2.1 Costs of the tertiary treatments
2.1.1 Ozonation cost
Ozone generators are supplied with air for low capacities (maximum of 75 kgO3 h-1 per ozone generator) and with oxygen for larger needs (200 kgO3 h-1 maximum by ozonator) (Baig and Mouchet 2017). Ozone generators operating with air have higher energy consumption than those operating with oxygen, 13 to 20 kWh kg-1O3 compared to 7 to 13 kWh kg-1O3 (Baig & Mouchet, 2017). The oxygen consumption of ozone generators supplied with oxygen are around 8.3 kgO2 kg-1O3 (Xylem, 2017) with an oxygen cost estimated at 0.1 € kg-1 (besnault et al., 2015).
The first cost of investment is the ozone generators. RECORD (2006) established this cost (excluding engineering) between 150 € g-1O3 h and 30 € g-1O3 h for the highest capacities, greater than 10 kg h-1 (RECORD, 2006). Similarly, Mendret et al. (2019) considered a ratio of 100 $ g-1O3 h for a capacity of 1.15 kgO3 h-1, based on feedback from suppliers and manufacturers of ozone generators, and Landry Carter (2017) gave a ratio of 54 $ g-1O3 h corresponding to an investment cost (Ex-works) of $ 2,500,000 for an ozone generator of 46 kgO3 h-1. This cost, including the injection, agitation and residual ozone destroyer system, was communicated by the company Primozone (Sweden) (Landry Carter, 2017).
2.1.2 Granular Activated Carbon (GAC) cost
GAC is implemented in a filter bed for the adsorption of organics on granular carbon. Process energy requirements are low for GAC and include both supply and backwash pumping (Hansen et al., 1979). The pollutants are eliminated by adsorption due to their affinity with the activated carbon and its high specific surface area of this adsorbent. The consumption of activated carbon can be first estimated from the COD load treated, typically in the range of 250 to 500 g COD kg-1AC or higher (Truc, 2007). In first approach, a consumption of 250 to 300 g COD kg-1AC is generally considered. Performances of this treatment are dependant of the organics compounds to be adsorbed and in particular the polarity, molecular weight, solubility and concentration. This can be evaluated in laboratory by adsorption isotherms. After the saturation, activated carbon must be replaced and reactivated in high temperature ovens. In France, the reactivation of coal is done in specialized centers, it is too expensive to be carried out on user sites (Bui et al., 2016).
The reactivation yield is dependent on the type of carbon and the nature of the molecule adsorbed. For charcoal made from softwood (pine) this yield is relatively low (70-90%), while for charcoal made from coconut it can reach 98% (information obtained from Chemviron 2019). Treatment of the spent GAC in a reactivation center will require a prior acceptance certificate with limits to be respected for certain parameters such as sulfur, chlorine and fluorine.
The cost of GAC is generally between 1 and 4 € kg-1 and the cost for the reactivation is 0.6-0.7 € kg-1 (excluding transport). The reactivation cost is slightly higher than the elimination cost (0.4-0.5 € kg-1), but leads to savings on the purchase of new GAC until it cannot be reactivated. Activated carbon treatment should not be used when treated fluxes have too high COD due to the costs associated with the carbon reprocessing. The investment costs were identified and estimated by the company IRH as part of a study carried out for the Rhône Mediterranean Corsica Water Agency. This preliminary design approach is exclusive of taxes and fees and doesn’t include supply (cost and mankind), the contracting authority staff and the project management mission (IRH, 2010). The investment costs are very dependent on the capacity and are here established at 50 € m-3 d and 625 € m-3 d for the highest and lowest capacities respectively (60 and 1 m3 h-1). Significantly higher costs have been estimated by Guo et al. (2014): 350 € m-3 d and 960 € m-3 d for the same highest and lowest capacities. For them, the investment cost covers the process, initial charge of activated carbon, piping, control and instrumentation and is linked to the flow quantity treated:
Log (Capital cost ($)) = 0.722 x log (flow rate (m3 d-1)1.023 + 3.443) Guo et al. (2014).
This equation was mainly defined using a simulation tool created by the Water Research Foundation and the USEPA.
Altogether, the investment costs depend on the flow treated and on its composition influencing the nature of the material used, the kinetic of filtration end the number of filters used.
2.1.3 Membrane filtration cost assessment
Reverse osmosis and Nanofiltration (NF) are implemented for water reuse or very strict constraints on discharges (very low threshold or low water flow rate authorized for discharge). They require more efficient pre-treatment than adsorption and ozonation. Energy consumption is higher and the elimination of retentate (10 to 30% of the initial volume of water treated) remains problematic impacting the operating fees. NF and RO investment costs, identified by the company IRH, are estimated near 1,500-1,750 € m-3 d (for units treating 200-300 m3 d-1) and 7,500 € m-3 d (for a capacity lower than 10 m3 d-1). In addition, an additional cost of 25 to 30% must be taken into account for the assembly, commissioning, etc. (IRH, 2010). In another study, Plumlee et al. (2014) have proposed an equation for capital cost estimation linking the overall investment (process and auxiliary costs) to RO and NF:
Capital cost ($M MGD-1) = 7.14 x flow rate (MGD)-0.22 Plumlee et al. (2014)
Where, capital cost is in $M MGD-1 and the flow rate is in MGD (Millions of Gallons per Day) with 1 US MGD = 3,785 m3 d-1. This equation can be used for flows higher than 1 MGD.
The overall capital cost was thus estimated at 1,500 € m-3 d for a unit treating 3,800 m3 d-1. This cost could be lower for the treatment of slightly polluted unsalted water.
For our simulations, the equation of Plumlee et al. (2014) will be used for a simulated unit with a flow at 0.5 MGD (assumption supported by the supporting information of (Plumlee et al., 2014) publication).
2.2 Cost assessment of the different wastewater treatment scenarios used for the comparison of costs
Studies comparing the costs of tertiary treatments are generally carried out for urban wastewater treatment plant applications (Bui et al., 2016; Choubert, 2018; Wahlberg et al., 2010). Only few works exist on these tertiary treatments on industrial wastewaters. The economic considerations drive the selection of the process implementation. Here, a simulation of different scenarios for is proposed to treat a conceptual effluent with a COD concentration at 500 mg L-1 (higher than urban waste concentration). A comparison of the 3 different treatment processes (ozonation, adsorption and membrane filtration) is carried out on a cost basis. Treatment channels considered are presented in Fig. 1. The channel with membrane filtration (ultrafiltration / reverse osmosis) is considered to study a scenario with water reuse.
In the different scenarios tested, the three post processes are described as follow:
- Membrane channels: wastewater is first filtered by ultrafiltration and reserve osmosis and retentate is post-treated by ozonation. Two scenarios are proposed here with permeate directly discharged (Channel 1) or reused (Channel 1 bis).
- Activated carbon channels: wastewater is treated by AC post-treatment, the obtained water can directly be discharged. Two scenarios here are considered using either reactivated AC (Channel 2) or new AC (Channel 2 bis).
- Finally, wastewater can be processed directly by ozonation treatment allowing direct discharge.
2.2.1 Hypothesis used in the simulation
General assumptions used for simulation are:
- A flow rate at 2 000 m3 d-1, 24h/24, 365 days per year;
- An electricity cost at 0.1 € kWh-1;
- A staff cost fixed at 50 € h-1.
The capital cost is amortized over 20 years (n) (Besnault et al., 2014) considering an interest rate (r) of 4.5% y−1 (Margot et al., 2013). The amortized capital cost (A) is given by the following equation (Mahamuni & Adewuyi, 2010):
Specific hypotheses of each scenario are listed in Table 1.
Table 1. Assumptions for cost estimation (APESA, RECORD 2020).
Item
|
UF + RO + O3, Channel 1 and 1 bis
|
GAC, Channel 2 and 2 bis
|
O3, Channel 3
|
UF
|
RO
|
O3
|
GAC Elimination
|
GAC Reactivation
|
O3
|
Performances
|
10% COD reduction
95% recovery ratio
|
95% COD reduction
80% recovery ratio (f)
|
> 90% COD reduction
90% O3 for transfer efficiency (l)
|
300 gCOD kg-1GAC
|
> 90% COD reduction
90% O3 for transfer efficiency (l)
|
Maintenance
|
0.026 $ m-3 x conversion rate €/$ (a)
|
3% capital cost y-1 (g)
|
1.5% of the process capital cost y-1 for the annual part replacement cost (m)
|
1.5% of the process capital cost y-1
|
1.5% of the process capital cost y-1 for the replacement (m)
|
Operating and maintenance staff
|
21h month-1 (b)
|
0.2 € m-3 (h)
|
150 h month-1 (n)
|
22 h month-1 (t)
|
170 h month-1 (n)
|
Consumables (chemicals, GAC, membrane)
|
0.025 $ m-3 x conversion rate €/$ for membrane replacement and 2% of capital cost y-1 for the cleaning agents (a,c)
|
0.1 $ m-3 for membrane replacement and 0.1 $ € m-3 for the cleaning agents 3 x conversion rate $/€ (g,i)
|
2.5 gO3 g-1 COD (o)
8.3 kgO2 kg-1O3 (p)
0.1 € kg-1O2 (q)
|
2 € kg-1 for a new one and 0.45 € kg-1 for the elimination
|
0.7 € kg-1 with a regeneration efficiency of 95% and 2 € kg-1 for a new one
|
2.5 gO3 g-1COD (o)
8,3 kgO2 kg-1O3 (p)
10 kWh kg-3O
0,1 € kg-1O2 (q)
|
Electrical consumption
|
0.3 kWh m-3 permeate (d)
|
2.8 kWh m-3 permeate (j)
|
10 kWh kg-1 O3 (r)
|
0.019 kWh m-3 (u)
|
10 kWh kg-1O3 (r)
|
Water saving
|
|
1.45 € m-3 (k)
|
|
|
|
Capital cost (€)
|
Conversion rate €/$ x Flow rate (MGD) x 3.57 x flow rate (MGD)-0.22 x 106 (e)
|
Conversion rate €/$ x Flow rate (MGD) x 7.14 x flow rate (MGD)-0.22 x 106 (e)
|
Process (30 € g-1O3 h)
+ piping (30%) + site work (10%) + contractors (15%) + engineering (15%) + contingencies (20%) (s)
|
Conversion rate €/$ x 10(0.722 x log(flow rate^1,023 + 3.443) + site work (10%) + contractors (15%) + engineering (15%) + contingencies (20%) (a,s)
|
Process (30 € g-1O3 h)
+ piping (30%) + site work (10%) + contractors (15%)+ engineering (15%)+ contingencies (20%) (s)
|
Reference used: (a) - (Guo et al., 2014); (b) - (Margot et al., 2011); (c) - (Andrade et al., 2015); (d) -(Guo et al., 2018) ; (e) - (Plumlee et al., 2014); (f) - (Bick et al., 2012); (g) - (Shouman et al., 2015); (h) - (Koroneos et al., 2007); (i) - (Andrade et al., 2017); (j) - (Burn et al., 2015) ; (k) (Pedro-Monzonis et al., 2016); (l) - (Margot et al., 2013), (m) - (Mahamuni & Adewuyi, 2010); (n) - (Mundy et al., 2018) ; (o) - (de Franceschi, 2018); (p) - (Xylem, 2017); (q) - (besnault et al., 2015); (r) - (Baig & Mouchet, 2017): (s) - (G.Melin (Ed.), 1999; Mahamuni & Adewuyi, 2010); (t) - (Hansen et al., 1979) ; (u) - (Nijdam et al., 1999).
2.2.2. Results of cost calculation and discussion
The O&M (Operating and Maintenance cost) consists of maintenance, operating and maintenance staff, consumables and electrical costs. The consumables include the membrane replacement costs, the chemical costs and the GAC replacement with elimination or reactivation of the spent activated carbon. It also took in consideration the water saved (channel 1 bis). The annual O&M and capital costs for all the scenarios have been calculated based on the aforementioned assumptions and are presented in Supplementary material.
The highest cost corresponds to the channel 1 which implements membrane filtration and ozonation for retentate treatment. This result confirms those of Wahlberg et al. (2010) and Choubert (2018) who showed that the reverse osmosis channels have the highest costs. The treatment cost can be revised downwards for the membrane filtration channel with water reuse. Considering that the water treated by RO is of very good quality and can be recycled, purchase of water and payment fees for withdrawals and rejections are saved. Indeed, the RO has the advantage of removing salts. Here an economy of 1.45 € m-3 of water recycled is considered (cost of surface water for industrial used (Pedro-Monzonis et al., 2016)). This cost can be higher or lower depending on the origin of water. Pedro-Monzonis et al. (2016), for example, considered for industrial water a cost of 0.18 € m-3 by employing groundwater. Thereby, RO with water saving can be a very interesting option depending on the regulatory of the sites and the price of water. When the activated carbon can be reactivated, the overall treatment cost is the lowest simulated. However, if GAC is eliminated and not reactivated, cost increases from 1.5 € m-3 to 4.2 € m-3 impacting the overall cost. In the case of ozonation, a cost of 3.6 € m-3 has been calculated.
This simulation provides a comparison of the costs for the different tertiary treatment plants and shows the impact of the various assumptions on the overall result. However, estimations are carried out on the basis of average treatment performance assumed for a fictitious effluent and not from available data for a real effluent. The treatment rates necessary for ozonation, the consumption of activated carbon as well as the performances of membrane filtration are specific to each effluent and could differ significantly from the assumptions made for this conceptual study.