention from operators. When limestone is loaded into a silo with trucks, the limestone generates some dust, so appropriate safety equipment should be used.

Backwashing filters with limestone filter layers will always yield to small loss of fine limestone particles, thereby increasing pH, alkalinity, and calcium levels in spent backwash water and in the sludge.

Another group of chemicals used to improve flocculation and filtration performance in water treatment is synthetic polymers. CO₂ emissions for synthetic polymers are estimated to be 3,250 kg CO_2-eq per ton product (Braun et al. 2022). These polymers are usually manufactured from raw oil in a process with high energy demand. The amount of polymer used in water treatment is small compared to other chemicals, and although the emissions per ton are substantial, the CO₂ emissions from polymer use are relatively low per unit of treated water.

Water treatment with DF-Al and CF-Fe requires different types and quantities of chemicals. The chemical dose is dependent on the raw water quality, the process design, and the type and properties of the chemicals used. The specific chemical dose data retrieved from the full-scale DWTPs used for this study are presented in Table 3. The higher head loss generated at 3-M filters for the CF-Fe alternative results in higher energy demand. To make a fair comparison between the two alternatives, pumping of filter effluent water is therefore added to the CF-Fe alternative, resulting in higher energy consumption.

CO₂ emissions originating from energy used at the treatment plant are similar for the two alternatives, i.e., 32- and 39-ton CO₂-eq for DF-Al and CF-Fe, respectively. The origin of energy in this study is Norwegian hydropower with low CO₂ emissions, but if, for example, a European energy mix is used, the carbon footprint connected to energy consumption would be 25 times higher (809 and 971-ton CO₂-eq for DF-Al and CF-Fe, respectively). Even though the CO₂ emission intensity of energy production has a large impact on the results, the treatment process design also has a great impact. Using the European energy mix, the DF-Al alternative has more than a 70% larger carbon footprint than CF-Fe – 850 tons CO₂-eq larger (Figure 3).

During 1 year of operation in the local conditions using low-emission energy, the total carbon footprint is estimated to be 1,000 tons CO₂-eq larger in the DF-Al process than for a plant using CF-Fe. The high CO₂ emissions connected to limewater manufacturing should raise awareness when choosing the process design and finding solutions/alternatives for existing DWTPs to apply more environmentally friendly chemicals for pH and corrosion control.

The energy consumption for heating, de-humidifying, and ventilation of the building mass that is part of the treatment plant also contributes to the total carbon footprint during the operational phase. The footprint is significantly larger for the DF process design, which in turn, requires a larger building. These factors are, however, not taken into consideration in the energy estimations for the operational phase.

An overdosing of chemicals during water treatment implies unnecessary costs, an increased load on the water treatment process, and increased carbon footprints. For example, a process operator who overdoses chemicals by 15% in the DF-Al process will increase the annual CO₂ emissions by 189 tons. This is close to the annual CO₂ emissions from the production of 80,000 m³ of water per day with the CF-Fe process. Similarly, a process operator who overdoses chemicals by 15% using the CF-Fe process would result in an increase in annual CO₂ emissions by 35 tons.

The carbon footprint derived from the operational phase of coagulation-filtration processes is dependent on the raw water quality, the selected water treatment process and process design, energy consumption, emissions intensity connected to the energy production, and the type and dose of chemicals used. Based on these results, environmental impact estimations should be included in the traditionally cost-driven evaluation of treatment alternatives, both when building new DWTPs and renewing existing ones. In addition, more emphasis should be devoted to finding alternative treatment chemicals with small CO₂ emissions.

Sensitivity analyses were performed for the operational phase by changing the inputs of energy and chemicals by 15%. The corresponding relative percentual changes were calculated for both CO₂ emissions and costs. The results showed that the most sensitive variable for the CO₂ emissions from the DF-Al process was limewater, with more than a 10% impact on overall emissions. The increase in CO₂ emissions was mainly connected to the manufacturing of the chemicals. The most sensitive variable for costs was energy, with more than a 6% impact overall. It should be noted that if a chemical dose (such as a limewater dose) is increased, other chemical consumption often increases as well, which makes the overall CO₂ emissions and cost impact larger than when calculated for a single variable.

For CF-Fe, the most sensitive variable (over 6%) was limestone in terms of CO₂ emissions, where the increase was connected mostly to the transport of limestone from the production site to the treatment facility. The most sensitive variable for costs was energy, with more than a 9% overall impact on the CF-Fe process.

As an additional sensitivity analysis, the energy cost input was changed by 70% due to the current fluctuations and increase in energy prices in Europe. This resulted in an overall energy cost impact of 29% for DF-Al and 43% for CF-Fe. However, a cost increase of 325% for the operational energy input is required before the DF-Al and CF-Fe would have the same annual operational costs. The price increases for other inputs, such as the manufacturing of chemicals and transport, are also influenced by the energy price. Manufacturing chemicals for the DF-Al alternative is more energy-intensive, and the price for manufacturing these chemicals is assumed to be more affected by the increasing energy prices.

Carbon footprints from a plant's operational phase are estimated based on full-scale DWTP data. The results show that the annual carbon footprint for production of 80,000 m³/d drinking water from a direct dual media (2-M) filtration plant using Al coagulant and limewater (DF-Al) is over five times (1000 tons CO₂-eq) larger than that produced by a three media (3-M) contact filtration plant using Fe coagulant and an alkalizing limestone filter bottom layer (CF-Fe).

The simplified carbon footprint and investment cost comparison for the construction phase for the direct filtration (DF-Al) and contact filtration (CF-Fe) enhanced coagulation process alternatives indicate a large difference, both in the carbon footprint and investment costs (both are significantly greater for DF-Al). This difference is explained mainly by the larger process footprint area (70% larger) for the direct filtration than for the alternative contact filtration process. The larger process footprint area means increased resource consumption during construction, resulting in higher investment costs and a higher carbon footprint in the construction phase.

This study reveals significant differences in the carbon footprint for two similar water treatment processes based on enhanced coagulation-filtration (contact and direct filtration). It is well-known that the carbon footprint and treatment costs are affected by the selected water treatment processes. The results from this study show that the carbon footprint and costs can also be significantly reduced by the proper design of similar treatment process alternatives and a careful selection of water treatment chemicals.

Therefore, to reduce environmental impacts and costs, the design of drinking water treatment processes should be carefully considered even for very similar processes. The results also indicate that the alternative with the largest carbon footprint also has the highest construction and operation costs, which should motivate both water professionals and decision-makers to include a carbon footprint evaluation as a routine step in the water treatment process selection and design phases.

We would like to thank Bergen Vann (Bergen, Norway) for funding this study. We would also like to thank Sandra McCarley for her valuable support and experience considering water operations and Elin Lavonen for her knowledge of water characteristics. Special thanks to the operators and engineers of the involved water treatment plants for supplying operational data.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.