Treated sewage effluent (water) potential to be used for horticultural
production in Botswana
Emongor * and Ramoleman, G. M.
Crop Science Department, Botswana College of Agriculture, Private Bag, 0027, Gaborone,
Botswana.
Abstract
Botswana being semi-arid and arid country, the provision of drinking water and water for
agricultural production is becoming increasingly scarce and expensive. Measures that can
augment the available sources of water or measures that can reduce the demand on potable
water should be given serious consideration. Horticulturists have incorporated new
technology into many of their production programs, which has enabled them to grow more
horticultural crops with less water; however, more effort is needed. Techniques such as drip
irrigation, sensors, growing plants with low water requirements, timing and scheduling of
irrigation to the growth needs of the plant, mulching, and establishing a minimum water
quality standard for horticultural crops must be used to stretch agricultural water supplies.
Recycling agricultural water and using treated municipal sewage effluent is a viable option
for increasing horticultures’ future water supply in Botswana. Agriculture wastewater and
sewage effluents often contain significant quantities of heavy metals and other substances
*
Corresponding author
Email address: vemongor@temo.bca.bw; (V. Emongor)
1
that may be toxic to people but beneficial to horticultural crops. However, before sewage
effluent can be used for commercial production of vegetables and fruits, research must be
undertaken to determine whether there is accumulation of heavy metals and faecal coliforms
in the edible portion of the horticultural produce which may be detrimental to human health
15 to 20 years later. Research must be undertaken to assess the impact of sewage effluent on
soil physical, chemical properties and environment after continued use.
Keywords: Botswana, heavy metals, horticulture, microorganisms, secondary sewage
effluent,
1. Introduction
Cool, clear water! More than two thirds of the earth is covered with water, snowfields,
glaciers, and icecaps. Water is so common that it has been treated with neglect, if not
contempt. Long droughts in Africa have caused a collapse in food production and brought
famine and death to the sahel region south of the Sahara desert. Agriculture has become the
major user of water (Gwynne, 1981). Water is used for irrigation inorder to make the desert
bloom and produce food and fibre.
If sewage effluent is to be recycled safely for agricultural production the problems associated
with using it on vegetables need to be known. Raw wastewater, or primary effluent, is not
considered in this paper because secondary effluent, is the type of wastewater generally used
for irrigation. The Gaborone sewage treatment plant has a capacity of 40,000 m3 per day. The
Gaborone sewage water is what drains from sinks, washing machines, bathtubs, and toilets,
2
combined with industrial wastewater (light industries).Primary sewage treatment in Gaborone
involves filtering debris such as pieces of paper, wood, plastics and any large objects that will
otherwise obstruct pipelines and pumps, jam equipment, interfere with proper functioning of
flow metering units, or create mechanical problems. Then grit removal is done, which can
cause excessive equipment wear and accumulate in deposits that can interfere with the
operation of treatment units. Next organic and inorganic solids are allowed to settle out in
large sedimentation tanks. Then there is the activated sludge process, where settled sewage
from the primary settling tank is treated biologically to remove dissolved organic materials,
mainly carbonaceous matter and nitrates. The aeration tanks comprise an anoxic zone
(denitrification) and aerobic zone for COD (chemical oxygen demand) removal. The level in
the tanks is adjusted by means of an overflow weir. The depth of immersion of the aerators
control their oxygenation capacity. At the end of the primary treatment 40-60% of the
suspended solids and 5-35% of the oxygen demanding waste have been removed. The
secondary sewage treatment removes up to 90% of the oxygen demanding wastes. There is
vigorous mixing of air and sewage and adding activated sludge (sludge from previous batches
of sewage that has developed a high population of microorganisms). The effluent from
secondary sewage treatment is then discharged to the maturation ponds, where the treated
secondary effluent is retained for about 21 days before it is pumped to the Phakalane holding
reservoirs. During the 21 days retention period an improvement of the bacteriological quality
of the treated secondary effluent takes place reducing the faecal coliform count. This paper
discusses treated secondary effluent. The objective of this review paper is to determine the
potential and problems associated with using treated secondary effluent for horticultural crop
production (vegetables and fruits) in Botswana.
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2. Potential of using treated effluent for horticultural crop production
In some countries most of the sewage sludge and sewage effluent generated in the cities is
applied to lands as a form of sewage disposal system. The waste can either be utilized for its
value as a fertilizer and soil amendment or as a landfill. Epstein et al. (1976) reported that
sewage sludge applications to soil, improve the nutrient status and cation exchange capacity
(CEC) of the soil. Sewage sludge contains high concentrations of calcium, sodium, zinc,
copper, lead and nickel (Oloya and Tagwira, 1996a). Oloya and Tagwira (1996a) reported
that sewage effluent (sewage water) contained only traces of cobalt, lead, chromium, silver
and cadmium. Phosphorus and nitrogen are wastewater pollutants or plant nutrients that have
been responsible for eutrophication of streams and lakes. Disposing of wastewater through
the irrigation of horticultural crops has practical applications. Plants use the nutrients,
reducing the pollution load that wastewater contributes to our surface water supply. Irrigation
instead of direct discharge may also help recharge the ground waters if industrial water
wastes can satisfy minimum horticultural standards. Application of sewage effluent in
Pennsylvania (USA) at a depth of 5 cm per week, applied at a rate no faster than the soil’s
infiltration capacity, was proved to be environmentally safe, disposed waste water and
produced the following positive results: crop yields were increased, forest tree growth of such
species as white pine and white spruce were increased, waste water replaced both chemical
fertilizers and usual irrigation waters, and ground waters were adequately recharged (Louis,
1970).
The suitability of soils for receiving waste waters without deterioration varies widely,
depending on their infiltration capacity, permeability, cation exchange capacities, phosphorus
adsorption capacity, water holding capacity, texture, structure, and type of clay mineral (Ivan
4
and Earl, 1972). Sandy soils will allow the greatest rates of water percolation but the least
adsorption and sieving action. Clay soils high in montmorillonite clay will be most subject to
structure breakdown when materials high in sodium salts are used (Donahue et al., 1977).
Phosphorus retention is greatest in well-weathered clay soils. Alkaline soils will remove most
heavy metals by precipitation.
3. Problems associated with using treated effluent for irrigation in horticultural crop
production
Concern has been raised about the possibility of toxic elements entering the food chain and/or
moving into the ground water (Larson et al., 1975). Extreme variations in heavy metal
content between sludges from different cities and treatment works have been observed (Page
et al., 1983; Sommers et al., 1976; Oloya and Tagwira, 1996a). For the purpose of this review
the problems associated with the use of sewage effluent for irrigation are divided into three
areas: physical, chemical and biological.
3.1. Physical problems
Soil aeration and excess water
Wasterwater is applied to land to dispose it, and large amounts are sometimes added to small
areas, resulting in flooding (Leeper, 1978). In regions where evapotranspiration is low the
excess water can remain on the land for several days. In Botswana due to the high
evapotranspiration rate in summer and the sandy soils, flooding would not be a problem.
Crops require at least 10% by volume of air in the soil (Wesseling and Wijk, 1957). This
value is lower, and it is reasonable to expect a different air-content limit with different
5
vegetables and fruits, and at different stages of growth. Some heavy metals such as
manganese and iron may become more available under waterlogged conditions than under
aerobic conditions (Leeper, 1978). Therefore application of sewage effluent in excess of the
water needs of a plant can be a problem unless the soil is adequately drained or
evapotranspiration rates are high.
Suspended solids and clogging
Suspended solids received by the soil are fine and mainly in the organic form because sewage
effluent from secondary treatment has been screened and settled. Suspended solids in
secondary effluent in Gaborone City Council (Botswana), Treatment Plant is in the range of
0-450 mg/litre (ppm), with an average monthly value of 55-60 mg/litre (Table 1). These
solids accumulate on the soil and form a layer of high hydraulic impedance that reduces the
infiltration rate. Hydraulic impedance of the clogged layer is directly proportional to the
concentration of solids in the sewage effluent. The layer is also an O2 sink, as it consists of
biodegradable organic material and can kill seedlings. Nitrogen can be immobilized initially
after the layer is worked into the soil (Bouwer and Chaney, 1974). Drying restores the
infiltration rate since the clogged layer decomposes. The suspended solids in sewage effluent
may also cause blockages of drip and micro-jet irrigation systems. Ravina et al. (1997)
reported that emitter clogging was a major problem incurred in the operation of drip irrigation
systems, when these systems utilized treated wastewater effluent stored in surface reservoirs.
3.2 Chemical problems
Salts
The total dissoved solids (salt) concentrations in domestic wastewater vary widely, according
to the salinity of the local water source and chemicals added during treatment (USAEPA,
6
1981). For example, the total dissoved solids content of the secondary effluent produced in
Gaborone City Council is in the range of 440-800 mg/litre (Table 1). Good drinking water
according to Berry et al.(1980), World Health Organisation (WHO) and Water Utilities
Centre guidelines in Gaborone (Botswana) should have
The conductivity of 800 mg/litre salt is about 1.3 mmho.cm-1 (USDA, 1954). Salinity effects
are mostly negligible at 0-2 mmho.cm-1 , and only tolerant crops yield satisfactorily at 8-16
mmho.cm-1 (Bernstein, 1964). Crops vary greatly in tolerance to salinity (Bernstein, 1964;
USDA, 1954). Salinity is generally of concern in semi-arid and arid regions in which
accumulated salts are not flushed from the soil by precipitation. Botswana being a semi-arid
to arid country may experience salinity problems with continuous use of sewage effluent for
irrigation, unless soil amendment procedures are carried out. There is therefore, need to carry
out research to determine the level of salinity that may occur with continued use of sewage
effluent, before it is recommended for commercial production of horticultural crops. Salinity
can also cause trouble in coastal areas because of salt intrusion.
There are 5 trace elements in domestic effluent which could be present in amounts potentially
toxic under certain conditions and should be periodically monitored. They are boron,
cadmium, copper, nickel and zinc.
Sodium and chlorine
High levels of sodium in drinking water is likely to cause hypertension and heart disease in
humans. The presence of calcium and magnesium may produce benefits by offsetting the
negative effects of sodium in hypertension and heart disease in drinking water. High levels of
sodium in the sludge and wastewater is not good for the soils to which it is applied because
sodium will adsorb onto the soil cation exchange sites, causing soil aggregates to breakdown
7
(disperse), sealing the pores of the soil, and making it impermeable to water flow. Leaves of
many crops show leaf burn due to excessive Na or Cl, especially under conditions of low
humidity and high evaporation. Levels of Na and Cl that cause leaves to burn depend on the
crop. Most fruit trees are quite sensitive to Na and Cl (Basiouny, 1982). The sodiumadsorption ratio (Na+.[(Ca2+ + Mg2+)½]-1) is an index of the effect of Na in reducing soil
permeability (Berry et al., 1980). A ratio of 6 or less is considered desirable. No adverse
impacts on the soil are expected unless the ratio exceeds 9 (USAEPA, 1981). The sodium
adsorption ratio (SAR) of the secondary sewage effluent of Gaborone City Council is in the
range of 4.63 and 6.19 (Table 1). This implies that the sewage effluent is suitable for
irrigation purposes provided the SAR does not increase with continued use to exceed 9. Li et
al.(2001) in a two-year study, in Botswana reported that use of treated effluent increased
maize yield by 8.62% compared to clean water. The use of treated effluent also caused an
increase of sodium concentration in the top soil (Li et al. 2001).
Chlorine used for disinfecting wastewater (to kill pathogens) could be a problem if used at
abnormally high rates. Some crops show injury when residual Cl in wastewater is greater
than 10 mg/litre.
Boron
Boron toxicity to plants can occur because it tends to be unaffected by treatment given to
wastewater (USAEPA, 1981). The problem of boron toxicity is accentuated because the
tolerance range between nutritionally deficient and toxic levels of boron is relatively narrow.
The concentration of boron recommended by the Botswana Bureau of Standards and the
South African General and Special Standards in secondary effluent sewage water is in the
range of 0.5 to 1 mg/litre. The high level of boron found in domestic effluents, usually
8
between 0.5 to 1.0 mg/litre is attributed to the use of boron compounds in the home laundry,
rather than phosphate compounds, in cleaning products (Berry et al., 1980). Boron-sensitive
plants such as fruits and citrus trees may show injury when boron is in the range of 0.5 to 1.0
mg/litre. Vegetable crops vary in their tolerance to boron (Chauhan and Powar, 1978). Boron
is very mobile in the soil and does not tend to build up on the soil as much as heavy metals,
but is leached through the soil profile. This could lead to another problem, that of
contaminated groundwater, which will have to be considered in any overall phase for water
reuse.
Lithium and selenium
Lithium and selenium at high concentrations are both toxic to plants and animals.
Concentrations of 0.05 mg/kg of selenium or less in feed and forage plants are in deficient
range, and 4 to 5 mg/kg or more are in the toxic range, thus animals have a fairly narrow
tolerance range for selenium (Kubota and Cary, 1982). Selenium toxicity is responsible for
certain livestock disorders such as ‘alkali disease’ and ‘blind staggers’ (Brown and Shift,
1982). For ruminants the toxic levels of selenium are above 5 mg/kg dry weight (Kubota and
Allaway, 1972). The selenium content of forage and grain should be between 0.05-0.1 mg/kg
dry weight (Kubota and Allaway, 1972).
Calcium and magnesium
The concentration of calcium and calcium carbonate in Gaborone effluent are 35 and 223
mg/litre, respectively, which are low (Table 1). The low calcium carbonate concentration in
Gaborone effluent implies that the sewage effluent is okay for sprinkler and drip irrigation of
fruit or vegetable crops, because it will not leave a white residue on fruits and leaves, and
minimum blockage of drip irrigation system. The magnesium content in Gaborone effluent is
9
relatively high (40 mg/litre). The magnesium content of Gaborone effluent compares with
that of Arizona (USA) effluent (36 mg/litre). However, the Arizona effluent has high calcium
(82 mg/litre) and bicarbonate (381 mg/litre) contents, which may not be suitable for sprinkler
irrigation of fruit and vegetable crops because it may leave a white residue on fruits and
leaves (Kirkham, 1986). The bicarbonate is otherwise not a problem (Bouwer,1981).
Nitrogen
Nitrates are of concern if present in food plants at concentrations that are toxic to people and
be toxic to people and animals. Young babies are especially sensitive. The maximum nitrate
concentration in drinking water recommended by World Health Organization (WHO) is 10
mg/litre. Water Utilities Corporation Botswana, and Botswana Bureau of Standards,
recommend nitrate concentration of 0 mg/litre in drinking water. While the WUC Botswana,
quality of the industrial effluent targeted for treatment should have maximum nitrate
concentration of 45 mg/litre.
Nitrates are of concern when untreated sewage is used on land (Schalsch et al., 1979). When
treated effluent is used, N in wastewater is usually beneficial, increasing yields and quality
(Kirkham, 1986). For example, N in the Gaborone effluent is mostly in the NO3-, and its NO3–N concentration of 6.7-29.3 mg/litre (Table 1). An application of 120 cm of effluent with an
average of 220 kg N/ha/m per year would add 270 kg N/ha to the soil (Bouwer, 1981). This
implies that application of Gaborone effluent in irrigation may provide adequate N to satisfy
the N requirements of some crops. For some crops, however, uptake off excess N can
increase succulence. Succulence is desirable in leafy vegetables, like lettuce. High
concentration of N also can reduce the sugar content of crops (USAEPA, 1981), which may
10
affect flavour and quality. In addition, high levels of N may induce vegetative growth and
delay flowering and fruiting.
Grasses such as sudan grass (Sorghum sudanese) (Shaw, 1962) and Rhodes grass (Chloris
gayana) (Vaisman et al., 1982) are reported to remove N efficiently from the soil and they
prevent deep seepage and groundwater pollution. These grasses are effective in mining
nitrates because of the following reasons: 1)they accumulate nitrate; 2) they can be cut more
than once in one season so that 2 or more crops can be grown; 3) their nitrate content does
not decrease with age; and 4) they are deep-rooted crops, their use is recommended to reduce
nitrate levels in soils (Stewart et al., 1968). Perhaps vegetables to be grown with wastewater
could be identified with similar characteristics. The vegetables would take up nitrate, and any
possible nitrate contamination of the groundwater would be minimized. For example, in
Gaborone, kale, rape, lettuce, etc can be harvested 4 times in winter, fall, summer and spring.
Harvesting these vegetables 4 times would remove much N from the soil. Ammonium
nitrogen in Gaborone sewage effluent is in the range of 0.06-0.26 mg/litre which is small
(Table 1). While the nitrites are in trace amounts or non-existent. Nitrogen-related problems
with crops, however, are not expected with typical municipal wastewater (USAEPA, 1981).
Phosphorus and potassium
Phosphorus and potassium in wastewater are not known to cause adverse health effects
(USAEPA, 1981). Like nitrogen, they are important nutrients for crops. The total phosphorus
content in Gaborone effluent is in the range of 1.1-12 mg/litre, with an average of 3.8 mg/litre
(Table 1). While the average potassium content in Gaborone effluent is 25 mg/litre. The use
of sewage effluent for irrigation of horticultural crops will supply some of the P and K
needed by plants. Therefore, the amount of fertilizer applied as part of the effluent will be a
11
plus factor and must be accounted for when considering the fertilizer requirements of
horticultural crops.
Heavy metals
The four heavy metals that are of concern to human health and may occur in wastewater in
potentially toxic concentrations are copper (Cu), nickel (Ni), zinc (Zn) and cadmium (Cd)
(Berry et al., 1980). Zingaro (1979) reported that low dosages of Cd were beneficial. The
International Lead Zinc Research Organization (1977) reported that significant positive
effects have been observed repeatedly when Cd salts were added to diets of rats.
Leeper (1978) reported that the heavy-metal content in wastewater was about 0.5 mg/litre for
Cu and Ni and 1 mg/litre for zinc. In the Gaborone effluent the Cu, Ni, Zn and Cd have not
been monitored. However, the Cu, Ni, Zn and Cd contents in the Gaborone composited
sludge are 2.0-4.7, 0.4-1.1, 10.2-23.1 and 0.04-0.08 mg/kg, respectively. While the
recommended amounts of Cu, Ni, Zn and Cd in treated sludges are 1000, 300, 2500 and 20
mg/kg dry matter (WHO, 1999).This suggests that due to the low levels of Cu, Ni, Zn and Cd
in Gaborone composited sludge, then the contents of the same heavy metals will be low in
sewage effluent, because heavy metal concentration is always higher in the sludge than the
effluent. Leeper (1978) further reported that the observed figures for Cd were too low to
attach meaning to. These figures exaggerate the danger, since contents of heavy metals in
effluent are usually well below these figures. Concentrations of heavy metals in municipal
effluents are often lower than the limits established for drinking water (WHO, 1999;
USAEPA, 1981). Most heavy metals end up in the sludge rather than in the effluent (Dean
and Smith, 1973). Heavy metals have not caused any adverse effects on crops grown at sites
in the United States of America that have received wastewater for as long as 38 years
12
(USAEPA, 1981). Analysis of crops grown at the Melbourne, Australia sewage farm, which
received raw sewage since 1893, showed little increase in heavy metal uptake, as compared
to crops irrigated with potable water (David and Willians, 1979; Leeper, 1978; USAEPA,
1981). Some scientists argue that the main problem with heavy metals in the soil is an ion
imbalance that results when they accumulate, rather than any direct effects they have on
plants.
If concentrations of heavy metals higher than now observed were to occur in wastewater,
they might not cause harmful effects, because the diet of man can be deficient in Cu and Zn
(Hambidge, 1981). Heavy metals taken up by vegetables grown with wastewater tend to
remain in the roots. Only a fraction of the heavy elements are translocated to the shoots, and
even a smaller fraction reaches the fruit (Berry et al., 1980). Many vegetables, however, are
not fruits. In some work, deficiencies of essential heavy elements, such as Fe, in hydroponic
culture have been reported because the pH of the wastewater was high enough to limit the
availability of Fe (Berry et al., 1977; 1980). It appears that heavy metals in sewage effluent
used for irrigation of vegetables are unlikely to cause adverse effects.
Fluorine
Some wastewater is enriched in fluoride because many cities add fluoride to the drinking
water. Fluoride is adsorbed by soil (Bouwer and Chaney, 1974). Calcium added to
wastewater keeps the concentration of Ca in the soil high enough to prevent fluoride injury.
The level of Ca and CaCO3 in Gaborone effluent is high enough to prevent fluoride injury to
horticultural crops. Fluoride injury occurs in acidic soils low in Ca but not in well-limed
soils. Cadmium, if present in trace amounts in wastewater, could negate the cariostatic
(decay-preventing) effects on teeth of fluoridated drinking water (Shearer et al., 1980).
13
3.3 Biological problems
Another concern associated with the use of wastewater for irrigation of vegetables is the
contamination of produce by human pathogens that may survive the sewage treatment. One
of the objectives of sewage treatment is to get rid of sewage borne pathogens so that the
effluent and sludge can safely be disposed off. Waste-borne human pathogens are destroyed
during the various stages of the treatment process through autolysis, predation, heat and ultra
violet light. The World Health Organisation has set up microbiological standards that must
strictly be complied with by sewage treatment plants. Wastewater in maturation ponds, just
like any surface water, can easily be re-contaminated by fresh faecal pathogens which are
introduced through surface runoff or by humans excreting near the ponds or from birds and
other animals. Untreated sewage effluents should never be applied on field or garden soils
where foods will go directly for human consumption. Before they are used, public health
services in the city, county, or state should be asked to certify that sewage effluents are safe
to use.
Systemic uptake of pathogens by crops and subsequent transmission through the food chain is
not a problem if the effluent has been treated (Cairns et al., 1978; USAEPA, 1981). No
incidents of infection resulting from crops receiving wastewater have been indentified in the
United States of America (Kirkham, 1986). Sewage farms in Paris (France) apply raw
wastewater to fruit and vegetable crops that are not eaten raw, which are approved for public
consumption by the Ministry of Health, with no reported health problems (USAEPA, 1981).
Use of untreated sewage in Mexico and Iraq (Barreno, 1971; Mutlak et al., 1980), however,
or of treated wastewater inoculated with pathogens (Van Donsel and Larkin, 1977) is
hazardous. It has been known in some countries, such as India, China, Kenya, etc that
14
untreated sewage is the cause of diseases such as cholera, typhoid, and dysentery.
Pathogens may survive on the surface of a plant when effluent water is applied by sprinklers.
Sunlight is a disinfectant, killing pathogens in a few hours to a few days, but any place that
stays warm, dark, and moist could harbour bacteria (USAEPA, 1981). Therefore, sewage
effluent should not be used to irrigate crops that are eaten raw, unless a high degree of
treatment is given to the sewage effluent. The National Technical Advisory Committee on
Water Quality in the United States of America recommends a standard of 1000 faecal
coliforms per 100 ml for water use in agriculture (USAEPA, 1981). The Water Discharge
Guidelines (WDG) in Botswana, recommends maximum allowable faecal coliforms of 100
per 100ml for perennial rivers and 500 faecal coliforms per 100 ml for ephemeral streams.
Serumola (1999) reported that for the Gaborone effluent (Botswana), an average of 1600 and
1990 faecal coliforms per 100 ml were present in final effluent and after maturation stage,
respectively (Table 1). World Health Organisation recommends a maximum limit of 100
faecal coliforms per 100 ml for water use in crop production.
When wastewater is used for sprinkler or central pivot irrigation of fruits and vegetables, it is
important that the microbiological quality of the water and the produce are regularly
monitored. Vegetables grown using wastewater from the maturation ponds of the Gaborone
Sewage Treatment Plant were found to be safe for human consumption (Li et al., 2001). The
samples of tomatoes and rape did not have detectable levels of faecal coliforms and E. coli.
While the levels of total coliforms were below the WHO standard.
Faecal coliforms are much more numerous than disease-causing bacteria (typhoid,
salmonella, cholera) in sewage effluent, and a certain number of pathogenic bacteria must be
15
ingested before they cause sickness. Therefore, faecal coliforms do not need to be eliminated
when irrigating fruit or vegetable crops (Bouwer, 1981). Ingestion of a virus, however, can
cause disease. Therefore, viruses in the sewage effluent should be reduced to zero when such
water is used to irrigate fruit or vegetables consumed raw.
Survival of viruses is affected by temperature (Lance, 1981). Most viruses live only for a few
days at temperatures of 35°C or more. They may survive for months at temperatures near
freezing or in frozen soils. Moisture of the soil has an indirect effect on viruses because
drying allows air to enter and to promote the growth of aerobic bacteria, resulting in viral
death. Thus it is important to let the soil dry between applications of sewage effluent to land.
Movement of viruses does not occur on coarse sands and gravels. The number of viruses
removed through adsorption may, however, approach the number removed by tertiary
treatment (Lance, 1981).
4. Conclusion
Apparently few physical, chemical, or biological problems are associated with secondary
sewage effluent applied to vegetables and fruits. However, there is evidence of accumulation
of heavy metals such as zinc, nickel and chromium in the food chain (Oloya and Tagwira,
1996b) when sludge is used as a fertilizer for growing crops. Most heavy metals end up in the
sludge rather than in the wastewater (Dean and Smith, 1973). Heavy metals have not caused
any adverse effects on crops grown at sites in the United States of America that have received
wastewater for as long as 38 years (USAEPA, 1981). Similarly the analyses of crops grown
at the Melbourne, Australia sewage farm, which has received sewage since 1893, show little
16
increase in heavy metal uptake, as compared to crops irrigated with portable water (David
and Williams, 1979; Leeper, 1978; USAEPA, 1981). Heavy metals taken up by vegetables
grown with wastewater tend to remain in the roots (Berry et al.; 1980; Kirkham, 1986). Only
a fraction of the heavy elements are translocated to the shoots, and even a smaller fraction
reaches the fruit (Berry et al., 1980; Kirkham, 1986).
Work done by the Chinees consultants in Botswana (Li et al., 2001) showed that there is
accumulation of sodium levels in the top soil following the use of sludge and treated sewage
effluent in the production of crops. The sodium concentration exceeded the acceptable levels
for crop production. Li et al. (2001) concluded that they had limitations on their study and
they recommended that their results needs to be verified. They also recommended that due to
the high sodium content in the top soil of sludge and effluent treated soil, experiments be
carried out to determine the harmful effects of sodium on crop growth and development.
Most of the experiments the Chinees experts carried out in Botswana were incomplete and
inconclusive.
Most work with wastes and vegetables has been done with sewage sludge (Kirkham, 1986).
More studies describing the benefits and limitations of wastewater on vegetables and fruit
would be valuable. However, there is scarce information on the use of sewage effluent
(water) as a source of water for irrigation and possible accumulation of heavy metals on
forage, cereals and horticultural crops especially vegetables and fruits in Botswana.
Therefore, there is need to determine heavy metal accumulation in forage crops and livestock
feeding on forage crops irrigated with sewage effluent. Heavy metal accumulation in
vegetable crops and fruits must also be determined to avoid heavy metal accumulation in
human beings and possibilities of heavy metal toxicities and disorders and to dispel any fears
17
of using sewage effluent for irrigation.
Before sewage effluent is used for irrigation it is also important to determine the level of
pathogenic helminths ova, protozoa, enteric viruses and bacteria in order to prevent the
development of infectious diseases on sewage water users and horticultural produce
consumers. The presence of pathogenic microorganisms in secondary sewage effluent creates
the potential for disease transmission where there is contact, inhalation, or ingestion of the
microbiological constituents of health concern.
Botswana being in the rain shadow, makes water the most limiting factor to crop production.
The rainfall is low, erratic, unreliable and poorly distributed, thus making dry land crop
production difficult. However, there is great potential in agricultural productivity through
irrigation and the use of modified growth environments (greenhouses and hydroponics).
Water being the most limiting factor to crop productivity, there is need to look for alternative
cheap water sources and types to be used for irrigation in order for this great nation to
achieve self-sufficiency in food production; and reduce foreign food imports and save on
foreign currency expenditure. Such alternative sources of water is the use of sewage effluent
(wastewater) for irrigation.
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23
Tables
Table 1
Properties of secondary effluent, tap water and irrigation water
Secondary Effluents
Property (mg/L)
Total
solids
Gaborone
Botswana
dissolved 450
Tap water
PhoenixX
Arizona,
USA
Standard
for
irrigation,
Botswana
Water
Utilities
Corporati
on
Botswana
EPAY
USA
WHOY
800
2000
500
500
1000
Suspended solids
84
-
100
0
0
0
Calcium
35
82
-
75
75
75
Magnesium
40
36
-
50
50
50
Sodium
81
200
< 100
200
75-150
200
SAR
4.63
-
10
-
-
-
Potassium
25
8.0
-
-
-
-
Ammonium-N
0.06
10-35
-
1.5
10.0
1.5
Nitrate-N
6.7-29.3
0.5
-
0
-
5.0
Nitrite-N
trace
trace
-
3.0
-
3.0
Carbonate
223
381
< 500
20-200
-
20-200
Chlorides
-
213
< 500
250
250
250
Sulphate
-
107
< 500
250
250
250
Total phosphorus
3.8
9.0
-
0.3
-
0.3
Iron
-
-
5.0-20
0.3
0.3
0.3
Boron
-
0.6
0.75-2.0
-
0.3
0.3
Fluoride
-
2.0
1.0-15
1.0
2.0-4.0
0.7-1.5
Cadmium
-
0.008
0.01-0.05
0.003
0.005
0.003
Copper
-
0.123
0.2-5
1.0-1.5
1.3
1.0-2.0
Zinc
-
0.193
2.0-10
3.0
5.0
3.0
Mercury
-
-
-
0.001
0.002
0.001
Nickel
-
-
-
0.02
0.1
0.02
Lead
-
-
-
0.015
0.05
0.01
pH
8.5-9.0
8.0
6.0-8.5
6.5-8.5
6.5-8.5
6.5-8.5
24
X
Y
Faecal coliforms
per 100 ml
16002000
105-106
102-5x104 0
0
0
Viruses
-
21
0
0
0
0
Turbidity (NTU)
50-136
-
-
0.1-1
0.5-5.0
5.0
Bouwer (1981)
Hach (2002)
25