Abstract

A study was performed based on the design of a new wastewater treatment plant (WWTP) to be built in Weesp, the Netherlands (about 46,000 Population Equivalents (PE)). The conventional activated sludge plant was considered among the alternatives, with and without primary sedimentation. This pre-treatment technique is considered a sustainability measure as it improves the energy balance of the WWTP. However, at the same time, the question arose about the cost effectiveness of this measure. The scope of the study was to assess whether other sustainability measures (like solar panels) can realise the same level of sustainability with lower costs. The outcome of the study indeed shows that, for a new WWTP, it is considerably cheaper to avoid primary sedimentation and focus on other measures like solar panels instead. This appeared not only to be the case for the scale of WWTP Weesp, but also for WWTPs with capacities higher than 500,000 PE. For existing WWTPs with primary sedimentation, the choice can be different as customisation is necessary.

INTRODUCTION

In the Netherlands, wastewater treatment and sustainability are strictly correlated: 67% of the Dutch water sector's global warming potential (GWP) is due to wastewater (Frijns et al. 2009). This is due to the fact that purifying wastewater entails the implementation of treatment techniques that have a high energy demand.

Several efforts have been made in the Netherlands to render the water sector more efficient in terms of energy use and production of sustainable energy, and therefore reduction of greenhouse gases (GHG) emissions (Frijns et al. 2013). Waternet, the public water utility of Amsterdam, has the ambition to become carbon neutral by 2020 (Waternet n.d.). For this reason, during the design of a new WWTP in the municipality of Weesp, an extensive study was conducted that took into account not only the financial aspects but also the carbon footprint of several plant configurations.

Among these configurations, two types of the typical activated sludge treatment plant (commonly referred to as University of Cape Town (UCT) process) were considered (Figure 1), one of which implemented primary sedimentation and one of which did not, and where the screened sewage water is fed directly into the biological reactor.

Figure 1

Typical Dutch UCT treatment scheme with the priority energy flows.

Figure 1

Typical Dutch UCT treatment scheme with the priority energy flows.

Primary sedimentation is normally applied to medium and large WWTPs (Gori et al. 2013). It is normally implemented in circular or rectangular tanks (usually concrete) and 50 to 70% of the suspended solids can be removed (Metcalf & Eddy 2003). Since less suspended solids and biodegradable organic matter has to be treated in the biological reactor(s), the aeration tank volume and the aeration requirements are smaller. Moreover, primary sludge (which settles in primary sedimentation tanks) has a high biomethanation potential (van Loosdrecht et al. 1997; Gavala et al. 2003; Gori et al. 2013). Essentially, the overall benefit is a better energy balance of the WWTP, therefore primary sedimentation is considered a sustainability measure.

A disadvantage of primary sedimentation is that the concrete tanks, the mechanical equipment, thickeners and sludge pumps are costly, and so is their maintenance. While in the past the advantages outweighed the disadvantages, nowadays new developments and opportunities force us to make new choices. First of all, the aeration equipment nowadays is more efficient than in the past. Secondly, other sustainability and energy producing methods (i.e. wind turbines or solar panels) are much cheaper today. Their share in the (overall) energy mix is greater which makes the energy from the grid both cheaper and ‘greener’ (Devabhaktuni et al. 2013).

The scope of this study is to assess whether the costs associated with the implementation of primary sedimentation can compete with other sustainability and energy production measures, such as solar panels.

METHODS

The new WWTP of Weesp will have a capacity of 46,000 Population Equivalents (PE) and it implements biological phosphorus removal. The thickened sludge is transported to a sludge treatment facility at the WWTP Amsterdam-West at a distance of 33 km. For this study, two of the configurations were considered: UCT without primary sedimentation (herein called configuration 1) and UCT with primary sedimentation (configuration 2). Figure 2 represents a simplified scheme of the two configurations. The influent characteristics and main plant design parameters were obtained from the static HSA Model (HochSchulGruppenAnsatz, STOWA 2007; van Nieuwenhuijzen et al. 2008). Table 1 presents the influent characteristics and main design parameters for both configurations.

Figure 2

Scheme of configuration 1 and configuration 2.

Figure 2

Scheme of configuration 1 and configuration 2.

Table 1

Main influent characteristics and design parameters

Parameter Unit Configuration 1 Configuration 2 
Load (150 g TOD) PE 46,000 
Max flow rate m3/h 1,800 
DWF flow rate m3/h 652 
Pre sedimentation tank surface m2  475 
Primary sludge production kg ds/d  2,674 
BOD/N before the activated sludge – 4.4 3.3 
Design temperature °C 10 10 
Sludge load kg BOD/kg ds/d 0.067 0.075 
Volume anaerobic tanks m3 1,304 
Volume anoxic tanks m3 836 1,024 
Volume aerobic tanks m3 6,480 3,314 
Secondary sludge production kg ds/d 2,426 1,424 
Aeration capacity kg O2/h 439 364 
Effluent nitrogen total standard yearly average mg/L 10 10 
Parameter Unit Configuration 1 Configuration 2 
Load (150 g TOD) PE 46,000 
Max flow rate m3/h 1,800 
DWF flow rate m3/h 652 
Pre sedimentation tank surface m2  475 
Primary sludge production kg ds/d  2,674 
BOD/N before the activated sludge – 4.4 3.3 
Design temperature °C 10 10 
Sludge load kg BOD/kg ds/d 0.067 0.075 
Volume anaerobic tanks m3 1,304 
Volume anoxic tanks m3 836 1,024 
Volume aerobic tanks m3 6,480 3,314 
Secondary sludge production kg ds/d 2,426 1,424 
Aeration capacity kg O2/h 439 364 
Effluent nitrogen total standard yearly average mg/L 10 10 

The investment costs of the two configurations of the Weesp plant, as well as the required amount of concrete, were calculated through the use of a standard system for cost estimates (CROW). The cost assumptions for the calculation of the yearly cost are specified in Table 2.

Table 2

Yearly costs calculations specifics

Useful life for civil engineering works 30 years 
Useful life for mechanical components 15 years 
Useful life for electrical components 15 years 
Useful life for process automation years 
Factor for foundation costs (*) 1.78  
Interest rate for capital costs 3.75%  
Energy costs 0.10 € per kWh, incl. VAT 
Personnel costs 100,000 € per fte per year 
Sludge handling costs (no prim. sed.) (secondary sludge) 492 € per ton ds dewatering + processing + sludge transport, incl. VAT 
Sludge handling costs (with prim. sed.) (primary sludge) 432 € per ton ds dewatering + processing + sludge transport, incl. VAT 
Sludge handling costs (with prim. sed.) (secondary sludge) 499 € per ton ds dewatering + processing + sludge transport, incl. VAT 
FeCl3 (40% solution) 120 € per ton excl. VAT 
FeClSO4 (41%) 124 € per ton excl. VAT (for orders >10 ton) 
AlCl3 (30,7%) 125 € per ton excl. VAT 
Polyelectrolytes (42%) 1,850 € per ton excl. VAT 
Maintenance costs   
Civil works/constructions 0.5 % of construction costs 
Mechanical components % of construction costs 
Electrotechnical % of construction costs 
Process automation 10 % of construction costs 
Maintenance devices/general services 10 % of construction costs/general services 
Useful life for civil engineering works 30 years 
Useful life for mechanical components 15 years 
Useful life for electrical components 15 years 
Useful life for process automation years 
Factor for foundation costs (*) 1.78  
Interest rate for capital costs 3.75%  
Energy costs 0.10 € per kWh, incl. VAT 
Personnel costs 100,000 € per fte per year 
Sludge handling costs (no prim. sed.) (secondary sludge) 492 € per ton ds dewatering + processing + sludge transport, incl. VAT 
Sludge handling costs (with prim. sed.) (primary sludge) 432 € per ton ds dewatering + processing + sludge transport, incl. VAT 
Sludge handling costs (with prim. sed.) (secondary sludge) 499 € per ton ds dewatering + processing + sludge transport, incl. VAT 
FeCl3 (40% solution) 120 € per ton excl. VAT 
FeClSO4 (41%) 124 € per ton excl. VAT (for orders >10 ton) 
AlCl3 (30,7%) 125 € per ton excl. VAT 
Polyelectrolytes (42%) 1,850 € per ton excl. VAT 
Maintenance costs   
Civil works/constructions 0.5 % of construction costs 
Mechanical components % of construction costs 
Electrotechnical % of construction costs 
Process automation 10 % of construction costs 
Maintenance devices/general services 10 % of construction costs/general services 

*incompleteness surcharge, insurance, taxes, permits, utilities, land survey, fees, installation costs, consultancy costs – and supervision, interest during construction, contingencies and VAT.

The total energy demands of both configurations were calculated from the gross energy demand of the technical processes and the energy production from the biogas that is produced from the sludge at the sludge treatment plant (WWTP Amsterdam-West).

For both the configurations the yearly CO2 footprint was calculated based on the net energy demand, the reinforced concrete used for construction, the chemicals used and the sludge transport. The CO2 equivalent factors were retrieved from the Ecoinvent database (Wernet et al. 2016). Direct CO2, CH4 and N2O emissions from the biological process were disregarded.

The cost of energy production with subsidised solar panels is 0.033 €/kWh (0.040 with VAT), while the cost for non-subsidised panels is 0.125 €/kWh (0.151 with VAT) (Lensink & Cleijne 2016). Subsequently, the ‘price of sustainability’ can be calculated by going through the CO2 reduction allowed for by solar panels (found as an avoided emission caused by the use of grey energy). However, this is only applicable if the solar panels are considered to be carbon neutral. This is not the case, but on average the CO2 footprint of solar panels is much lower than that of fossil fuels (Nugent & Sovacool 2014). Thus, the price of sustainability for subsidised solar panels is 0.059 €/kgCO2; for unsubsidised solar panels it is 0.225 €/kgCO2.

RESULTS

Table 3 shows the costs for both configurations. Table 4 shows the results of the calculation of the CO2 footprints.

Table 3

Investment and yearly costs and yearly net energy demand for the two configurations

Parameter Configuration 1 Configuration 2 
Investment costs € 22,933,000 € 26,096,000 
Yearly costs € 2,593,000 € 2,942,000 
Net energy demand* 705,695 kWh/y 274,621 kWh/y 
Parameter Configuration 1 Configuration 2 
Investment costs € 22,933,000 € 26,096,000 
Yearly costs € 2,593,000 € 2,942,000 
Net energy demand* 705,695 kWh/y 274,621 kWh/y 

*Net energy demand, gross energy demand of all equipment minus sustainable production from biogas, sludge incineration, etc, (see Giorgi 2017).

Table 4

CO2 footprints of the two configurations

Parameter Factor Unit Configuration 1 Configuration 2 
Energy in total 0.67 kg CO2/kWh 829,798 794,273 
Energie out total −0.67 kg CO2/kWh −355,994 −609,893 
Energy (net) 0.67 kg CO2/kWh 473,804 184,380 
Reinforced concrete 0.057 kg CO2/kg concrete 11,746 11,853 
Polyelectrolytes (PE) 2.13 kg CO2/kg PE active 22,964 21,992 
AlCl3 0.537 kg CO2/kg AlCl3 14,976 
Sludge transport 0.115 kg CO2/ton.km 54,276 73,914 
CO2 footprint  kg CO2/year 562,789 307,116 
Parameter Factor Unit Configuration 1 Configuration 2 
Energy in total 0.67 kg CO2/kWh 829,798 794,273 
Energie out total −0.67 kg CO2/kWh −355,994 −609,893 
Energy (net) 0.67 kg CO2/kWh 473,804 184,380 
Reinforced concrete 0.057 kg CO2/kg concrete 11,746 11,853 
Polyelectrolytes (PE) 2.13 kg CO2/kg PE active 22,964 21,992 
AlCl3 0.537 kg CO2/kg AlCl3 14,976 
Sludge transport 0.115 kg CO2/ton.km 54,276 73,914 
CO2 footprint  kg CO2/year 562,789 307,116 

Detailed tables with cost components and energy use can be found in Giorgi 2017.

As expected, configuration 1 is cheaper but has a higher CO2 footprint (it is less sustainable). Therefore, a ‘price of sustainability’ was calculated by dividing the difference in yearly costs between the two configurations and the difference in yearly footprints. This amounted to 1.40 €/kg CO2. By dividing the difference in yearly costs by the difference in yearly energy demand, this is 0.83 €/kWh.

When comparing these calculated key figures with the key figures of solar energy (0.225 €/kgCO2 and 0.125 €/kWh (non-subsidised)) they appear to be very high.

This means that it would be much cheaper to be equally sustainable by not applying primary sedimentation, but matching the energy demand difference with solar panels.

To get an indication of the economy of scale the same calculations were performed for a plant designed for 500,000 PE, with the same characteristics. The cost estimations of the 500,000 PE plant were made by linear and square root upscaling. The investment costs of the several civil and mechanical/electrical parts of the WWTP's were estimated individually. Upscale units are cubic meters per hour, cubic meters or square meters depending on the facility (e.g. the secondary clarifiers are scaled with square meters, civil from the cost sheet (linear) and mechanical/electrical squared from the capacity (surface 500,000 ie/surface 46,000 ie)). The yearly costs were calculated with the cost rates as the 46,000 PE WWTP, shown in Table 2 (see Giorgi 2017). The results are summarised in Table 5.

Table 5

Differences in costs, energy consumption and CO2 footprint between the two configurations for a capacity of 500,000 PE

Parameter Unit Value 
Additional investment costs € 18,247,000 
Additional yearly costs € 2,566,000 
Delta energy use kWh/y 4,686,000 
Delta CO2 production kgCO2/y 2,781,000 
Euros per kWh saved €/kWh 0.55 
Euros per kgCO2 saved €/kgCO2 0.92 
Parameter Unit Value 
Additional investment costs € 18,247,000 
Additional yearly costs € 2,566,000 
Delta energy use kWh/y 4,686,000 
Delta CO2 production kgCO2/y 2,781,000 
Euros per kWh saved €/kWh 0.55 
Euros per kgCO2 saved €/kgCO2 0.92 

The values of the key figures were indeed reduced, but are still high compared with the key figures for solar panels. Figure 3 shows the results in a graph with the relation between the plants' scale and the key figures for energy cost and of CO2 footprint for primary sedimentation and solar panels.

Figure 3

Graph relating the scale of the plant with the cost of energy production (above) and with the cost of CO2 reduction (below).

Figure 3

Graph relating the scale of the plant with the cost of energy production (above) and with the cost of CO2 reduction (below).

An exponential trend line was hypothesised, because when building a treatment plant the investment costs versus the plant capacity exponentially decrease due to the principle of the economy of scale. This line flattens due to the fact that, at bigger scales, more separate unit operations will be built (because of the constraints on the maximum diameter), therefore the upscaling is not squared anymore but linear. Gratziou et al. (2006) demonstrated that this is the case for several types of wastewater treatment systems. Since the costs of energy production and CO2 reduction are based on the yearly costs, that are in turn proportional to the investment costs, the trend line is expected to assume an exponentially decreasing shape. Therefore, as it can be seen from Figure 3, primary sedimentation is not expected to compete with solar panel at any realistic scale for the Netherlands.

A sensitivity analysis was performed both on the calculations for the small scale plant and for the larger scale plant. It was performed in such a way that the considered factors were tuned in the direction in which they could improve the primary sedimentation's business case. Even when all the factors were tuned at the same time, primary sedimentation still was more expensive than solar panels, for both the scales. For more details about the sensitivity analysis, the reader is referred to Giorgi 2017.

DISCUSSION

An important remark to mention here is that the results are specific for the situation in Weesp in the Netherlands and they cannot be taken out of their context. Both wet weather flow (WWF) and dry weather flow (DWF) can be different for other regions depending on water consumption behaviour, precipitation regimes, etc., resulting in smaller sedimentation tanks, changing in turn the financial aspects connected to the implementation of primary sedimentation. Moreover, the outcome of the calculations will change accordingly with organic loads, characteristics of the sludge, the prices, etc.

Therefore, to evaluate the application of a primary sedimentation for a completely different WWTP in another country, a customised cost assessment should be performed. Nonetheless, the results do give an indication of the fact that it is fairly likely that constructing primary sedimentation for a new WWTP is now an obsolete process to apply (in terms of energy and CO2 reduction costs).

There are two pros to primary sedimentation that have not been considered in this study: grease and sand removal. However, the savings represent few percentage points of the total investment, which again does not change the fact that configuration 2 would still be too expensive.

This study was performed on the case of a plant to be newly built. A special case is represented by a plant that needs to be retrofitted. In that case, if primary sedimentation tanks are already present, a calculation should be performed taking into account what the yearly costs would be to repair the tanks versus their demolition and the implementation of a treatment scheme like the one of configuration 1.

Finally, it is important to remember that there is no one-size-fits-all solution. For instance, for a water authority that has no option of matching the carbon dioxide emissions with solar panels or wind turbines, primary sedimentation should still be implemented if CO2 reduction is the goal.

CONCLUSIONS

Primary sedimentation can be an expensive sustainability measure when compared with solar panels, even at larger scales. This means that for the same carbon footprint it is much cheaper not to implement primary sedimentation, but to make up for the energy difference through the installation of solar panels.

The recommendation that stems from this study is that, for future plants that will be built (or retrofitted), the option of not implementing primary sedimentation should be considered: a thorough financial and sustainability analysis should be performed in order to make sure that no other cheaper sustainability measures are available.

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