During the course of this century, the influence exerted by expanding urban populations upon the naturalenvironment has highlighted one of humanities most pressing problems - growth within a finite system.
53
D. Bt.m.ER and J. PARKINSON
More recently, the development of mega-cities has initiated many concerns associated with the quality oflife in the urban environment (Black, 1994) and the environmental impacts upon the hinterlands that citiesrely upon to support urban life. Recognition of these problems and the mounting concerns about thesustainability of city life has initiated much research interest and activity in investigating the means to amore sustainable existence. The advent of the concept of a sustainable city (Cooper, 1994) calls for areappraisal of the many diverse elements of structural form and human activity that constitute a city. Urbantransport systems and the provision of water have been identified as being the two most critical factorsdetermining the future of cities in the next century (Dalhammar and Mehlmann, I996). This paper isspecifically concerned with the need for the provision of more sustainable urban drainage services.
Agreeing on detailed, specific, and workable definitions of sustainability, often perceived purely inecological terms, proves difficult due to the complex interactions between an enormous number of social,economic, and technical variables that influence the environment (Brooks, 1992). One definition proposessustainable development to be that which improves the quality of life while living within the carryingcapacity of supporting ecosystems (IUCN et al., 1991). Here the emphasis is placed on mankind's demandfor and impact upon earth's resources and the environment. However, it is evident that sustainablepractices must Incorporate elements of making something last (to sustain is "to support", ''to endure" or ''tocontinue"). The widely accepted definition of sustainability, from the Brundtland Report (WCED, 1987),states that "sustainable development is that which meets the needs and aspirations of the present generationwithout compromising the ability of future generations to meet their own needs". Therefore, sustainablesolutions must include components of achieving goals "now" whilst not neglecting needs of "the future".However, locational peculiarities and specific requirements associated with different stages of developmentmake exact definitions difficult and generally inappropriate in most cases. Terms such as "sustainabledevelopment", "sustainable cities" and "sustainable growth" are likely to remain ambiguous and with noagreed absolute definition.
urbandrainage
Fig. I. Urban drainage systems are intrinsically linked to other components of the water cycle
The drainage systems serving tOOay's urban communities range from rudimentary, unlined, open-channeldrains to highly engineered, expensive, piped sewer systems. In both cases, water provides an apparentlyconvenient mechanism for transporting waste away from areas of human habitation where it wouldotherwise cause illness and disease or disrupt the operation of other urban services. Sufficient flows ofwater are required to convey particulate waste matter from the urban environment back into the naturalenvironment and, consequently drainage systems are highly dependent upon the availability of naturalwater resources. Their effective operation is therefore intrinsically linked to other components of the watercycle (see fig. I) and must not be considered in isolation. Therefore, the safe disposal of urban wastewatersmust lie alongside the demands for the adequate provision of water supplies, maintenance of natural waterresources, and the prevention of adverse environmental impacts (Andoh, 1994).
Towards sustainable urban drainage
It is evident that, in many parts of the developed world, this approach has been highly successful at :
• virtually eradicating common illnesses associated with the faecal-oral transmission route,• eliminating frequent flooding by urban runoff,
• reducing the localised and short-term environmental impacts of pollution.
Although it is arguable whether all these achievements have reached equal level.
However, the fundamental nature of the system relies upon the availability of large amounts of water as themechanism of transport of wastes away from areas of human habitation. Subsequent treatment prior todisposal minimises localised environmental damage in the short term, but the process transfers waste fromone medium to another causing irreversible flows of substances, nutrients and synthetic chemicals thataccumulate in the aquatic environment. According to Marsalek et al. (1993), the effectiveness ofconventional technical drainage concepts has reached a limit and the sustainability of such practice is inquestion particularly with respect to depletion of natural water reserves and degradation of the globalenvironment (Niemczynowicz, 1993; Van der Graaf, 1993). [t is necessary to reconsider the overallapproach.
The aim of this paper is to identify the current objectives of urban drainage and how these might beextended to incorporate elements of sustainability. Realistic strategies are proposed through which this newobjective should be pursued under the current levels of commitment to invest in conventional drainage.Given the constraints of specific locational requirements and demands of different communities, no oneparticular type of drainage system is advocated as being sustainable, but approaches and technologieswhich will potentially enable us to move towards more sustainable urban drainage systems arerecommended.
CURRENT & PROPOSED OBJECTIVES
The primary objective of urban drainage systems is to protect and maintain the health and safety ofcommunities. In practice this amounts to the draining away of flood waters and the removal of humanwaste to maintain a sanitary environment. The management of flood waters is required to avoid structuraldamage and interference with movement and activity within the city. The second major objective of urbandrainage systems is to protect the natural environment including flora and fauna, This is currentlyapproached by maintaining environmental standards involving limits on the pollution of naturalwatercourses, land and the atmosphere. In addition, the quality of the environment contributes to thequality of life and the well-being of people insofar as human populations rely upon the environment tosupport their basic daily needs of food, fuel, and shelter. The United Nations Environment andDevelopment UK Committee (UNED-UK, 1994) concluded that the protection of health and the quality ofthe environment are intimately connected.
Current practice attempts to satisfy these two objectives but, in the spirit of the current debate, it is nowsuggested that there should be a third objective; that systems should be "sustainable", Of course, asmentioned earlier, we currently do not have an agreed definition of this term in general. let alone adefinition suitable for application to urban drainage. Figure 2 helps to illustrate a number of pointsconcerning this third objective. Firstly. we would see consideration of the sustainability of practice asunderpinning the first two objectives and absolutely not replacing them. Secondly, we suggest that thereare strong interactions between the three objectives. In the same way that health and environment areintimately interconnected, so are environmental protection and sustainability. However, they are notinterchangeable (Brooks, 1992). Thirdly, figure 2 illustrates the need to take into consideration the longterm and widespread consequences of existing or proposed practice. This proves to be difficult but
S6 D. BU11..ER and J. PARKlNSON
important as the impact of unsustainable practices in many cases is relatively long term as opposed tohealth or indeed some environmental problems which tend to be observable over a much shorter timeperiod. The extent of the effects of such practice may also be far reaching, even up to a global level, andthe effects are more subtle as a consequence of the extremely long term build-up rate and the ability ofecosystems to adapt to change.
In order to attain any of these objectives of a drainage system, the efficient, effective and on-goingoperation of the system is necessary. Drainage systems fail more frequently as a consequence of a lack ofappropriate maintenance procedures rather than as a result of poor engineering design. This may be due toa lack of resources, a lack of skilled personnel or an inadequate economic system to provide the financesnecessary to operate the system. A system will only be truly sustainable if its financing is compatible withthe long-term ability and will of the community to pay for it. Solutions must also be practicable within thesocial context of the community that is expected to use the technology. The limits of the effectiveness oftechnology without the requirements of the community must be recognised and thus. changes in publicperception may be required prior to attempts to introduce technology that requires a change in the socialpractices or habits of a community. Operational sustainability and socio-economic sustainability are thus,without doubt, critical to the attaining the ultimate goals of sustainable development.
Sustainable development should therefore be viewed as a much broader concept than environmentalprotection (CEC, 1994), or indeed health and safety protection, and hence sustainable solutions should seekto address the sustainable utilisation of water resources respecting both social and economic as well asenvironmental interests (Maksimovic, 1996). Consequently, recommendations for specific types oftechnology cannot be considered in isolation from their economic and social influences and a successfulstrategy to sustainable urban drainage must in principle require an integrated planning procedure.
li'E.aIONAI
Sustainability
Fig. 2. Hierarchy of objectives of drainage systems showing increasing extent of spatial and temporal effects
STRATEGIES TOWARDS REDUCING UNSUSTAINABILITY
By definition and history the function of an urban drainage system is to deal with unsustainable problemsassociated with human over-consumption. A realisation of this fact requires us to accept that a trulysustainable urban drainage system. fully satisfying a multitude of worthy criteria. is surely an idealised andunattainable goal. Consequently, we must identify those elements of "unsustainability" associated with theoperation of conventional drainage systems and develop new strategies for dealing with urban wastewaters.The investigation of iMovative drainage designs and their application is without doubt critical topromoting sustainability. However. given the current levels of investment in existing infrastructure and
Towards sustainable urban drainage ~7
their highly developed o~rating systems: radical modifications to drainage systems. particularly within theconfines of dense urban mfrastructure, wIll not occur and an incremental approach is therefore advocated.
Three strategies fundamental to the promotion of more sustainable drainage systems are proposed :
• reduce the reliance on water as transport medium for waste• avoid mixing industrial wastes with domestic sewage• separate storm runoff from flows of polluted wastewater
If priority is given to these strategies, many alternatives to the adoption of conventional drainagetechnology are opened up and benefits would be immediately realised even if they are only introducedsingularly and incrementally. The strategies and their potential benefits. summarised in Table I, aredescribed in detail below.
Water transport
Conventional drainage systems utilise water extremely inefficiently. Large quantllles of water areabstracted and treated using expensive treatment technology to drinking standard yet are subsequently usedto flush faeces and urine down the water closet. For example, in the UK, approximately 33% of totalhousehold consumption is squandered on this non-potable application (Butler et aI., 1995). Not only Is thiswasteful of a precious resource but it also promotes unnecessary mixing and dilution of wastes andcontamination of previously unpolluted water. End-of-pipe technology is then needed to abstract the solidwaste components from the liquid component of the waste flow.
Household water consumption can be reduced substantially with minimal investment (NRA, 1995). Thismay be achieved by installing low-flush toilets or possibly domestic greywater recycling systems. Thechallenge is to ensure. for example, that existing systems will operate effectively with much lower dryweather flows (Lim, 1996). Alternative no-flow technologies for the conveyance of sanitary waste such asdry sanitation systems or vacuum sewerage should be investigated as feasible technical options for thefuture.
Mixing of wastes
The mixing of industrial effluents with domestic wastewaters can cause problems for conventional sewagetreatment, spoils the usability of valuable components contained in wastewater resulting in a loss ofresources and the creation of water pollution problems. Synthetic organic chemicals and heavy metalsaccumulate in the environment causing problems to ecosystems as they move through the food chain. There-use potential of nutrients and minerals contained in sludge is diminished by small concentrations ofindustrial contaminants. This results in many cases in the disposal of sludge to landfill sites or byincineration both wasting a valuable resource and accounting for up to 50% of the total cost of operatingwastewater treatment plants (Liessens et al.• 1996).
Removing the industrial waste component of municipal sewage is therefore critically important tosustainable drainage strategies. Dealing with the waste as close to the location of production as possiblereduces the problems associated with treating a highly complex mixture of substances and increases the reuse potential of the more valuable components of waste (Hvitved-Jacobsen et al., 1995). Where thecomplete isolation of an industrial waste stream is impractical. pre-treatment using best availabletechnology to reduce concentrations of the undesirable wastes (be they refractory chemicals or excessivelyhigh concentrations of biodegradable waste) is reqUired prior to discharge into the sewer.
D. BlTT1.ER and J. PARKINSON
In more general terms, we believe that the extent of mixing of all elements of waste should be reducedwherever practicably feasible. This allows exploration of the many opportunities to deal with waste with aview to utilising it as a reusable resource as opposed to a waste disposal problem. Options for sourcecontrol should be considered rather than automatically relying upon end-of-pipe treatment facilities.
Separation of stormwater
We would concur with Kollatsch (1993) who identifies the removal of the stormwater component fromcombined urban wastewater as being one of the most important measures to strive towards more efficientdrainage systems. By disconnecting stonnwater from overloaded combined systems a number ofconcurrent benefits are achieved; combined sewer overflow discharges are decreased, thus reducing theextent of environmental damage and the problems caused at treatment works by peak, transient flows, andthe viability for use of rainwater for abstraction or recreation is increased. Generally, drainage systems failto exploit storm water as a resource. There are a number of opportunities for the drainage engineer toutilise stormwater as a resource including reuse for toilet flushing and garden watering.
Isolating storm runoff from sewage is not a new idea. Separate systems, advocated since the 1950's, wereregarded as being the drainage systems of the future and indeed are still the current practice in mostdeveloped countries. Despite this, the expense and the lack of environmental improvements realised by theinstallation of two totally separate piped systems, does not commend this as a sustainable approach.However, it seems that the overall strategy of separating the two components of wastewater is not at fault,but more the method through which the strategy has been implemented.
More recent research again advocates separation of stonn runoff from other urban wastewaters (Andoh,1994; Kollatsch, 1993 ; O'Loughlin, 1990) but, instead of routing urban runoff directly into a pipedsystem, the alternative approach recommends utilising the natural drainage patterns of the catchment or onsite infiltration into the soil. Where the soil and the quality of the runoff permits, direct infiltration into theground is preferable in order to recharge groundwater reserves. Where infiltration is not possible, thedevelopment of natural drainage patterns offers a range of opportunities for conservation, recreation, andamenity, as well as providing basic flood and pollution control (Ellis, 1995). In denser urban environments(particularly in developing countries), road carriageways may be utilised as a drain to convey large stormflows (Kolsky et al., 1996).
The way forward
Niemczynowicz (1993) and Beck et al. (1994) identify two possible future development scenariospertaining to drainage infrastructure. Either, a continuation and further refinement of the present approachof the high-tech, centralised, end-of-pipe approach or the widespread provision of low-cost, low-energysolutions based upon maximisation of source control procedures (waste minimisation, recycling, re-useetc.) and the application of alternative decentra1ised biological systems. However, natural scepticism andopposition to change are natural parts of human nature and, at the present time, only the most forwardthinking and ecologically minded entrepreneurs have got to the stage of actually implementing alternativesystems and these tend to be on an individual or, at best, small-scale basis. Evolution as opposed torevolution seems to us to be the most likely development scenario in which elements of both the "hightech" and the "low tech" approach will play complementary roles in tackling the problems associated withthe sustainable urban drainage.
REQUIRE~mNTS FOR SUSTAINABLE TECHNOLOGY
Site-specific solutions
Past experience shows us that there is no unique technological solution to the problems of urban drainagewhich can be applied to all situations (Geiger, 1990, von Sperling, 1996). Beck et al. (1994) conclude thatthe manner in which we judge a part in isolation may be quite different from an evaluation of its merits as a
Towards sustainable urban drainage ~9
part of a holistic and interactive system. Therefore, with each drainage problem must come a review ofavailable technology and its suitability for application to a particular locality and situation. Von Sperling(1996) ranks efficiency, reliability, sludge disposal. and land requirements of wastewater treatmenttechnology to take precedence over environmental impacts in the selection procedure in already developed
TABLE 1 Strategies towards sustainable urban drainage
COMPONENT OF PROBLEMSWASTEWATER
PROPOSEDSTRATEGIES
POTENTIALBENEFITS
POTENTIALD1SADVANTAGES
• unnecessary Wilier
consumptIonCARRIAGE
WATER • dilution of wasles
• requires expensive endof pIpe Ireatment
• introduce waterconservationlechn,ques
• rc-usc water
• seek alternative meansof waste conveyance
• conserves waterresources
• improves efficiencyof treatment
processes
• ,ncreases POSSIbilityof sedimenlation
in sewers
• heallh hazardsassociated wllh
• disrupts conventional
hiologtcaltrealment• remove from domestic. Improves treatability. costs associated
wasle streams of waslewaters with implementingnew practice
countries. However, in developing countries construction costs are recognised to be the most importantcriteria followed by operational sustainability, operational costs, and the simplicity of the technology.
Adaptability
Brooks (1992) concluded that a suitable policy for the selection and deployment of technologies is one thatforecloses as few future options as possible compatible with a given level of present benefit. To fulfil thecriteria of sustainability, the system needs to be adaptable to meet future environmental quality objectivesand demands. Therefore, sewerage infrastructure needs to be structurally adaptable to accept changes in itsmode of operation or the effects of modifications to associated treatment processes. These changes may benecessary to accommodate increasing urbanisation and industrialisation or possible changes to currentindustrial practice in already industrialised areas.
Centralisation versus decentralisation
Technological advancements are generally in response to social need as opposed to in its anticipation(Meadows et al., 1972). The development of urban drainage technology is linked to the need for a potablewater supply which historically has come before any investment in drainage or sanitation. History indicatesthat the development of cities has lead to the installation of a centralised system following on from thefailure of many small decentralised systems.
Centralised systems tend to suit the dense urban environment of inner cities where space is at a premium.Centralised systems are easier to monitor than for many smaller, local systems covering an equivalent area.However, it must be remembered that the consequences of failure of a centralised system are much moresevere (Beck, 1996) as the eventuality of the simultaneous failure of many small decentralised systems isextremely unlikely. Therefore. the introduction of end-of-pipe tech.nology to a centralised collection systemrequires an increasingly sophisticated framework of procedures in order to ensure the safe and effectiveoperation of the system. A skilled and motivated staff is necessary to implement these procedures and theapproach becomes Increasingly expensive as It becomes reliant upon high-tech electronic monitoring andcomputerised decision making tools.
Decentralised systems have several advantages in addition to those already mentioned including. in somesituations, reduced costs associated with the installation of sewers and the pumping of waste waters overlong distances. However. they are not the universal panacea due to issues of lack of adaptability into theurban environment, lack of manageability & control, maintenance of environmental protection standardsand loss of the economy of scale.
Hence, in considering the merits of centralised or decentralised treatment, it seems appropriate to look atthe existing level of development within the city and. in particular, the density of the urban environment;both existing and projected future development. The transition from small, local systems to largecentralised systems and the extent of the centralisation has until recently posed the challenges (Abd ElGawad and Butler, 1995), but now is the time for a re-cvaluation oUrls whole approach.
CONCLUSIONS
Sustainable management of water as a resource must seek to satisfy current demands and environmentalobjectives whilst purporting to satisfy the projected future demands of groWing populations and the need toprotect the long term availability of natural resources.
Towards sustainable urban drainage 61
Sustainable urban drainage is an unattainable goal. Therefore. the approach that we advocate is to reduceunsustainable practices wherever possible. Although the tenns are poorly defined. we suggest thed~velopmen! of "less unsust~n~ble" drainage systems requires both an understanding of the long-tenn andWIdespread Impacts of contmumg current practices and an understanding of the implications of makingproposed changes.In this quest for impro~ement there should be no compromise on the fundamental objectives of publichealth & safety and environment protection. Sustainable urban drainage should:
••••
maintain an effective public health barrier and provide sufficient protection from floodingavoid local and more distant pollution of the environment (air, land. and water),minimise the utilisation of natural resources (e.g. water, energy, materials), andbe operable in the long-tenn and adaptable to future requirements.
It is proposed that progress towards sustainable urban drainage can be made by initially following 3strategies:
• reduce inappropriate "use" of potable water as carriage medium in sewers• separate handling of industrial waste to enable the reuse of sewage sludge• separate handling of stonnwater to restore natural drainage patterns
These strategies may potentially be realised through the adoption of the following techniques:
• domestic water conservation,• small-scale recycling of greywater and rainwater,• on-site storage or infiltration of stonnwater,• utilisation of natural drainage patterns, and• local sanitation technology.
Whatever approach is adopted, an integrated and multi-disciplinary framework is required. Each case mustbe assessed on its individual merits and no one particular technique or technology has all the answers. Anincremental approach containing both high-tech and low-tech solutions is the most likely developmentscenario. Many of the solutions demand the provision of drainage technology appropriate for small-scaleapplications. but in tenns of cost and environmental protection the debate between centralised versusdecentralised treatment has not ended. However, it is clear that if more local solutions are to be adoptedthen the active participation of the users is essential and "out of sight, out of mind" can no longer be anacceptable attitude. The education process must start now.
ACKNOWLEDGEMENTS
The authors wish to acknowledge Peter Kolsky of the London School of Hygiene and Tropical Medicinefor his helpful comments. The second author is in grateful receipt of a research studentship from the UKEPSRC's Clean Technology Programme.
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