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To centralise or to decentralise: An overview of the most recent trends in

wastewater treatment management

Giovanni Libralato

*

, Annamaria Volpi Ghirardini, Francesco Avezzù

Department of Environmental Sciences, Informatics and Statistics, University Cà Foscari Venice, Campo della Celestia 2737/b, I-30122, Venice, Italy

a r t i c l e i n f o

Article history:

Received 6 November 2010 Received in revised form 3 May 2011

Accepted 15 July 2011

Available online 19 September 2011 Keywords:

Centralisation Decentralisation Wastewater treatment

a b s t r a c t

An overview of recent trends in wastewater management is proposed concerning the role of central-isation and decentralcentral-isation in wastewater treatment. The main advantages, criticisms and limitations considering social, economic and environmental issues have been summarised. It resulted that none of the approaches could be excluded a priori, but were generally shown to integrate one another on the basis of the specific required situation.

Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The need for sustainability in the management of water resources is becoming day-by-day more necessary due to their levels of contamination and the frequency of shortages. Indeed, the environment is repeatedly experiencing highly stressing phenomena related to deficient or non-existent wastewater and waste treatment plants, compromising the accessibility to water and sanitation with the resulting health troubles. To cope with this problem, decentralisation, in association with local governance, is increasingly recognised as a potentially suitable way to contribute towards reducing the world’s population with no access to a clean water supply or lacking proper sanitation (Bieker et al., 2010; IDRC, 2010; Larsen and Maurer, 2011), as well as increasing the efficiency of wastewater treatment and treated wastewater recovery and reuse. The goal of environmental sustainability should be pursued to reduce all discharge dilution phenomena, maximise treated wastewater reuse and by-products recovery. The treatment tech-nologies should be efficient and reliable, with low costs for construction, management and maintenance that support self-sufficiency and acceptance by stakeholders and the general public (Chung et al., 2008; Massoud et al., 2009; Afferden van et al., 2010).

Wastewater is nowadays considered as a renewable resource from which potable/non-potable water and energy (e.g. from anaerobic digestion processes) as well as fertilisers could be derived (Pettygrove and Asano, 1985; Mann and Liu, 1999; Lazarova et al., 2001, 2003; Metcalf and Eddy, 2003; Lazarova and Asano, 2004; Guest et al., 2009; IWA, 2011).

Decentralisation seems to increase the possibility of achieving some of the United Nations Millennium Development Goals, i.e. mainly to halve, by 2015, the proportion of the population without sustainable access to safe drinking water and basic sanitation, ensuring environmental sustainability and reversing the loss of environmental resources. Increasing the accessibility to water and sanitation does not imply overexploitation of the existing resources, but improving their management by reducing, recycling and reusing as well as identifying new water sources such as stormwater and reclaimed wastewater (UN, 2010). This topic has also recently gained the attention of the general public thanks to a reader friendly book discussing the importance of decentralisation in both developed and underdeveloped countries (George, 2008).

Centralisation in the urban context is the norm in the developed world, whereas in the developing world the opposite is generally the case. In the latter, wastewater from houses, businesses and industry remains untreated or is frequently treated on-site, and discharged (whether treated or not) into the ground or nearby drains and watercourses. The question facing communities in the developing world is, however, the same, i.e. whether they should install a centralised or decentralised system if they want to deal

* Corresponding author. Tel.: þ39 0412347737/8596; fax: þ39 0415281494/þ39 0412348596.

E-mail address:giovanni.libralato@unive.it(G. Libralato).

Contents lists available atScienceDirect

Journal of Environmental Management

j o u r n a l h o m e p a g e : w w w . e l s e v ie r . c o m / l o c a t e / j e n v m a n

0301-4797/$e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvman.2011.07.010

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with their wastewater. Indeed, nowadays, decentralisation processes seem to be able to satisfy all traditional centralised treatment requirements, with some added values mainly related to the ability of minimising potential residual effluent contamination as well as ecosystem disruption by removing emerging micro-pollutants such as metals, pharmaceuticals and personal care products (Borsuk et al., 2008).

Some years ago,Jefferson et al. (2000)highlighted the fact that small wastewater treatment plants have begun to play an important role at global level in the management of water quality of rivers, lakes, estuaries and aquifers, with a greater numerical growth compared to centralised systems (IPPC, 2003). Indeed, in some countries the total number of small plants can treat a greater volume of wastewater than the existing centralised ones (Deininger and Widerer, 2000). In Italy, more than 9000 wastewater treatment plants present<2000 person equivalent (p.e.) (one p.e. corresponds to a biodegradable organic load of 60 g BOD/day) and in some areas (ISTAT, 1999), mainly due to morphological conditions, decentral-isation is the only suitable option. As an example, in Venice (Italy) historical centre, there are more than 140 small decentralised bio-logical wastewater treatment plants as well as a huge number of septic tanks (Tromellini, 2008; MAV, 2007; IWA, 2011).

The international debate has evinced the existence of various economic, social, technological and environmental constraints in the centralisation/decentralisation dichotomy, and that it is not possible to accept or refuse one of them a priori, being necessary to proceed on a case-by-case basis. The main outcome, is that, apparently, no economic connotation can be defined in general terms, except for the fact that the major costs in centralisation are absorbed by the collection system (Bakir, 2001) and, conversely, in decentralisation by the treatment technology (Hong et al., 2005).

This paper tries to highlight the main aspects of decentral-isation, pointing out the fact that it is not at all in contrast with centralisation, but is a way to integrate and increase the general performance of wastewater treatment.

1.1. Who is involved?

The importance gained by and given to decentralisation can also be monitored through the high and increasing number of govern-mental and non-governgovern-mental institutions that are currently researching on this topic. Among the most important of these are the World Bank, the United Nations, the US Environmental Protection Agency, the Eigenössische Anstalt für Wasserversor-gung, Abwasserreinigung und Gewässerschutz (EAWAG), the Swedish Environmental Protection Agency, the Deutsche Gesell-schaft für Technische Zusammenarbeit GmbH (GTZ), the Stichting Toegepast Onderzoek Waterbeheer (STOWA) and the Rijksinstituut voor Kust en Zee (RIKZ).

At the International Water Association (IWA), a Specialist Group for Small Water and Wastewater Systems has been constituted in order to deal with small water and wastewater systems serving individual households, clusters of households or communities. The emphasis is on localised systems that lend themselves to the recycling and reuse of water and the removal and recycling of nutrients, because it is expected that sustainability can be achieved through the closing of the water and nutrient cycles. More recently the Group’s interest has also included package plants and other systems serving small-scale industry, as well as treating industrial wastewater.

1.2. Definitions

How can decentralisation be defined? Certainly, it is an approach related to wastewater treatment in a not centralised way,

meaning that there is not just one wastewater treatment plant (WWTP) serving a population in a defined area, but surely more than one and, probably, with an assortment of treatment technologies.

The treatment ability classification at European level is defined by the Directive 91/271/EEC and is mainly based on the treated organic load expressed as person equivalent. According to the English Institute of Water Pollution Control a WWTP can be deemed as small with less than 1000 p.e. treated, whereas the US Environmental Protection Agency fixed the same threshold at 10,000 p.e. (De Fraja Frangipane and Pastorelli, 1997). Moreover, these authors considered a threshold level of 2500 p.e. for small WWTPs, evidencing that the same WWTP configurations could be applied until 5000 p.e. (De Fraja Frangipane and Pastorelli, 1997; Avezzù et al., 2010). A decentralised system has been assumed by

Ho and Anda (2004)to supply<5000 p.e., which is one order of magnitude greater than the definition for small systems arbitrarily set by the IWA Specialist Group for Small Water and Wastewater Systems as systems treating less than 100,000 l day1, but still two orders of magnitude smaller than for a centralised system. More-over, it confuted the previous definition for small systems given by the IWA Specialist Group on Design and Operation of Small Wastewater Treatment Plants reporting a treatment ability <2000 p.e. or having an average daily flow <200 m3 (Ødegaard, 1997).

Finally, it can be stated that decentralisation and small plants do not have a one-to-one correspondence, because small WWTPs act locally in a decentralised way, but at the same time decentralised WWTPs cannot always be deemed as small. Moreover, as a general statement it can be said that decentralisation cannot be associated either to small plants or to a defined threshold of p.e. due to the fact that it definitely involves a general matter of scale, as also sug-gested byGikas and Tchobanoglous (2009).

1.3. Treatment scaling

Decentralised treatment is principally defined by the fact that raw wastewater is treated next to the source (Wilderer and Schreff, 2000). Wastewater still requires to be collected, but the use of large and long pipes is avoided, as well as the related excavation works to create a more or less composite collection system network.

Decentralisation in wastewater treatment can consist of from one to several decentralised systems: from individual on-site systems, to a series of larger clusters or semi-centralised plants, going to a range of various potential alternative options that are summarised in Fig. 1. Starting from the extreme level of decen-tralisation, there is the individual based treatment such as in the

STP • Satellite Treatment Plant facilities are integrated with centralised systems for solids processing

SESATS • SEmi-centralized Supply And Treatment Systems Great Block • Wastewater from individual buildings ( can be managed with complete recycle systems e.g. schools)

Cluster • Typically, 4 to 12 or more houses are grouped to form a cluster system for improved wastewater management

Individual • The extreme scenario: treatment systems vary from conventional to advanced Centralisation

• Consisting of a sewer system collecting wastewater that is conveyed to a wastewater treatment plant generally located outside of the limits of the city

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case of the NoMix approach where urine and faeces are separated directly at source via an ad hoc WC (McCann, 2010). Indeed, it seems to allow a better treatment efficiency (Ho, 2005) and great energy saving (Otterpohl et al., 2003). In 2003, the NoMix dry toilet was already used by 700,000 Chinese households, as reported by

Larsen et al. (2009). Moreover, besides the source collection of Anthropic Nutrient Solutions (ANSs), the possibility has been shown of a mix between local decentralised diversion offlows and their centralised treatment based on a pulse recovery of urine into the existing sewers that are practically empty during the night time, eliminating the expensive morning peak of ammonia at treatment plants, as well as the need for any treatment of urine on-site or its truck transport to a plant for centralised treatment (Larsen et al., 2009; Larsen and Maurer, 2011). Further examples of streams separation at source have also been proposed byOtterpohl et al. (2003), Nolde (2005)andPeter-Fröhlich et al. (2007)mainly about the diversion of grey waterflows from the black ones.

A second level is a cluster of typically 4e12 more or less isolated houses, although it might be possible tofind groups of clusters. A third level could be attributed to great blocks such as school, hospital and shopping centre buildings that could also support the direct treated wastewater reuse that is generally on-site. In Japan, this kind of application has gained a broad success with about 2500 decentralised WWTPs in the great block configuration, mainly addressed to single building wastewater treatmentfinalised at on-site effluent reuse (Yamagata et al., 2002; Kimura et al., 2007).

Dimension-wise, the following levels have the potential to be defined as a SEmi-centralised Supply And Treatment System (SESATS) or Satellite Treatment Plant (STP) that could be also integrated within the existing centralised system even if only for solid sludge processing. Finally, there is the most common cen-tralised WWTP with a wide range of treatment ability and ef fi-ciency supported by the relative sewage collection network.

According toOrth (2007), decentralisation could also be classi-fied in three main categories: (1) simple sanitation systems (i.e. toilets) (e.g. pit latrines, pour-flush toilets, composting toilets and aquaprivies), (2) small-scale mechanical-biological treatment plants and (3) recycling systems. The purpose of simple sanitation systems (i.e. toilets), which have simple technology and relatively low cost, is mainly to minimise sanitary problems by retaining faecal matter and discharging the liquid phase, with control of water pollution being of minor significance. Small-scale mechan-ical-biological treatment plants have at least a mechanical and a biological treatment stage or, alternatively, might offer a natural-like treatment such as ponds and wetlands. They are designed to limit water pollution and are generally upgraded with facilities enhancing nutrient removal, disinfection or solids removal, also by means of membranes. In the case of recycling systems, environ-mental protection has top priority. This approach can also support flow diversion while complying with modern hygienic standards, the production of high-quality fertilisers and, eventually, biogas, as well as the possibility of treated wastewater reuse for non-potable purposes. Indeed, there still remains a great potential for waste-waterflows separation besides urine (yellow wastewater) and faecal matter (brown wastewater) diversion, which are both components of black wastewater. Indeed, it is possible to organise the separa-tion, treatment and reuse of white (rain/storm water) and grey wastewater (e.g. kitchen, bathtub, washing machine) (Otterpohl et al., 2003; Nolde, 2005; Peter-Fröhlich et al., 2007; Li et al., 2010). 2. Centralisation or decentralisation: which is the best?

The analysis of more recent trends in wastewater management inevitably led to identifying the major advantages and disadvan-tages of both centralised and decentralised treatment approaches.

This section summarises some statements reporting their pros and cons mainly referring to the essential information that, according to

Brown et al. (2010), should be provided: life span of system elements, estimated capital and operating costs, periodic mainte-nance and operation costs, energy use, residuals, water and nutrient budgets and water reuse potential. Due to their impor-tance, economic and social issues are discussed in the following relative sections. On centralisation, some general statements may be provided from a series of authors such as that:

-the wastewater treatment cost per unit volume is still

competitive compared to decentralisation where the waste-water collection system already exists (Crites and Tchobanoglous, 1998; Bakir, 2001; Ho and Anda, 2004; Ho, 2005; Maurer et al., 2006);

-about 80e90% of the capital costs are related to the collection system with potential economies of scale associated to densely populated areas (Otis, 1996; Bakir, 2001; Maurer et al., 2006);

-it is predicted that the whole collection system or of part of it has to be renewed every 50e60 years, besides the required periodic maintenance, potentially generating disruptions to traffic and other public utilities (Maurer et al., 2006);

-wastewater treatment generally means “to sanitise”, but nutrients and other micropollutants might not be removed (Van Lier and Lettinga, 1999; Tidaker et al., 2007; Crites et al., 2006);

-potential eutrophication phenomena may occur in the receiving water body due to the large volumes of treated wastewater discharged (Wilderer and Schreff, 2000; Ho and Anda, 2004; Ho, 2005; Crites et al., 2006; Libralato et al., 2008);

-rainwater is frequently drained from residential areas by infiltration into the collection system, potentially causing the lowering of the aquifer (Crites and Tchobanoglous, 1998; Ho and Anda, 2004; Ho, 2005);

-diluted wastewater requires more expensive treatment

approaches (Ho and Anda, 2004; Ho, 2005; Maurer et al., 2006; Libralato et al., 2008);

-heavy rainfall events or contamination by industrial

waste-water may generate overflow phenomena (Ho and Anda, 2004; Libralato et al., 2008);

-natural disasters such as earthquakes and terroristic attacks

may cause disruptions to the system generating strong pollu-tion phenomena in the receiving water body (Wilderer and Schreff, 2000; Ho and Anda, 2004);

-diseconomies of scale are possible where long distances have

to be covered or as a consequence of rainwater infiltration (Bakir, 2001; Maurer et al., 2006);

-there is a strong dependency on electrical energy supply that

might not be adequate due to an economic or political crisis (Wilderer and Schreff, 2000; Bakir, 2001; Maurer et al., 2006);

-huge volumes of potable water are required to keep the sewage

system clean (Wilderer and Schreff, 2000; Bakir, 2001; Maurer et al., 2006).

On decentralisation, other general statements may be high-lighted from various authors such as that:

-it may respond to suburban areas and rural centres, industrial,

commercial and residential areas (re)development, as well as to population growth in rural areas and developing countries (Wilderer and Schreff, 2000; Tchobanoglous, 2003; Tchobanoglous et al., 2004; Ho and Anda, 2004; Ho, 2005; Lamichhane, 2007; Weber et al., 2007; Brown et al., 2010);

-it tends to stop the decrease of surface water quality

(Tchobanoglous, 2003; Tchobanoglous et al., 2004; Brown et al., 2010);

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-it may be of some help in the case of great block construction in metropolitan areas, pre-treating/treating and reusing waste-water, even if in part, thus limiting the volume of discharged wastewater into the existing sewage collection system and obviating its upgrading to support greater volume loads (Ho and Anda, 2004; Ho, 2005; Weber et al., 2007; Brown et al., 2010);

-it may contribute in the planning of isolated communities

development (Hong et al., 2005; Lamichhane, 2007; Borsuk et al., 2008; Brown et al., 2010);

-it supports treated wastewater recovery and reuse (Rauch et al., 2003; Tchobanoglous, 2003; Tchobanoglous et al., 2004; Ho and Anda, 2004; Ho, 2005; Hong et al., 2005; Ronteltap et al., 2007; Weber et al., 2007; Borsuk et al., 2008; Brown et al., 2010);

-it reduces or excludes the inconveniences related to discharges collection, with much smaller and shorter pipes compared to centralisation (Rauch et al., 2003; Tchobanoglous, 2003; Tchobanoglous et al., 2004; Ho and Anda, 2004; Ho, 2005; Brown et al., 2010);

-it is applicable to various levels from individual to community

(Bakir, 2001; Tchobanoglous, 2003; Tchobanoglous et al., 2004; Borsuk et al., 2008);

-small WWTPs are considered as viable if a medium-high

technological level is implemented that is efficient, robust, easy to manage and maintain (Tchobanoglous, 2003; Tchobanoglous et al., 2004; Ronteltap et al., 2007), although some unexpected bad performances were experienced mainly due to their managing (Liang and van Dijk, 2010);

-small WWTPs are eligible to be easily remote controlled

facilitating their management (MAV, 2007);

-much of the cost could be related to possible economies of scale that could be achieved organising wastewater treatment on a cluster basis such as in Australia (Fane and Fane, 2005);

-the cost of technologies in decentralisation is becoming comparable to that of centralisation per unit of treated organic load (Fane and Fane, 2005);

-small WWTPs may assure a greater level of environmental sustainability by supporting the potential reuse of treated wastewater as well as nutrients recovery (Rauch et al., 2003; Tchobanoglous, 2003; Tchobanoglous et al., 2004; Fane and Fane, 2005; Ronteltap et al., 2007; Borsuk et al., 2008; Libralato et al., 2008; Brown et al., 2010);

-the potential contamination of nutrients to be reused by metals and xenobiotics in general could be greatly reduced (Ronteltap et al., 2007; Libralato et al., 2008);

-it is possible to reduce eutrophication events (Wilderer and Schreff, 2000; Rauch et al., 2003; Ho and Anda, 2004; Fane and Fane, 2005; Ho, 2005; Crites et al., 2006; Ronteltap et al., 2007; Borsuk et al., 2008; Libralato et al., 2008; Brown et al., 2010);

-it allows urine source separation and to reduce/remove micropollutants such as metals and other emerging compounds (e.g. pharmaceuticals and personal care products) (Rauch et al., 2003; Tchobanoglous, 2003; Tchobanoglous et al., 2004; Ronteltap et al., 2007; Borsuk et al., 2008);

-small WWTPs allow the separation of domestic wastewater

and rainwater, avoiding dilution phenomena (Ho and Anda, 2004; Ho, 2005);

-it is possible to operate a separation of contaminants at source,

easing their treatment and potential reuse and at the same time increasing treatment efficiency and saving energy (Rauch et al., 2003; Tchobanoglous, 2003; Tchobanoglous et al., 2004; Borsuk et al., 2008; Brown et al., 2010);

-it is possible to exclude the possibility of domestic wastewater contamination by industrial wastewater as well as the relative sludge produced (Bakir, 2001; Borsuk et al., 2008);

- it is possible to maximise the in situ reuse of treated waste-water, as a consequence of diminishing the final discharge volume and the potential cumulative impacts on the receiving water bodies (Tchobanoglous, 2003; Tchobanoglous et al., 2004; Brown et al., 2010);

- it is possible to considerably reduce the health risk for the

population, also by preventing catastrophic events (Bakir, 2001; Tchobanoglous, 2003; Tchobanoglous et al., 2004; Libralato et al., 2008; Brown et al., 2010);

- small WWTPs are suitable for isolated or scattered settlements

or in the case where only a small amount of space is available for the installation (Bakir, 2001; Brown et al., 2010);

- small WWTPs are generally compact, with highly flexible

operating conditions and reduced aesthetic impact (Bakir, 2001; MAV, 2007; Libralato et al., 2008; Brown et al., 2010).

2.1. Economic issues

In centralised systems, it is well recognised that most of the financial costs are related to the construction and maintenance of the sewage collection system. Conversely, most of the decentral-isation costs are related to the treatment unit (Hong et al., 2005), but some economies of scale could be achieved mainly on the basis of a cluster treatment organisation (Ho and Anda, 2004). In Japan,

Kimura et al. (2007)stated that the recovery and reuse of treated wastewater in small decentralised treatment plants is comparable to that of tap water. In particular, having a discharge with a daily flow of between 50 and 200 m3, the specific costs in both

circum-stances were estimated to be about 7 US$ m3.

Taking into account the Australian urban context,Ho and Anda (2004)affirmed that considering similar building and operational techniques as well as pollution removal abilities (20 mg l1BOD and 30 mg l1SS) decentralised and centralised systems present very similar costs. It has been estimated that the connection to a public sewage collection system could vary between 4500 and 10,000 US$ per property, with routine maintenance between 500 and 1000 US$ y1per property. In Italy,Rocca (2010)found that the energy consumption of decentralised plants is not so significantly different from that of centralised ones considering both the volume (m3) of treated wastewater and the kg of COD removed per unit time.

Maurer et al. (2006)tried to estimate in Europe and USA when the cost of decentralisation technologies would be competitive compared to centralisation, showing its competitiveness only when savings are also able to cover the costs related to the abandonment of the previous wastewater treatment system. Nevertheless, it has been highlighted that decentralisation costs tend to diminish when considering a scenario implying the use of an existing sewage collection system with a rate of 262e679 US$ per person in the case of a city with more than 50,000 and less than 10,000 inhabitants, in that order. Vice versa, the total absence of an existing collection system network coupled to urine source separation make costs oscillate between 665 and 2179 US$ per person, considering a city with >50,000 and <10,000 inhabitants, respectively (Maurer et al., 2006).

Brown et al. (2010)reported that on-site wet composting systems (i.e. similar to a waterless composting toilet with worms degraded compost to be removed about every 5 years) and cluster greywater treatment in Australia are about 10,000 AU$/household in capital cost. Options with an individual on-site urine storage tank have capital costs about 25% higher than equivalent options without urine separation due to the provision of an individual urine storage tank for each household. For activity centres, it has been estimated that the increase is only about 1000 AU$/household due to the economies of scale.Brown et al. (2010)compared analysis on 18 decentralised and on-site WWTPs in Melbourne (AU), showing that total costs per

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household are significantly lower for the cluster option than for the on-site one and that they are higher for wet composting systems than for dry ones. Anyhow, the urine separation option provided the greatest opportunity to recover and reuse nitrogen.

2.2. Social implications

Social repercussions of small WWTPs, and of decentralisation processes in general, are frequently underestimated compared to the economic and environmental ones. Actually, the general feeling is that centralisation has no reason to be substituted by decen-tralisation where it is already in force (Ho and Anda, 2004). This sounds rather obvious, but sometimes it may not be. Centralised systems are already accepted by the general public. Indeed, the public is aware that treatment processes are ongoing in continuum and are generally guaranteed by a public authority that is in charge of the management of WWTPs, as well as of the sewage collection system requiring a corresponding pay per unit volume tariff. This means that wastewater treatment is operated by third parties: the wastewater producers are not directly involved in wastewater treatment and probably most of them wish not to be involved (Lienert and Larsen, 2006). In the case of decentralisation, apart from the authorisation and implementing processes, it is the end user who is in charge of its management: this is the most important aspect to take into account. Moreover, it has been verified that some NoMix pilot projects developed in Austria, Denmark, Germany, Luxembourg, The Netherlands, Sweden and Switzerland have been widely accepted, thanks to the environmental awareness of contributing to nutrient recycling in agriculture. However, some inconveniences have been reported mainly about clogging and smelling (Larsen et al., 2009).

Centralised WWTPs satisfy the demand of highly populated areas, but they do notfit with the new expectations about water recycling and reuse as well as nutrients recovery and elimination of emerging pollutants. At the moment, the new concept of urban village could strongly favour decentralisation. This approach is characterised by the presence of alternating urbanised and unurbanised areas that could permit the reuse of treated waste-water for both waste-watering and fertilising the green zones (Ho and Anda, 2004). Anyway, some criticisms have been highlighted about regulations, which are generally lacking for decentralisation processes, as well as about management and performance rates that do not always reach very high-quality standards (Fane and Fane, 2005).

3. Decentralisation in practice

In the urban context of developed countries, centralisation is sometimes the unique wastewater treatment solution, thus it is surely the most applied approach to treat wastewater. Conversely, in developing countries decentralisation has been assuming great importance (Ho and Anda, 2004). Anyway, decentralisation processes are already recognised worldwide and accepted by both water professionals and lawmakers. As an example, 25% of the USA population is served by small decentralised WWTPs, mainly in rural areas or where the construction of a sewage collection system is not economically viable (UNEP, 2002).

Small end-of-pipe decentralised WWTPs are fast growing in USA due to their generally low costs and to the fact that the perception related to their existence does not differ from the traditional centralised ones (Bakir, 2001). Analogously, in Colombia, the costs related to one big centralised WWTP have been reduced in favour of two decentralised WWTPs (Bakir, 2001), which could also be seen as a lower level of local centralisation.

Japan is in the avant-garde of decentralisation processes, where

Kimura et al. (2007)reported the presence of 2500 decentralised WWTPs, principally devoted to wastewater treatment and reuse in great blocks of commercial and residential buildings. It has been estimated that decentralised WWTPs are installed by 26% of public offices, 13% of private buildings and another 15% in schools, hospital and sporting centres.

In Australia, the city of Melbourne with an approximate pop-ulation of 3.9 million is studying a portfolio of decentralised and on-site design concepts of WWTPs. This is a strategy to cope with the uncertainty in future sewage production and its reuse and the need to prepare for integrated water cycle planning. The existing Melbourne sewerage system is largely centralised, thus about 90% of sewage discharges are conveyed to two major centralised plants. However, several small satellite treatment plants service local urban areas generally more distant from the centralised system. The use of decentralised WWTPs in Melbourne is still rare, but the aim of the future integrated water planning is to combine cen-tralisation with various levels of decencen-tralisation as well as on-site operations (Brown et al., 2010).

In Italy, 6% of the population is served by WWTPs with less than 2000 p.e., which represent 73% of existing Italian WWTPs (Avezzù et al., 2010): this is mainly for reasons of morphology. Moreover, an interesting case study is represented by Venice. This well-known ancient city that was built on a series of 119 islands located in the middle of a 540 km2lagoon with an average depth of 0.5 m does not have a real sewage collection and treatment system due to it peculiar urban characteristics, but has a huge number (4493) of on-site decentralised WWTPs (MAV, 2007). Their installation, sup-ported by policy and lawmakers, has been reducing the total load of inorganic and organic contamination, enhancing the general health and environmental status of Venice lagoon.

4. Increasing potentiality

Today, the growth of urban fringes, rural centres and the refurbishment processes of industrial, residential and commercial areas have imposed the consideration of alternative scenarios to traditional wastewater treatment systems (Bakir, 2001; Ho and Anda, 2004). There are several variables that might influence the future development of decentralisation processes such as the population growth in rural areas and developing countries, the decrease in surface water quality, the construction of great blocks of buildings in metropolitan areas, the planned development of iso-lated communities, the growing scarcity of water resources as well as the attention paid to water recovery and reuse (Randall, 2003).

The approach to be followed to support and integrate other than centralised options should be assessed attentively, examining management, administrative and environmental protection issues. Some variables that should not be underestimated are the pop-ulation density, the morphological characteristics of the area to be served as well as the scale factor, the costs of investment, mainte-nance and management, environmental protection and long-term sustainability, the conservation of energy and water resources, the possibility to reuse treated wastewater, the protection of public health, the policy of human settlements development and the role of technological content that is strictly related to treatment ef fi-ciency (Ho, 2005; Chung et al., 2008).

The dichotomy centralisationedecentralisation is nowadays at the centre of the debate in wastewater treatment science, but it can be said as a consequence of the present analysis that one approach cannot exclude the other and vice versa. The possibilities for wastewater treatment are quite huge and a sort of transition exists in decentralisation processes moving from individual on-site treat-ment, to cluster or community type (e.g. great blocks), to satellite

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treatment and semi-centralised WWTPs. Each type is substantially related to the characteristics and volumes of wastewater to be treated as well as to the possibility offlow separation at source.

It is evident that the adoption of decentralised strategies is not in contrast with centralised ones, as shown byBrown et al. (2010). Indeed, highly dense populated areas are in general historically served by a sewage collection system and one or more centralised WWTPs, thus decentralisation would not represent a suitable and viable economic alternative (Avezzù and Anselmi, 2007). Anyway, the general approach would be to support a true coexistence between centralised and decentralised systems with various levels of applicability, which appears to be more realistic especially in the case of great blocks such as commercial centres, hospitals, airports or in new developing areas where treated wastewater reuse might be effectively planned (Guest et al., 2009). However, it is frequently verified that the theoretically more suitable approach results as inadequate in the real world principally because of regulatory requirements and urban and industrial models of development, which may have led to the creation of situations that are hard to manage especially on the economic side. Under this point of view, decentralisation might be supported by a new hypothesis of legislative requirements related to population density or to the total surface of developed areas.

Historically, the collection and treatment systems have been set up to manage wastewaterflow generated in urban contexts that are typically conveyed by gravity to the central treatment facility. In the meantime, the development, redevelopment and growth of urban and peri-urban areas have made the centralised WWTP frequently unable to satisfy the treatment requirement of the grown waste-waterflow. Moreover, most of the time the existing WWTPs often cannot be enlarged because the urban and industrial development has occupied the areas that could have been used for this. Besides, the extension of collection systems may lead to the interruption of roads and other public services and this is an option not well accepted by policymakers. As a consequence some alternatives should be introduced, such as decentralised and satellite treatment systems (Gikas and Tchobanoglous, 2009). The continued growth of urban areas has determined an increasing demand for water resources from both surface and groundwater for potable and non-potable purposes. Indeed, as a consequence they are already over-exploited in several areas. High quality treated wastewater might be a very interesting solution in order to cope with water scarcity, representing a stable and sustainable clean and controlled water source. Decentralised as well as satellite WWTPs may allow actions to be taken in this direction, obtaining at the same time a reduction in potable water demand and increasing awareness of sanitary and environmental issues. Indeed, the recent discoveries about the importance of water resources are making policymakers particu-larly attentive to tightening water legislation, which is becoming increasingly strict about concentrations limit values, and support-ing recovery and reuse policies under this viewpoint. Treated wastewaters arefinding a lot of applications from agricultural and landscape irrigation to industrial, environmental and recreational uses (Gill and Rainville, 1994). Direct potable reuse is still extremely unusual (Harrhoff and Van der Merwe, 1996), but some indirect-potable reuse such as groundwater recharge and surface water augmentation are becoming increasingly frequent (Crites et al., 2006; Crites and Tchobanoglous, 1998). Treated wastewater reuse could be particularly strengthened in those areas that are histori-cally affected by drought phenomena or are expected to suffer from water scarcities in the near future as a consequence of global warming. Afirst reuse classification could be provided on the basis of reuse application scales into two major categories: reuse from on-site treatment and reuse from cluster treatment units (Brown et al., 2010). In relation to thefirst option, reuse is mainly focused

on a local basis, such as in-house cold water washing machine inlets, toiletflushing, car washing at home, garden/lawn irrigation or subsurface irrigation, compost from dry toilet used in the garden, except for urine that can be transported to remote commercial farm sites. Concerning the reuse from cluster units, this is mainly focused on in-house cold water washing machine inlets, toilet and car washing at home through a third pipe from cluster units to houses, public open space irrigation or subsurface irrigation, release of effluents to an environmental buffer/raw water storage for further water treatment as part of an indirect-potable reuse scheme. Moreover, urine can be collected in on-site storage tanks and transported to remote commercial farm sites (Brown et al., 2010).

For urine treatment, it has been estimated that in Europe an investment of 260e440 US$/person would cover the difference between the NoMix approach and traditional treatment systems (Oldenburg et al., 2007). According toLarsen et al. (2009), this could be verified only when the NoMix becomes really widespread. Moreover, considering the need to remove micropollutants (e.g. pharmaceuticals), the source separation approach still appears to be more stringent, as traditional WWTPs are not capable or efficient enough in treating this new category of pollutants. Conversely, small decentralised WWTPs, especially those operating at source, have shown a high efficiency in their reduction/removal (Larsen et al., 2004; Kujawa-Roeleveld and Zeeman, 2006; Joss et al., 2008). Indeed, 60e70% of pharmaceutical and hormone-like substances tend to concentrate in urine (Lienert et al., 2007a), although a certain amount is present in faecal matter, determining a distribution of the ecotoxicological potential that seems to be equally split between the liquid and solid fractions (Lienert et al., 2007b). Nevertheless, the elimination of even half of the organic micropollutants load would in itself represent an interesting success due to the environmental impact that they generate to the target environmental compart-ments and their relative biota (Lienert and Larsen, 2006; Burkhardt et al., 2007; Larsen et al., 2009).

Other aspects related to decentralisation also involve national security concerns. Centralised WWTPs can be seen as an easy-to-attack target that could seriously affect life in some urban areas, due for example to the physico-chemical and microbiological contamination of surface water preventing its use as a drinking water source. A series of decentralised WWTPs may considerably reduce the risk and potential impact to the receiving water body without compromising the system functions. Moreover, decen-tralisation processes can reduce the impact of natural disasters such as aflood, tornado, hurricane, volcanic eruption, earthquake, or landslide that could in this way affect only a limited part of the territory keeping the rest safe.

Wastewater source separation and decentralised treatments are in general more appealing when there is no sewage collection system. Traditionally, source separation has mainly been considered as more appropriate in rural areas (Nelson and Murray, 2008), but recent trends have demonstrated how this kind of approach may also be of some interest in highly dense populated areas, especially in developing countries (Nhapi, 2004; Nhapi and Hoko, 2004).

Larsen et al. (2009)evidenced how in the Chinese city of Kumming, characterised by rapid industrialisation combined with water shortages, local experts tend to support the separation offlows at source (Medilanski et al., 2006). In coastal areas with no wastewater treatment facilities, the containment of N-compounds in order to prevent water eutrophication phenomena has been prioritised by urine diversion at source (Larsen et al., 2007). Indeed, small decen-tralised WWTPs are able to retain and variously dispose of up to 80% of N-based compounds, i.e. 20e30% more than a traditional cen-tralised activated sludge WWTP (Larsen et al., 2009).

A frequent criticism about decentralisation is the absence of economies of scale, and thus specific regulations about (Fane and

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Fane, 2005). This means that the decentralised option is taken into account only when the construction of a sewage collection system is not economically viable. However, the technological innovations in wastewater treatment processes have increased the competi-tiveness of decentralisation, an example of this is membrane technology (Di Giano et al., 2004), which is becoming less expen-sive year by year (Tewari et al., 2010). Anyhow, this is not only a technical matter, but also an issue related to economy of numbers: today large amounts of membranes are produced and their cost is decreasing. Moreover, other technologies for source separation are already economically viable, making some decentralisation processes more easy to realise (Oldenburg et al., 2007).

5. To plan within a decentralised perspective

Four elements exist that may influence the decision-making process in decentralisation such as cost, flexibility of land use, maintenance and environmental protection, especially in the case of small communities (Engin and Demir, 2006).Chung et al. (2008)

proposed a model to assess the suitability of decentralisation, showing that itfits best to territories with mixed morphology and scattered urban centres. Comparing a centralised WWTP and a series of satellite WWTPs for a community of 1.2 million inhabi-tants, it has been observed that economies of scale would surely favour centralisation, except in the case of an area with a range of significantly different heights above sea level that would make the latter option more suitable. Innovators must tackle the costs related to initial infrastructural investments and concerted actions are required between the various stakeholders involved to support the overall costs. On the contrary, the centralised model has received huge public capital investments making it sustainable due to the economies of scale that are created. Moreover, it is possible that the major costs that could be required to terminate the collection system would be saved by reducing the technological investment compared to initial expectations. There is also the risk that the support offered to innovators able to create high value content niche products in the wastewater treatment sector could be in contrast with the interest generated by new and economically viable options, even though the definition of a risk assumption policy is compulsory.

In Europe, the water sector management is already oriented towards considering the full costs of the resource. The effects of water policy changes will become evident in the near future. However, they will not be overestimated because the benefits obtained from a good quality water environment are still consid-ered as intangible, which is the main reason for the lack of support for innovation in this sector. The financial support could be strengthened if the water resource received greater development mainly in the domain of public health, tourism, education and research. These uses requiring a high water quality would justify the support for innovation and innovators (Krozer et al., 2010).

6. Conclusions

This paper provided a general overview about the new trends in wastewater treatment, highlighting how the traditional centralised approaches are changing in favour of new decentralised treatment levels. When talking about decentralisation, it is not possible to refer to just the NoMix toilet or any other extreme individual treatment system, but to a range of wastewater treatment facilities presenting a strong scaling transition. It is evident that in most cases the adoption of a decentralised approach in highly dense populated areas with an already existing sewage collection system could not be a viable alternative to the centralised treatment. It is not possible to give a general clue on how to approach decentral-isation due to the high number of conditioning factors (e.g. social,

economical and environmental) and the creativity of engineers puzzling their brains to make alternatives to traditional wastewater treatment modes. The suggestion is to support the coexistence of various level of centralisation and decentralisation in WWTPs considering the potentiality of the full series of decentralised approaches that are currently showing a highly realistic appeal, mainly in the case of great blocks (hospitals, shopping centres, airports, schools) and refurbished urban areas, especially in relation to the new trend of treated wastewater recovery and reuse.

Moreover, it could be of some interest to speculate upon the upcoming future necessity of current centralised WWTPs substi-tution as well as refurbishment and upgrading of their collection system due to their ageing. Major interventions are estimated to be required every 50e60 years, but how to tackle this problem? Would it be better to have the pristine centralised facilities, generating traffic and other public utilities disruption? Or would it be possible to introduce some kind of decentralisation in the system? Today, this second option appears to be sometimes more suitable, mainly due to the continual growth of urban areas as well as the increasing demand for water resources. The use of decen-tralised and satellite systems will allow treated wastewater recovery and reuse, making them a stable and sustainable water source, especially for those areas that historically suffer or have recently been suffering from water scarcity.

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