Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
6.11 Case studies (Topic k)
6.11.1 Case study 1: real time control of urban drainage and sewerage system
in Bolton, UK*
The existing Bolton Town Centre sewer system now serves a
population of 90 000 in an area of some 4 500 Ha, or approximately one third of
the total Bolton population. The network is comprised largely
of brick sewers which were constructed
between 1870 and 1930 to take both foul and surface water. The Croal
Valley/Middlebrook trunk sewer was initially constructed in the 1930s to
intercept the direct discharges and thus reduce pollution of the Middlebrook
and River Croal. However to minimize the size and cost of this sewer numerous
overflows were retained to restrict the flows passed to the newly constructed
sewer. Following development of Bolton as a town with a population rising to
250 000, river pollution of the Croal caused by the 42 crude overflows has
gradually worsened and condition have deteriorated to an unacceptable level. In
a study undertaken in 1987, possible solution were identified to rationalize
the number of overflows in the catchment
and significantly reduce pollution.
Two off-line retention tanks with a capacity of
respectively 9000 and 2000 cu. m were built and completed in 1992. Two more
tanks were commenced in 1992, one being a 1250 cu. m on-line oversized gravity
sewer, the other a 10 000 cu. m off-line sump style tank from which a pump
returns the sewage on cessation of the storm. This construction programme has
been designed to alleviate flooding and pollution. The next step was to
rationalize the operation of the scheme so that:
- the flow will be accommodated in the downstream
- no downstream flooding will occur;
- no spillage takes place at downstream overflows;
- the receiving treatment plants will be able to
handle the additional load;
- and sewage will not stand in any of the tank for more than 24 hours,
taking into account that no sewage can be passed from tank to tank.
The overall objective
of the scheme was to maximize the use
of the storage facilities during storm events to eliminate flooding, minimize
pollution to local rivers and to optimize the use of the treatment works.
The system for
controlling the operation involves the adaptation of existing telemetry
equipment collecting data on a daily basis from ten raingauges located around
Bolton together with sewers depth monitors, major overflows and treatment
works. This links to a second system providing "alarm data" required
for reactive maintenance of small pumping stations and to monitors installed
above-mentioned tanks. Other monitors cover remote pinch points on the trunk
sewer. The system collate all data at a single System Control And Data
Acquisition (SCADA) Masterstation based at the receiving treatment works. The
control computer interfaces with the computer to extract relevant data for
simulation. Simulation and optimization of the system in real time is
undertaken using MOUSE ON-LINE and control decisions sent back to the SCADA to
enable activation on site.
MOUSE ON-LINE is based
on a modular design with two blackboards, an external to exchange information
with SCADA and an internal for the exchange of information between the modules
constituting MOUSE ON-LINE. The main modules are:
- The rain forecasting module. Based on on-line
rain measurements and a description of a growth and decay time profile for the
rain, a rain forecast is computed.
- The runoff forecasting module. Based on the rain
forecast and a description of the
network topography, a forecast of the sewer load is computed.
- The control module. Based on the forecasted sewer load,
a control strategy is selected and the corresponding control actions set out.
The system works
in three modes, viz. monitoring, forecasting and controlling. It is based on
close monitoring of the performance of the tanks, treatment works and network
overflows and will provided archive data of the implications of various storms
and control actions. A set of criteria specify when the mode is changed. The
change from monitoring to forecasting is needed when the current situation in
the sewer indicates that information about the expected situation is necessary.
This could be the case when the intensity of the rain or the inflow to the
treatment plant or levels in overflow structures exceed biological capacity ;
in this case, the system changes back to monitoring.
* Excerpted from: Sharman , B.J. & Tidswell, R.G., 1993
6.11.2 Case study 2: optimization of nutrient removal in the wastewater
treatment plant Zürich-Werdhölzli, Switzerland*
The plant at Werdhölzli is the largest treatment plant in Switzerland. It
handles wastewater from about 500 000 pe. A second plant receiving wastewater
from the city of Zürich is located at Zürich-Glatt and handles about
100 000 pe. The plant at Glatt will cease operation by the year 2001 and the
wastewater it handles now will be transferred to Werdhölzli by tunnel and
the plant will be upgraded. The total volume of wastewater was 225 300 m3
per day in winter of 1997 and the total chemical oxygen demand of the primary
effluent was 53 500 kg per day, and, respectively, the total Kjeldahl–load
6 410 (of which 1050 was digester supernatant), total nitrogen 4 700 and phosphorous
895 kg per day. It should be noted that the COD/N ratio shifted from 7.5 to
8.5 in the last 10 years which allows a significant improvement of the denitrification
capacity. Since phosphate was banned from detergents in 1986, the phosphate
load did not change significantly during that period and the slight increase
of the P-load might be due to polyphosphates used in dishwashers. Simultaneously,
the COD-load increased substantially due to increase discharges of industrial
wastewater, enzymes used in households and grease removal tanks.
In 1986, Werdhölzli
was enlarged for permanent nitrification
with an average total nitrogen concentration in effluent of less the
2000 mg/m3 at 10 °C. The initial activated sludge system operated as
a partial two stage treatment with a fully aerated pre-step and consists of two
lanes each having six parallel activated sludge tanks (5000 m3 each)
and six secondary clarifiers (6000 m3 each).
Combining the flows
received at the Werdhölzli and Glatt plants and, at the same time reaching high
levels of nutrient removal required careful studies and the optimization of
process and structural design and operation of the plant at Werdhölzli
Hydraulic capacity: During the 1980s and 90s, the amount of
infiltration was substantially reduced. The dry weather peak flow of both
plants is now about 3 cbm per second which allows a substantial reduction of
storm peak flow from 9 to 6, taking into account the 40 000 m3
stormwater tank located before the plant at Werdhölzli
which reduces the overflow of untreated
stormwater to less than 0.5% of the total annual wastewater flow.
The existing aeration tanks can, thus handle
the inflow from Glatt, even with the provision of anoxic zones (see below) and
if the concentration of activated sludge is increased by some 50%. Reserve
capacity is still 10%.
Installation of anoxic zones and reduction of oxygen input
during primary treatment:
During a pilot operation, anoxic zones of 28 vol% were installed
in two parallel aeration tanks in one of the lanes at Werdhölzli, dived into
two compartments with a volume of 700 m3. This reduced the nitrogen
concentration in the secondary effluent by about 40% to an average of 10 g
nitrate-nitrogen per cbm. Simultaneously, the energy for oxygenation was
reduced by about 15%. Average effluent alkalinity increased by about 0.6 mM to
above 3 mM which reduces corrosion of cement and improves nitrification. In the
light of these positive results, anoxic zones were also installed in the
remaining lanes. Additional measures to improve denitrification included:
- Improved sludge blanket by a reduction of
scraper speed in the secondary clarifiers by 25 to 50%.
- Reduction of oxygen input and degradation of
readily degradable COD during primary treatment by reducing air flow in the
grit removal tank, conducting excess sludge directly into the primary
clarifiers, and reducing weir height of the primary clarifiers.
- Reduction of oxygen input in anoxic zones by
more frequent controls of return sludge pumps.
Digester supernatant treatment:
A pilot study is under way to treat digester
supernatant with anaerobic ammonium oxidation. In the process, half of the
ammonium will be oxidized to nitrite and the remaining ammonium with the
nitrite in the anoxic reactor. No organic carbon is required for
denitrification; the costs of chemical
and energy will be lower than for conventional nitrification and
denitrification of the supernatant. If the pilot study is successful, the
overall nitrogen removal of the plant will be 75%.
Internal recirculation: Recirculation
from the last section of the aeration
tanks to the first or second compartments of the anoxic zones is also under
study with several objectives in mind:
- To cope with occasionally high concentrations of
COD and resulting partially anaerobic conditions in the anoxic zones.
- To supply sufficient nitrate to the anoxic zones when concentrated
organic industrial wastewater is to be handled by the plant.
- To enhance phosphorous removal.
Enhanced biological phosphorous removal: If the treatment of
digester supernatant can be successfully installed, the first anoxic
compartment could be kept in anaerobic conditions which would enhance
biological removal of phosphorous.
* Excerpted from: Siegrist et al., 1999
6.11.3 Case study 3: the reuse of treated effluent in Spain*
Spain is one of the
European countries where water resources are scarce, especially along the
Mediterranean cost South of Barcelona and on the Balearan and Canaries islands.
Direct and planned reuse of effluent is considered a valid option for
augmenting the natural resource though a number of constraints have limited the
extent of reuse in the past, among them the degree of treatment of the
wastewater and the cost of water conveyance from STPs to the point of use. On
the other hand, the following benefits of the reuse of effluent are
- Treated effluent is a valid incremental
resource, especially where without reuse, it is discharged into the sea.
- Treated effluent is a valid source for reuse
within human settlements where strict criteria will not apply such as those for
the quality of drinking water.
- Reduction of water pollution.
- Reduction of energy consumption when reuse is
intended in the vicinity of STPs.
- Recovery of nutrient contained in treated effluent.
- Reuse as a major water resource in arid zones, such as on the Balearan and Canaries
In this context, it is
recognized that the amount of treated effluent will increase considerably when
the implementation of EU directive 91/271 concerning the treatment of urban
wastewater is achieved in 2005 (Section 6.3). At that time, a volume of more
than 350 000 m3 will be produced annually of which perhaps one third
may be feasibly available for reuse by the year 2012. The major consumer will
always be agriculture but other uses are also potentially valid, i.e.:
- Municipal use, e.g. for the irrigation of parks,
fire fighting, street cleansing.
- Recreational use, e.g.: irrigation of golf
courses, and artificial lakes.
- Recharge of aquifers, combating seawater
intrusion and other "ecological" issuses.
- Industrial use, e.g.: flushing and cleaning of
materials or use as cooling water.
The need to assure a
safe water quality is fully understood and is a major factor in the planning of
the reuse of effluent. Strict quality criteria are still needed though it is recognized
that in any case, wastewater treatment will involve tertiary technology,
especially filtration, microfiltration, physico-chemical
processes, disinfection, and/or desalination
whenever exposure of people to the treated effluent is possible.
El Cedex, an
independent research and development organization under the Ministries of
Education, and Environment, has established a data base on the reuse of
effluent containing information on the schemes for reuse, the volume of
effluent used, application of the effluent, and wastewater treatment provided.
There are currently 124 schemes in operation which use a volume of 2 320 000 m3
treated effluent annually. The data base of El Cedex provides detailed
information on 41 of the schemes which use 2 080 800 m3 annually or
89% of the total volume reused. Agriculture is the main user with 88.7%. The
total volume of effluent is used as follows:
* Excerpted from: Catalinas & Ortega, 1998
Table 6.26: Reuse of treated effluent in Spain
|Type of reuse
||Volume 1000 m3/year
|Source: Catalinas & Ortega, 19XX
The 41 schemes referred to above vary considerably:
- 25 schemes use effluent solely for agricultural
purposes. For all of these, the wastewater is treated by the activated sludge
process. It should be noted, however, that tertiary treatment of wastewater is
now considered essential prior to the reuse of effluent for the irrigation of
certain crops (see below).
- 8 schemes serve agriculture and golfing. Only
three of these treat wastewater by activated sludge followed by tertiary
treatment by filtration and chlorination or ozonization while the three
remaining use activated sludge.
- 4 schemes serve municipal purposes and in a few
cases, are combined with agricultural use. All of these apply tertiary
treatment in addition to activated sludge.
- Of the remaining 4 schemes, 3 use the effluent
for "ecological" purposes and one for cooling. They apply the activated
The above information
confirms that in the sample of 41 schemes, tertiary treatment of the wastewater
is practiced whenever people may have contact with the treated effluent.
For the future, however, tertiary treatment is also considered essential
as follows although legislation is not yet in place:
- Tertiary treatment plus disinfection for
irrigation of ornamental plants in recreational areas with potential contact by
people and products eaten raw. In these cases, faecal coliform (MPN) less than
10/100ml and residual chlorine higher than 0.6 and 0.5 mg/l, respectively
(after 30 minutes).
- Secondary treatment plus disinfection for the
irrigation of cereals, fruit trees etc. with faecal
coliform less than 500/100ml and 200/100 ml, and residual
chlorine higher than 0.1 and 0.3 mg/l, respectively.
- Tertiary or secondary treatment prior to the
reuse for recreational areas with/without contact of the public with the
effluent, and faecal coliform less than 200/100ml and 10000/100ml, respectively.
- For cooling, faecal coliform less than 200/100ml
and 10000/100ml for, respectively, closed and open systems.
The lack of
legislation concerning quality standards for the reuse of effluent is a major
problem which constrains the planning of additional schemes for reuse. It has
been proposed therefore, the European Union establish a Directive. Other
constraining factors include the following:
- The absence of a comprehensive water resources
plan with a clear indication of priority areas where the reuse of effluent
would be promoted.
- The logistical and financial problems associated
with the construction of many STPs with tertiary treatment which would be
Nevertheless, 8 new schemes are under
construction and for 7 more, tendering is under way. At least 20 more schemes
are being planned. Some of those under construction are big schemes with an
annual volume of effluent reused of up
to 150 000 m3 per year and most are for agricultural irrigation. The
scheme under construction for Madrid will reuse 90 000 m3 per year
for municipal greenery. Many of the smaller schemes will reuse effluent for
parks and golfings.