areas all over the world (Bouma 2016b), central
sewage systems in rural areas may contain excess
nutrients or pesticides that are also difficult to
remove. Quality control of both purified
wastewater and compost are therefore very
important before application to land is considered
and threshold data are crucial to define quality.
As illustrated in the following case studies, such
threshold and quality data related to health and
safety aspects of the waste application are not
only widely available but also widely applied.
Also, the engineering of wastewater treatment
facilities follows standardized practices. Unanswered questions arise when waste or wastewater
is applied to a wide variety of soils in different
climate zones with different soil moisture
regimes.
2 A Brief Review of Case Studies
on the Water-Soil-Waste Nexus
Hettiarachchi and Ardakanian (2016b) edited an
interesting UNU-FLORES sponsored book with
17 worldwide case studies focusing on good
practice examples of safe use of wastewater in
agriculture. The case studies allow an evaluation
of the state-of-the-art in terms of the input of
hydrology and soil science into systems that
operate in the real world, “warts and all”.
Examples describe the application of untreated
liquid wastewater for irrigation purposes, intended to overcome water shortages while also
nutrient contents are beneficial for plant growth.
Clearly, health risks are substantial when applying untreated wastewater and, increasingly,
treatment by Wastewater Treatment Plants
(WWTPs) occurs before applying to the soils is
considered. But this results in many financial and
operational problems. For example, in Bolivia
(case 12) 31 of 84 WWTPs don’t work, and the
remaining plants have efficiencies below 50%.
Different systems are described: (i) wastewater,
either untreated or treated, is directly applied to
the soil; (ii) an enclosed “sandfilter” with growing plants on top is used to purify the effluent
before it is discharged and collected; and
(iii) constructed wetlands guide wastewater
through an open area with plants and treated
water is collected at the point of outflow.
Regarding the water-soil-waste nexus, a
number of general observations can be made:
(i) Excellent analyses are reported of
wastewater in many of the cases in terms
of its chemical composition, including
heavy metals and various organic compounds, contents of pathogenic viruses
and bacteria. Measurements of groundwater and surface water quality are widely
reported as well. Obviously, standard
analytical procedures recommended by the
WHO are not only widely available but
also applied in practice.
(ii) A strong emphasis is on the importance of
wastewater irrigation for agriculture with
major concerns for health effects when
consuming irrigated crops or vegetables,
which is, of course, justified (see, for
example, case 4, Lima, Peru). The case
studies show that significant economic
advantages obtained by farmers applying
wastewater for irrigation are a prime driver
leading to acceptance of these practices.
(iii) Little data on soil water regimes are provided that are crucial for wastewater
purification. Case 1 reports infiltration
rates of sands where wastewater is infiltrated but purification is associated with
the unsaturated flow and travel times as
wastewater moves down to the groundwater (Bouma 1979, 2016b). No data are
provided on these processes. Virus
removal rates are reported but these values
are difficult to interpret because data on
the flow system are lacking. None of the
other studies report data on the movement
of wastewater into and through the soil.
Water use efficiency by applying drip
irrigation is emphasized in several cases
(e.g., case 9 on irrigation of sugarcane in
Colombia applying rates on the basis of
plant demands and activities in Mexico,
reported in case 16). But lack of soil
physical data does not allow a judgement
as to the efficiency reached. Lack of
information on flow regimes implies that
18
J. Bouma
sewage systems in rural areas may contain excess
nutrients or pesticides that are also difficult to
remove. Quality control of both purified
wastewater and compost are therefore very
important before application to land is considered
and threshold data are crucial to define quality.
As illustrated in the following case studies, such
threshold and quality data related to health and
safety aspects of the waste application are not
only widely available but also widely applied.
Also, the engineering of wastewater treatment
facilities follows standardized practices. Unanswered questions arise when waste or wastewater
is applied to a wide variety of soils in different
climate zones with different soil moisture
regimes.
2 A Brief Review of Case Studies
on the Water-Soil-Waste Nexus
Hettiarachchi and Ardakanian (2016b) edited an
interesting UNU-FLORES sponsored book with
17 worldwide case studies focusing on good
practice examples of safe use of wastewater in
agriculture. The case studies allow an evaluation
of the state-of-the-art in terms of the input of
hydrology and soil science into systems that
operate in the real world, “warts and all”.
Examples describe the application of untreated
liquid wastewater for irrigation purposes, intended to overcome water shortages while also
nutrient contents are beneficial for plant growth.
Clearly, health risks are substantial when applying untreated wastewater and, increasingly,
treatment by Wastewater Treatment Plants
(WWTPs) occurs before applying to the soils is
considered. But this results in many financial and
operational problems. For example, in Bolivia
(case 12) 31 of 84 WWTPs don’t work, and the
remaining plants have efficiencies below 50%.
Different systems are described: (i) wastewater,
either untreated or treated, is directly applied to
the soil; (ii) an enclosed “sandfilter” with growing plants on top is used to purify the effluent
before it is discharged and collected; and
(iii) constructed wetlands guide wastewater
through an open area with plants and treated
water is collected at the point of outflow.
Regarding the water-soil-waste nexus, a
number of general observations can be made:
(i) Excellent analyses are reported of
wastewater in many of the cases in terms
of its chemical composition, including
heavy metals and various organic compounds, contents of pathogenic viruses
and bacteria. Measurements of groundwater and surface water quality are widely
reported as well. Obviously, standard
analytical procedures recommended by the
WHO are not only widely available but
also applied in practice.
(ii) A strong emphasis is on the importance of
wastewater irrigation for agriculture with
major concerns for health effects when
consuming irrigated crops or vegetables,
which is, of course, justified (see, for
example, case 4, Lima, Peru). The case
studies show that significant economic
advantages obtained by farmers applying
wastewater for irrigation are a prime driver
leading to acceptance of these practices.
(iii) Little data on soil water regimes are provided that are crucial for wastewater
purification. Case 1 reports infiltration
rates of sands where wastewater is infiltrated but purification is associated with
the unsaturated flow and travel times as
wastewater moves down to the groundwater (Bouma 1979, 2016b). No data are
provided on these processes. Virus
removal rates are reported but these values
are difficult to interpret because data on
the flow system are lacking. None of the
other studies report data on the movement
of wastewater into and through the soil.
Water use efficiency by applying drip
irrigation is emphasized in several cases
(e.g., case 9 on irrigation of sugarcane in
Colombia applying rates on the basis of
plant demands and activities in Mexico,
reported in case 16). But lack of soil
physical data does not allow a judgement
as to the efficiency reached. Lack of
information on flow regimes implies that
18
J. Bouma