Urban Storm Water Practice in Germany Hansjrg Brombach UFT
Umwelt- und Fluid-Technik Dr. H. Brombach GmbH, Steinstr. 7, 97980
Bad Mergentheim, Germany; [email protected] Abstract The paper
reflects the overall European history of urban storm water
practice, with a specific focus on Germany. The European and German
Water Policy is described in brief. By now, there are 31,000
CSO-tanks in operation in Germany. They represent a total storage
of over 33 billion m3 or 400 litres per German citizen. About
another 20,000 old CSOs shall be rehabilitated in the next 20
years. The investment into CSO-control in the past 25 years was
about 400 (Euros; European Dollars) per capita. Seven different
layouts of storm water tanks with sizes ranging from 50 to 17,600
m3 are shown in detail and described concisely. The paper closes
with a short discussion of latest trends in technical equipment to
reduce the impacts of CSOs. Historical Look Back Urban storm water
management has a long history in Europe. The trunk sewers in Rome,
Italy, called Cloaca Maxima, were a size that ships could pass
through. The first systematic sewer flushing action in Rome was
arranged and paid by Agrippa (64 B.C. 12 A.C.) in 32 B.C. Some
parts of the Cloaca Maxima are today still in operation. In the
dark Middle Ages, the technical know-how of Mediterranean and
Oriental urban drainage was lost. During the industrial revolution
urban drainage management got a fresh start in England in the 19th
century. It was a type of a wild mixed system that just carried the
sewage out of the city in the shortest distance to a receiving
water. In Leamington the first real separate system was implemented
in 1870. At the end of the century British design rules spread to
Continental Europe. The famous English engineer William Lindley
(1808 1900) planned the general layout of the sewer systems for
Frankfort and many other German cities, but also for Ble, Prague
and Warsaw and even for Sydney, Australia. Other great European
names in sanitary engineering included William Phillips Dunbar
(1863 1922), Robert Koch (1843 1910), Max von Pettenkofer (1818
1901), Karl Imhoff (1876 1965) and others, see ATV (1998).
Following the British roots of sewer planning, by 1914 very nearly
all middle sized to large cities in Germany got combined sewer
systems. Small towns, which are most common in Germany, only got
adequate collecting systems much later often systems of the
separate type. In order not to surcharge the mechanical treatment
plants of these early works at the end of the collecting network,
combined sewer overflow (CSO) devices were found to be necessary to
allow large inflows to be spilled directly into the receiving
waters. At that time, it was assumed that the spilled water was
sufficiently diluted. Soon, however, it was discovered that the
spilled water carried a large pollutant load. This load could be
significantly reduced by providing some extra storage volume at the
overflows and/or to allow to retain some runoff volume and to
settle heavy sediments before overflowing into the receiving
waters. The first Rainwater Treatment Plant within a combined sewer
in Germany was built in 1913 and reported from Engberding (1915),
see Figure 1. Engberding died in World War I and his pa1
per was published posthumously. As a result of the World-War I
and II, the technological lead of Germany in continental Europe in
sewerage was more or less lost. However, the German
Wirtschaftswunder triggered a fresh start in the 1970s.
Figure 1. Engberdings first Combined-Sewer-Overflow Tank.
Off-line arrangement with clarifier overflow. The facility went
into operation 1913. The new European Water Policy Until about the
year 2000, each European country looked for national solutions for
river pollution control separately. An exception is the region of
Lake Constance. The lake is Germanys most important fresh water
resource and supplies more than 10 million people. The Rhine is the
stream feeding Lake Constance, see also Figure 2. In the 1970s, the
lake showed an alarming increase in phosphorous concentrations,
Michelbach et al (1999). The three surrounding nations/countries
Germany, Switzerland and Austria founded the International
Commission for the Protection of Lake Constance and set up the
first mutual guideline for the design of combined sewer overflow
tanks in 1973. Together with improved wastewater treatment plants
and the prohibition of phosphorous in detergents it took about 10
years to stop the increase in concentration at 80 g/l. Today we are
back to the level of the 1960s, with less than 20 g/l, and we hear
complaints from fishermen similar to the situation at the Great
Lakes in the USA and Canada of reduced fish stocks due to a
decrease in Lake productivity. Since 2000 a new framework in the
field of water policy of the European Community (EC), is in force:
directive 2000/60/EC (2000). There are remarkable parallelisms to
the USClean-Water-Act, but 28 years later! Water is declared to be
not a commercial product like any other but, rather, a heritage
which must be protected, defended and treated as such. Here are
some quotes from the directive 2000/60/EC: Member States should aim
to achieve the objective of at least good water status by defining
and implementing the necessary measures within integrated programs,
taking into account existing Community requirements. Where good
water status already exists, it should be maintained ( 26, intro).
With regard to pollution prevention and control, community water
policy should be based on a combined approach (emission plus source
control) using control of pollu-
2
tion at source through setting of emission limit values and of
environmental standards ( 40 intro). Pollution through the
discharge, emission or loss of priority hazardous substances must
cease or be phased out ( 43 intro).
The objective of the plan is to get to similar levels of water
protection all over Europe. The EC-directive is in force in 12
European states since October 2000. Action plans set fixed and
rigid deadlines for all members. The programmes of measures shall
be established latest in the year 2009 and all measures shall be
operational latest 2012. The programmes of measures shall be
reviewed, and if necessary updated, latest 2015 (quotes from
article 11, 7 and 8 directive 2000/60/EC (2000)). Looking at the
figures for some selected European nations, see Table 1, it is
obvious, that the numbers are not homogenous at all. Exact sewer
figures are hard to get, even under the new EC-administration, so
some of them are my private estimates. Figures for connection
grades are worse in Southern and Eastern Europe and somewhat under
discretion. The data for the USA are added, but for me as a
European, some question marks are remaining, too. The larger
average density of the population in Europe is obvious in
comparison with the USA. The density in Europe is 4 to 10 times
higher then in the USA. This very different scenario should be
always taken into account, when comparing figures or policies from
both sides of the Atlantic! One parallelism, that is often not
realized, can be seen in the British and German figures in table 1.
Is that the late effect of the traditional preference of British
sewer design philosophy in Germany? Table 1. Statistical sewer
figures from selected states in Europe in comparison with the USA
USA for comparison Great Britain Denmark Germany Belgium
Country
Total area in 1000 km2 31 357 43 544 244 41 9,363 Population in
million people 10 82 5 58 58 41 281 Averaged density of population
325 230 121 106 237 371 30 in heads/km2 Connection to public sewers
in 60 93 94 80 98 92 70a % of population Connection to public owned
20 91 92 77 82 88 70a WWTP in % of population Connection to
combined sew70 63 47 75 70 85 15 erage in % of population a. About
30% of the U.S. population in rural areas is served by septic
systems, not connected to WWTPs. The EC-water framework sets a
totally new and demanding water scenario for Europe, which will
affect nearly all urban drainage systems within the next 20 years -
or even longer. The powerful lever will be source control, and the
main sources of urban drainage are the storm outlets from separate
systems, the CSOs from combined systems and all treatment works.
3
Netherlands
France
There are doubts and critiques and wonderful new ideas from all
parties involved. New candidates from Eastern Europe are knocking
at the doors of the EC, not quite realizing yet which requirements
they would have to fulfil one good day. On the other hand, some
European states are still ignoring the chance for a mutual approach
to the global water problem. Thats Europe! The technical
standardization in the field of urban collecting systems is just
beginning. In 1995, the first European standard Drain and sewers
outside buildings was released; see EN 752-1 (1995). The most
important performance requirement, which brought most city councils
in Europe into new problems, is the setting of a minimum basement
flooding risk in publicly owned collecting systems in EN 752-2
(1996). The minimum return frequency for design storms is one in 2
to 5 years for residential areas, and one in 10 years for
underground traffic structures. This corresponds to an acceptable
return period for basement flooding from 10 to 50 years. These
limits were adopted by 18 European nations now. However, this
socalled drainage comfort is not assured, neither in the entire
country of Germany, nor in the other countries. The traditional
English design storm used in Germany in the past 150 years was a 15
minute storm with a return frequency of once per year. Another
strict regulation is to be found in EN 752-4 (1997). At CSOs,
floatables and un-aesthetic pollutants have to be retained by scum
boards, screens and other measures. This is a quite new demand and
it will take decades to fulfil it. German Water Legislation with
Regard to Urban Drainage The legal principle is that European right
will overlay the existing national right step be step. Germany is a
regular member of the EC and will respect the directive 2000/60/EC
(2000) in the future. The Federal Republic of Germany is
politically organized in a way similar to the USA one good result
of World War II. The federal government sets only the general
outlines for the water policy. The actual federal water legislation
in Germany is based mainly on two policies: The Abwasserverordnung
AbwV (Waste Water Act), AbwV (2001) and the Abwasserabgabengesetz -
AbwAG (Waste water emission fees), AbwAG (2001). Any outlet from a
wastewater treatment plant (WWTP), each Combined Sewer Overflow
(CSO) and CSO-tank in combined systems, each storm outlet from a
separate system and any major industrial or private water outlet to
public receiving waters in Germany traditionally needs a
state-permit by water law. The conditions to get that permit are
within the sovereignty of the water authorities of the 16 German
Federal Countries and differ slightly, but cannot be lower than the
federal minimum requirements. There is not enough space in this
paper to quote all of the German states water regulations. The
reality is, that for historical and political reasons, thousands of
non-registered or non-permitted emergency outlets exist. The
unexpected political reunification of Germany in 1990 brought a lot
of confusion into the German water administration. It will take a
generation of engineers to fill up the omissions of the former GDR
(German Democratic Republic, East Germany). Due to the sovereignty
of the Federal Countries in water affairs, a strong federal
organization, like the US-EPA (US-Environmental Protection Agency),
does not really exist in Germany. The design and construction of
new CSOs is subsidized from the Federal Countries in various
manners. The city or community has to present a proper design of
any proposed new or rehabilitated sewage structure. The water
authority will check the design according to existing legislation
in the State and to technical standards. If everything is in due
order, the state will subsidize the investment in environmental
protection with subsidies from 20 to 80 %. This scheme provides
opportunities for very clever communities! This subsidizing
procedure is informally called the golden rain. 4
Today, the average water consumption in Germany is 130 litres
per capita and day. Every household has a water meter. The average
water supply fee is 2 , the sewer fee is another 2 per m3 water.
This is about 0.5 or about 0.5 US$ per day per person to get water
and to get rid of it. This equals the cost of one can of beer from
the supermarket per day. Sewerage Systems in Germany In combined
sewer systems, the domestic sanitary outflow and the runoff from
roofs, streets, parking lots, and industrial sewage (after
pre-treatment) are collected together in one single sewer. Separate
systems use two independent pipe systems for sewage and storm
water. Separate systems have been used frequently in areas where
the sewer design gradient is low for topographical reasons. This is
the case along all German coasts to the North Sea and to the Baltic
Sea.
Figure 2. Distribution of separate and combined sewerage systems
in Germany in percent of population served
5
For more then 150 years there has been a permanent and lively
discussion going on in Germany about the advantages and
disadvantages of the separate and combined collecting systems. The
discussion sometimes reaches sometimes excessive and ridiculous
stages. The arguments, such as efficiency, double sewer systems,
problems with sewer sediments, costs, wrong connections,
inflow/infiltration, and so on, are the same as discussed
worldwide. Ideology dominates, and usually, once the decision for
one specific system has been made, there is no comparison of cost
and benefit afterwards. Since the 1980s, more confusion arose with
the introduction of so-called modified collecting systems. For
instance, less polluted roof runoff in residential areas shall be
infiltrated directly on private ground. But even this good idea has
unexpected drawbacks. Some of that water later shows up in the
sanitary sewer as inflow; see Brombach, Weiss, Lucas (2002). Today,
the result of the 150 years of discussion and practice can be seen
in Figure 2. In the North, the separate system is dominant with 90
%. In Southern Germany, the combined system holds a top 90 %. On
the average, 63 % of the German population is served by combined
systems, Brombach (2002). Ten years earlier, the combined system
rate was still at 67 %. The separate system has re-conquered 3 % of
all Germans. The Equator of combined sewage has moved about 20 km
to the south. When reading German papers on sewer systems, please
be aware from which part of Germany the author comes from! From the
low lands or the high lands?
Figure 3. Typical combined sewage system with CSO tank
The German Philosophy of Urban Storm Water Management Since
about 1970, the use of CSO tanks has become common practice,
starting from the state Baden-Wuerttemberg (Southwest Germany)
where at that time the pollution of the Lake Constance had
developed into a severe problem (see above). Today, in Germany
around 20,000 6
CSO tanks are in operation. Together with the installation of
biological treatment plants with nutrient removal all over the
country, river quality has improved dramatically. Figure 3 shows a
typical combined sewage system. To protect the river, the pollutant
loads spilled into it from all sources must be taken into account.
During dry weather, all sewage is fed to the treatment plant where
pollutants are efficiently removed by mechanical and biological
treatment. The effluent is continuously monitored by samplers.
Averaged outflow concentrations from WWTPs in Germany range
depending upon the size and type of plant from 15 to 100 mg/litre
chemical oxygen demand (COD). The maximum threshold set by German
water law, AbwV (2001) for COD, is 75 mg/litre for larger plants
and 150 mg/litre for smaller plants. So the limiting margins are
typically well met. This is a good result and treatment plants have
surely been a good investment. During storms, the treatment plant
inflow must be limited by flow control devices at the CSO tanks.
The biological treatment processes do not allow for more than about
three times of the calculated peak dry weather flow. Moreover, the
final clarifiers must not be surcharged. During storms, much larger
volumes of water than during dry weather are entering the combined
sewer system. Pollutants are washed off from the surface into the
sewers, sewer sediments are entrained, which have settled there
during the previous dry weather periods. Combined sewage frequently
shows surprisingly high pollutant concentrations up to several
1,000 mg/litre of COD. Sediment remobilisation processes in the
sewer may be highly dynamic. A so-called first flush, a high
concentration peak at the beginning of a storm event, is probable
in some cases, especially in low gradient sewers and small
catchments. The inrushing storm water enters a CSO storage tank. As
soon as the inflow gets larger than the outflow to the treatment
plant, the storage volume fills. Smaller storms will be completely
captured by the tank volume; nothing will overflow. Overflow occurs
only at medium to larger storm events. The volumes of the CSO
tanks, the flows to the treatment plant and the volume of
overflowing water at a given storm event and thus also overall are
in close dependency, see Figure 4. In short, the German sewer
philosophy has the following objectives: Limit the overflow volume
and overflow frequency in the collecting system to an acceptable
minimum. Protect the waste water treatment plant from overload
during wet weather. The inflow to the treatment plant during dry
weather is limited to about 2 to 3 times the peak dry weather flow
by installing effective flow controls in the collecting system.
Extra storage volume is to be added to or to be activated in the
collecting system. Source control in the collecting system,
minimization of inflow from less polluted storm water. Bypassing of
wet weather flow or release of sewage between primary and secondary
treatment at the WWTP is prohibited.
This is achieved by the following measures:
7
Fig. 4. Idealized hydrograph of the inflow to a CSO tank due to
a typical storm event. The spilled combined sewage volume can be
reduced by using a larger storage volume (B) or by increasing the
discharge which is fed downstream (C).
Technical Standards for Storm Water Treatment After World War
II, for good reasons, the rehabilitation of German sewer systems
focused on combined systems in large cities first. However, the
required technical standards were not set by the water authorities,
but by a non-government-organization, the ATV-DVWK (German
Association for Water, Wastewater and Waste). Similar to the US-WEF
(Water Environment Federation), any consultant, constructor,
researcher, sewer department, or plant operator can apply for
membership. Today, the association has about 20,000 members. For
the states water authorities it is only optional to design to the
ATV-DVWK-standards. However, most of them choose to do so. The
standards are continuously updated by ATV-DVWK and adjusted to the
newest available technologies. There is one standard for design of
CSO-tanks that is obligatory throughout Germany: The required
guideline ATV-A 128 (1992) was first introduced in 1977 and updated
in 1992. The basic idea behind A 128 was, to give the system
combined sewer plus WWTP (Waste Water Treatment Plant) the same
efficiency in pollutant control as a perfect separate system plus
WWTP. From this concept, a criterion for a required storage volume
is derived, assuming mean COD concentrations for all flow
components. 8
The basic principle of the A 128 standard is shown in Figure 5.
The x-axis shows the specific peak rainwater inflow to the WWTP
during wet weather. This specific inflow corresponds to the peak
outflow from the last CSO tank upstream of the WWTP. The vertical
axis shows the required specific extra storage volume to be
activated or added to the CSO-station. The mussel-shaped curves
represent the long-term overflow volume in % of the effective storm
runoff. Typical solutions are to be found close to 1 litre per
second and hectare of storm run off to the WWTP and a comparatively
large CSO tank volume of some 20-30 m3 per hectare of impervious
catchment area. This corresponds to 2 to 3 mm of effective rainfall
(or 1/10 inch). Forty percent of the long-term average of effective
rainfall will escape from the combined system via the overflows of
the CSO-tank. The emptying time of the CSO tank should not exceed
24 hours. The A 128 cooking recipe can be applied straightforward.
It cannot be cited here in detail for brevity. Moreover, A 128 also
recommends the application of modern methods like numerical
quantity-quality simulation. It is essential to know, that ATV-A
128 is a purely emission-oriented approach. In this procedure, the
sensitivity of the river or even the degree of amenity is not
accounted for. Nor are there any requirements included for the
allowable frequency of spills. It is, however, recommended that the
requirements should be enhanced. Some ATV-DVWK commissions are
working hard to implement the European Combined Approach. The
latest standard published for modern CSO tank dimensioning is
ATVDVWK-M 177 (2001). German storm water treatment standards have
led to well-performing solutions. In the past 25 years, Germanys
river quality has improved considerably. The first few salmon have
showed up again in the Rhine after 50 years of absence! However,
our standards are not the cheapest, if compared with other nations.
But perhaps the heavy industrialization and dense population
justify the expenses. Since public money is always short, solutions
are sought that are able to lower the costs for control. Latest
research has shown that there is a good potential for optimisation
in systems that have about one dozen or more of de-centralized
CSO-stations. Optimisation can be achieved here by fine-tuning of
the flow rates and storage capacities; see Haller and Weiss (2001).
These investigations have also shown that some additional rainwater
treatment at the WWTP will give better results than excessive extra
storage. Perhaps the German decision of the 1970s to limit the wet
weather inflow to the treatment plants to 2 to 3 times of the peak
dry weather flow was too rigid?
Figure 5. Design diagram specific storage of CSO-tanks versus
rainfall discharge rate from ATV-A 128 (1992) 9
One irony in the development of standards for storm water
management in urban collecting systems was that the research,
legislation and action have been primarily focussed on bad combined
systems. Up to now, no single conclusive standard exists on what to
do with the storm runoff from the separate systems in Germany. New
measurements indicate that the runoff in storm sewers may be not as
clean as assumed. This open question seems to be the next challenge
in urban drainage! Tanks in operation The latest statistical census
from 1998 showed, that Germany had 82,038,000 inhabitants and
10,312 WWTPs in operation. This corresponds to an average of 8,000
people served per WWTP. This relatively small number indicates the
typical German polycentric settlement pattern. Most parts of the
population live in these middle-sized towns. 61,376 outlets from
collecting systems and WWTPs hold a permit (see Table 2; Brombach,
2002). In relation to the population this means, that for
approximately every 1,250 people there is one permit. Over the past
25 years, new tanks have been continuously added to the collecting
systems. In 1998, a total number of 31,044 tanks and reservoirs
were in operation. The total storage was 33,143 000 m3, which
corresponds to 400 litres storage capacity per person. With the
German water consumption of 130 litres per day, the sewer system
could theoretically hold back the sanitary run off of 3 entire days
- if it will not rain! Table 2 indicates clearly that the vast
majority of tanks are within the combined systems. But the task is
not completed yet. Over 20,000 old CSOs still exist and need to be
rehabilitated. If we continue the way we did up to now, the
rehabilitation of CSOs will go on for another 25 years in other
words, we have only completed half of the task. The costs for
adding extra storage varies enormously from case to case. But
averaging from a large number of tanks, a good figure is that about
1,000 are needed to implement 1 m3 of extra storage. Or expressed
another way, up to now, for every German citizen about 400 has been
invested into the rehabilitation of mostly combined systems. This
sums up to a national total investment of 33 billion . Table 2.
Numbers and volumes of tanks in operation in Germany, census 1998
Symbol CSO-tank CSO RT CTT WWTP Number of units All types of
CSO-tanks in combined systems 20,080 Combined sewer overflows with
no signifi- 20,020 cant extra storage Retention reservoirs with out
overflow in 9,392 combined and separate systems Clarifier type
tanks in storm outlets from 1,572 separate systems Waste water
treatment plants for combined 10,312 and separate systems Total
61,376 Type of structure Storm storage capacity in m3 13,104,000 0
18,169,000 1,871,000 0 33,143,000
10
Examples for design and construction of typical CSO and storm
retention structures Following a former period of very individual
planning by consultants, there is now a catalogue available that
illustrates proven standard designs for various tank sizes and
types, ATVA 166 (1999) and ATV-DVWK-M 176 (2001). These standard
designs should not be simply copied one for one by the consultants,
but should inspire or guide individual designs. Altogether the
standard shows 21 different CSO structures varying from 50 m3 to
17,600 m3 of storage volume and two stormwater retention basins in
separate systems. For the sake of brevity, merely a few selected
examples can be shown here. Whoever is interested in the full
catalogue can consult ATV-DVWK-M 176 (2001). The most frequently
applied design and construction standard for a CSO tank is shown in
Figure 6. An oversized pipe made from prefabricated concrete is the
only element in the design to provide the storage volume. The
typical diameter is 1.8 to 2.4 metres. With a length of 50 meters
of storage pipe plus some volume in the control shaft, the volume
totals 170 m3. Such a small structure is capable of providing the
CSO control for a village with 400 to 500 inhabitants. The tank may
be arranged under the street, so no extra space is required. At the
lower end, there is a flow control that limits outflow during wet
weather to 25 litres per second. The storm overflow is situated at
the upstream end and simply consists of a fixed weir, 3 meters
long. The whole structure has no moving parts at all. It needs no
electricity and is selfcleansing by the continuous flow.
11
Figure 6. First flush pipe-type in-line CSO-tank with upstream
storm overflow, storage volume 170 m3, design UFT 2001 Figure 7
shows an open in-line circular tank. The tank empties by gravity.
To improve selfcleansing performance, the tank is fed tangentially.
This will result in swirl flow action of the water body. The tank
features a clarifier overflow. This overflow weir is arranged at a
position that the water body has to perform nearly one full
rotation before being discharged. Settleable solids thus are
settled in the tank. To support the rotation during emptying of the
tank, two stirring propellers are installed. The wet weather
outflow to the WWTP is 36 litres per second.
12
Figure 7. In-line circular CSO-tank with upstream storm overflow
and inner clarifier overflow according to M 176 (2001), storage
volume 500 m3 (example M 176 D.1.3.2) For bigger volumes,
rectangular tanks are used frequently. They are usually split into
several parallel lanes to ease cleaning and to secure good
sedimentation; see Figure 8. To achieve a uniform inflow to the
tank chambers, a lamella wall is arranged behind the diversion
weir. Research has shown sedimentation efficiencies of up to 80 %
of settleable solids. Two scum boards at the storm and clarifier
overflow will retain floatables. The wet weather outflow to the
WWTP is 118 litres per second. Tanks of this off-line design can
also be arranged such that the tank bottom is at a lower level than
the incoming sewer. In this case, emptying is done by a couple of
small pumps (not shown here). This allows more compact structures
and saving of footprint area. The energy costs for pumping are
negligible.
13
Figure 8. Off-line rectangular CSO-tank with clarifier overflow
according to M 176 (2001), storage volume 3 000 m3 (example M176
C.1 and C.2) Research has shown that not only combined sewage is
considerably polluted. The runoff from heavy-duty traffic areas,
e.g. motorways, requires stormwater treatment too. For this reason,
all new motorways in Germany are now equipped with storm water
detention facilities similar to Figure 9. The discussion about
whether to use wet or dry ponds is still open, but in practice most
structures are of the wet-pond type. The objective of these ponds
is to minimize the hydraulic stress in receiving waters and,
moreover, to retain sediments and oil and petrol in the case of a
traffic accident. So far, however, there is limited experience with
these ponds..
14
Figure 9. Retention pond for motor way storm runoff, upper part
wet, lower part dry pond version, storage volume 3 000 m3, design
UFT 2002 Recent developments in technical CSO tank equipment
Typically, some 10 to 15 % of the construction costs of a CSO tank
are spent for mechanical and electrical equipment necessary for
proper and effective operation, such as flow controls, tank
cleaning devices, backflow prevention, flow meters and water level
control devices, see Weiss and Janovsky (2001). In flow control,
there is a trend towards electronic solutions that allow for
measuring of the actual discharge rates. Inductive flow meters are
now available that are able to measure flows even when the pipe is
only partially filled. Today, most new CSO tanks have some form of
automatic cleaning device, either tipping flushers pushing out the
sludge after the tank has been emptied, or stirring propellers that
mix up the sludge during tank emptying. All these components are
controlled electronically, usually by programmable logical controls
(PLC). Many CSO tanks feature remote controls; the current state of
the tank and, alarms, etc. are displayed on a computer at the
treatment plant. 15
Figure 10. Self-regulating movable weir for water level
control
Figure 11. Horizontal fine screen for removal of gross solids
from overflowing water at a CSO-tank, opening width 4 mm, courtesy
of ROMAG, Switzerland
Rather new products for stormwater tanks are movable weirs in
various designs. Their task is to allow larger overflow discharges
at smaller variations of the water level in the CSO tank in order
to save construction costs by a reduced overflow weir length and by
more efficient use of the tank volume. Figure 10 is an example for
a self-regulating weir operated by a spring. Such devices can also
be used for backflow prevention in the case of a flood in the
river. Another recent development is the use of sieves or screens
to remove gross solids from the spilled combined storm water,
particularly at high-amenity river locations (a river promenade,
for example). This solution is promoted mainly for aesthetic
reasons, e.g. because of complaints about traces of toilet paper
and other debris on the riverbanks. Investigations have shown that
there is also a small reduction in COD loads, but it is known that
sieves will reduce the pollutant load on the river only slightly
since the major part of the pollutants is associated with small
particles that pass through the sieve. Up to now, however, the use
of such devices is not demanded by any standards. Figure 11 shows a
horizontal fine bar screen at a CSO. It is cleaned by a sliding
rack that is operated by a hydraulic piston. The clearance between
the bars is 4 mm. Figure 12 shows a sieve/filter. A large
perforated drum that is partially submerged is located inside the
CSO-tank. The perforation slots have a width of 3 mm. The water
enters the drum from the outside. Within a short time a filter
mattress builds up. When the water level is rising due to the
increasing hydraulic resistance of the filter mattress, the drum is
rotated slowly and an over-water brush clears off the accumulated
solids.
Figure 12. Rotating drum sieve at the overflow of a CSO-tank;
sieve opening width of 3 mm, design UFT 16
Figure 13. Pre-fabricated vortex separator, factory-produced in
polyethylene, lifted into position. Ready for operation next
day.
For small communities prefabricated vortex separators made of
polyethylene such as in Figure 13 are a cost-effective CSO-measure
(see Weiss, Brombach and Bauer, 1996). The tangentially incoming
flow forms a swirling flow. Similar to the teapot effect,
settleable solids are pushed towards the underflow. Floatables will
collect in an air cushion under the top plate. The device is
self-cleansing. It may go into operation the next day after
installation. Summary The paper reflects the European history of
urban Storm Water practice with a focus on Germany. One of the
oldest CSO-facilities of 1913 is shown. As a result of World War I
and II, the know-how and momentum in sewer technologies was lost in
Germany more or less until the Wirtschaftswunder triggered a fresh
start in the 1970s. The present European Water Policy is described
in brief. The directive 2000/60/EC has remarkable parallels to the
US-Clean-Water-Act and sets rigid deadlines. By 2009, all 18 member
countries involved shall have established programmes or measures,
and all measures shall be operational at the latest by 2012. This
new policy will affect nearly all existing urban drainage systems
in Europe. The sewer situation in Europe is demonstrated with some
statistical figures. The technical standardization in the field of
urban collecting systems is still advancing today. For instance,
the basement flooding risk has just recently been legally defined
for 18 European Nations to the same level. The German Water
Legislation and the philosophy behind urban storm management are
reported in brevity. Furthermore, water consumption and costs of
water supply and sewer fees are discussed. A map shows the current
distribution of combined and separate drainage systems in the
country. The combined system serves 63 % of the population, but the
separate system re-conquered 3 % of the population in the last 10
years. The technical standards set by the ATV-DVWK, the German
Association for Water, Wastewater and Waste, are very comprehensive
and detailed in regard to urban drainage and CSO-control. The
principle of adding or activating extra storage in the collecting
system during wet-weather is explained as well as the general
design rules. Up to now there are 31,000 CSO-tanks in operation in
Germany. They represent a total storage of over 33 billion m3 or
400 litres per German citizen. About another 20,000 old CSOs shall
be rehabilitated within the next 20 years. The public investment
into CSO-control in the past 20 years equals about 400 per
capita.
17
Some selected layouts of storm water tanks of different sizes
are shown in detail and described in short. The paper closes with a
short discussion of latest technical trends in the instrumentation
of CSOs, such as automatic self-regulating weirs, fine bar screens,
drum filters, and vortex separators. References Brombach, H.
(2002): Abwasserkanalisation und Regenbecken im Spiegel der
Statistik. Korrespondenz Abwasser, in print AbwAG (2001):
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(2001): Verordnung ueber Anforderungen an das Einleiten von
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Michelbach, Weiss, Brombach (1999): Nutrient Impact from CSOs on
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