**Table  5.1 Progressive wastewater treatment options by level of economic development**

|**Characteristics by**<br>**level of economic**<br>**development**|**Low-income countries**<br>**(43 countries)**<br>**<$975 GNI/cap**|**Lower-middle-**<br>**income countries**<br>**(55 countries)**<br>**$976-3,855 GNI/cap**|**Upper-middle-**<br>**income countries**<br>**(46 countries)**<br>**$3,586-11,905**<br>**GNI/cap**|**High-income countries**<br>**(66 countries,**<br>**27 OECD)**<br>**>$11,906 GNI/cap**|
|---|---|---|---|---|
|Scale of interventions|Household gardens;<br>Small-scale communal<br>plots;<br>Large urban or peri-<br>urban plots with<br>decentralized simplified<br>sewerage systems;<br>Large peri-urban plots<br>with city-wide sewerage<br>systems|Small-scale communal<br>plots;<br>Large urban or peri-<br>urban plots with<br>decentralized treatment<br>systems;<br>Large peri-urban plots<br>with city-wide<br>treatment systems|Large urban or peri-<br>urban plots with<br>decentralized treatment<br>systems;<br>Large peri-urban plots<br>with city-wide<br>treatment systems;<br>Watershed level water<br>management systems|Municipal reclamation<br>and reuse projects for<br>crop and landscape<br>irrigation;<br>Basin-wide water<br>management systems|
|Evolution of typical<br>treatment technology<br>options<br>(see WHO, 2006 for|Household gardens:<br>On-site latrine<br>alternatives<br>Septic tank|Large plots or<br>medium-size city:<br>WSP systems<sup>28</sup><br>UASB|Large plots or<br>medium-size city:<br>WSP systems<br>UASB|Conventional treatment<br>+ tertiary treatment<br>Membrane technologies<br>(MBR, MF, UF, RO)<sup>30</sup>|
|description of basic<br>treatment options)|Greywater collection<br>Communal plots:<sup>_26_</sup><br>Imhoff tank<br>Storage ponds<br>Three tank system<br>Large plots or medium-<br>size city:<sup>_27_</sup><br>Waste stabilization<br>ponds (WSP)<br>Upflow Anaerobic<br>Sludge Blanket Reactors<br>(UASB)|Wastewater Treatment<br>and Storage Reservoirs<br>(WSTR)<br>Chemically enhanced<br>primary treatment<br>(CEPT)<br>Large city centralized<br>system:<br>Conventional<br>treatment<sup>29</sup>+ polishing<br>ponds|WSTR<br>CEPT<br>Large city centralized<br>system:<br>WSTR<br>CEPT + sand filtration<br>+ disinfection<br>Conventional<br>treatment + polishing<br>ponds<br>Soil aquifer treatment<br>(SAT)systems|SAT systems|

_Sources:_ WHO (2006); Bixio _et al._ (2006); Libhaber (2007); Income data from World Bank (2010). WSP = waste stabilization ponds; UASB = upflow anaerobic sludge blanket reactors; WSTR = wastewater treatment and storage reservoirs; CEPT = chemically enhanced primary treatment; SAT = soil aquifer treatment system; MBR = membrane bioreactors; MF = microfiltration; UF = ultrafiltration; NF = nanofiltration; RO = reverse osmosis 26 Assumes communal drains or sewerage system available (e.g., small-bore sewers, condominial sewers, conventional sewers) with localized treatment plant.

27 Assumes sewerage networks, interceptors, and decentralized treatment plants.

28 Waste stabilization pond (WSP) systems typically include anaerobic + facultative + maturation ponds.

29 Conventional treatment, for the purposes of this report, normally includes primary treatment (e.g., primary sedimentation) followed by secondary treatment process such as activated sludge and its variants (e.g., aerated lagoons and oxidation ditches, and trickling filters). Note that conventional systems do not meet microbiological quality requirements for unrestricted irrigation use without additional tertiary treatment processes (e.g., filtration, disinfection, or a polishing pond).

30 Membrane technologies applicable to wastewater treatment and use include membrane bio-reactors (MBR), microfiltration (MF), ultrafiltration (UF), and reverse osmosis (RO).

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