Cost comparison of full-scale water reclamation technologies with an emphasis on membrane bioreactors

Posted on 21 June 2018

Raquel Iglesias^a, Pedro Simón^b, Lucas Moragas^c, Augusto Arce^d, Ignasi Rodriguez-Roda^e

^aDepartment of Water Technology, Centre of Hydrography Studies (CEDEX), Spain
^bEntidad de Saneamiento y Depuraciónde Aguas Residuales de la Région de Murcia (ESAMUR), Murcia, Spain
^cAgencia Catalana Del Agua (ACA), Barcelona, Spain
^dDepartment of Chemistry and Food Processing, Technical Agriculture Engineering of Universidad Politécnica de Madrid (UPM), Madrid, Spain
^eCatalan Institute for Water Research (ICRA), Girona, Spain; Laboratory of Chemical and Environmental Engineering (LEQUiA), Girona, Spain

1. Introduction

MBR installation in Spain began in 2002, increasing from four 400−6,000 m³d^-1 facilities by 2005 to more than 50, with a total capacity of ~244,000 m³d^-1, by 2014 (Fig. 1). As such, the country has an unusually large number of MBRs compared with other EU nations, in part associated with its commitment to water reuse (Fig. 2). Elsewhere in Europe, water reclamation is typically for agriculture or indirect potable reuse such as aquifer injection or surface water discharge.

Despite the well-known advantages offered by MBRs, specifically the high treated water quality coupled with the small footprint, the costs associated with the technology are not as well documented.

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It is therefore useful to consider cost information available from the installations across Spain, and to compare this with corresponding information for the more conventional activated sludge (CAS) technologies. The CAPEX and OPEX − capital and operating expenditure − information is used to generate cost curves as a function of flow capacity or population equivalent (p.e.) for both technology types.

Figure 1. MBR technology implementation in Spain, 2002−2014

Figure 2. Water reuse applications in Spain

2. Methodology

The 14 installations considered (Table 1) were all of the immersed process configuration (iMBR) based on the Suez ZeeWeed (hollow fiber, HF) or the Kubota (flat sheet, FS) membrane. These installations are reasonably representative of those employed for water reuse in Spain. The information was provided by public wastewater management institutions in Catalonia and Murcia (ACA and ESAMUR).

Table 1. Capacity and configuration of MBR installations considered for cost analysis
MBR	Code	Capacity (m³d^-1)	Pretreatment	System type	Membrane configuration	Start-up
HF: hollow fiber FS: flat sheet S: fine screening PC: Primary clarification CAS: Conventional activated sludge process IF: Integrated fixed-film activated sludge process D (MBR−IF): two parallel treatment lines with different technologies, an MBR and an IF H (MBR−CAS): hybrid configuration including an aerated tank and membrane filtration tank that have the identical pretreatment and maintain a secondary settler to treat peak flows.
Aledo	AL	288	S	MBR	FS	2008
Calasparra	CL	2,000	PC+S	H (MBR−CAS)	HF	2006
El Valle	EV	1,400	S	MBR	FS	2007
La Bisbal	LB	3,225	PC+S	H (MBR−CAS)	HF	2003
Los Cañares	LC	3,750	S	MBR	FS	2008
Mar Menor	MM	1,880	S	MBR	FS	2009
Riquelme	RQ	1,575	S	MBR	FS	2007
Riells Viabrea	RV	2,116	S	H (MBR−CAS)	FS	2004
Sabadell	SB	35,000	PC+S	H (MBR−CAS)	FS	2008
San Pedro	SP	20,000	S	MBR	HF	2007
Terrassa	TR	15,000	PC+S	D (MBR−IF)	HF	2008
Vacarisses	VC	1,320	S	MBR	HF	2010
Viladecans	VD	32,000	PC+S	D (MBR−IF)	HF	2010
Vallvidrera	VV	1,100	S	MBR	HF	2008

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Available data were used to determine CAPEX (in € per m³d^-1 capacity and per p.e.) and OPEX (€ per m³ produced or € per kg of BOD removed). Six MBR facilities were selected for the CAPEX study:

EV, MM, RQ, SP, VC and VV,

and 11 selected for OPEX:

AL, CL, EV, LB, LL, MM, RQ, SP, VC and VV.

CAPEX calculations ignored local variations in land, equipment and materials costs; investment costs were obtained from the liquidation budget and excluded overheads and profit. VAT was excluded from all costs, and start-up assistance for 3−6 months assumed. Costs were updated to 31/12/2011 for data published between 2002 and 2009, using the CPI (Consumer Price Index) to adjust costs to the baseline date.

In some cases, the MBR was retrofitted to an existing CAS process by placing the membranes in either the secondary clarifier or within the biological tank. These facilities were excluded from the analysis for determining cost curves, though implementation costs were logged.

OPEX is significantly impacted by optimization, and specifically (a) the period of time over which the installation operates at full capacity, and (b) the membrane air scour management strategies during under-capacity periods. Some installations (e.g. LB) divert the feed to the secondary clarifier when the MBR capacity is exceeded. In 2011, this plant treated no more than 30% of the total flow capacity from the conventional treatment line: the OPEX data for this line was therefore excluded from the analysis of total OPEX for this plant.

Key OPEX components (Table 2) were broken down into fixed (A) and variable costs (B), with global energy use normalized against permeate flow (kWh per m3) and load (kWh per kg BOD) with energy costs ranging from €0.096 to €0.121 per kWh. Although membrane replacement was excluded, an average membrane life of 10 years would be expected to add 10−12% to the OPEX.

Other OPEX contributors include sludge management, waste disposal, chemical prices, and management by private or public entities. The latter can subcontract services with different agreement conditions. Sludge disposal contributed between 20% and 30% to the OPEX. Average costs per ton of wet sludge were €15−20 for agricultural use, €30−35 for composting, and €40−60 for thermal drying.

Table 2. OPEX items delineated by fixed and variable costs
Costs	Items included
A. Fixed costs, € year^-1
Operators	Annual salary/maintenance engineer
General expenses	Accounting, fees, insurance, supplies
Maintenance and repairs	Equipment and infrastructure
Energy	Potential term (€ kW^-1 year^-1)
Chemical analysis	Royal Decree 11/1995
B. Variable costs, € year^-1
Overall energy consumption	Pre-treatment, membrane, biological and sludge
Waste and sludge	Disposal management
Chemicals & replaceable components	Membranes

CAPEX is influenced both by the required effluent quality and the footprint. The land area required by the MBRs studied ranged from 0.15 to 0.30 m²per p.e.

The MBR facilities data were compared with those from 75 CAS installations constructed between 1993 and 2007 based on conventional water reclamation treatment (CRT, coagulation followed by sand filtration and disinfection), and advanced water reclamation treatment (ART, with membrane polishing). Capacities of these WWTPs range from 2,000 to 48,000 p.e., or 360 to 12,300 m³d^-1. All facilities operated under extended aeration (EA), and many included biological nitrogen removal. The CAPEX and OPEX for ART and CRT were obtained from published data and converted to €.m^-3 treated water for OPEX and € per m³.d^-1 installed capacity for CAPEX. The values were added to the EA costs as mean values of the range of each type of treatment.

In addition, new OPEX and CAPEX data for EA, CRT and ART were gathered from CYII, ACA and ESAMUR facilities in 2015, comprising 35 CAS−EA, 16 CRT and 5 ART facilities having flow capacities of 5,000−10,000 m³d^-1. All cost data were normalized to the same date of December 31, 2011.

3. Results and discussion

Investment costs (Table 3), excluding retrofits and dual installations, ranged from 2,000 to 3,200 €.m^-3.d and 420−650 €.p.e.^-1 for facilities with capacities of 1,000−2,000 m³d^-1. For capacities exceeding 10,000 m³d^-1 the cost decreased to as low as 625 €.m^-3.d and 94 € p.e.^-1 (Fig. 3).

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A comparable range of OPEX can be established by considering the six plants operating regularly in 2011 (Table 4), including the specific aeration demand (SAD) for air-scouring the membrane. SAD can be expressed in terms of the air flow per m2 membrane area (SADm) or per m3 permeate (SADp). According to these data, the FS configuration has SADm values predominantly in the 0.45−0.5 m3h-1m-2 range, compared with 0.26−0.3 for the HF modules.

Table 3. Investment cost of MBR facilities
MBR	Capacity m³.d^-1	Capacity p.e.	Type of process⁽¹⁾	Configuration	Membrane area m²⁽²⁾	Cost €.m^-3.d	Cost €.p.e^-1
⁽¹⁾N−D+P: biological nitrogen removal with chemical precipitation of phosphorus. EA−N−D: Extended aeration with biological nitrogen removal. ⁽²⁾Total membrane area
EV	1,400	4,666	EA-N-D	FS	2,560	1,982	595
MM	1,880	7,800	EA-N-D	FS	3,200	2,002	482
RQ	1,575	5,250	EA-N-D	FS	3,200	2,074	622
SP	20,000	133,333	EA-N-D	HF	48,537	625	94
VC	1,320	6,600	N-D+P	HF	3,030	2,130	426
VV	1,100	5,500	N-D+P	HF	2,526	3,277	655

Feat Spanish Cost Comparison Fig 3Cd2 — Figure 3. Estimated CAPEX: (i) per unit flow, and (ii) per p.e., and OPEX (iii) per unit flow, and (iv) per p.e. for the MBR facilities

Table 4. Principal operational parameters of six selected MBR plants
	AL	CL	MM	RV	SP	VV
Fd: Design flow Ft: Treated flow SRT: sludge retention time SP: Sludge production MLSSb: mixed liquor suspended solids in the biological tank MLSSm: mixed liquor suspended solids in the membrane tank HRT: hydraulic residence time LMHm: liters per square meter and hour at medium flow LMHp: liters per square meter and hour at peak flow TMP: transmembrane pressure K: Permeability SADm: specific aeration demand per membrane area R: recycle ratio F: filtration B: backflush R: relaxing
Biological system
Fd/Ft (%)	36	89	21−37	51	41	77
SRT (days)	52	38	40−80	13	43	16
MLSSb	10,000	5,429	9,000	8,900	3,250	7,298
HRT (h)	60	24	25−12	12	24	15
SP(kg MS.kg BOD−1 )	0.99	0.60	0.49−0.75	0.81	0.89	1.12
Membrane system
Configuration	FS	HF	FS	FS	HF	HF
T (ºC)	22	22	24	19.5	22	18
LMHm (L.m−2.h)	25	17.5	25	10.5	14−25	28
LMHp (L.m−2.h)	42	33	49	27.2	41	30,9
TMP (mbar)	−65	−155	−120	−250	−120	−110
K (L.m−2.h−1.bar−1)	385	112	250	42	206	294
MLSSm (mg.L−1)	12,000	6,000	11,000	13,500	6,000	7,298
R (n x Ft)	4	4	0.9	1.4	3	3
Cycle (F−B−R; F−B−F−R) (min.)	9−0−1	10−0,5	9−0−1	9−0−1	17−1−1	10−0,5−10−0,5
SADm (Nm3.h−1.m−2)	0.8	0.3	0.45	0.51	0.26	0.27
SADp (Nm3.m−3)	45	24.32	31	24.18	18.3	19.27

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The energy consumption significantly increases for biological tank mixed liquor suspended solids (MLSS) above 6 g.L^-1 due to the decreased oxygen transfer coefficient and increased recirculation rate required to maintain a low of suspended solids concentration in the membrane tank. Low MLSS concentrations, and the associated low energy demands, are possible for oversized facilities providing flexibility in operation, in terms of the number of lines, decks and blowers, as well as the membrane configuration. The SP and RV facilities have such flexibility.

The specific OPEX in plants treating >50% of their flow capacity was between 0.2 and 0.5 €.m^-3 at a BOD concentration of 150−600 mg.L^-1, and €0.5−18 per kg BOD removed depending on the load (in kg BOD.day^-1) and flow (Table 5). OPEX indicators across the selected installations were highly variable due to the non-optimal flows and/or loads of the WWTPs and the differing plant capacities, waste sludge disposal options and costs, and the financial and contractual arrangements at each site. Subsidies are sometimes provided from the reuse of the treated water and waste sludge, which then impacts on the OPEX since the volume of sludge generated can be altered through adjusting the operating conditions.

Table 5. OPEX of MBR treatment lines
Site	Capacity m³.d⁻¹	Treated m³.d⁻¹	Specific cost €.m⁻³	Specific cost €.kgBOD⁻¹	Sludge treatment⁽¹⁾
⁽¹⁾Sludge disposal costs are included in OPEX ⁽²⁾1400 m³.d^-1 were not treated by the MBR treatment line. COM: Composting TD: thermal drying AGR: Agricultural use
AL	288	104	2.590	4.3	AGR
CL	2,000	1,793	0.330	0.5	COM
EV	1,400	219	1.600	44	AGR
LB	3,225	4,626(2)	0.215	0.8	COM−TD
LC	3,750	161	2.180	17	AGR
MM	1,880	552	0.480	3.8	AGR
RQ	1,575	362	0.970	18	AGR
RV	2,116	1,116	0.397	3.2	COM
SP	20,000	8,247	0.241	1.5	AGR
VC	1,320	683	0.577	4.9	COM
VV	1,100	849	0.460	2.9	COM

Energy consumption was similarly variable (Table 6), and mainly associated with aeration (Fig. 4). The blowers contributed 19−22% to the total energy consumed on site, the energy demanded by the MBR representing 28−34% of the site total energy consumption and accounting for 41% of the total OPEX on average. The specific energy consumption was in the range 0.6−1.2 kWh.m^-3 for facilities treating >50% of their flow capacity with regular feed water quality. At an average energy cost of €0.11 kWh^-1, OPEX attributable to energy consumption ranged from 0.06 to 0.13 €.m^-3 in facilities treating more than 2,000 m³.d^-1 (Fig. 5).

Table 6. Energy costs of MBR treatment lines
MBR	kWh.m⁻³ WWTP⁽¹⁾	kWh.m⁻³ MBR⁽²⁾	€.kWh⁻¹ WWTP	€.m⁻³ WWTP	% MBR
⁽¹⁾Overall WWTP energy consumption ⁽²⁾Biological treatment blowers, membrane air cleaning blowers, recirculating pumps, permeate pumps and excess sludge pump
AL	3.27	1.710	0.118	0.387	52.29
CL	0.98	0.898	0.096	0.100	85.85
EV	2.60	0.860	0.108	0.420	33.05
LB	0.550	0.190	0.115	0.063	34.55
LC	2.51	1.950	0.097	0.340	77.69
MM	1.27	0.790	0.117	0.111	83.16
RQ	1.63	0.900	0.110	0.180	55.21
RV	0.680	0.460	0.105	0.071	67.65
SP	0.75	0.534	0.104	0.079	69.99
VC	2.00	1.10	0.121	0.241	55.00
VV	1.097	0.714	0.118	0.129	65.09

Figure 4. Mean energy consumption breakdown by membrane configuration

Figure 5. Estimated specific energy costs against effluent flow in MBR plants

Overall, waste and sludge disposal contributed an average of 16% to the OPEX for the installations studied (Fig. 6). Chemical costs for membrane cleaning were around 3% of the OPEX. Membrane replacement costs do not feature in this study: there had been no membrane replacement in the 5−8 years of operation of the installations. Replacement at two of the facilities was attributable to design flaws rather than degradation of the membrane through routine operation. However, for a membrane life of 8−10 years, membrane replacement contribution to OPEX can be assumed be €0.02−0.04 m^-3.

Figure 6. Overall mean OPEX contribution to MBR plants

A comparison of the MBR system costs with those for EA(ND) (extended aeration with nitrification/denitrification), with ART (physical−chemical treatment with a lamella settling system, depth filtration, ultrafiltration and disinfection) and CRT (physical−chemical treatment with a lamella settling system, depth filtration and disinfection) indicates significant differences (Fig. 7). The CAPEX for EA(ND)−CRT ranged from €730−850 per m³.d^-1 flow for plants having installed capacities of 8,000−15,000 m³.d^-1. The costs were between €1,250 and €1,050 per m³.d^-1 if followed by ART and between 700 and 960 €.m^-3.d for MBR treatment.

Membrane filtration costs are compensated by savings in civil construction compared to CAS with water reclamation treatment. However, this differs for capacities over 8,000 m³d^-1 for retrofitting with ART and for capacities over 15,000 m³d^-1 for retrofitting with CRT. Membrane system investment costs are decreasing, so the cost effectiveness of MBR is expected to improve in the future. Moreover, MBR footprint is 40−60% lower than that of classical treatment technologies, reducing land costs.

The OPEX of the studied MBR ranging from €0.2 to €0.45 per m³ treated, and the costs of BOD removal from €0.5 to €4.5 per kg BOD removed. An MBR facility, when operating in conjunction with EA(ND), can achieve a specific energy consumption of ~1 kWh.m^-3 if based on a flexible design and operated close to its capacity at a low MLSS. The MBR has a similar energy consumption to the CAS−ART compared to CAS with water reclamation technologies and is 20% higher than CAS−CRT (Table 7).

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Feat Spanish Cost Comparison Fig 7B V3 — Figure 7. CAPEX and OPEX functions for MBR, EA-CRT and EA-ART

Table 7. Energy demand and costs of different WWTPs
Type of WWTP	Energy consumption (kWh.m⁻³)	Energy cost⁽¹⁾ (€.m⁻³)
⁽¹⁾at €0.11 kWh^-1
MBR (EA-N-D)	0.8−1.2	0.06−0.11
CAS (EA-N-D)	0.4−0.8	0.04−0.08
CAS (EA-N-D)+ CRT	0.6−1.0	0.06−0.1
CAS (EA-N-D) +ART	1−1.2	0.1−0.12

The CAPEX for the 16 CRT-based facilities considered was around €115 per m³d^-1at capacities of 5,000−10,000 m³.d^-1compared with €100 per m³.d^-1for facilities with a capacity of 20,000 m³.d^-1. Data from five facilities provided by Spanish local water agencies ACA and CYII indicated the ARTs to have an average CAPEX of €430 per m³d^-1 for facilities with capacities of 5,000−10,000 m³.d^-1. A comparative CAPEX exercise performed for EA−CRT, EA−ART and MBR in facilities in this range of flow capacities, based on 13 of the WWTPs operating with EA, indicated an average investment cost of €600 per m³d^-1, giving an average CAPEX of 715, 1,030 and 960 €.m^-3d for EA−CRT, EA−CRT and MBR respectively.

The OPEX for reclaimed water was approximately €0.08−0.12 and €0.18 per m³ for CRT and ART-based facilities respectively, excluding conveyance to the reuse zone (€0.02 to €0.04 per m³ transported). The average OPEX for the 22 EA facilities studied with capacities of 5,000−10,000 m³d^-1 was 0.22 €.m^-3 (including sludge disposal management), giving a mean average total OPEX of 0.31, 0.4 and 0.32 €.m^-3 for the EA−CRT, EA−ART and MBR technologies respectively.

4. Conclusions

An analysis of the projected and real data for the CAPEX and OPEX of 14 MBRs with capacities of 288−35,000 m³d^-1 based on HF and FS membrane configurations has provided cost curves for both CAPEX and OPEX. Data were supported through comparison with similar data from other WWTPs and water reclamation technologies providing identical or similar water quality.

The CAPEX of the MBR technology was found to be around 10% lower than that of a reuse installation based on conventional extended aeration with tertiary membrane polishing. MBR costs were, however, 30−34% higher than those associated with classical tertiary clarification by coagulation followed by sand media filtration and disinfection at capacities below 10,000 m³.d^-1.

The cost benefits of MBRs increase with increasing plant size due to the decreased labour requirement, though the energy demand is higher than for conventional processes.

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