At each stage of the process, countries should seek to implement those measures that provide the maximum net benefits, or, in the absence of sufficient information for a cost-benefit analysis, choose the most cost-effective measures for achieving a desired level of risk reduction (WHO, 2006). Examples of a low-income country, an upper-middle-income country, and a high-income country are provided below.

# _Phased improvements in wastewater treatment_

To progress from unplanned to planned wastewater use for agriculture, the introduction of wastewater treatment is an essential strategic goal. Low-income countries should seek to introduce non-treatment options and simpler low-cost treatment options as a first step, and progressively move toward expanded sewerage systems and costlier treatment technologies as financial and operational capacity grows. An important part of this first step is to clearly define responsibilities for household, community, and public sanitation service provision, and to put in place the capabilities to monitor operations and verify that treatment targets are met (for example, by implementing the HACCP system described in Chapter 4 and Annex B). A progressive array of treatment options is shown in Table 5.1, along with possible pathways to expand sewerage and treatment as development occurs.

When planning for appropriate treatment systems, particularly those that can positively impact food safety, scale is an important factor (Table 5.1). Taking into account the common characteristics of cities in developing countries, four levels of scale are of interest:

- _On-site wastewater treatment and use,_ i.e _._ wastewater management systems used at the site of wastewater generation. On-site systems may handle greywater and blackwater separately or combined. Typical non-sewered sanitation options can range from simple pit latrines, VIP latrines, and dry (composting) toilets, to septic tanks with gravity-fed leachfield, horizontal flow filter or natural treatment system. Ideally, only greywater from household storage tanks will be used for household gardens, or wastewater that has been processed by soil infiltration, and only composted human waste for adding nutrients.

- _Communal wastewater treatment and use,_ i.e. systems where wastewater from a cluster of homes is collected and diverted for treatment close to the point of use on small agricultural plots. Communal systems might include unconventional collections systems like small-bore sewers or condominial sewers, and the recovered wastewater used for irrigation of communal plots. Appropriate treatment options can include septic tanks, Imhoff tanks, anaerobic ponds, storage ponds, or three tank systems.

- _Decentralized wastewater treatment and use_ , i.e. the collection, treatment and use of wastewater from isolated medium-size communities, as well as from portions of larger urban areas contained within a minor watershed, at or near the point of wastewater generation, generally to irrigate larger urban or peri-urban plots. Appropriate low-cost decentralized treatment options can include waste stabilization pond systems (e.g., Lima’s San Augustín project described in Box 2.6), upflow anaerobic sludge blanket reactors (UASB), wastewater treatment storage reservoirs (WTSR), or chemically enhanced primary treatment (CEPT).

- _Centralized wastewater treatment and use,_ i.e. the collection and drainage of wastewater, and sometimes stormwater, from a large urban area using an extensive network of pumps and piping for transport to a central location for treatment and reclamation, usually near the point for convenient irrigation use. While waste stabilization pond or WTSR systems may be feasible for larger cities, due to land constraints large city systems will often be based on conventional treatment options, or appropriate technology options such as CEPT followed by sand filtration and disinfection (Libhaber, 2007). Membrane technologies are emerging as preferred technologies in high-income countries (Bixio _et al_ , 2006). Within a large river basin encompassing multiple urban areas, many centralized facilities may be required.

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