CZECH TECHNICAL UNIVERSITY
IN PRAGUE
FACULTY OF MECHANICAL ENGINEERING
DEPARTMENT OF PROCESS ENGINEERING
Waste Water Treatment – Lisbon (Frielas)
Dr. Ing. Pavel Hoffman
Department of Process Engineering
Faculty of Mechanical Engineering
Czech Technical University in Prague
Alexandre Rio
1
Index
1. Introduction .............................................................................................................. 3
2. GENERAL DESCRIPTION OF THE WASTE WATER TREATMENT
IN FRIELAS ...................................................................................................................... 4
3. Pre-Treatment.......................................................................................................... 6
3.1 Reception of the Affluent Waste Waters by pumping: ...................... 7
3.1.1 First Stage of Initial Elevation: ........................................................... 7
3.1.2 Second Stage of Initial Elevation: ..................................................... 7
3.2 Mean Bar-Screen: ............................................................................................ 7
3.3 Fine bar-screen: ............................................................................................... 7
3.4 Water-Flow measure of the screened waters: ..................................... 8
3.5 Traps for sand and Degreasing: ................................................................ 8
4. Primary Treatment ................................................................................................ 8
4.1 Primary Decantation: ......................................................................................... 8
5. Secondary Treatment .......................................................................................... 9
5.1 Intermediate Elevation: ................................................................................ 9
5.2 Homogenization and Equalization Tank: ................................................ 9
5.3 Biological Treatment – Activated Sludge: .............................................. 9
6. Control Treatment ............................................................................................... 10
6.1 Final Elevation: ............................................................................................... 10
6.2 Bio-Filtration: .................................................................................................. 10
6.3 Final Fine Water-flow Measurement: ..................................................... 10
6.4 UV Treatment: ................................................................................................ 11
7. Sludge Treatment ................................................................................................ 11
7.1 Thickening: ....................................................................................................... 11
7.2 Conditioning by hydrated whitewash addition:.................................. 11
7.3 Anaerobic Digestion: .................................................................................... 11
7.4 Sludge dewatering: ....................................................................................... 12
8. Biogas Valorisation ............................................................................................. 12
9. Dewatering Sludge Project ............................................................................ 12
10. Conclusion ............................................................................................................. 13
11. Terminology.......................................................................................................... 14
2
1.Introduction
Lisbon - also known as the city of the seven hills, has always been highly
dependent on the River Tagus and its estuary. Its foundation, growth and
history are intimately linked with the Tagus. Much of what characterizes and
distinguishes Lisbon in Europe and throughout the world is linked to this
special geographical position and to the privileged natural conditions which it
enjoys on the largest estuary in Europe (320 km2 and 50 km long). However,
the environment and ecosystem of this beautiful and enormous estuary have
been degraded in recent decades as a result of urban and industrial expansion.
Today, almost one-fourth of Portugal's population (almost 2 million
inhabitants) live and work in Lisbon and a variety of industrial complexes - the
chemicals industry, the oil industry and ship-building and repairs - are based
on the banks of the Tagus estuary, creating sources of pollution and
environmental degradation for a significant part of it. The only reason that the
level of pollution in the Tagus and its estuary has not reached even more
worrying levels is because of the capacity of its tide to cleanse itself and
regenerate the enormous volume of water. It is calculated that twice a day six
billion cubic metres of water enter the Tagus estuary with the tides, thus
reoxygenating the water and reducing, through dilution and other chemical
changes, some of the effects of existing industrial and domestic pollution.
The importance of Wastewater treatment stations is irrevocable to the
maintenance of the Environment’s Health. Next it will be describe the
functioning of one Water Treatment Station located near of Lisbon: Frielas.
3
2.GENERAL DESCRIPTION OF THE WASTE
WATER TREATMENT IN FRIELAS
The Waste Water Treatment Station in Frielas, created in 1999, receives
the domestics and industrials effluents of a large geographic area that cover
part of Lisboa, Loures, Odivelas, Amadora, and, in the near future, also Mafra
and Sintra.
The ETAR of Frielas daily receives about
40.000m3 of water, what it gives an average
of 2 million litres/hour = 0,56 m3/s. This
represents the total of 350 000 hab.eq
(water flow) or 700 000 hab.eq (BOD5).
The construction of this station had in mind
the future increase in three phases:

1st Phase: year 2001, to 700.000 hab.eq

2nd Phase: year 2011, to 900.000 hab.eq

3rd Phase: year 2021, to 1.050.000
hab.eq
The
general
architecture
after
the
equalization has three independents lines of
treatment.
The expected water flows for the three
different phases are: (1)
Table 1 - Water flow Inlet Values
(1) - The translation to English is on the last page of this work, on the Chapter 11.
4
And for the inlet pollutants content: (2)
Table 2 – Inlet Pollutants content
(2) – Again, the translation is on the page 14 o f this work.
5
To aim the goals for the quality of the final effluent the station is divided
in 3 main ways for the liquid line:
1.
2.
3.
4.
Pre-Treatment
Primary Treatment
Secondary Treatment
Control Treatment
For the treated effluents, the preview characteristics are shown in the
next tables. The values are considered like MVA (Maximum Values Admitted).
Table 3 – Effluent quality after the bio-filtration
Table 4 – Quality for the effluent after the UV disinfection
To obtain these values the station has the following treatment:
3.Pre-Treatment

Receptions of the sewage in the gravity way:
The waste water arrives in the station by four collectors:

Frielas Collector with Ø 400mm (at the station entrance it turns in
200 mm) and the Industrial Park collector

Rio da Costa collector with Ø 1500mm and P collector ovoid with 600
x 900 mm (they enter on the 2nd stage of the initial elevation).
In both stages of initial elevation, the effluents are unloaded in an out-let drain
on the base of the screws to limit the effluents velocity.
6
3.1 Reception of the Affluent Waste Waters by pumping:
By pumping, arrives to the station the affluent from the elevation station 3 (EE3) that
are unloaded on the repartition channel of the bar screen.
3.1.1 First Stage of Initial Elevation:
The elevation is made by Archimedean screws (2 + 1 of reserve), on the
out-side and with a security covering to limit the smells release.
3.1.2 Second Stage of Initial Elevation:
The elevation is made by Archimedean screws (3 + 1 of reserve), on the
out-side and with a security covering to limit the smells release.
3.2 Mean Bar-Screen:
To the repartition channel of the bar-screen arrive a huge water-flow by
the Archimedean screws from the primary elevation and the water-flow from
the EE3.
The water-flow is divided by four channels, going by a first stage of vertical
bares with automatic clean and 40mm between each bare.
The solids residues recuperation is made by transportable rug. They put the
residues in containers.
3.3 Fine bar-screen:
The water-flow goes to a second stage with vertical bars (10mm between
each bare) and also with automatic clean.
The solid waste is recuperated by a second unity of transportable rug which
routed the solid waste into the containers.
7
3.4 Water-Flow measure of the screened waters:
After the bar-screen the effluent flow rate is measured by a ultrasonic
probe installed in a Parsha II channel.
3.5 Traps for sand and Degreasing:
The sand, oils and graisse removal is made in the same combined
equipment, with three treatment tanks. The oils and graisses are removed by
air injection in shrouded of membranes, allowing the flotation.
After that, they are scraped in the surface and routed to a boiler, downstream
the tanks, where are through, by water injection, until the graisse well.
After they can be routed to a flotation separator or to a graisse tank if the
separator is damaged.
The sands are deposited in the bottom by gravity where are removed by “sand
removal pumps” to “sand classifiers” witch deposited them in containers.
4. Primary Treatment
4.1 Primary Decantation:
The effluents are submitted to a lamellar primary decantation in
rectangular tanks, with the possibility of the reagents introduction (coagulants
and flocculants).
The system is divided in 4 lamellar decantation lines, equals between them:
 One tank of fast mixture to coagulant addition
 One tank of fast mixture to pH correction by the milk of lime
addition
 One tank of slow mixture to flocculant addition
 Lamellar sedimentation tank in the final of each line
8
After the decantation the effluents proceed to the intermediate elevation by
one pipe where the water flow is measured. The settled sludge is took by
pumps of eccentric rotor type and routed to the thickeners.
5. Secondary Treatment
5.1 Intermediate Elevation:
Upstream of the equalization tanks there is the intermediate elevation,
made by 4 Archimedean screws (3 + 1 reserve), witch allows elevate the total
water-flow for the equalization tanks.
5.2 Homogenization and Equalization Tank:
The three equalization tanks have the total volume = 16 465 m3 and
they communicate between them by the surface unloader and by a motorized
wall valve located near from the each tank floor. They limit the affluent waterflow to the biological treatment (4.650 m3/h), and they also regulate the
effluent contents.
To preserve the solid material waste in suspension, and also to homogenize
and refresh the residual waters each tank is equipped with agitation and
aeration systems.
5.3 Biological Treatment – Activated Sludge:
This treatment has the followings equipments:

Six aeration tanks with unitary volume = 4000m3 where, with the
presence of O2, it will occurs the degradation of the organic matter by
aerobic microorganisms

Six pairs of secondary settlers witch allows separate the treated water of
the activated sludge – in a way to re-use the biomassa, depuration base.

Sludge Recirculation, allowing another route through the aeration tanks in
activated sludge. Allows also the maintenance of an environment where
9
the relation between the BOD affluent and the activated sludge mass
corresponds to the mass content preview to be functional.

Excess Biological Sludge extraction to the flotation
6. Control Treatment
6.1 Final Elevation:
Allows elevate the effluent from the secondary treatment to the biofiltration by 3 centrifugal pumps (2 + 1 reserve), installed in pipes.
6.2 Bio-Filtration:
At the end of secondary settler, to obtain the demanded quality (BOD and
TDS), the effluents are submitted to a bio-filtration treatment.
This treatment phase occurs in BIOSTYR filters, filled with a floater material
composed by polystyrene balls denominated BIOSTYRENE, colonized by a
depurated biomass. The secondary effluent inlet in the filters is made by an
ascendant flow, accompanied with air. This air is delivery in co-current by
perforated distributors net, located in the bottom of the filter. The
BIOSTYRENE are kept in the filter by a concrete flag-stone cover equipped
with mole cricket.
In a single equipment occurs also the elimination of the soluble pollution and
the effluent clarification by the filtration through the biomass waterbed. This is
the Bioreactor of piston type, to the liquid and gaseous phase.
6.3 Final Fine Water-flow Measurement:
In the pipes, that connect the treated effluent from the bio-filtration to
the UV disinfection, is installed one electromagnetic water flow meter.
10
6.4 UV Treatment:
Before the effluent rejection on the reception environment, to guarantee
the biological fixed quality (200 E. coli. / 100 ml), the effluent is disinfected by
UV radiation.
The system is composed by 3 parallel channels, equals between them, each
one with 3 benches of 23 modules with 8 lamps each, placed paralleled to the
flow. It means 552 lamps by channel and a total of 1656.
The treated effluent is redirected to “Real Ditch” that is connected to the river
bank (“Ribeira da Póvoa”) by a tide gate system that allows the isolation of the
ditch and the effluent discharge even if the surface-liquid level of the river
bank is higher.
7.Sludge Treatment
7.1 Thickening:

The primary sludges are thickened by gravity in two circular thickeners
with 20m of diameter.

The excess biological sludges are thickened by flotation (with or without
addition of polyelectrolyte), in two circular recipients with 11m of diameter.
7.2 Conditioning by hydrated whitewash addition:
After thickening, the sludges (primary and biological) are mixed in a
mixed sludges tank where they suffer conditioning by hydrated whitewash
(lime milk) addition.
7.3 Anaerobic Digestion:
After conditioning, the mixed sludges are submitted to a stabilization by
mesophilic anaerobic digestion, in 4 digesters of 400m3, mechanically agitated.
11
7.4 Sludge dewatering:
Sludge, in the digestion phase, must be kept in a constant temperature
(35 ≈ 39ºC), that’s why is necessary to warm them. In this way, the mixed
sludge at the digestion entrance is pre-heated in a heat exchanger, by the
digested sludge that goes out to the dewatering. After that, the sludge stay in
a permanent recirculation, passing through heat exchangers where are heated
by hot water, to compensate the thermal lost of the digesters.
There are 2 boilers, using the produced biogas as fuel to the water heating that
will circulate in the referred heat exchangers.
8.Biogas Valorisation
The produced biogas during the digestion is treated to the impurities
removal and deposited in two gasholders of 1050m3 each and valorised by
combustion in 2 cogeneration engines. These engines can work with biogas
and with fuel and they convert part of energy from the biogas combustion in
electric energy.
9.Dewatering Sludge Project
In association with Engineering University of Lisbon (Instituto Superior
Técnico), the Waste Water Treatment Station in Frielas realized one important
study of sludge dewatering:
“Dewatering process of mixed primary and waste activated, anaerobically
digested, sludge using an advanced membrane filter press system”
The experimental work was conducted at the pilot-plant from the “Centro de
Processos Químicos da UTL” using an integrated filtering and dewatering
system.
The global dewatering cycle involved an initial filtration step, followed by the
membrane squeezing using hot-water and a final step of vacuum drying.
12
The results showed that it is possible to achieve competitive values of final
humidity when comparing to other more common technologies such as beltfilter presses and centrifuges.
The final humidity of the filtration cakes ranged from 47% to 76% comparing
to the maximum expectable value at Frielas of 74% (using centrifuges).
To obtain these results, diatomite was used as precoat and, in some cases, as
body feed. The same cationic floculant used for dewatering with centrifuges at
Frielas (ZETAG 51) was used at various concentrations as body feed.
These results show that is possibly not only reduce the humidity rates in the
sludge but also decrease the volume, the unpleasant smells and increase the
biological stability.
Figure 1 –Advanced membrane filter press system
10.Conclusion
The effluent treatment scheme is almost the same in all WWTP, at least in
the liquid line and in the first stages of sludge treatment. The problem is the
final destiny of the sludge. The direct application on the soil requires one
exigent quality control to avoid contamination, the deposition in sanitary land-
13
fills is too expensive, and last possibility is the incineration, which also requires
attention with the gaseous emissions.
In all those possibilities, it’s better to have the less water content possible in
the sludge to decrease de volume, weight and also the price with the
transportation or to facility the combustion in the incinerators.
That’s why is so important to invest in systems more efficient in the sludge
dewatering.
11.Terminology
Caudal = Water Flow
Caudal médio diário = Diary Average Water Flow
Caudal de ponta = Instantaneous Water Flow
CBO5 = BOD5 (biochemical oxygen demand during a 5 days period at 20ºC
(mg O2/l))
CQO = COD (chemical oxygen demand (mg O2/l))
Coliformes fecais = Faecal Coliforms
N-NTK – Total Kjeldahl Azote
P Total = Total Phosphorus
Tempo Pluvioso = Wet Weather
Tempo Seco = Dry Weather
SST = TUS (total undissolvent solids (g/l))
SSV – TVSS (total volatile suspended solids (g/l))
14