As a solution to the shortcomings of centralized systems, over the last two decades large numbers of decentralized wastewater treatment plants of different technology types have been installed all over the world. This paper aims at deriving lessons learned from existing decentralized wastewater treatment plants that are relevant for smaller towns (and peri-urban areas) as well as rural communities in developing countries, such as India. Only full-scale implemented decentralized wastewater treatment systems are reviewed in terms of performance, land area requirement, capital cost, and operation and maintenance costs. The results are presented in tables comparing different technology types with respect to those parameters.

INTRODUCTION

Rapid growth in population, urbanization, industrialization, and demand of energy has drawn attention of many researchers towards the scarcity of clean water. Globally, billions of people are suffering due to inappropriate sanitation and wastewater treatment and unavailability of useable water. The situation is particularly grave in smaller towns (or peri-urban areas) and rural communities in developing countries. Worldwide, around 40% of the population lacks basic sanitation and 25% of the developing country urban dwellers lack access to sanitation services, with a much higher percentage for the rural populations of developing countries reaching up to 82% (Ho 2005; Massoud et al. 2009; Chong et al. 2012). The adverse effect of this situation on hygienic, environmental and ultimately social aspects is well documented (Abegglen & Siegrist 2006; Fach & Fuchs 2010).

Limited financial resources demand environmental engineers to design environmentally and economically sustainable wastewater treatment systems. From an economic perspective in particular, the differentiation in centralized and decentralized systems is of relevance. A definition of both types of system as well as an overview of their respective advantages and disadvantages can be found in Starkl et al. (2012). This paper focuses on decentralized wastewater treatment systems, which are often considered as more sustainable options as compared with centralized alternatives, in particular in small towns and peri-urban areas as well as rural communities in developing countries (Nanninga et al. 2012). Further, many centralized treatment plants have been found to be unable to cope with stringent environment legislation in developing countries (Schories 2008). Consequently, over the last decade many researchers have studied various decentralized options for wastewater management, for example, Beausejour & Nguyen (2007), Galvao et al. (2005), Fane & Fane (2005), Starkl et al. (2007) and Meuler et al. (2008). Some of the typical advantages attributed to decentralized systems are that they can be installed without requiring a huge budget especially at isolated locations or that they save money otherwise required for a sewerage network and increase the possibility of reuse of treated water without extra expenditure required for the water supply network (Massoud et al. 2009; Wang 2014).

The objective of this paper is to summarize the information available in literature on full-scale decentralized wastewater treatment plants worldwide. The review will help in identifying current knowledge gaps with respect to decentralized wastewater management.

CLASSIFICATION OF DECENTRALIZED WASTEWATER TREATMENT SYSTEMS

For structuring the review, the following classification of (decentralized) wastewater treatment systems has been used:

1. Natural treatment systems

2. Aerobic systems

• (a) Suspended growth

• (b) Attached growth

• (c) Combined suspended and attached growth

3. Anaerobic systems

• (a) Suspended growth

• (b) Attached growth

4. Combined (aerobic/anaerobic/natural) systems

• (a) Anaerobic–aerobic

• (b) Anaerobic–natural

• (c) Anaerobic–aerobic–natural

The following sections give a brief overview of those systems.

Natural treatment systems

Natural wastewater treatment systems can provide cheap solutions for wastewater treatment. The simplest system is a pond system, where the algal–bacterial symbiotic relationship is used for wastewater treatment. Other common systems are designed to work with natural media like soil and plants for filtration and biochemical reactions. These systems are generally adopted in developing countries due to their low operational and maintenance cost and can be used either as a secondary treatment or as a combination of primary and secondary treatment. Some have also been used for tertiary treatment, such as duckweed pond systems (DPS), waste stabilization ponds (WSP), facultative pond, and constructed wetlands (CWs) (Vymazal 2010). These systems involve combined treatment by earthen material, plants and microorganism. Besides the chemical and biological action, these systems are supported by some physical and chemical methods like precipitation and adsorption (Surampalli et al. 2007). Like other treatment systems, these systems are capable of removing the nutrient and organic load from wastewater. Natural systems are generally used in those places where there large space is available and funds are limited. Despite their low cost and satisfactory wastewater treatment, natural systems require a large space and longer hydraulic retention time (HRT) than other treatment systems, and may produce bad odors not suited for hot climates where evapotranspiration from plants takes place. Natural systems are also capable of coping with shock load and load fluctuation by using the combinations of two or more natural systems. Availability of raw material makes these processes a good choice for wastewater treatment in low income countries (Surampalli et al. 2007; Yoon et al. 2008). An overview of natural wastewater treatment systems in India can be found in Starkl et al. (2013).

These systems are more appropriate for warm tropical and subtropical regions. Treatment capability of natural treatment systems, specifically CWs, are limited by various components such as sunlight, wind, soil characteristics and geology, hydraulics, health and sustainability of vegetation, and seasonal variations in water surface elevation (Thullen et al. 2005). Adoptability of these systems is also limited due to lack of awareness, lack of knowledge of tropical wetland ecology and native wetland species, prevalence of mixed domestic/industrial wastewaters, and limited knowledge and experience with design and management (Kivaisi 2001; Smith & Moelyowati 2001). Major problems associated with adoptability and faced by designers and operators of natural treatment systems include disordered vegetation growth, nuisance control (e.g. insect vectors, nuisance animals), slow treatment rate, wastewater exposure, and fast macrophyte growth rate (Verhoeven et al. 1999; Vymazal 2005; Jing et al. 2008).

Aerobic treatment systems

Aerobic treatment methods involve the use of oxygen utilized by microorganisms for degradation of organics into the simplest degradation products, i.e. carbon dioxide and water. Further, these systems can be classified as those systems which requiring forced aeration or mechanical aeration equipment. The footprint of these systems is small in comparison to natural systems but energy consumption is high. In comparison with natural systems, aerobic systems are capable of providing good quality effluents which can easily meet the effluent discharge standards. The biggest advantage of aerobic systems is that they require only semi-skilled personnel, which make them a good choice of wastewater treatment technology in low income and developing countries. These systems are considered as high rate systems and can be categorized as attached growth processes, suspended growth processes, and hybrid processes. These systems are also capable of satisfying the discharge standards of various developed countries. Sometimes aerobic systems are practiced as a post-treatment option for the effluents of anaerobic systems. Some well-documented and full-scale implemented aerobic systems include extended aeration process (EA), moving bed biofilm reactor (MBBR), oxidation ditches, membrane bioreactor (MBR), submerged aerobic fixed film (SAFF) reactor, rotating biological contactor (RBC) and sequential bioreactor (SBR). These systems are generally practiced for the treatment of domestic wastewaters (having chemical oxygen demand (COD) concentration of less than 1,000 mg/L). Aerobic treatment systems are also able to produce high quality effluent with good sludge settling characteristics (Seghezzo et al. 1998; Fane & Fane 2005; Abegglen & Siegrist 2006; DeCarolis & Adham 2007; Meuler et al. 2008).

Usage of these systems requires more sophisticated operation control, knowledgeable operators, higher level of maintenance, semi-skilled personnel, high specific energy consumption, high biomass production resulting in frequent sludge disposal (Eckenfelder 1988; Al-Rekabi et al. 2007). Major problems associated with these systems include plugging of aeration devices during operation, difficult scale-up, and mechanical failures (Cortez et al. 2008; Singh & Srivastava 2011).

Anaerobic treatment systems

These treatment systems govern the biological treatment by the production of methane and biomass through a basic mechanism involving hydrolysis, acidogenesis, acetogenesis and methanogenesis in the absence of oxygen. Anaerobic treatment systems are low energy consuming biological treatment systems. Despite the low organic and nutrient removal, anaerobic treatment systems are cheap and simple and can be an energy provider. However, these systems achieve only poor to moderate effluent quality and take a long time to start up (around 3–4 months), while aerobic systems have much shorter start-up time (around 2–4 weeks) (Alexiou & Mara 2003; Melidis et al. 2009). To meet the discharge standard most of the anaerobic treatment systems are followed by aerobic systems (Yeoh 1995; Chan et al. 2009). Examples of these types of system include upflow anaerobic sludge blanket (UASB), anaerobic baffled reactor and septic tank, but most of the anaerobic systems are practiced with other aerobic/natural processes. Many anaerobic systems are also practiced in combined treatment systems. Further, these systems are most suitable for treating high strength wastewaters (having COD >4,000 mg/L).

Treatment efficiency of anaerobic systems is limited by low temperatures zones, long HRT and effluents of these systems requiring post-treatment for removing the remaining COD, nutrients and pathogens (Al-Rekabi et al. 2007), while potential problems of these systems include slow start-up and potential odor problems and corrosion (Eckenfelder et al. 1988; Cortez et al. 2008).

Combined (aerobic/anaerobic/natural) systems

These systems incorporate the features of all three natural, anaerobic and aerobic systems. Such systems often are based on a combination of various technical components such as septic tank, Imhoff tank, anaerobic filter, baffled septic tank, trickling filter, hybrid and some natural systems.

TECHNICAL AND ECONOMICAL EVALUATION OF WASTEWATER TREATMENT SYSTEMS

The present review focuses on full-scale implemented decentralized wastewater treatment systems in terms of cost (capital, land and operation and maintenance (O&M)) and performance in terms of their targeted pollutant removal.

Technical evaluation of decentralized wastewater treatment systems

Tables 1,23 summarize the common performance parameters of various wastewater treatment systems implemented at full scale across all over the world. The performance of each technology is presented in terms of removal of their targeted pollutant.

Table 1

Performance inventory of full-scale natural treatment systems

Removal (%)
Treatment processCapacityUnitBODCODTSSTNTKNNH3-NTPTC/FCCountryReferencesRemarks
DPS 5,000 p.e. ∼ 95 ∼ 90 – 87 – – 43 – Zimbabwe Nhapi et al. (2003)  Treatment scheme includes anaerobic and maturation pond
10,000 p.e. – – – 56 – – 11 –
2,000–3,000 p.e. – – – – 74–77 > 90 – – The Netherlands Alaerts et al. (1996)  –
WSP 2,000 p.e. 75 70 60 51 – – 51 – Spain Rodríguez (2009)  –
30–60 m3/d 94 – 63 – – 72 – – Greece Papadopoulos & Tsihrintzis (2011)  WSP utilizes duckweed plants
3,000 p.e. 50.6 48.9 44.3 – – – – 98.8 and 95.6 Egypt Ghazy & El-Senousy (2008)  The scheme comprises anaerobic, facultative and maturation ponds
5,000 m3/d – 6.7 (sol) 16.3 – – 3.6 18.1 – Israel Avsar et al. (2008)  Scheme comprises two sedimentation tanks followed by a pond
– – 75 55 48 44 – – 46 1.6 log unit FC Brazil Sperling & Oliveira (2009)  –
Horizontal flow constructed wetland (HFCW) 1,750 p.e. – 98.7 93.1 94 – 91.9 92.4 – Ireland Dzakpasu et al. (2012)  Combined treatment of domestic sewage and mountain water river
– – 96 – – – – 88.4 87.8 – China Wu et al. (2011)  Preceded by a settling tank
< 2,000 p.e. > 78 – > 78 40–60 – – 40–60 – Spain Vera et al. (2011)  Treatment scheme includes septic tank as pretreatment followed by HFCW and WSP as post-treatment unit
350 p.e. > 90 > 90 95.6 – – – – – France Merlin et al. (2002)  Septic tank followed by HFCW
100 p.e. 97 94.5 99.4 – – – 62.5 – Czech Republic Vymazal (2011)  Pretreatment by septic tank and screen followed by HFCW
20 p.e. – 79.2 64.7 – – – – – Italy Pucci et al. (2000)  Imhoff tank followed by HFCW
Vertical flow constructed wetland (VFCW) 1,000 p.e. 92.3 91.7 93.2 – 80.3 87.5 61.3 99.9 France Gikas et al. (2007)  Treatment scheme includes screening, primary sedimentation tank (PST) and sludge tank followed by VFCW
72 p.e. > 60 > 55 > 80 – – – – – Tunisia Sellami et al. (2009)  Septic tank followed by VFCW
– – – 93 96 – – 86 75 – Nepal Bista & Khatiwada (2004)  Septic tank followed by reed bed based vertical and horizontal wetland
Removal (%)
Treatment processCapacityUnitBODCODTSSTNTKNNH3-NTPTC/FCCountryReferencesRemarks
DPS 5,000 p.e. ∼ 95 ∼ 90 – 87 – – 43 – Zimbabwe Nhapi et al. (2003)  Treatment scheme includes anaerobic and maturation pond
10,000 p.e. – – – 56 – – 11 –
2,000–3,000 p.e. – – – – 74–77 > 90 – – The Netherlands Alaerts et al. (1996)  –
WSP 2,000 p.e. 75 70 60 51 – – 51 – Spain Rodríguez (2009)  –
30–60 m3/d 94 – 63 – – 72 – – Greece Papadopoulos & Tsihrintzis (2011)  WSP utilizes duckweed plants
3,000 p.e. 50.6 48.9 44.3 – – – – 98.8 and 95.6 Egypt Ghazy & El-Senousy (2008)  The scheme comprises anaerobic, facultative and maturation ponds
5,000 m3/d – 6.7 (sol) 16.3 – – 3.6 18.1 – Israel Avsar et al. (2008)  Scheme comprises two sedimentation tanks followed by a pond
– – 75 55 48 44 – – 46 1.6 log unit FC Brazil Sperling & Oliveira (2009)  –
Horizontal flow constructed wetland (HFCW) 1,750 p.e. – 98.7 93.1 94 – 91.9 92.4 – Ireland Dzakpasu et al. (2012)  Combined treatment of domestic sewage and mountain water river
– – 96 – – – – 88.4 87.8 – China Wu et al. (2011)  Preceded by a settling tank
< 2,000 p.e. > 78 – > 78 40–60 – – 40–60 – Spain Vera et al. (2011)  Treatment scheme includes septic tank as pretreatment followed by HFCW and WSP as post-treatment unit
350 p.e. > 90 > 90 95.6 – – – – – France Merlin et al. (2002)  Septic tank followed by HFCW
100 p.e. 97 94.5 99.4 – – – 62.5 – Czech Republic Vymazal (2011)  Pretreatment by septic tank and screen followed by HFCW
20 p.e. – 79.2 64.7 – – – – – Italy Pucci et al. (2000)  Imhoff tank followed by HFCW
Vertical flow constructed wetland (VFCW) 1,000 p.e. 92.3 91.7 93.2 – 80.3 87.5 61.3 99.9 France Gikas et al. (2007)  Treatment scheme includes screening, primary sedimentation tank (PST) and sludge tank followed by VFCW
72 p.e. > 60 > 55 > 80 – – – – – Tunisia Sellami et al. (2009)  Septic tank followed by VFCW
– – – 93 96 – – 86 75 – Nepal Bista & Khatiwada (2004)  Septic tank followed by reed bed based vertical and horizontal wetland

BOD: biochemical oxygen demand; COD: chemical oxygen demand; TSS: total suspended solids; TN: total nitrogen; TKN: total Kjeldahl nitrogen; TP: total phosphorus; TC/FC: ratio of total coliforms to fecal coliforms; p.e.: population equivalent.

Table 2

Performance inventory of full-scale aerobic treatment systems

Removal (%)
Treatment processCapacityUnitBODCODTSSTNTKNNH3-NTPTC/FCCountryReferencesRemarks
Oxidation ditcha – – 91.3 81.6 79.7 – – 41.3 51.5 – Nepal Sah (2004)  –
Conventional and extended aeration activated sludge processa – – 85 81 76 50 – – 46 2 Log unit Brazil Sperling & Oliveira (2009)  –
SBRa – – 90–98 – 84.7–97.2 – – 90.8–96.8 – – USA Surampalli et al. (2000)  –
Aerated lagoon (AL)a 637 p.e. 75 – 73 – – 59 – – USA Surampalli et al. (1999)  Aerated lagoon followed by polishing pond
MBRa 95 m3/d > 98 > 91.7 – – – > 97.7 – – California DeCarolis & Adham (2007)  –
– – > 99 > 91 – – > 96.8 – > 24 – The Netherlands Krzeminski et al. (2012)  –
240 m3/d – 94.8 – – – 98.3 – – – Zhang et al. (2003)  –
RBCb 1,000 m3/d 76.2 – 70 41.2 – – – – Japan Tanaka et al. (1987)  Grit chamber and primary clarifier followed by RBC
MBBRc < 2,000 p.e. > 90 > 90 > 92 – – – – – Norway and Sweden Rusten et al. (1997)  Treatment scheme includes PST and MBBR is followed by chemical precipitation
SAFF reactorc 250 p.e. 87–95 67–73  60–90 – > 94 > 65 – – Nabizadeh & Mesdaghinia (2006); Pramanik et al. (2012)  –
Removal (%)
Treatment processCapacityUnitBODCODTSSTNTKNNH3-NTPTC/FCCountryReferencesRemarks
Oxidation ditcha – – 91.3 81.6 79.7 – – 41.3 51.5 – Nepal Sah (2004)  –
Conventional and extended aeration activated sludge processa – – 85 81 76 50 – – 46 2 Log unit Brazil Sperling & Oliveira (2009)  –
SBRa – – 90–98 – 84.7–97.2 – – 90.8–96.8 – – USA Surampalli et al. (2000)  –
Aerated lagoon (AL)a 637 p.e. 75 – 73 – – 59 – – USA Surampalli et al. (1999)  Aerated lagoon followed by polishing pond
MBRa 95 m3/d > 98 > 91.7 – – – > 97.7 – – California DeCarolis & Adham (2007)  –
– – > 99 > 91 – – > 96.8 – > 24 – The Netherlands Krzeminski et al. (2012)  –
240 m3/d – 94.8 – – – 98.3 – – – Zhang et al. (2003)  –
RBCb 1,000 m3/d 76.2 – 70 41.2 – – – – Japan Tanaka et al. (1987)  Grit chamber and primary clarifier followed by RBC
MBBRc < 2,000 p.e. > 90 > 90 > 92 – – – – – Norway and Sweden Rusten et al. (1997)  Treatment scheme includes PST and MBBR is followed by chemical precipitation
SAFF reactorc 250 p.e. 87–95 67–73  60–90 – > 94 > 65 – – Nabizadeh & Mesdaghinia (2006); Pramanik et al. (2012)  –

aSuspended growth systems.

bAttached growth systems.

cHybrid growth systems.

Table 3

Performance inventory of full-scale anaerobic and combined treatment systems

Removal (%)
Treatment processCapacityUnitBODCODTSSTNTKNNH3-NTPTC/FCCountryReferencesRemarks
UASBa 250–500 p.e. 67 60 78 – – – – – Brazil Vieira et al. (1994)  –
– – 72 59 67 –13 – – –1 0.6 log FC Brazil Sperling & Oliveira (2009)  –
35 m3 80 66 69 – – – – – Colombia Schellinkhout & Collazos (1992)  UASB followed by anaerobic pond system
Anaerobic suspended followed by aerobic attachedb 250–500 p.e. 82 79 83 – 40 40 – – Brazil Vieira et al. (2013)  UASB followed by trickling filter
Aerobic suspended followed by aerobic attachedb 10.8 m3/d 88 89 – – – 70 – Mexico Norouzian & Martinez (1985)  AST followed by RBC
Anaerobic suspended followed by anaerobic attachedb – – 59 51 66 24 – – 30 1 log unit Brazil Sperling & Oliveira (2009)  Septic tank + anaerobic filter
Removal (%)
Treatment processCapacityUnitBODCODTSSTNTKNNH3-NTPTC/FCCountryReferencesRemarks
UASBa 250–500 p.e. 67 60 78 – – – – – Brazil Vieira et al. (1994)  –
– – 72 59 67 –13 – – –1 0.6 log FC Brazil Sperling & Oliveira (2009)  –
35 m3 80 66 69 – – – – – Colombia Schellinkhout & Collazos (1992)  UASB followed by anaerobic pond system
Anaerobic suspended followed by aerobic attachedb 250–500 p.e. 82 79 83 – 40 40 – – Brazil Vieira et al. (2013)  UASB followed by trickling filter
Aerobic suspended followed by aerobic attachedb 10.8 m3/d 88 89 – – – 70 – Mexico Norouzian & Martinez (1985)  AST followed by RBC
Anaerobic suspended followed by anaerobic attachedb – – 59 51 66 24 – – 30 1 log unit Brazil Sperling & Oliveira (2009)  Septic tank + anaerobic filter

aAnaerobic–suspended growth systems

bCombined systems.

Table 4

Economic evaluation of natural treatment systems

Treatment processCapacityUnitCapital costUnitLand requirementUnitAnnual (O&M) costUnitCountryReferenceRemarks
WSP 100–100,000 m3/d 854 US$/(m3·d) 16 m2/m3 19 US$/(m3·d) Thailand Singhirunnusorn & Stenstrom (2010); Comas et al. (2003)  1 THB = 0.0231 US$(2003), 1 THB = 0.03 US$ (2010)
1,500 PEa 0.298 M€ – – 0.011 M€/year Spain Senante et al. (2012)  1 € = 1.3197 US$(2013) Facultative lagoon along with WSP < 18,930 m3/d 1–4$/GPDb 49–161 acre/MGDc 0.15–0.75 $/MGD USA Muga & Mihelcic (2008) – – – 0.54$/1,000 GPD 25 m2/m3 0.21 $/1,000 GPD USA Tecle et al. (1988) – Anaerobic lagoon < 18,930 m3/d 1–4$/GPD 49–161 acre/MGD 0.15–0.75 $/MGD USA Muga & Mihelcic (2008) – Intermittent sand filter 1,500 PEc 0.172 M€ – – 0.022 M€/year Spain Senante et al. (2012) 1 € = 1.33 US$ (2013)
HFCW 1500 PE 0.36 M€ 30 m2/m3 0.026 M€/year Spain Nogueira et al. (2009); Comas et al. (2003); Senante et al. (2012)  1 € = 1.0503 US$(2003), 1 € = 1.3955 US$ (2009), 1 € = 1.3197 US$(2013) 124 PE 503 €/PE – – 58 €/(PE·year) Spain Puigagut et al. (2007) 1 € = 1.3146 US$ (2007)
100 PE 20,834 € – – 2,576 €/year Spain Nogueira et al. (2009)  1 € = 1.3955 US$(2009) VFCW 98 PE 295 €/PE – – 28 €/(PE·year) Spain Puigagut et al. (2007) 1 € = 1.3146 US$ (2007)
Treatment processCapacityUnitCapital costUnitLand requirementUnitAnnual (O&M) costUnitCountryReferenceRemarks
WSP 100–100,000 m3/d 854 US$/(m3·d) 16 m2/m3 19 US$/(m3·d) Thailand Singhirunnusorn & Stenstrom (2010); Comas et al. (2003)  1 THB = 0.0231 US$(2003), 1 THB = 0.03 US$ (2010)
1,500 PEa 0.298 M€ – – 0.011 M€/year Spain Senante et al. (2012)  1 € = 1.3197 US$(2013) Facultative lagoon along with WSP < 18,930 m3/d 1–4$/GPDb 49–161 acre/MGDc 0.15–0.75 $/MGD USA Muga & Mihelcic (2008) – – – 0.54$/1,000 GPD 25 m2/m3 0.21 $/1,000 GPD USA Tecle et al. (1988) – Anaerobic lagoon < 18,930 m3/d 1–4$/GPD 49–161 acre/MGD 0.15–0.75 $/MGD USA Muga & Mihelcic (2008) – Intermittent sand filter 1,500 PEc 0.172 M€ – – 0.022 M€/year Spain Senante et al. (2012) 1 € = 1.33 US$ (2013)
HFCW 1500 PE 0.36 M€ 30 m2/m3 0.026 M€/year Spain Nogueira et al. (2009); Comas et al. (2003); Senante et al. (2012)  1 € = 1.0503 US$(2003), 1 € = 1.3955 US$ (2009), 1 € = 1.3197 US$(2013) 124 PE 503 €/PE – – 58 €/(PE·year) Spain Puigagut et al. (2007) 1 € = 1.3146 US$ (2007)
100 PE 20,834 € – – 2,576 €/year Spain Nogueira et al. (2009)  1 € = 1.3955 US$(2009) VFCW 98 PE 295 €/PE – – 28 €/(PE·year) Spain Puigagut et al. (2007) 1 € = 1.3146 US$ (2007)

aPE: population equivalent.

bGPD: gallons per day.

cMGD: million gallons per day.

Table 5

Economic evaluation of aerobic treatment systems

Treatment processCapacityUnitCapital costUnitLand requirementUnitAnnual (O&M) costUnitCountryReferenceRemarks
ALa 100–100,000 m3/d 0.392 €/m3 m2/(m3·d) 27 US$/(m3·d) Thailand Singhirunnusorn & Stenstrom (2010) 1 THB = 0.03 US$ (2010)
SBRa 400 m3/d 0.632 M€ 353 m2/MLDd 0.020 M€/year Spain & India Kalbar et al. (2012a), (2012b); Comas et al. (2003); Senante et al. (2012)  1 € = 1.33 US$(2013), 1 € = 1.0503 US$ (2003), 1 Rs = 0.0188 US$(2012) EAa 400 m3/d 0.358 M€ – – 0.059 M€/year Spain Comas et al. (2003); Senante et al. (2012) 1 € = 1.33 US$ (2013), 1 € = 1.0503 US$(2003) Oxidation ditcha 3,785–25,740 m3/d$2.5–$4 Per GPD – – – – – USEPA (2000) – MBRa 400 m3/d 0.440 M€ 300–800 m2/MLD 0.048 M€/year Spain Kalbar et al. (2012b); Senante et al. (2012);Khalil et al. (2008) 1 € = 1.33 US$ (2013)
RBCb 1,500 PE 0.533 M€ 3,266 ft2/MGD 0.025 M€/year Spain Comas et al. (2003); Senante et al. (2012); Williams & Williams (2011)  1 € = 1.0503 US$(2003), 1 € = 1.3385 US$ (2011), 1 € = 1.33 US$(2013) Trickling filterb 1,500 PE 0.522 M€ 1,620 m2/MLD 0.026 M€/year Spain Comas et al. (2003); Senante et al. (2012); Khalil et al. (2008) 1 € = 1.0503 US$ (2003), 1 € = 1.33 US$(2013), 1 Rs = 0.0254 US$ (2008)
MBBRc 1,500 PE 0.533 M€ 450 m2/MLD 0.025 M€/year Spain & India Senante et al. (2012); Khalil et al. (2008)  1 € = 1.33 US$(2013), 1 Rs = 0.0254 US$ (2008)
Treatment processCapacityUnitCapital costUnitLand requirementUnitAnnual (O&M) costUnitCountryReferenceRemarks
ALa 100–100,000 m3/d 0.392 €/m3 m2/(m3·d) 27 US$/(m3·d) Thailand Singhirunnusorn & Stenstrom (2010) 1 THB = 0.03 US$ (2010)
SBRa 400 m3/d 0.632 M€ 353 m2/MLDd 0.020 M€/year Spain & India Kalbar et al. (2012a), (2012b); Comas et al. (2003); Senante et al. (2012)  1 € = 1.33 US$(2013), 1 € = 1.0503 US$ (2003), 1 Rs = 0.0188 US$(2012) EAa 400 m3/d 0.358 M€ – – 0.059 M€/year Spain Comas et al. (2003); Senante et al. (2012) 1 € = 1.33 US$ (2013), 1 € = 1.0503 US$(2003) Oxidation ditcha 3,785–25,740 m3/d$2.5–$4 Per GPD – – – – – USEPA (2000) – MBRa 400 m3/d 0.440 M€ 300–800 m2/MLD 0.048 M€/year Spain Kalbar et al. (2012b); Senante et al. (2012);Khalil et al. (2008) 1 € = 1.33 US$ (2013)
RBCb 1,500 PE 0.533 M€ 3,266 ft2/MGD 0.025 M€/year Spain Comas et al. (2003); Senante et al. (2012); Williams & Williams (2011)  1 € = 1.0503 US$(2003), 1 € = 1.3385 US$ (2011), 1 € = 1.33 US$(2013) Trickling filterb 1,500 PE 0.522 M€ 1,620 m2/MLD 0.026 M€/year Spain Comas et al. (2003); Senante et al. (2012); Khalil et al. (2008) 1 € = 1.0503 US$ (2003), 1 € = 1.33 US$(2013), 1 Rs = 0.0254 US$ (2008)
MBBRc 1,500 PE 0.533 M€ 450 m2/MLD 0.025 M€/year Spain & India Senante et al. (2012); Khalil et al. (2008)  1 € = 1.33 US$(2013), 1 Rs = 0.0254 US$ (2008)

aSuspended growth systems.

bAttached growth systems.

cHybrid systems.

dMLD: million litres per day.

From Table 2, it can be stated clearly that aerobic treatment systems are capable of providing the highest level of efficiency among all systems. Aerobic systems are efficient to produce treated effluents which can meet the discharge standards, and anaerobic processes are energy providers but less efficient than aerobic and advanced aerobic processes. Based on the performance efficiency data of natural systems (Table 1), it can be clearly stated that natural treatment systems can be a good option and are comparable to aerobic processes.

Economic evaluation of decentralized wastewater treatment systems

Tables 4 and 5 provide an economic evaluation of these systems in terms of capital or investment cost, and O&M cost.

Capital costs include cost incurred in land acquisition, and construction and design cost of facilities.

The presented inventory shows that performance is independent of size of plant, whereas total cost is directly proportional to size of the community.

DISCUSSION AND CONCLUSIONS

It can be seen that there is a large number of technologies available, which have been successfully implemented in a decentralized context. Whereas performance data and land requirement give a good comparison of the different technology types, it is more difficult to compare the technologies with respect to their costs as these depend on the local economic conditions. The following key lessons can be derived from the tables presented in this review.

• Very limited information is available on the performance and economic analysis of decentralized wastewater systems operating in developing countries. However, some information is available from developed countries, which are highlighted in the tables. Moreover, it may be possible that data may exist in governmental reports but not easily accessible to the scientific community. So there is a strong need for the performance and economic evaluation of decentralized treatment systems for both developed and developing countries like India and other Asian countries.

• Developing countries generally use anaerobic and combined treatment systems for small communities; however, only limited performance evaluation data are available, and economic evaluation is almost non-existent.

• It is very difficult to compare the cost of different wastewater treatment systems because commodities prices and cost incurred in laying of sewers is much higher in developed countries. So it is essential to include cost of commodities and sewer systems in economic evaluation.

• On the basis of performance, it was found that aerobic systems provide better quality in terms of organics and nutrients removal, and can be placed in a much smaller area, but higher O&M cost limits its use. However, natural systems require a larger area and are limited by their performance especially in nutrient removal.

Further, it can be seen that data are mainly published from developed countries, and few literature studies could be found that report data from Asia, in particular India. Hence, there is a need to conduct more evaluations of the performance of such technologies in these regions.

ACKNOWLEDGEMENTS

Co-funding of the project leading to parts of these results by the European Commission within the 7th framework program under grant number 308672 and the Department of Science and Technology, Government of India, is kindly acknowledged.

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