Integrated management of urban water systems to …...Integrated management of urban water systems...

Integrated management of urban water systems to reuse waste waters

0. Giustolisi, D.Laucelli & M. Mastrorilli Dipartimento di Ingegneria Civile ed Ambientale, II Facolta di Ingegneria di Taranto, Politecnico di Bari, Italy

Abstract

The present paper relates all that has been conceived and realised in different parts of the world in dealing with the integrated management of urban drainage systems. The old and new trends about planning and managing of the urban drainage systems are analysed here with a specific care for the real time control. The complexity of the interactions between urban drainage systems and the hydrological cycle leads to have a global view of this problem and so, by the awareness of water resources finiteness, to preservation by resources management. According to this point of view, a further analysed application is the reclaimed rainwater reuse by water tanks connected to the main drainage network, with the aim to minimise the environmental consequences caused by pollutant discharges carried by sewer pipelines. Hence, we report the Japanese experiences about this new concept of urban drainage systems. Finally, it is reported a concrete example, complete with economic considerations and numerical evaluations of the water demands for many non drinkable water urban uses.

1 Introduction

The correct discipline of meteoric waters and waste waters is one of the important issue in the politic of environment safeguard and in general of the quality of life in the urbanised areas. The problem of waste and meteoric waters, in fact, is not exhausted in removing them from the urbanised areas in way that they do not interfere with social and economic activities, but in the control of their qualitative and quantitative impact to the goals of the environmental guardianship in general and particularly of the water resources safeguard. It concerns a problem whose unitary solution currently does not seem easy also for political and administrative motives:

Transactions on Ecology and the Environment vol 49, © 2001 WIT Press, www.witpress.com, ISSN 1743-3541

- are separated in different institutional corporate body the competencies about natural receptors (rivers and lakes), irrigation channels, sewerage systems and so on;

- laws and rules about hydraulic infrastructures are sometimes in contradiction or they face in autonomous way, and not co-ordinated, different problem list;

- in the large scale planning the interactions between the urban sewerage system and the external hydraulic systems are not faced and resolved. The control of the impact of the urban waste waters is made more complex by continuous evolution of the urban areas and by the interaction of different, not only technical, but also political, social and economical factors. The solutions are difficult in the case of ex novo planning of the urban waste water system, but also in situations of limited improvement, adjustment or reconstruction of existing nets. However, to solve the problem of waste waters becomes impossible if the planning is not inserted in a global approach of environmental guardianshp problem. Particularly, the individual drlnkable water endowments are already elevated and characterised by an increasing trend; thls is often caused by an old cultural presupposition that water is an inexhaustible resource and therefore to void cost. Today it is known that the individual drlnkable water endowments have an important influence on the correct management of the water resources because the increasing of urbanisation causes the increasing of volumes of water supply and waste waters. However, the development of the technological science allows to think realistically to a different way to resolve the problem of removing waste waters from urban areas. The way is the construction of integrated hydraulic systems and their real time control through monitoring nets that allow the best management also to the goal of environmental guardianship. For this reason, in the last decade we assisted to the increasing of the scientific researches in the field of the hydrological, hydraulic and transport of pollutants phenomena modelling and also a continuous evolution of the waste waters treatment technologies. The principal motivations of such studies are to put in relationship with: 1. the continuous tendency to the extension and intensification of the urbanised

areas, that causes the progressive increase of waste water volumes often not compatible with the dimensions of the existing nets and with the natural receptors;

2. necessity to simulate correctly the superficial urban outflows that originate during extreme meteoric events and that can determine the overload of the sewerage system with harmful effects on the economy and on the environment guardianship;

3. the increase of the environmental sensibility and the consequently necessity to limit the human impact on the environment, in accord with the parallel evolution of the local and national laws;

4, necessity to optimise the use of the boundless water resource. For aforesaid reasons, the present paper, inside to the study of sustainable management of the water resources thanks to monitoring nets, proposes to reconsider the waste water urban system in an organic and more general context


Water Pollution 17 89

going out from the simple parameters of traditional civil engineering (more interested to the constructive aspects) to pursue also the goal of environment guardianship, in terms of small impact of these hydraulic systems and of drastic reduction of the individual drlnkable water endowments to optimise the natural water resources. So it has to be reconsidered the planning of a different urban waste water system based on the followings general principles: 1. separation of the draihage system from sewerage system for domestic waste

waters; 2. realisation of the urban drainage system to remove road waters only; 3. realisation of system to collect and reuse the meteoric waters of roofs,

courtyards etc.; 4. realisation of local treatment plants for meteoric waters that allow domestic

reuse from a sanitary and t echca l point of view (washers, gardening, cleaning, etc) when it is not required drinkable water;

5. integration of the local post-treatment net with the drlnkable water net; 6. realisation of distribution net, for non drinkable scopes, of waste waters after

their treatment in the city plant and its integration with the local post- treatment net;

7. realisation of the interconnection between the drainage system and the local post-treatment system to decrease the hydraulic risk of the fust;

8. real time forecasting by compact and flexible mathematical models that allow, through monitoring, the best management of the integrated hydraulic system.

The positive aspects that should compensate the managerial complication of such urban water system should be the reduction of the individual drinkable water endowments and of the waste water volumes to digest in environment, with a lower environmental impact of the urban areas, a lower dimension of the urban drainage system and of the drinkable water system, the reduction of the hydraulic risk of the hydraulic systems and a consequent reduction of the economic damages of floods in the urban areas. In short, the flexibility of the water system increases, allowing a general guardianship of the water patrimony because water completes more cycles before returning to the environment. Insofar, the analysis of the benefits derived from costs is useful to associate synthetic performance indicators (for instance a simple relationshp cost- benefits) and to represent the convenience of the project considering its general utility. A more deepened analysis, economical and financial, would be able to determine flows of box products in the time thanks to specific initiatives to front of the necessary investments. Such analysis would be conclusive to the goals to evaluate the possibility to finance such projects through innovative procedures of project financing. This approach has been applied rarely to real experiences. Projects, finalised to a correct management of the urban water system, introduce characteristics of complexity to suggest with strength the analysis of the project financing practicability.


90 Water ~ o I / ~ r t r o n 1'1

2 Urban drainage systems and numerical flow simulation models evolution

The hydrogeological water cycle is more and more interacting with the structures of anthropization which man builds to takes advantage of them. The main traditional facilities representing such interactions are Urban Sewerage Systems. At the moment the design of pipelines and flow control systems starts choosing a hydraulic risk lasting 2-10 years, renouncing to control more critical storm. Today is more frequent the choice of 2-5 years because of a realistic observation of the urban flooding features and in order to try to control flooding with an optimisation of the drainage network. Ths approach needs the setting up of such forecasting methods for the critical discharge, characterised by intuitive physical parameters, good statistic reliability and practical availability. In the urban environment the description of the surface runoff and the evaluation of the critical flood discharge are fit to application of logic model and hydraulic algorithms, such as rainfall/runoff models, that apart from evapotranspiration simulate the surface and underground infiltration processes by the evaluation of only two parameters: the surface runoff coefficient c$ and the characteristic flooding time T, referred to the control section [l]. Now, if we sum up these kinds of rainfalhnoff models for the meteoric simulations, we can have: - Physically based models: they reproduce every process dynamics in a

deterministic way; they are released model since they make allowances for spatial distribution of physical and meteorological phenomena; they are very complex and difficult to tuning;

- Global models: they simulate hydrogeological process globally by global parameters, which represent mainly the basin behaviour; it will be considered only one critical discharge from only one compared storm; they give a basin global information;

Among these last models, the linear methods (Impulse Unit Hydrogram - WH) and the other ones based on the linearity and stationarity hypothesis are the most spread. Moreover, without a wide knowledge about all the physical basin process, IUH can be evaluated by a "black box" method; in fact, because of the hard complexity of the problem it is better attaining the solution without a physical comprehension. By this way in the last years various authors modelled hydrological systems by means of neural networks. It is to underline that the Input-Output Dynamic Neural Networks are a flexible mathematical tool to realise a compact model useful, for example, in online prediction for real time control. However, before selecting one specific neural network, it is necessary to study the various structures from a mathematical point of view, and then their relationship with physical behaviour. In fact, a common idea about neural networks as "magic tool" that is able to solve every modelling problem without physical insight is not realistic. Physical insight about the phenomena to model always remains important in order to choose the best structure [2]. Hence, we have to put in evidence that a carehl application of physically based methods allows an optimisation, with respect to global methods, from technical and economic point


of view, of the UDS designing. It means that they carefully elaborate the overcharges amount and location, that they define a more objective actions priority and work better on mesh networks. Ultimately, to design sewerage systems it is wrong to choose high hydraulic risks to reduce the flooding risks, whle by supporting the pipelines with other overflow control structures (weirs, sluice gates etc.) these risks would be reduced. The recent, but consolidated American experience, has conceived a very efficient design method called "double drainage system" and made up of:

Minor system: traditional sewer designing with 2-10 hydraulic risks; - Major system: formed by surface runoff flows and reservoirs which

operated only with the most critical storm (50-100 years); it requires further city-planning constraints about the road slopes, the gutters, the traps etc.

Their main purpose is the full use of the system water containing capacity by sluice gates able to real time control or by an appropriate use of reservoirs. They are used above the sewer pipelines (roof, parkmg area), or inside the system, by underground reservoirs (wide diameter conduct) placed on the side of pipelines, or with the classical solution of open reservoirs. Some interesting experiences in this way are the Scarden Park Storm Water Management (Toronto) and Trehorningen Lake (Sweden)[ l 1. The control of the UDS also consists of a reduction of discharge to treatment plants, specially with the critical storm, so as to prevent the overflow discharge pollution arrived at the final catcher. Many American and European experiences (France, Holland, Germany, England, Switzerland) [ l ] proved that the rainwater reservoirs have a very efficient control on the pollutant flow by a temporary keeping of pollutant overflow, whch then are directed to the treatment plants.

3 Real-time control of urban drainage systems

Civil engineers all over the world designed as many different collection systems and treatment plants as there are different perceptions about security and appropriate cost-effective solutions. Most of these systems have one common feature in that they are static systems for which neither the connection of the drainage system with the treatment plant and the receiving waters, nor the possibility of temporarily activated storage has been considered. So, the limited efficiency in reducing flooding, environmental pollution and health hazards is caused by the lack of flexibility in the operation of the static urban drainage systems (UDS) under dynamic loading. The historical solution of this problem of designing, constructing, operating and managing UDS is challenged by real-time control (RTC). A new concept of RTC is emerging to improve UDS performance and to reduce these hazards. The objective of this active operation of the UDS is to prevent flooding of the catchment and to prevent overflows to receiving waters before the capacity of existing storage is used up. At the same time, optimum flow rates to the treatment plant, depending on its capacity aid operational state, are to be maintained. By this means each particular storm and transient pollution load can be controlled using improved regulator devices and remote monitoring and control systems. RTC is essential for the full use of


transport and storage capacities under all operational conditions. Without RTC a UDS can only work optimally for one loading, namely the design storm [3]. RTC brings about: - better use of storage instead of transport only; - loolung beyond the collection systems and drainage, towards the treatment

plant, receiving waters, urbanisation, pollution sources, and safety of sewer maintenance staff;

- combining the necessity of lower investment costs and more operational efficient y;

- using the capacity of all facilities of the UDS to master environmental and drainage problems in the service area.

Moreover, since storm discharges only occur during short periods their impact on receiving waters is not well described by annual loads. It is the intermittent pattern of combined sewer overflows (CSO) that mostly impacts on aquatic ecosystems. Hence, transient flows into treatment plants can cause disruption of the treatment process. Hydraulic shock loads might cause high turbulence in the final clarifier with the result that, in the extreme case, the activated sludge is lost. RTC of UDS has the potential to reduce these transients to a minimum and, hence, indirectly improves wastewater treatment [3]. However, a number of obstacles to the use of thls promising t echque might hinder or even prevent the implementation of RTC, as follows: - administrative boundaries w i t h a physical UDS which constrain possible

action w i t h these political limits, separation of tasks (pollution control, drainage) in the operation of combined sewer systems, which are in fact two-purpose systems;

- funding arrangements that strongly favour high investmentllow maintenance solutions;

- inflexible UDS regulations and standards which prescribe static solutions; - disregarding the potential of RTC for less costly design; - the belief that RTC should solve all UDS problems immediately; - the lack of a motivated management and engineering team; - the lack of skilled maintenance and operation crews.

3.1 Real-Time control concepts for urban drainage systems

UDS is operated in real time if process data, which is currently monitored in the system, is used to operate flow regulators during the actual process. RTC assumes continuous monitoring and controlling of the flow process. In principle, the control of a process can be schematised to a simple control loop. This control loop consist of elements such as measuring, signalling, communication, presentation, control operation. In a real-time control system (RTCS) at least one of each of the following elements applies: - a (measurement) sensor that is used to monitor the ongoing process (e.g.

water level gauge); - a (corrective) regulator that manipulates the process (e.g. sluice gate); - a controller that causes the regulator to bring the process back to its desired

value (e.g. a PID controller);


- a communication system that carries the measured data from the sensor to the controller and the signals of the controller back to the regulator; e.g. telemetry system.

In RTC of UDS several hlerarchlc levels of control can be distinguished, llke the following ones: - local level (direct level); - unit process level (regional level); - global level (overall level, system level, integrated level, strategic level,

central level, supervisory level); - management level. Local operation of a pump, for instance, requires other information than determination of the amount of storage to be used during a storm. Depending on the requirements of a specific UDS, the control can be completely decentralised, fully centralised, or some mixed arrangement such as decentralised local control with central global control. If a RTCS is more complex (requiring information on the general environment of the system) global control has to be applied; in general, it can be required if one of the following occurs: - several unit processes exist that affect each other; - loading patterns are temporally or spatially variable and advantage can be

taken of these phenomena; - strict performance criteria are to be kept; - management information and operatiodmaintenance of the UDS has to be

improved. Only with global control can one react flexibly to the rainfall runoff process in every operational situation [3].

3.2 Modelling the state of an urban drainage system.

A UDS does not have a constant output. Its tasks and operating conditions change with the risks involved: flooding, pollution construction work, safety, etc. Therefore a RTCS design should start with a thorough selection and quantification of the decision variables which are output of the system, and of those which are controlled. Examples of desired output variables are: - construction site throughflow below sump pump capacities whenever the

risks of storms are low; - removal of a construction site protection barrier whenever the risk of a

storm is hlgh; - no overflows to the river when the risk of a storm is low; - basement or street flooding volume to be minimised during heavy storms; - regulated variables as dose as possible to the predetermined set point

values. A real-time control system (RTCS) can be thoroughly planned if numerical simulation models are used. The following processes have to be modelled in


order to obtain a comprehensive overview on the performance of the system: - the input to the system; - the system's response to the input; - the total output to the environment; - the response of the environment to the output from the system. The measurement of runoff at the entry points of the hydraulically controlled network may often prove that the reaction time is greater than response time. In other words, flow time is too short to react properly. In these cases areal rainfall models for subcatchments of which the outlet is an entry point to the controlled network are useful tools. This estimation can be done: - from point rainfall measurements; - from indirect surface rainfall measurements such as radar reflectivity

measurements coupled with ram gauges; - from forecasted movement of rain cells using a sequence of radar images. The sensitive part of the input modelling is the rain input, where phenomena like spatial distribution of rainfall, and growth, movement and decay of rain cells play a major role. However, with a fairly dense network of rain gauges (it is estimated that a density of about 1 rain gauge per km? is enough for this purpose) adequate rain input to a surface runoff model can be obtained. Another way of obtaining rain input data is the use of the radar techmque which also provides a possibility of rain forecasting and a proper description of the spatial distribution of rainfall [4]. Runoff models are needed in conjunction with areal ralnfall models to estimate the flow at entry points of the hydraulically regulated networks. They include models for surface runoff and uncontrolled pipe flows. Although physically based models are used for, say, roof runoff, surface flow, gutter flow, etc., nowadays mostly black box deterministic models are used for UDS analysis [5] [6]. These models feature a loss approach to estimate the effective rainfall and an impulse response function to compute sub-catchment runoff from effective rainfall. The impulse response function could also be of several types, usually assumed to be time invariant: - single linear reservoir model; - cascade of linear reservoirs; - triangular-shaped unit hydrograph (of which the base time is the calculated

longest flow time withn the sub-catchment). Experiments in urban areas have proved the validity of using a time variable loss function and a directly estimated triangular unit hydrograph [7]. Pollution input to the system is more difficult to model and further work is necessary to establish a fair basis for ths. Pollution transport in the system is also a subject on which further work is needed in order to develop models which can simulate overflow pollutographs or treatment plant inflow loads. Presently no urban drainage models exist which also include simulation of the treatment plant. Named "system" the part of whole UDS which is controlled by the regulators, input to this system can be surface runoff, flow from upstream branches with no RTC possibilities, sanitary sewage flow and infiltration. Modelling the state of the system for RTC has two purposes: - in the planning (off-line) phase, modelling is used to design the drainage


Water- Pollution VI 95

system and to determine control strategies and to predict system performance;

- in the operational (on-line) phase, modelling can be used to supervise and modify control strategies during the flow process.

Based on measurements (and forecasts) of rain, inflow to the controlled system can be computed. These data, together with the computed andlor measured state of the system are used to control the system. The advantage of both on-line and off-line modelling is that the state of the entire system can be determined, although measurements are only available for some points of the system [3].

3.3 Basic control techniques and control strategies

From the point of view of basic control techniques a UDS can be regarded as a system which is surrounded by its environment, with certain interaction with it (rainfall, overflow). Thls interaction can be described in a control model as the action of: - input variables (input signals, independent variables: representing the

influences of the environment on the system); - output variables (output signals, dependent variables: representing the

influences of the system on its environment). Several types of control techniques are applicable in RTC of UDS, and closed loop control is the most common (continuous and discrete, open and closed, sequence, feedback or feedforward control). In practice it is often very difficult to formulate the required control performance in advance, especially where the dynamics of the controller are concerned. Hence, in the design of a control system one has to rely heavily on experience gained in similar systems. In a typical RTCS, pumps, sluice gates, weirs, etc., have to be operated to store wastewater and route it to treatment and receiving waters. The major objective in the operation of these regulators is to avoid flooding while minimising combined sewer overflows (CSO) and operation and maintenance (O&M) cost. It is important to recognise that neither measurements nor computed flows are accurate. Measurements include measurement errors. Model computations include uncertainties which are due to unknown input, unknown parameters and model simplifications. It is therefore important to check control strategies with respect to measurement and modelling errors. Practically speaking, control strategies have to be 'cautious' to avoid 'surprises'. These could be unexpected storm development, inflows from non-monitored tributary sewers, etc. Modelling uncertainty might be a reason why simulation is rarely applied on-line, during the ongoing process. Existing RTC strategies are rather based on measurements alone, but it is useful to develop them using off- line simulation of the UDS processes [3] . Neither static flow restrictors (e.g. orifices) nor locally controlled regulators (e.g. vortex valves, float-regulated gates) can guarantee good systems performance. This would only be achieved if each regulator (e.g. sluice gate) is operated according to the flow process in the whole system. This operational mode is termed global control. Global control allows flexible reaction with respect to every operational situation since the set points of the control loops can be


96 Water Pdiurion W

continuously adjusted in accordance with the actual state of the entire UDS. The determination of a control strategy can be either automatic or manual. The strategy can be found through mathematical optimisation, search, decision matrices, control scenarios, trial-and-error (heuristically), or through a self- learning expert system. The most rigorous approach to finding a control strategy automatically is to use mathematical optimisation techmques. One of the better known techniques is linear programming where all decision variables, i.e. state and control variables, are linear. Once a control problem is formulated as a linear programming problem (LPP) it can be easily solved with commercially available software packages [3]. Multiple objective optimisation techques, however, cannot be fully automatic. They require interaction with a decision maker to find best compromise (Pareto optimum) solutions. This involves spatial and temporal aggregation, and linearization. The effects of these simplifications no final control performance, although probably not very important, have not been fully investigated yet. Search is a technique that can be carried out intuitively (like a chess player does) or, similar to optimisation, be formulated as a mathematical problem. Automatic search techmques require some insight into the search algorithm (e.g. steepest descent search, Newton-Raphson search, etc.) to reduce the number of required iterations. In practical applications such a large number of iterations might be required that the necessary computer time would not allow on-line applications. However, search techmques allow for a more flexible formulation of the objective function and the constraints (e.g. o non-linear). All this operation can be done either during the ongoing flow process (on-line) or beforehand (off-line). After the control strategy has been defined it is executed by controllers which are usually distributed in the field at the regulator sites. In most existing RTCS, operators adjust regulator set points based on their experience. T h s control mode is called supervisory control. It is flexible in that operators can use any kind of information that is available. The robustness of the control performance has to be checked either by modelling the controlled process with a detailed (and more realistic) model or by careful 'fine-tuning' in the real UDS. The better the inflow volumes and pollution loads are known in advance, the better the process can he controlled. It is desirable to know future inflows as accurately as possible for the whole control horizon. In a UDS this control horizon is the remaining storm duration plus the time required to empty the system. Currently available rainfall and rainfalVrunoff models may yield inflow forecasts which are considered to be not accurate enough. It has to be evaluated how inflow forecasting errors affect the quality of the optimised control strategy and, hence, in which cases inflow forecasts should be used [3].

4 The case of Japanese water recycling

Because of its rapid economic growth and the concentration of its population in urban areas, water demand in large Japanese cities has stretched the reliability of the water supply systems and necessitated developing new water resources, with considerable economic and environmental costs. To alleviate these situations, since the post-Second World War construction


Warer- Pollution 1'1 97

booms of the late 1950s, the government of Japan has invested heavily in building the nation's mfrastructures, including flood control, drainage and sewerage systems, water and wastewater treatment facilities, environmental protection and the creation of water amenities in the urban environment. For example, in the periods of industrial expansion in 1960s, the Tokyo metropolitan government promoted industrial water supplies using reclaimed municipal wastewater to prevent over-abstraction of groundwater in the coastal areas of the Tokyo Bay. Reclaimed water has been promoted by those municipalities as a safe, dependable, and aesthetically pleasing new water resource for water for toilet flushing, in-stream flow needs, industrial reuse, urban aquatic amenities and restoration of the environment. By 1997, 163 publicly owned wastewater treatment plants (POTWs) in Japan provided water reclamation and reuse across 192 areas. In addition, 1475 on-site individual building and block-wide water reclamation and reuse system provided toilet flushmg water in commercial buildings and apartment complexes, and water for landscaping. The annual volume of reclaimed water use was approximately 206 million cubic metres [8].

4.1 Water reuse schemes

Closed-loop water recycling systems The water recycling systems discussed below are implemented on a relatively small scale, such as in a single building or several buildings to form a block-wide water recycling system without the benefit of public sewerage systems.

Individual building recycling systems Individual wastewater reclamation and reuse is mainly for toilet flushng on individual sites, such as in large office buildings or apartment complexes with an on-site wastewater treatment plant. in some cities such as Tokyo and Fukuoka, dual distribution systems are mandated for newly constructed buildings of a certain floor area, generally greater than 3,000 -5,000 m2 andlor with an installed water supply pipe diameter greater than 50mrn. The reason for this requirement is that local water supply facilities, trunk sewer mains, pumping and wastewater treatment facilities are limited and cannot accommodate increased water supply demands and wastewater flows and treatment in large cities.

Block-wide water recycling systems In these systems, several buildings are connected to a block-wide wastewater treatment facility and their reclaimed water distributed back to the buildings via block-wide urban distribution pipelines, mainly for toilet flushing. There were 1475 installations of individual and block-wide water recycling systems in total and the annual volume of reclaimed water used was approximately 71 million cubic metres.

Large area water recycling systems Large area water recycling systems are implemented via the public sewerage system and POTWs. Tertiary or advanced wastewater treatment processes are


98 Water Pollution 1'1

normally used for further treatment prior to water reuse. The reclaimed water is distributed through a network of pipelines to large water reuse areas. The main uses of reclaimed water for these systems are toilet flushing and environmental water, but other uses include irrigation and melting snow. Use of thls type of large scale water reuse scheme has been increasing, with showcase installations such as the Tokyo metropolitan government

Of-site water recycling systems for other applications The off-site water recycling system is an open loop system in which reclaimed water is supplied for off-site locations such as industries, agricultural land, aesthetic and environmental uses, and snow melting facilities. The reused waters are not generally returned to the POTWs apart from some industrial wastewater, and are discharged to the environment instead. There are hundreds of such installations in Japan.

4.2 Characteristics of Japanese water reuse

As in other countries, there have been no uniformly enforceable national water quality standards for wastewater reclamation and reuse so the water quality criteria for toilet flushing were established jointly by the Ministries of Construction, Health and Welfare and International Trade and Industry in 1981. In Japan, water use efficiency is relatively low because of the heavy rainfalls in the typhoon and monsoon seasons, and the mountainous topography, which means rivers flow rapidly to the oceans. In these physical circumstances, water reclamation and reuse are viewed in Japan as new water resources located right on the doorstep of the urban areas. Water reuse for environmental purposes can be characterised as follows: - reclaimed water is treated by tertiary treatment consisting of chemical

coagulation, granular-medium filtration, and often ozonation to remove colour and the musty smell;

- reclaimed water is normally transported a short distance from POTWs to a point of discharge;

- unlike large-scale toilet flushing reclaimed water distribution systems, no complex pipeline networks are employed;

- maintenance work is normally conducted by POTW personnel, keeping operational and maintenance costs low.

Furthermore, an aquatic park or water gardens are often located near or within the POTW, so that the cost of water reuse can be much lower than using conventional potable water supply. The water quality criteria for this use are to have equal to or less than lOOO/lOOrnl faecal coliforms, compared to California's 2,21100rnl for similar applications. Water reuse in Japan is not cheap. Although the yardstick price for reclaimed water of about 80% of the drinking water price generally applies in Japan, the reported production cost for reclaimed water in Fukuoka city is $2.01/m3 compared to a drinking water cost of $1.88/m3 .The consumer price of reclaimed water averaged $2.99/m3 compared to the drinking water price of $3.73/m3. Judging from Fukuoka city's experience during more than 20 years, water reuse



for toilet flushlng can be economically justified in many water-scarce urban areas. Another reason for the expense of water recycling in Japan is the cost associated with the installation of dual distribution systems in buildings and for the installation of pipelines in built-up and congested areas. These reclaimed water prices reflect competition for new water resources, and these expenses are the necessary cost of doing business in highly urbanised metropolitan areas [g].

4.3 Other applications in the world

The fust comprehensive approaches to RTC in UDS were initiated in the United States at the end of the 1960s. In the 1970s a number of demonstration projects were implemented and are mostly still in operation. In many North American cities (Detroit, Seattle, Cleveland, etc.) the drainage system are managed with a RCTS by sluice gates. Besides, it is more difficult to obtain an overview on RTC applications in Europe because of the many different countries with their respective administrative regulations. For example, some German states enforce specific CS0 regulations. These resulted in the construction of literally thousands of small detention tanks to catch the fust flush of pollutants in CSS. Some 20% are equipped with controllable regulators. Quite a few employ some form of regional control, mainly to avoid downstream CSC) caused by upstream tank releases. As for large supervisory real-time control systems, examples can be found in cities such as Bremen, Hamburg, and Munich. Large parts of The Netherlands are below sea level. There, virtually every drop of water in UDS has to be pumped, mostly from CSS. Hence, in a strict sense, all drainage systems in The Netherlands are under RTC integrated systems, of course, are not so frequent, e.g. Rotterdam, Wervershoof, Utrecht, and Eindhoven. The county of Seine-St-Denis, close to Paris, has operated a large real-time control system for more than a decade. Other existing or planned real-time control systems in France include the counties of Hauts-de-Seine, Val-de-Mame, and the cities of Nancy and Bordeaux. After the re-organisation of water resources management and administration in England, the number of integrated real-time control systems is steadily increasing. For example, solely dedicated to waste water control, are the drainage systems in Newcastle-upon-Tyne, Binningham/Wolverhampton, and GrimsbyICleethorpes.

5 A water reuse application

The main principles of this example are the same which we are t a l h g about: - The need to reduce the maximum meteoric discharge at the final catcher; - An optimised real time management of the system reservoirs to reduce the

environmental impact caused by the frequent skimming in the final catcher; - The treated wastewater reuse. The designing of our urban drainage system (UDS) regards the town of Crispiano, near the city of Taranto in the south-east of Italy, whose urban basin has been subdivided in 1900 built or building lots; it has been realised in four different manners: - Hydraulic risk of 15 years designing;


- Hydraulic risk of 15 years designing, with rainwater collection tanks; - Hydraulic risk of 2 years desigmng;

Hydraulic risk of 2 years designing, with rainwater collection tanks; In the first part of this work it will be designed the sewer pipelines and the water collection tanks. In the second part it will make hypothesis about the reclaimed water reuse, pointing out, in every kind of reuse the needed quantity and the chemical and physical features. There are two different designing methods: the standard one, instead the second includes the reclaimed water tanks. The rain sewer pipelines start from the Crispiano's outskirts, delimiting the current Piano di Fabbricazione areas. They are provided of inspection traps at a maximum distance of 25 m or in any case on every crossroads. There are a lot of grating traps along all the plan, mostly on the compluvium points and on the crossroads, in such a way as to intercept and drain the flowing water rapidly. From the result of dimensioning it is observed that the 15 years planning with tanks and 2 years without tanks determine similar capacities and diameters (fig. 1). Our preference falls on the first solution because, to parity of digging costs and for the pipes, it contains heavier critical events for the network with the primary environmental possibility of containing, treating and reusing a big part of the rainwater. The next diagrams compare the four solutions:

/ 15 years 0 15 years with tanks U 2 years 2 years with tanks/

Figure l : comparison of the four solutions diameters

The dimensioning of the tanks and its consequent risk of insufficiency are related to a rain length derives from an empirical cost-benefits analysis: in fact, to higher rain length correspond greater construction costs; counter we have greater water reclaimed water, smaller risks of insufficiency, with consequent economic and sanitary-environmental benefits. A rain length of 1 hour for the dimensioning of the tanks is chosen, such to guarantee a good volume of reservoir (25 m3). Such dimensioning starts by localisation, for every branch of the network, of surfaces through which it is possible to reclaim and contain the rainwater. Such surfaces


Warer. Pollution 17 10 1

are 58 hectares approximately. Therefore, from the medium monthly rainfall (ISTAT annals) for our station, we have estimated the expectable monthly rain volumes for the total of the collection areas. Putting in evidence the effective rain days (with rain height > 1 mm) it can be obtained useful information about the number of times in which our reservoirs will be filled up. In conclusion, the next diagram has been obtained comparing the water volumes for every monthly rain and those containing in the network tanks:

I B3 Rainfall volumes Tanks volumes /

Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov

Figure 2: comparison of the volumes

So, can be observed that the system allows to contain rainwater for a lot of days, considerably reducing environmental impact caused by the direct discharge of rainwater at final catcher. Therefore, it can be concluded that a single rainfall reclaimed water can be totally contained in the network tanks and be reused.

5.1 Reclaimed water reuse

The water collection tanks are situated on the beginning of the network (tanks on the roofs, in the parking, etc.), underground or open tanks. The choice of the optimal solution is a problem of remarkable interest and often not has easy solution. The requirements of qualitative control of the flows take part in the decision also, because of the tanks allows: a) reuse of not drinkable waters: remote heating, irrigation and agricultural

uses, fue protection, flushing toilet, roads washing, washing traps; b) treatment before discharge at the final catcher; c) the provisional retaining for subsequently discharge at the final catcher. The emptying of reservoirs can also be started in advance to the rainfall: using the meteorological forecasts (which have a four days reliability) for forecasting the operation of a urban drainage network. The percentage of rainwater that is possible to reclaim and reuse with various purposes, as the next, has been estimated in 26%. The operation pattern is based on the synergic use of two


tanks of the volume of 25 m3 each (fig. 3). The rainwater can be reclaimed in new tanks or in old readapt reservoirs, through a communication system, made up of conduits and pumping systems between the tanks, to guarantee the constant level of one of them at least. The rainwater feed the two tanks; an overflow weir and bottom drain avoiding overflows and allows the direct use for roads washmg, in case the tanks were full. Moreover, it can be guaranteed an fire protection volume of 10 m3 in one of the two tanks. Many uses of waters can be fed from a tank or from both, with the needs of being connected to the supply water system, to guarantee water supplying for all the uses, also in drought periods.

Drinkable water supply

water

Remote l~enting Roads naahing N'r~hing traps Toilet fluslling

Fire Protec-

Figure 3: basic water reclamation and reuse pattern

Passing to the specific uses, we consider first the remote heating. It is based on a centralised system, usually fed to methane, which supplies, at a distance, warm water necessary to heat houses, hospitals, schools and places of job. With this system the emission of polluting substances into the atmosphere is reduced strongly, because in place of the emissions of thousands of boilers, there are only the smoke of one central plant, whose pollution can easily be controlled. If the heating costs are lower than the transport system and its exertion costs, the operation convenience is demonstrated. However, water for boilers has to be purified by some particular substances: materials in suspension (clay, sands, various kinds of soil), emulsified substances (oils), O2 and CO2, dissolved salts (bicarbonates, sulphates, chlorides and



silicates of Ca and Mg), while the pH will have to be included between 7.5 and 9.5. The most common irrigated uses in a urban environment are those relative to the private and public gardens; through a global evaluation of water volumes really used, it can be observed that the employed water distribution facilities seldom exceed the 50 l/h for square meter. The quality of water for gardening use does not demand particular physical or chemical characteristics; naturally all the bacteriological restrictions must be respected. Currently, in the different kinds of buildings there are fire protection means such as the fire hydrants connected to an appropriate water supply network and fixed systems of extinction that operate automatically. In order to contain the fire protection reservoir, the ancient drinkable water reservoirs could be readapted for the old buildings, using meteoric waters. For the new ones it would be sufficient to have only a 25 m3 volume of reservoir for two or three built lots (B and C areas of the P.D.F.). Moreover, a reservoir of 10 m3 volume can support the fire protection provided for Italian regulations (Fig. 3) in order to guarantee both fire protection aims and other just suggested uses, increasing the system efficiency. As far as the private users we can consider the toilet flashmg made by the following devices: flush tanks, fluxrneters and short stroke taps. The toilet flushmg does not need water with particular physical, bacteriological or chemical characteristic. The roads washlng is carried out by tankers, whde the public water closets are weekly washed and disinfected. The washing of black sewers with low slope and also of the initial pipelines of higher slope but with little discharges is necessary because of water full of (sedimentabili) solids causing possible obstruction. A high purity is demanded in order to avoid obstructions of the sections, caused by limestone or soil residues, mucilages. At last, the presence of a 50 m3 public tank every lulometre is suitable to solve daily problems and to allow more immediate extraordinary actions (washing roads to high pressure, washing roads under the cars in pause, fire protection service, innaffiamento, service disintasamento, monuments washing). Finally, we can state that the reclaimed rainwater quality is under the lawful limits provided for Italian regulation for the discharges at the sewerage systems and at water stratum. In fact, such water is not different from the road rainwater, which are more polluting since their flow drag heavy metals and other chemical elements produced by smog and our roads carelessness.

Conclusions

It is well known that the limited efficiency in reducing flooding, environmental pollution and health hazards is caused by the lack of flexibility in the operation of the static UDS under dynamic loading. In addition, the control of the impact of the urban waste waters is made more complex by continuous evolution of the urban areas and by the interaction of different, not only technical, but also political, social and economical factors. The solutions are difficult in the case of ex novo planning of the urban waste water system, but also in situations of


104 Water Poiiutiot~ V7

limited improvement, adjustment or reconstruction of existing nets. However, to solve the problem of waste waters becomes impossible if the planning is not inserted in a global approach of environmental guardianshp problem. This paper want underlines that a new concept of hydraulic designing is emerging to improve UDS performance and to reduce this hazards. A combination of RTC and waste water reuse seems to be a better solution for the environmental issues. In many cases it is also an economical way to solve UDS problems. But neither technical nor economical criteria are sufficient for successful application. Above all, it is necessary to have a dedicated and enthusiastic core group of engineers, usually small in number, but eager to communicate with the public, the regulatory agency, the executives and the operating personnel. They are from different divisions such as planning, designing and operations and are willing to "look over the borders" of UDS towards treatment and receiving water quality. The operating and maintenance crews must be able to understand features such as basic hydraulic processes or rain gauge records and they must have an idea of the impact flows and overflows on the treatment plants and the receiving waters. Operating personnel need strong motivation and backing. Moreover, the crew's experience will, after a while, create new ideas for further system improvements and the RTC system will not become an alien element.

References

Giustolisi O., Input-output neural networks simulating inflow-outflow phenomena in an urban hydrological basin, Journal of Hydroinfonnatics, IAHR-IWA, 02.4, 2000, pp.269-279. Paoletti A. et al., Sistemi di fognatura e drenaggio urbano, Ed. CUSL, Milano, 1996. Jaquet G. et al., The use of radarprecipitation measurements and forecasts in urban hydrology. Colloque Eau et Informatique, ENPC, Paris, 1986. Jaquet G., Une politique ambitiuse, la protetion d'un cows d'eau, La Selle au Cateau (Nord), eflort sur les Eaux pluviales, SHF, Proceedings, XVII Journee de l'Hydraulique, Nantes, 1982. Wisner P., Consuegra D., Testing of OTTHYMO on twenty watershed, Proceedings of the International Symposium on comparison of Urban Drainage Models with Real Catchments data, UDM '86, Dubrovnik, 1986. Bertilotti R., Guillon A., Jaquet G., Evaluation of three rainfall/runoff models for the control logic of RCTS in the Seine Saint Denis County, Proceedings of the International Symposium on comparison of Urban Drainage Models with Real Catchments data, UDM '86, Dubrovnik, 1986. Jaquet G., Schilling W. et al., Real time control of urban drainage systems, Scientific and Technical Report 2, IAWPRC, London, 1989. Asano T. et al., Non-Potable urban water reuse - a case of Japanese water recycling, Water 21, IWA Publishing, June 2000, pp.27-30.


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