Sustainability of decentralized wastewater treatment technologies

About half of the world population currently lives in rural areas. In the EU, almost 30% of the overall population of the former CEE (Central and Eastern European) countries (42 million people) lives in settlements with less than 2,000 inhabitants, while the percentage is less than 20% in the western part. Many other areas of the world show a preponderantly rural character, although this tendency is slowly decreasing, due to the ongoing urbanization phenomenon. A large part of this population is still waiting for proper sanitation systems, or is aiming to improve the efficiency of existing ones and scale-up environmental protection and resources recovery. In most cases, centralized treatment systems for rural communities or peri-urban areas in low income countries would result in long-term debt burdens for the population (Parkinson & Tayler 2003), therefore system decentralization appears as a logical solution to tackle the problem, as these facilities can usually be built to fulfil current needs and be expanded as need arise. Even in developed countries, cities are gradually losing their character of densely concentrated settlements and are gradually sprawling to the countryside: the urban area of Paris now counts over 11 million inhabitants (up from about 4.5 millions in the early 1900's) and extends well beyond the original urban administrative boundaries (over 17,000 km² versus the Ville de Paris’s initial 2,850 km²). In areas where construction of a sewage collection system is not considered economically viable, decentralization is becoming quite popular: as an example, 25% of the population in the US is already served by small, decentralised wastewater treatment plants (WWTPs) (UNEP 2002).

Sustainable decentralized sanitation focuses on on-site treatment and on recycling of resources contained in domestic wastewater, in primis, water itself. Other resources that can be readily recycled are: bio-energy (from transformation of organic material), and nutrients (mainly nitrogen and phosphorus). Decentralisation could therefore contribute to the continuing progress and completion of the Millennium Development Goals, promoting environmental sustainability and reversing loss of environmental resources. This tendency will be more relevant in the next future, thanks to new global pressures towards water management paradigms change, from waste-oriented approach to resource-recovery and water reuse ones. Such a move to decentralized water management even in developed countries’ urban areas, could be essential in order to improve system resiliency, efficiency, lost or diminished environmental functions, and resource recovery (Novotny & Brown 2007). In this context, the term decentralized also qualifies systems serving small portions (clusters) of the urban area, according to hydrology, landscape and local ecology considerations.

With most of the developed world urban water infrastructure close or past its useful design lifespan (usually 50–60 years), and thus due to undergo substantial rehabilitation/refurbishment in the next decade, switching to smaller, cluster-base systems could not only be a sustainably-wise sensible solution, but, in the long term, a financially sound one, as well.

The purpose of this paper is to discusses merits and drawbacks of the various illustrated technologies, in view of their sustainability potential, and according to the new development paradigms for urban water systems, that encourage the development of local water-cycle clusters with local reuse and recycle of the resource, and possible local recovery of energy and/or materials.

The two main objectives of wastewater management systems are, and have been since their development: first, to protect and promote human health, by breaking the cycle of disease, and, later, to provide water quality and ecosystems protection, by avoiding the negative effects of excessive pollutants discharge into the environment. These objectives hold their validity whichever service modality (centralized or decentralized) is implemented. The difference between centralized and decentralized wastewater treatment (DWT) is that the latter is used to treat and dispose, at or near the source, relatively small volumes of wastewater, originating from single households or groups of dwellings located in relatively close proximity (indicatively, less than 3 km maximum), or in peri-urban development clusters, not yet served by a central sewer system connecting them to a regional WWTP. DWTs still need a local collection system, yet this will be much smaller and less expensive than those used for conventional centralized treatment. DWT can be a sensible solution for communities of largely different sizes and demographics in countries at any level of development, but, like any other WWTP system, DWTs must be properly designed, maintained, and operated to provide optimal benefits. In general, almost all current wastewater treatment technologies could theoretically be applied into a decentralized setting, considering also that the definition of decentralized may vary in size from a few to a few thousand people served; not all of these technologies constitute, however, sensible choices. Advantages of DWTs are several: they can effectively and efficiently treat domestic sewage to protect health, water quality, and support local water supplies, since wastewater treated by decentralized systems is more likely to remain in the local watershed. Using decentralized systems may thus make it easier for a community to implement local water reuse schemes for nondrinking purposes, and hence reduce inappropriate demand for treated drinking water. Energy and resources local reuse can also be facilitated by such systems, sometimes in combination with other waste (e.g. organic solid) disposal facilities (Capodaglio et al. 2016a).

The simplest form of DWT consisted historically of a simple underground septic tank (cesspool), which both settled suspended solids, and achieved some degree of anaerobic digestion. In hot climates, septic tanks can remove up to 50% of the organic load of ‘normal strength’ sewage, but usually they achieve little in the way of pathogen reduction, requiring post-treatment (adding cost and complexity to the system) to achieve environmental standards. In some Eastern EU Countries, this technology still supplies as much as 70% of wastewater processing, sometimes as a pre-treatment (Istenic et al. 2015). As a result of strict EU legislation, existing infiltration or percolation system (sand/soil non-planted filters) formerly used as post-treatment for such installations, are gradually being dismissed in these countries.

Another common class of DWT systems are waste stabilization ponds, that include anaerobic ponds, facultative ponds (combining aerobic and anaerobic processes), and purely aerobic maturation ponds. The obvious advantage of pond systems is their simplicity, while another is the long retention times that favour pathogen abatement. Ponds could also produce secondary economic benefits, as maturation ponds may provide a good environment for growing fish, such as tilapia, which would be an advantage in rural, developing areas. Effluent from these ponds may have fairly high algae concentrations, so it is also good for irrigation, and minimize the level of nitrates entering the ground water. The main disadvantages of waste stabilization ponds is that they require relatively large land areas (US EPA 2015). For this, and other reasons, more compact, aerobic DWT technologies have often been adopted where land availability is an issue.

Most decentralized systems take advantage of gravity flow, rather than pumping, may incorporate septic tanks at the source, resulting in reduced costs and energy demand, and can easily be scaled up to the needed size in communities with rapid growth, thus using wisely energy, money and land. Advanced DWTs can achieve treatment levels comparable to centralized wastewater treatment systems, and can be designed to meet specific treatment goals, handle unusual and peculiar site conditions, and address local environmental protection requirements. Therefore, DWTs mostly comply with new paradigms for sustainable urban development (Capodaglio et al. 2016b).

In addition to the traditional Imhoff tanks and small activated sludge (AS)-based plants with sludge separation traps, used in extensively in earlier times, and still adopted in many rural/developing countries situations, several other process technologies are being preferentially developed, among all the available ones, for use in DWT systems. Research in decentralized applications has recently focused on a few classes of processes, including natural systems (i.e. constructed wetlands (CWs)), improved aerobic, or anaerobic biological systems (both suspended or attached growth) (Singh et al. 2015). These, therefore, will be addressed in the present paper. Applicability of these technologies may depend on the characteristics and climate of their proposed location: general climatic conditions, wastewater strength (dilution) and variability, land requirements and availability, and local recycle/reuse requirements may strongly influence the feasibility and operational outcome of an installation. The main influencing factors in a DWT's technology choice are: specific treatment efficiency in that specific condition, low O&M requirements, operational reliability and future, gradual expansion possibilities, favourable economics.

CWs are engineered, natural treatment systems that are being used throughout the world as DWT systems, with a diversity of design and operational features that can be adapted to treat domestic, agricultural and industrial (mostly agro-food) wastewaters. The simplest CW system is a pond, where algae and bacteria are used for wastewater treatment. More complex systems are designed to work with soil and plants for filtration and biochemical reactions. Since they can provide simple and cheap solutions for wastewater treatment, use of CWs for small to medium size settlements is increasing sharply in Mediterranean and tropical countries due to favourable climatic conditions (Machado et al. 2016), although even in northern EU countries, such as Poland, Estonia and Lithuania, positive experiences with CWs have been reported (Mander et al. 2001). These systems, contrary to common prejudice, when properly designed and maintained, can be operated even under cold Baltic climate conditions with good treatment efficiency of organic substances.

CWs have several inherent advantages compared to traditional systems, including: very low capital costs, less infrastructure, lower operating costs, simplicity of design and ease of operation. They include surface-flow (SF-CW), subsurface-flow (SSF-CW, either vertical VF-CW, or horizontal HF-CW flow), and hybrid (multi-stage) systems (HCW). SF-CWs can be further classified by the dominant macrophyte type in the system. Removal efficiencies in CWs depend mostly on the following parameters: hydraulic conductivity of the soil, type and amount of microorganisms, available oxygen supply, substrate chemical characteristics, latitude regional climate. Temperature can play an important role even though SSF-CWs generally show less sensitivity to this factor. SF-CWs are more sensitive to solar radiation, promoting higher degradation rates, and thus tend to be more effective in warmer climates. In order to build facilities that combine the best operating concepts of the existing technology, HCWs have been proposed, consisting of a succession of stages such as: primary treatment by Imhoff tank, 1st stage horizontal subsurface flow (HF-CW) system, 2nd stage vertical subsurface flow (VF-CW) system, and possibly additional HF/VF stages (Behrends et al. 2006). Such multiple stages systems are now becoming more common, due to higher tolerance to flow, load and waste characteristics variations, and their generally lower footprint. HCWs have shown to provide excellent secondary and tertiary treatment for municipal and domestic wastewater with variable operative conditions in small-to-medium size installations (500–5,000 P.E.), in different climates. An eventual final, open, shallow-water stage may also be adopted, to enhance effluent oxygenation prior to discharge, and provide solar UV disinfection.

CWs offer reliable and steady removal of TSS and organic matter (in the long-term, over 97%), allowing to obtain very low concentrations in the effluent both in low and high inlet concentrations situations. Removal of nutrients has been observed at about 70–86% for ammonia, and 60–70% for Total N, with unit area (U.A.) requirements as low as 1.5 to 2 m²/P.E. in warm climates (like Italy and Spain). In cold climate countries (e.g., Poland, Estonia, Lithuania) the required U.A. ranges usually from 5–12 m²/P.E. Reported operating costs are quite low, about 0.1€ /m³ treated wastewater, with construction costs related for the most part to the land surface needed (Masi et al. 2013). CWs therefore can be considered an interesting solution for DWT, especially in locations with warm temperatures, extensive radiation hours, and available land extensions.

Table 1 summarizes some the main features (design values, applied loads, average efficiencies) of CWs reported in literature.

Aerobic treatment processes involve the use of oxygen by microorganisms for degradation of organics. Their footprint is much smaller in comparison to natural systems, but their energy footprint (consumption) is much higher. In comparison with natural systems, they are capable of providing good quality effluents, which can easily meet effluent discharge standards directly. A typical example of aerobic system is the AS process. Although requiring, at the simplest implementation level, only semi-skilled personnel, which makes it a good technological option in DWT for low income and developing countries, AS systems require gravity-driven separation phase of the solids produced in the process, that needs relatively large settling tanks. A new process to overcome this limitation was developed with membrane bio-reactors (MBRs).

MBR technology integrates biological degradation of wastewater pollutants with membrane filtration, ensuring effective removal of organic and inorganic contaminants and biological material from domestic and/or industrial wastewaters, and has become a proven, more compact alternative to traditional AS systems. The filtration component (pore size is typically <1 μm) dispenses the need for gravity clarification of the effluent, that could constitute a critical treatment bottleneck in small systems under highly varying hydraulic loads and even induce process failure (Capodaglio 2002).

Given their smaller space requirements and more effective operation (filtration allows to obtain higher biomas concentration within the process, which further lowers active aeration volume requirements) MBRs represent an ideal solution for DWT in peri-urban or urban areas under renovation, where only small free spaces could be available for new WWTPs (or these would have to be built underground).

Use of membrane systems in decentralized treatment of household (domestic) wastewater was described by several researchers (Meuler et al. 2008; Blstakova et al. 2009; Pikorova et al. 2009; Matulova et al. 2010; Chong et al. 2013). MBRs, when properly operated, have also shown the capability to effectively remove nutrients and, to some degree, micropollutants, from a waste stream (Abegglen et al. 2008).

Generally, treatment of residential wastewater by MBR systems would produce effluent with non-detectable TSS, BOD concentration (less than 2 mg/L), ammonia-nitrogen concentration of less than 0.5 mg/L, fecal coliform count of less than 20 per 100 mL and, with proper design, total nitrogen concentration of less than 5 mg/L. MBR application to domestic wastewater can remove more than 96% COD, 90% TSS and 90% TN. Application to segregated domestic wastewaters (black, with COD_ave 1,220 mg/l, and grey, with COD_ave 250 mg/l) from a housing neighbourhood showed removal efficiencies greater than 96% COD, 99% TSS, 89% TN and 100% coliforms for black water treatment, and greater than 95% COD, 94% TSS, 92% TN and 100% coliforms for grey water treatment, with specific energy demands of 2.3 and 1.7 kWh/m³ treated wastewater, respectively (Atasoy et al. 2007).

Comparative advantages with respect to traditional treatment techniques include smaller footprint, high loading rate capabilities, modularity and disinfected/highly clarified effluent immediately suitable for reuse. Consequently, MBR technology could play a prominent role in DWT systems. Limitations inherent to these processes are the cost of membranes themselves, high maintenance and energy requirements, and the progressive loss of filtration capacity due to medium fouling.

A technology quite similar to MBRs, consisting of a reactor with suitable filters for biomass separation, was proposed by Capodaglio & Callegari (2016), after successfully testing it with poorly-treatable organic contaminants (Capodaglio et al. 2010; Capodaglio & Callegari 2015). The biomass concentrator reactor (BCR) consists of an aerobic reactor vessel, in which a mixed liquor formed by wastewater (in this case, domestic) and biomass is kept in suspension by means of fine bubble aeration, positioned at the bottom of the vessel. The treated effluent is filtered by a membrane-like medium, with pore size of about 20 μm for solids separation purposes, just like a MBR. In this case, however, due to the coarser characteristics of the filter, effluent filtration occurs by gravity only with a maximum head loss in the order of 2–3 cm.

Laboratory tests with these systems showed average COD removal efficiencies of 93–97% In the same tests, specific flux through the membrane was approximately constant at 22 L/m²d, a very low value compared to the rated filter capacity. Consequently, the filtration capacity of the membrane remained substantially constant during the tests and no backwashing became needed. It has been observed that similar systems with gravity flow can sustain continuous operation for up to 1 year without incurring in operatively dis-habilitating fouling (Zhang et al. 2006). This is contrary to similar trials conducted with actual membranes (pore size 0.1 μm) in similar conditions, where the medium filtration capacity decreased by 77% after just 3 months, requiring membrane substitution and/or regeneration (Pikorova et al. 2009).

The system, similarly to MBRs, can be modified to achieve nitrogen removal. Such systems have shown to achieve 75–79% N removal, figures absolutely comparable to those of MBRs (Scott et al. 2013). Table 2 summarizes some the main features (design values, applied loads, average efficiencies) of MBRs and BCRs reported in literature. It should be noted that actual values may differ significantly from those summarized here, depending on actual medium type and characteristics, wastewater composition and operational parameters.

Anaerobic treatment processes achieve biological treatment with production of methane (however, hydrogen can also be a process product) and biomass through basic mechanisms involving a sequence of steps (hydrolysis, acidogenesis, acetogenesis and methanogenesis) in the absence of oxygen. These systems are therefore classified as low-energy consuming processes due to this feature. As anaerobic bacterial kinetics are slower than aerobic, these systems in practice achieve only moderate effluent quality and are usually followed by another treatment step in order to meet the most stringent discharge standards. Despite this, anaerobic treatment systems are cheap to implement, simple to run, and can be an excellent energetic recovery mean. Today, anaerobic digestion is traditionally used to process residual sludge from large centralized WWTPs, with recovery in the form of a methane mixture (biogas). Anaerobic digestion, however, was originally the first technology used for DWTs, in the form of septic tanks (i.e., Imhoff tanks), with usually poor treatment performance (30–50% COD and 60% TSS). More modern forms of anaerobic treatment are currently considered an attractive, sustainable and suitable technology for on-site wastewater treatment due to low energy consumption, relatively small space requirement and simple reactor design. As no oxygen is needed, high costs of aeration (more than 50% of the overall process cost of aerobic treatment processes) are avoided, and sludge handling costs are dramatically lower, as its production is 3–20 times lower than in aerobic systems. Efficiency and effectiveness of anaerobic processes are enhanced by concentrated substrates (COD > 4,000 mg/L), and higher operating temperatures, better suited to take advantage of the lower process kinetics characteristic of anaerobes, these processes are actually applicable to many types of wastewater and environmental conditions, even to diluted wastewater at low process temperatures.

Today, UASB (upflow anaerobic sludge blanket) systems are among the most used high-rate anaerobic digesters for treatment of wastewaters. Originally developed for industrial wastewater treatment (Lettinga et al. 1983), UASB design required several adaptations for practical application with domestic wastewater, that has typically low COD concentrations. This resulted in lower methane production, insufficient to heat the process reactors to the more favourable mesophilic temperature range (35–45 °C). Full scale UASB applications initially showed excellent results under tropical conditions (T > 20–25 °C), with COD removal efficiencies around 75% at 6 h HRT (van Haandel & Lettinga 1994). UASB are nowadays widely used in Brazil and other countries in South America, India, Indonesia and Egypt due to low construction and operational costs (Kalogo & Verstraete 2000), even though their nutrient removal capability is low.

UASB application at temperatures below 10 °C is still feasible, in these conditions however, low hydrolysis rates were found to cause deterioration of the overall anaerobic reactor performance. Soluble COD can still be efficiently converted to methane at temperatures as low as 5 °C but, for successful application of anaerobic treatment of domestic sewage under such conditions, incoming solids must be separated from the waste stream before entering the methanogenic reactor. This can occur purely by physical pre-treatment (i.e., primary clarifiers, or mesh filtration), or by applying a sequence of two reactors in series, in which the first is designed to entrap and (partly) hydrolyse solids, or by providing surface area for biomass attachment and growth in the reactor above the sludge blanket (Lew et al. 2004). In these conditions, biogas generation diminishes considerably with decreasing temperature, and about 50% of it may escape the system with the effluent (Uemura & Harada 2000), making its recovery unprofitable, save for local use of small isolated communities. This, however, is of secondary importance compared to the general economic benefits of the process under these terms, somehow undermining the intrinsic merits of anaerobic processes as energy recovery technologies, consisting of low initial investment, low energy for operation, lower sludge production and easier maintenance than conventional aerobic processes. Table 3 presents the main features (design values, average efficiency values, etc) of UASBs. Table 4 summarizes experimental results from UASB applications.

Sustainability, although not explicitly mentioned in the relevant EU or national legislations, it is key to implement efficient wastewater systems. The main objectives of these systems are to protect and promote human health by providing a clean environment and breaking the cycle of disease. It should be remembered that the ‘most appropriate technology’ in any situation is that (or those) turning out to be economically affordable, environmentally protective, technically and institutionally consistent, and socially acceptable for the specific application. In other words, sustainable, under all viewpoints. As an additional score point, decentralized systems are generally compact, with highly flexible operating conditions and reduced aesthetic impact, however, other local impacts (i.e. odours, traffic) should be considered (Capodaglio et al. 2002; Torretta et al. 2016). When improving an existing and/or designing a new sanitation system, sustainability criteria related to the following aspects should be considered:

• (1)

  Health and hygiene: minimizing risk of exposure to pathogens and hazardous substances that could affect public health from the toilet to the point of disposal (or reuse);

• (2)

  Environment and natural resources: considering energy, water and other resources required for construction and operation, as well as potential emissions resulting from use. This should include the degree of recycling and re-use practiced and their effects (e.g. returning water, nutrients and organic material to agriculture), and the protection of other non-renewable resources (e.g. production of renewable energy, like biogas);

• (3)

  Technology: maximizing functionality, and ease with which the entire system can be constructed, operated and monitored by local utilities. Its robustness and vulnerability towards power cuts, water shortages, floods, etc., and flexibility/adaptability to existing infrastructure and demographic or socio-economic developments are also important aspects;

• (4)

  Financial and economic issues: relating to the capacity of households/communities to pay for the system, including construction, operation, maintenance and necessary reinvestments;

• (5)

  Socio-cultural and institutional aspects: socio-cultural acceptance, convenience, perception, impact on human dignity, compliance with the legal framework and institutional settings must be considered.

From the purely economic viewpoint, any affordable technology could find application almost anywhere; however, for a system to be environmentally sustainable, it must ensure protection of environmental quality, conservation of resources, and allow reuse of water as well as recycling of nutrients and resources. Understanding the receiving environment is crucial for technology selection and should be accomplished by conducting comprehensive site evaluation processes, with determination of the carrying capacity of the receiving environment. Generally speaking, decentralized treatment schemes would allow tighter, more natural-like water-use cycles without long-distance flow transfer, less water bodies depletion and minimizing effluent-domination phenomena, reducing both ecological and hydrological negative impacts of anthropic water use.

Social acceptance is also a critical factor in the selection process. Centralised systems are already accepted as a de-facto necessity by the public, who is aware that treatment processes are continuously ongoing ‘out of sight’ under the supervision of an authority in charge of their management. This is not always so clear-cut in decentralized systems. In case of very small systems, it is the end user(s) who is in charge of management, and probably wish not to be. Decentralized systems generally require more awareness, involvement and participation from local residents. Decentralized systems may be however be very well accepted by residents when clearly made aware of their objectives and advantages, including economical ones. In a few EU countries (Germany, The Netherlands) demonstrative decentralized systems serving up to 1,000 people have been implemented in urban areas with positive results. As an additional score point, decentralized WWTPs are generally compact, with highly flexible operating conditions, and reduced aesthetic impact.

Sustainable decentralized sanitation strong points are on-site treatment and recycling of resources contained in domestic wastewater. Centralised systems satisfy the demand of highly populated areas, but do not fit with these new expectations (paradigms). At the moment, the new concept of ‘City-of-the-future’ could strongly favour decentralisation.

In traditional systems, household discharge streams are combined and transported by an extended sewer systems, to a centralised WWTP. Hence, to collect and treat wastewater, centralized wastewater treatment requires more pumps, longer pipes and more energy than decentralized ones, therefore increasing the infrastructure cost of the system. About 80–90% of capital costs in such systems are related to the collection system itself, with some economy of scale in densely populated areas (Libralato et al. 2012). Wastewater treatment cost per unit volume in centralized systems is competitive compared to decentralisation, however, as it is estimated that any collection system (whole or of part of it) needs to be renewed every 50–60 years, besides the required periodic maintenance, a decision on the type of system to adopt when substantial upgrades are due should be introduced in the planning process in due time. Decentralized systems respond well to suburban areas, rural centres, industrial, commercial and residential areas development changes, as well as to population growth in rural areas and developing countries, since infrastructure investments can be gradual and put in place as needed. Decentralization may be quite helpful in the case of large block redevelopment in metropolitan areas with sewage collection systems, since local treatment and reuse of wastewater can limit the strain of additional discharges into the existing sewer, that may disrupt or overwhelm or service during peak load events.

Conventional wastewater management is also based on a disadvantageous approach, depending on high water flows, and thus waste dilution, for its continued operation. This not only increases the cost of treatment diluted wastewater requires more expensive (due to larger volumes) and less energy-efficient treatment approaches, but also increases operational costs for users, since relatively large volumes of drinking water, requiring significant pumping effort, are usually employed for transporting waste and flushing the system periodically. It has been estimated that by enacting source control and differential water usage, new decentralized technologies could manage wastewater systems with just around 20% of the current drinking water demand (Otterpohl et al. 2002).

Further diseconomies of scale are possible where long distances have to be covered by the collection system, or as a consequence of ground/rainwater infiltration, or both, requiring considerable additional pumping energy. Strong dependency on electrical energy supply for pumping might put these systems at risk in economic/political crisis times, making the system poorly resilient in such instances. In addition, heavy rainfall events or contamination by industrial wastewater may generate overflow phenomena, impairing ecological status. In addition to water resources transfer and creation of effluent-dominated water bodies, eutrophication phenomena may occur in receiving water body due to the large volumes of treated wastewater discharged at centralized WWTP outlet points. Decentralized system, on the other hand, tend to lessen degradation effects of surface water quality and reduce eutrophication events (Brown et al. 2010).

The cost of the more advanced process technologies in DWTs, such as MBRs, is rapidly becoming comparable to that of centralisation per unit of treated organic load (Fane & Fane 2005). Results from a study by the City of San Diego Water Department showed that MBR costs were approximately 60 percent lower than conventional (AS) treatment for the smaller capacity (Adham et al. 2004) This can even be lower if low-tech, low-impact technologies are selected, considering also that small WWTPs can now be easily remotely controlled, facilitating their O&M without the need for resident personnel. In the past, the lack of reliable monitoring technology constituted a serious obstacle to the adoption of DWT systems, often resulting in intensive personnel requirements and unreliable treatment results. The now common availability of reliable remote monitoring technology, dramatically reduces such requirements, allowing remote-control of distant facilities and demand-actuated on-site maintenance when needed (Capodaglio et al. 2016c).

Decentralization separates domestic wastewater and rainwater, avoiding dilution phenomena, and could allow additional solutions such as source separation, extremely difficult to implement in centralized systems, reducing dispersion of micropollutants such as metals, and other emerging compounds (e.g. pharmaceuticals and personal care products) in the environment. Also, potential contamination of reusable nutrients and sludge by these compounds could be greatly reduced (Libralato et al. 2012). Separation of contaminants eases their treatment efficiency, saves energy and enhances potential reuse.

It is evident that adoption of DWT strategies is not in direct contrast with centralised ones, in fact, highly dense populated areas in developed countries are historically served by extended sewage collection systems and centralised WWTPs. In these cases, decentralisation would not represent an immediately suitable and viable economic alternative, and a balanced approach would consist of supporting the coexistence between centralised and satellite decentralised systems, especially in the case of new large developments such as residential and commercial complexes, hospitals, where treated wastewater reuse could be effectively planned.

Some of the most common decentralized technologies have been illustrated in the previous sections.

Table 5 summarizes sustainability-related considerations for each of these.

Furthermore, from a technological point of view, while introducing DWTs, concomitant separation of black water (possibly jointly with kitchen waste), and grey water would maximize the total recycling potential of the local system: these wastes, representing a small volume of the overall flow, contain the largest fraction of COD and nutrients of domestic wastewater, and also the almost totality of pathogens and micropollutants. It is an established principle that concentrating treatment needs to a smaller volume with higher loads enables better and more efficient control, and can limits negative environmental effects. A high concentration of black water would make anaerobic treatment with subsequent nutrient recovery a very attractive option. Sewage concentrations in these cases could be as high as 10,000 mg/l COD (if using vacuum toilets technology) and reach easily 3,000 mg/l using extremely low-flush devices. With these concentrations, UASB technology would generate enough biogas to warrant its recovery. Post-treatment will be still required for anaerobic effluent to fulfil standards for reuse or discharge (especially pathogens).

Besides factors such as land availability (in particular, for CWs), costs and environmental requirements, that can be overcome with the choice of a more suitable DWT, several non-technological barriers stand before a wider implementation of DWT systems in developed countries. Somewhat unsurprisingly, DWTs are more readily accepted in developing countries, since no technological pre-existence, favourable economics and the influence of younger, more novelty-prone engineers (local or from ONGs) all favour that choice.

In developed countries, mostly pre-existing centralized settings and traditional engineering approaches favoured by senior public-system water technologists are the main obstacles to a more general diffusion of DWT systems. Often, economics ‘it would cost too much to change the entire system’, of lack of specific consolidated experience ‘it is not feasible/acceptable by the users, it won't work, it cannot possibly be done’ and fear of unknown/undetermined problems are brought up as irrefutable reasons as why system design must go on unchanged. The promoters of DWT systems are mainly among progressive young professionals, who have difficulty getting these concepts accepted by decision makers and traditional wastewater professionals with a ‘Business as Usual’ mentality. It is also possible that the odd, old decentralized system had been either inappropriately dimensioned, was born technologically obsolete, or had been hampered by ineffective operation and maintenance, and is still pointed as an example of a bad idea, regardless of the number of existing successful examples reported nowadays.

Decentralized or cluster wastewater treatment systems designed to operate at small scale, not only can reduce the effects of wastewater disposal on the environment and public health, but may also increase the ultimate reuse of wastewater, depending on community type, technical options and local settings. However, when both centralized and decentralized systems are viable, the ‘most appropriate technology’ should be selected, case-by-case, as the one that is economically affordable, environmentally sustainable and socially acceptable; management strategies should also be site specific.

Implementing decentralised technologies could give planners a chance to consider whether to also introduce source separation (urine/black water, and possibly grey water) systems toilet or other extreme water saving systems (very low flush/vacuum) in order enhance resources and energy recovery. Furthermore, in view of the necessity to reconstruct/refurbish/upgrade current centralised systems due to ageing, planners could find of interest to speculate upon alternatives to traditional wastewater treatment modes, possibly supporting the coexistence of various degrees of centralisation/decentralisation (satellite systems).

Currently, there is a good level of knowledge regarding implementation and performance of DWTs at the experts’ and scientific levels, however, technological transfer into practice is still insufficient, and low awareness and recognition of DWTs benefits and a ‘business as usual’ mentality still persist at the institutional and administrative levels.