The authors are grateful to Professor Simon Judd & Claire Judd for the opportunity to publish this article on their renowned MBR-focused web page (www.thembrsite.com).

1. 1 Experiences from the commissioning of the Arenales del Sol WWTP
Commissioning of a 10 MLD WWTP with flat sheet MBR technology: The Arenales del Sol case
Román Gasull1
, Héctor Rey2
.
1
Abengoa Water - Processes & Systems.
Campus Palmas Altas. C/ Energía Solar, Nº 1. 41014 Seville. Spain.
E-mail: r_gasull@hotmail.com
2
Prointec. Water Treatment Department. Avda de Burgos 12.Madrid 28036. Spain
E-mail: hjrey@prointec.es
1. INTRODUCTION
The Arenales del Sol WWTP, located in Alicante, in
the Mediterranean region of Spain, it combines
biological treatment with nutrient removal along with a
submerged MBR system fitted with flat sheet (FS)
ultrafiltration membranes, with an average treatment
capacity of 10.000 m3
/d (10 MLD). The plant provides
high quality treatment to highly variable seasonal
sewage inflow due to the nature of this popular coastal
region, where second homes and golf courses attract
visitors during the summer months and winter and
Easter vacation periods. This seasonal feature became
a major challenge both during commissioning and
during the design of the O&M strategies of the plant.
Time constraints and the little room available for
contingencies required the employment of site-
specific operational strategies in order to commission
this relatively large MBR plant.
Table 1. Arenales del Sol WWTP characteristics
Flow (AWF) 10,000 m
3
/d No.treatment lanes 2
Peak Flow (WWF) 12,000 m
3
/d No. MBR lines 8 lines, 9 modules/line
Influent / effluent characteristics Process characteristics
BOD5 700 / 10 SRT 18 d
SS 500 / 5 HRT 16,8 hrs
NT 80 / 10 F:M 0.084 kgBOD5*d
-1
/kgTSS
PT 10 / 1 Average Design Flux (J): 22 LMH
Turbidity - / 2 NTU [MLSS] MBR/oxic 12.5/ 10.5 g/l
Pretreatment
1.Coarse Screening 3.Sand & Grit removal
2.Medium Screening (6 mm) 4. Fat & Grease removal
5.Fine Screening Rotary Drum punch hole sieve (2 mm)
Process tanks MBR system
Balancing tank (Vt = 2.130 m
3
) Total of 72 double-deck MBR modules with 280 m
2
/unit filtration area
Model: TORAY TMR-140-200D
2 x anoxic tanks (Vt = 1.380 m
3
) Total filtration area: 20.160 m
2
2 x oxic tanks (Vt = 3.616 m
3
) Design Flux values: 0,5 m/d (mean) , 0,69 m/d
2 x MBR tanks (Vt = 1.854 m
3
) SADm: 0,42 – 0,6 m
3
/m
2
/d

2. 2 Experiences from the commissioning of the Arenales del Sol WWTP
2. TYPICAL COMMISSIONING CHALLENGES OF A
MBR
2.1. Background
Many WWTPs fitted with MBR technology have
experienced severe difficulties during the
commissioning and start up phases of the Project,
specifically because operational problems are
normally manifested as failure to meet the hydraulic
load, rather than achieve the required level of
purification. This demands that membrane fouling
and clogging is minimised as much as possible,
through removal of fine solids and reduction of EPS or
dissolved organic carbon levels in the sludge. Also, the
variation of operational parameters such MLSS
concentration, peak fluxes, and aeration volume are
known to exacerbate fouling and clogging, as well as
the sludge floc structure.
Various challenges arise during MBR
commissioning, including:
a) rapid membrane fouling due to insufficient
biomass concentration, unhealthy
microorganism populations, excessive EPS
formation or membrane blocking;
b) overflowing tanks from heavy foaming events
(with subsequent biomass loss) or membrane
clogging;
c) under-spec treated water quality prior to the
biological conditions reaching the design
values, and specifically the MLSS
concentration;
d) impaired treatment from incomplete tasks
from commissioning, such as clean water
testing, PID control adjustment, etc;
e) membrane clogging/ragging due to
unscreened seeding sludge;
f) membrane damage due to abrasion from
construction materials, such as metal shards
from thread tapping, not removed from the
MBR tanks;
g) sludge settling due to blower failure;
h) membrane over/under-aeration due to
incomplete PID blower testing or incorrect
blower design.
All the activities planned for the commissioning
of this WWTP aimed to minimise or avoid the above,
and from applying knowledge available from a
number of published case studies.
2.2. Commisioning requeriments, hollow fibre Vs flat
sheet systems
Sludge seeding in biological and MBR tanks can
become challenging in medium to large municipal
WWTPs. Typically, the anoxic and oxic reactor
volumes are usually too large and un-chambered, and
are to be filled with rather concentrated sludge from
other WWTPs in a single step. Equally difficult and
expensive is to truck in excess recirculation sludge
from other secondary settlers to achieve a high MLSS
concentration in the MBR tanks. On-site biomass
augmentation is thus unavoidable for any medium to
large MBR plant, and the target MLSS concentration
will depend on the membrane configuration: 5-8 g/l
(or 10-12 g/l for short periods) for an HF configuration
compared with 12 to 15 g/l for the FS. It follows that
target biomass conditions are more rapidly attained
for HF systems. Moreover, as often observed in MBR
literature, FS membranes experience major fouling at
lower MLSS from small pin-floc structures causing
rapid pore blocking more rapidly. At higher MLSS
levels and correspondingly larger flocs, the sludge-
scouring air mixture provides greater higher shear
and so an improved cleaning efficiency at higher MLSS
concentrations (up to 18 g/l) than at lower.
In the case of HF systems, the initial MLSS
concentration is not considered as important during
commissioning, since they demonstrate good best
filtration performances at sludge seeding
concentrations (2 to 6 g/l).

3. 3 Experiences from the commissioning of the Arenales del Sol WWTP
0
200
400
600
800
1000
1200
1400
1600
2 4 6 8 10 12 14 16
Flux
MLSS concentration (g/l)
Typical Flux vs MLSS in sMBR
Flat Sheet
Hollow Fiber
Fig. 1- A) Comparison between Flat Sheet and Hollow Fiber
It was important to reduce the risk of initial
fouling at the low MLSS concentrations to avoid
downtime for chemical cleaning. However, since
the existing WWTP was not meeting effluent
discharge standards the new plant had to be
ready for work with full flow treatment
warranties in a shortened time. Given the time
constraints the project was under, a minimum
MLSS concentration of only 6-7 g/l was deemed
necessary prior to operation with the FS MBR
membranes.
All the ancillary equipment to the MBR (i.e.
blowers, pumps, PLC, etc) was fully tested with
clean water.
3. FOULING CONTROL STRATEGIES DURING
START UP
3.1. Biomass development
Biological degradation of the sludge in the
reactors is required not only to meet effluent
discharge standards but also to avoid membrane
fouling by excessive unoxidised BOD levels. Time
restrictions associated with the commissioning
required both a stable biomass and a ready-for-
business MBR system at the design conditions of
12.5-15 g/l within 8-10 weeks. Unfortunately,
since only about 30% of design inflow was being
received at the time, there was a shortage of
carbon for natural bio-augmentation. Biomass
augmentation was thus achieved using the anoxic
and oxic tanks in one (of two) of the treatment
lines in an sequencing batch mode, with
supernatant being regularly withdrawn so as to
increase the MLSS without discharging the solids,
(thus increasing the SRT). The plant was fed both
with untreated sewage pumped from the existing
plant at a reduced loading rate and sugar cane
residues as an additional carbon source.
Additionally, small volumes of lyophilized
bacteria were added in the first few days to
rapidly increase the available microorganism
population in the tanks. This strategy allowed for

4. 4 Experiences from the commissioning of the Arenales del Sol WWTP
a controlled seeding process, while closely
examining biomass species under the
microscope.
All sludge fed into the new plant was mesh
filtered in the new rotary drum screens (punch-
hole, 2 mm) for fail-safe operation of the
membranes.
Modelling software (WEST) was used to
simulate biomass growth in the biological tanks.
As it turned out, WEST predictions were quite
accurate in assessing the actual biomass growth
rate during start up. The time employed in
seeding and concentrating the sludge was used
to calibrate the input data for the modelling
software.
Fig. 2- A) Comparated growth of MLSS (simulated and experimental)
3.2. Flux control
Flux control is crucial for fouling control. It is
well known that if flux is maintained below a
certain critical value (known as critical flux), it is
possible to ensure stable operation with little or
negligible increase in TMP for longer periods, and
hence reducing cleaning frequency. It is also true,
however, that this critical flux is unique to every
MBR plant and process water characteristics, and
it is usually only ascertained through trial and
error experimentation under design operational
conditions (in this case, 12.5 g/l at 22-28 LMH).
Strict flux control during commissioning stage is
thus essential to avoid accelerated fouling,
particularly when using FS system configurations.
During start-up at around 6-7 g/l MLSS, it
was not deemed necessary to try find the critical
flux value for the MBR system; flux values
permitted until design conditions were kept
conservative and well below design conditions.
Average biomass temperature at start-up was
slightly above 16º C.
Since commissioning time was getting closer
and filtration had to be resumed promptly prior
to the Easter vacation period, filtration was
started with a low MLSS of 6.5 g/l, and a flux
below 8 LMH, with a maximum allowed
emergency flux of 10 LMH. The operational flux
allowed at each of the commissioning phases
(see below) were chosen according to the
previous experiences with the chosen membrane
technology.

5. 5 Experiences from the commissioning of the Arenales del Sol WWTP
Table 2. Commissioning phase operating parameters
Start up
conditions
Transitional
phase
Design Conditions
[MLSS] (g/l) 5 to 7.5 8 to 11 12 to 15
Aver. Flux allowed (LMH) 6 to 8 8 to 14 20 to 28
Max. Flux allowed (LMH) 10 16 41
This filtration time prior to commissioning,
even at these low fluxes (design flux was 22 LMH)
allowed for both biomass concentration and
biomass acclimatization while reaching the more
acceptable MLSS levels above 10 g/l. Again, the
WEST modelling software was employed to
forecast biomass growth and hence inform the
initiation of full-flow start-up of the plant.
3.3. Air scouring control and optimization
The beneficial effects of air scouring to
control fouling at the membrane surface of flat
sheet membranes is well documented. Typically,
it is assumed that there is a linear relationship
between membrane flux (Lwater/m2
/h) and the air
scouring rate (Nm3
/h/m2
), within some limits
above which this positive influence of higher air
volume per unit membrane area is no longer
observed. The designed air scouring range for the
membranes was from 0.42-0.6 Nm3
/h/m2
–
typical for a double-deck Toray system.
Initially, the MBR control system had been
programmed so as to reduce the energy
consumption related to air scouring according to
a flux/TMP related algorithm; that is, employing
the lowest air scouring rate possible as to
maintain a flux below the critical flux value. This
type of control assumed a steady sludge
concentration within the design limits (12-15 g/l),
as opposed to the low solids concentration
operation required at the start-up phase. For this
reason, it was decided to re-program the SCADA
control software so that it would allow for an
additional Operational Mode with Low MLSS,
where the highest air scouring rate could be
employed when [MLSS] in the MBR tanks was
below a critical value of 8 g/l.
Also, during low or no inflow periods to the
plant, the MBR would allow for intermittent
aeration every 30 minutes to maintain aerobic
conditions, avoiding sludge settling at the bottom
of the tanks and between the membrane plates.
When not in operation, all membrane lanes were
aerated for 2.5 minutes every 25 minutes. During
low inflow periods, the eight MBR lanes were
operated alternatively in pairs.
Finally, a submersible mixer was installed in
each MBR lane not fitted with MBR modules
(until second commissioning phase) to keep
biomass in suspension, while a minimum 200%
recirculation was maintained.
3.4. Polymer dosing
To minimise potential fouling at initial low
MLSS, it was also decided to dose a synthetic
cationic polymer, (MPE 50, Nalco) designed to
suppress membrane fouling by colloids. This
product has shown good results in increasing
critical flux at other existing MBRs worldwide at
low temperatures, as per the MBR consultants’
experience, and also suppresses foaming. It was
not possible at the time of commissioning to fully
evaluate the beneficial effects of the addition of
this polymer, since there was a simultaneous
temperature increase in the sludge at the time of
dosing (see graph). The dosage of MPE 50 was
initiated at 4 g/l MLSS, and was stopped shortly
before reaching 9 g/l. The use of this product

6. 6 Experiences from the commissioning of the Arenales del Sol WWTP
was considered, however, a viable option to
reduce fouling - or enhance flux - and it was
subsequently included in the plant's O&M
guidelines as an emergency strategy to improve
plant's overall performance during storm events,
load changes or significant temperature
decrease.
Fig. 3- A) Flux Vs TMP during the start-up
3.5. Foaming
Foaming is a common issue in
biotreatment processes and there are a large
number of management strategies, yet these are
not always put into practice. Historically, there
have been many MBR plants installed without
foaming removal or control systems in their
designs which subsequently experienced
problems during foaming events, with foam
sometimes overflowing into the biological tanks.
The occurrence of foam at start-up and
operational stages differs both in its origin and
also in its magnitude. The release of the
hydrophilic agent the membranes are coated
with for preservation during storage may cause a
very light and clear foam, which disappears
within hours once filtration conditions start.
Another type of foaming that can happen at the
same time is of biological origin and can arise
from low temperatures, insufficient organic
loads, a non-acclimatized biomass or excessive
chemical cleanings. Foaming may be
exacerbated when the biological process in the
aerobic/anoxic tanks and the MBR is interrupted
due to the typical PID and equipment
adjustments that take place during start-up. As a
result, extensive inactivation of microorganisms
will lead to foaming due to proteinaceous DNA
release from dead cells. Finally, foaming
occurrence during normal and stable operation of
MBRs is also common due to the long SRTs, yet
this foam type tends to be naturally controlled at
large municipal plants as long as foam is not
"trapped" in the reactor.
At Arenales del Sol, foaming issues were
taken in consideration when designing the
biological tanks, ensuring that there was always
an overflow from anoxic tanks all through to the
MBR chambers and into the recirculation
chamber. Any foaming event eventually ends up

7. 7 Experiences from the commissioning of the Arenales del Sol WWTP
at the MBR lanes, where an overflow weir direct
all recirculating sludge (and foam) into a channel
fitted with a simple foam removal system that
operates with two manually operated gates. All
foam removed is accumulated in a sump fitted
with submersible pumps, which can then be sent
to the sludge dewatering system, or back into the
recirculation pipeline onto the anoxic tanks.
4. CONCLUSIONS
1. The Arenales del Sol WWTP was, at the
time of commissioning, under some of
the typical pressures often found in
municipal WWTPs in coastal regions.
There was an urgency to have the plant
fully operational at certain dates.There
was no room for errors in either the
water quality to be provided, nor the
chance to rely on the existing WWTP for
treatment, as it was being
decommissioned.
2. The previous MBR knowledge of the
Contractor, MBR system integrators and
technical assistance, ensured a rapid
commissioning with minimal deviations
from the planned schedule over a period
of 8 weeks. The whole WWTP was
designed with a great amount of
redundant equipment, and fully
orientated towards MBR's best practices.
3. The WEST modelling software was
helpful in forecasting potential biological
process pitfalls, even prior to the plant
attaining design conditions. It aided
greatly in the planning of sludge seeding
and biomass acclimatation. It also
provided useful strategies for operation,
as the software was also used to simulate
different inflow scenarios throughout the
year.
4. Finally, a great deal of care and planning
was put into the commissioning and start
up phases of the project. Various
preventive fouling control techniques
were enforced, with the sole aim of
reducing plant downtime for
maintenance cleaning. This provided full
flow treatment capability from the very
beginning, enabling smooth handover to
the end user.
ACKNOWLEDGEMENTS
The authors are grateful to Professor Simon Judd
& Claire Judd for the opportunity to publish this
article on their renowned MBR-focused web page
(www.thembrsite.com).
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8. 8 Experiences from the commissioning of the Arenales del Sol WWTP
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