Effective wastewater treatment to meet water reuse quality standards, and efforts to protect public health, are the basic preconditions for wastewater reuse systems. Urban wastewater treatment consists of a combination of physical, chemical and biological processes for the removal of solids, organic substances, pathogens, metals and sometimes nutrients from wastewater. The general terms used to describe the different treatment stages are primary, secondary, tertiary and/or advanced treatment. Disinfection to control pathogen populations is the final treatment stage and takes place shortly before the storage or distribution of the treated wastewater.
While reuse projects in Europe typically have very high standards for wastewater treatment, in lower-income countries, raw sewage is often used directly. It is estimated that 20 million hectares (10% of all irrigated land) are irrigated with raw, partially treated, or fully treated wastewater (United Nations, 2003). The criteria of wastewater treatment in guidelines related to the use of treated wastewater effluent for irrigation purposes as shown in Table 2.4. When wastewater is treated with the intention of using the effluent for agricultural irrigation and not disposal in receiving waters, the important quality criteria are those relevant to human health rather than environmental criteria and those related to the health of fish in receiving waters.
Table 2.4 Wastewater treatment criteria for irrigation of reclaimed water according to several guidelines.
Country Irrigation of Treatment required
Cyprus
All crops Secondary + tertiary +
disinfection Crops for human consumption - Amenity
areas of limited public access
Secondary + storage >1 week and disinfection
Industrial crops Secondary + storage >30
days
Spain
Irrigation of crops for human consumption not avoiding direct contact of regenerated water with edible parts
Filtration + disinfection
Localized irrigation of ligneous crops impeding contact of regenerated water with food for human consumption. Irrigation of ornamental flowers, greenhouses and nurseries with no direct contact of regenerated water with crops
Filtration + disinfection
Greece
All crops Secondary + tertiary +
disinfection Fodder, industrial crops, pastures, seed
crops, crops that produce products which are processed before consumption.
Secondary + disinfection
Mediterannean1
All crops Secondary, filtration and
disinfection Cereals, fruit trees, plant nurseries,
ornamental nurseries
Secondary + few days storage
Cereals, fruit trees, plant nurseries,
ornamental nurseries using trickle irrigation systems
Primary treatment
US EPA
Food crops Secondary + tertiary +
disinfection
Processed food crops and Non Food crops Secondary + disinfection
WHO
A. Vegetable and salad crops eaten uncooked, sports fields, public parks
stabilization ponds, sequential batch-fed wastewater storage and treatment reservoirs B.Cereal crops, industrial crops, fodder
crops, pasture and trees
Retention in stabilization ponds for 8-10 days C.Localised irrigation of crops in category B
if exposure of workers and the public does not occur
Pre-treatment as required by irrigation technology, but not less than primary treatment
1 Recommended guideline for water reuse in the Mediterranean Region (Bahri and
2.2.1 Primary treatment
The term primary treatment refers to the initial treatment of wastewater to remove specific substances. In conventional treatment systems, primary treatment consists of screening, de-sanding and the removal of large particles. Conventional wastewater treatment is effective in removing solids over 50 μm. Generally, 50% of suspended solids and 25-50% of ΒΟD5 are removed during primary treatment (Metcalf and Eddy, 1991). Nutrients, hydrophobic particles, metals and microorganisms associated with the removed particles can also be removed by primary treatment. Approximately 10-20% of organic nitrogen and 10% of phosphorus is also removed by conventional primary treatment. For most wastewater reuse systems, primary treatment is not sufficient to achieve the required quality of treated wastewater.
Despite this, a large number of wastewater treatment plants, especially in the Asian and African part of Mediterranean comprise solely primary treatment. Currently it is considered as the absolute minimum level of treatment before water is discharged. Some countries (Palestine, Syria, Libya) are still struggling to achieve this minimal level of wastewater treatment (Kellis et al., 2013).
2.2.2 Secondary treatment
Secondary treatment systems consist of a series of biological processes combined with the separation of the liquid and solid phase.
The biological processes are designed to provide effective microbiological metabolism of the dissolved or suspended organic substrate present in the wastewater.
Conventional treatment systems include an aerobic biological reactor combined with secondary sedimentation for the dissolved or suspended organic substrate produced by the treatment of the wastewater components. Conventional treatment systems result in suspended solids and ΒΟD5 levels ranging both from 10 to 30 mg/l. Depending on the process, 10-50% of the organic nitrogen is removed during conventional secondary treatment and phosphorus is converted into phosphoric ions (ΡO4-3
). The resulting solids are treated using aerobic or anaerobic digestion, composting or other types of treatment technology. There is only partial removal of pathogens, trace elements and pathogens combined with biological filtration and physical separation.
For many wastewater treatment and reuse systems, secondary treatment results in the satisfactory removal of organic substances from wastewater. Secondary treatment is often combined with filtration for further removal of particles and disinfection.
2.2.3 Tertiary and/or advanced treatment
Tertiary and/or advanced wastewater treatment is applied when specific wastewater components must be removed but this cannot be achieved by secondary treatment. Advanced treatment refers to the removal of specific substances such as ammonia or nitrates, using nitrification/denitrification processes, ion exchange or removal of the total dissolved solids by reverse osmosis. Tertiary and/or advanced wastewater treatment usually follows the other biological treatment processes.