Water Resources Management 13: 303–314, 1999. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.
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Wastewater Irrigation-Economic Concerns Regarding Beneficiary and Hazardous Effects of Nutrients? NAVA HARUVY1∗, RAM OFFER1, AMOS HADAS1 and ISRAELA RAVINA 2 1 Institute of Soils and Water, Agricultural Research Organization, The Volcani Center, PO Box 6, Bet Dagan 50250, Israel (∗ author for correspondence, e-mail:
[email protected]) 2 The Technion, Israeli Institute of Technology, Haifa, Israel
(Received: 12 August 1997; in final form: 2 September 1999) Abstract. The optimal wastewater treatment level is affected by costs, hazards and benefits. Lowering the wastewater treatment level decreases fertilization costs because of the increased levels of available nutrients left in the water, and irrigation costs decrease if water prices reflect the lower treatment costs. Agricultural yields and/or prices may decrease according to differences between levels of nutrients needed by crops and those available in wastewater. The present article focuses on determination of monthly optimal treatment levels and of the mix of crops calculated to maximize agricultural incomes, according to farmers’ point of view. It does not reflect the national pointview focusing on maximization of net national benefits considering also environmental hazards. The methodology appears in Haruvy (1994) and application will be presented in another article (Haruvy et al., 1999). Key words: economics, irrigation, nitrogen, nutrients, wastewater.
1. Introduction Israel is characterized by scarce water resources, which limit agricultural production possibilities. The annual water consumption of 1900 million m3 is allocated among urban, industrial and agricultural uses, and approximately 1200 million m3 (63%) is used for current agricultural consumption (Israel Water Commission, 1995). Good-quality water resources, available for agricultural use, tend to decrease as the population growth enhances domestic water use, and is being replaced with unconventional water sources, including effluent, to maintain agriculture. By the year 2040, treated sewage effluent will become the main source of water for irrigation in Israel and the Palestinian autonomous regions, supplying 1000 million m3 (70%) out of the 1400 million m3 , that will be used for irrigation (Israel Water Commission, 1995). ? Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan,
Israel, No. 2222-E.
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Wastewater is a preferred unconventional water source, of which supply is increasing because of the population growth and enhanced awareness of environmental quality, and its costs are relatively low (approximately 10–25 cents m−3 as compared to production costs of good water – 35 cents m−3 ). Urban sewage must be treated to enable agricultural uses, but treatment is also essential for safe environmental disposal, so that the relevant costs of wastewater for agricultural reuse comprise just the additional costs needed for further adaptation to agriculture (Sadan and Haruvy, 1994). Wastewater can serve as a source of both, water and nutrients, thus reducing the fertilization costs. Benefits of agricultural reuse of wastewater are expressed in the maintenance of agricultural production with scarce water sources. But, at the same time, wastewater irrigation may be hazardous to the environment since the influent and, therefore, also the effluent, contain pollutants. They include macro-organic matter (including biochemical oxygen demand (BOD), chemical oxygen demand (COD) and total suspended solids (TSS)), microorganic pollutants, trace elements, pathogenic micro-organisms, macro-nutrients (nitrogen, phosphorus) and salinity. These constituents may harm the environment, health, soil, aquifers and crops (Feigin et al., 1990; U.S. Environmental Protection Agency, 1992). The wastewater treatment level and irrigation and fertilization practices should be adapted to agricultural uses (Pettygrove and Asano, 1985; Shelef, 1991) to restrain possible hazards. Any optimal decision-making procedure at the national level aims at maximization of net national benefits, e.g., benefits minus costs and minus the value of environmental damage (Haruvy and Sadan, 1994). National benefits include the value of agricultural output produced by recycled water and affected by modified crop-mixes and improved yields, the value of aquifer recharge resulting from irrigation, the value of open areas, the value of maintaining agricultural settlements in specific regions, and saving in wastewater disposal costs. Costs include wastewater treatment, storage and conveyance costs, and agricultural production costs (fertilization and irrigation). Damage includes environmental quality degradation, aquifer pollution due to nitrate leaching, additional salinity and other pollutants (Nielsen et al., 1986; Phillips, 1994; Haruvy et al., 1997), health injuries, and soil structure deterioration. Farmers generally do not consider environmental benefits or hazards and are interested at maximizing agricultural profits (Haruvy, 1994). For them, wastewater serves as a source of both water and nutrients, but although wastewater irrigation may save fertilization and water costs, it may cause damage to crops because of wastewater constituents such as salinity (Maas, 1986) and excess nutrients. In the present article we focus on the farmers’ considerations. They are interested at maximizing agricultural profits, i.e., revenue minus production costs, with the latter including fertilization (with fertilizer consumption including the supplied amount and the remainder of the crop requirement, obtained in the wastewater) and irrigation costs (according to water prices and irrigation amount).
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2. The Model The target function of farmers is maximization of net agricultural profits (Equation (1)). Maximize 5 =
X i
where 5 i j Si Yi Cij
= = = = = =
Si (Yi −
X
Cij ),
(1)
j
Net agricultural profit ($), Crop type, Growing month, Cultivated area of crop i (ha), Revenue of crop i ($ ha−1 ), Production costs of crop i in month j ($ ha−1 ).
Production costs comprise fertilization, irrigation and other costs (Equation (2)). Cij = Fij ∗ P Fi + Wij ∗ P Wj + F Ci , where Fij P Fi Wij P Wj F Ci
= = = = =
(2)
Amount of N fertilizer used for crop i in month j (kg ha−1 ), Price of N fertilizer for crop i (affected by all used fertilizers), ($ kg−1 ) Amount of water used for crop i in month j (m3 ), Price of water in month j ($ m−3 ), Other production costs for crop i ($).
The amount of fertilizer (Fij ), is affected by the difference between needed and supplied quantities (Equation (3)). Fi = NDij − NSj if NDij > NSj , otherwise, Fij = 0, where NDij NSj
= =
(3)
N amount demanded by crop i in month j (kg ha−1 ), N amount available in wastewater in month j (kg ha−1 ).
The price of water (P Wj ), is either constant (P W ) or reflect treatment costs in month j (CWj ) ), which increase with higher treatment level, reflected by lower nitrogen level in effluents (NSj ) (Equation (4)). P Wj = P W + CWj (NSj ) and where
dCWj /dNSj < 0,
(4)
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Yl
=
Revenue of crop i is affected by a damage factor, which depends on the nitrogen excess in wastewater (Equation (5)).
Yi = Ri ∗ (1 − Di ) where Ri Di
= =
(5)
Maximal revenue to crop i ($) Rate of damage to crop i due to excess nitrogen; it is defined as
Di = MDi ∗
X
CDj ∗ [(NSj − NDij )/NDij ],
(6)
j
where MDi
=
CDij
=
Rate of maximal damage for crop I (for multiplying fertilization amount) Damage coefficient for crop i in month j (CDij = 1 if damage occurs, otherwise CDij = 0).
The optimization procedure is restricted by existing amounts of cultivated area for each crop, and water supply for each month. Solutions of this model give optimal cultivated area for each crop (Si ) and treatment level for each month (NSj ). 3. Case Study These considerations are demonstrated for major agricultural crops grown in ‘Emek Heffer’, located in central Israel. This case study is an exercise, however, results of this research were partially applied regarding treatment plant in this region, which changes nitrogen level throughout the year. The crops are orchards as citrus (orange, grapefruit, etc.) and mango, and field crops as cotton and corn. Total irrigated area is 4500 ha and area irrigated by treated effluents accounts to 1750 ha. The chosen crops include 70% of the irrigated area. The irrigation amounts are 4500– 7320 m3 ha−1 ; nitrogen fertilization amounts are 150–300 kg ha−1 and the needed nitrate concentration is 30–40 mg L−1 (Table I). The economic significance of these inputs is indicated in Table II. Fertilization costs are 1.3–8.7% of the total revenue, while irrigation costs are 7.2–28.6% of the revenue. Net profits account to 22–55% of the revenue, and fertilization costs are 3–25% of the net profit. These profits can be increased by adapting wastewater treatment levels to the crops’ needs (optimal value added) as will be described later. The farmers can save by decreasing fertilization costs according to available nitrogen levels in wastewater (Equation (3)). We will demonstrate this saving by referring to secondary treated wastewater with 40 mg L−1 N as presented in Table III (raw sewage in this region originates with a nitrogen concentration of 60 mg L−1 and secondary treatment without nitrification–denitrification yields effluent with
WASTEWATER IRRIGATION-ECONOMIC CONCERNS
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Table I. Fertilization and irrigation – crop needs Crop
Water (m3 ha−1 )
Nitrogen (m3 ha−1 )
Nitrogen (mg L−1 )
Cotton Corn Avocado Mango Orange Grapefruit
5050 4500 7320 6270 6600 6600
200 150 240 180 270 300
40 33 34 30 31 30
Source: Field experience.
20–40 mg L−1 N). Later on, we will estimate the saving when nitrogen levels in wastewater are optimally adapted to crop needs. The saving estimation is based on the nitrogen needs of crops through the growing season, in addition to the constant concentration in effluents. This saving in fertilization costs amounts to 0.012–0.022 $ m−3 (Table III, row 2.2), and it increases if nitrogen is accumulated in the crops. Now, we compare saving in fertilization costs with damage to crops caused by excess nitrogen during specific periods. For example, cotton is fertilized during June and July and irrigated through April–September, and avocado is fertilized through June–August and irrigated for 8 months (Table III, row 1). Excess nitrogen promotes vegetative growth, therefore, if production is based on fruits or seeds, as with cotton, reproductive growth will suffer. Many experiments are needed to estimate damage to crops caused by excess nitrogen in wastewater (Steinhardt et al., 1996). Nevertheless, we estimated the damage on the basis of interviews with experts. Our study is preliminary in generalizing economic damage coefficients, which can be improved later on. For each crop, we asked what was the maximal percentage of damage for multiplying regular fertilization amount- MDi . Then, we defined the damage coefficient for each month- CDij , to be 1 in the season when damage prevails and 0 otherwise. The total damage- DI was calculated as the maximal damage multiplied by the sum of damage coefficients being multiplied by the relative fertilization surplus (Equation (6)) (Table III, row 3). For example, in cotton, the maximal damage was estimated as 2% of the revenue. The damage coefficients were 1 in August and September, when intensive vegetative growth affects profits by decreasing the quantity and quality of cotton, and 0 otherwise. Nitrogen excess of 1.4 kg ha−1 in August is divided by fertilization quantity of 200 kg ha−1 and multiplied by 2%, and then added to nitrogen excess of 17.9 kg ha−1 in September, multiplied by the same coefficients; this leads to a total damage of 1.09% of the total revenue. Regarding corn, there is assumed to be no damage with existing nitrogen levels (Berkovitz, 1985). In orchards, we assumed
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Table II. Economic meaning of inputs Crops Cotton
Corn
Avocado
Mango
Orange
Grapefruit
Revenue ($ ha−1 )
4030
2406
12263
14323
4806
11900
Nitrogen fertilizer ($ ha−1 ) (% of revenue)
153 3.8%
210 8.7%
167 1.3%
183 1.3%
250 5.2%
300 2.5%
Water ($ ha−1 ) (% of revenue)
793 19.6%
690 28.6%
1383 11.2%
1037 7.2%
1210 25.1%
1210 10.1%
Added value ($ ha−1 ) (% of revenue)
1363 33.8%
820 34.0%
2717 22.1%
6443 44.9%
–120 negative
6527 54.8%
Optimal added value ($ ha−1 ) (% growth)
1493 9.5%
1027 25.2%
3030 11.5%
6657 3.3%
200 positive
6860 5.1% NAVA HARUVY ET AL.
Source: Israel Ministry of Agriculture, 1994.
Cotton
Corn 1
1 1.1 1.2 1.3
Months of Irrigation Fertilization Damage
2.1
Saving in fertilization costs ($ ha−1 ) $ m−3 % of maximal fertilization costs
2.2 2.3
3
Damage ($ ha−1 )
4
4–9 6–7 8–9
Avocado 2
4–6 4–6
Mango
Orange
Grapefruit
3
5–7 5–7
8–10 8–9
86
54
57
76
0.017 54.5%
0.012 60.2%
0.013 63.6%
0.016 86.3%
4–11 6–8 4–11
4–11 5–7 4–11
92
138
0.013 49.2%
0.022 81.3%
4–11 4–6 4–11 128 0.019 49.1%
4–11 4–6 4–11 128 0.019 43.3%
–29
0
0
0
–285
–157
–66
–162
Accumulative saving ($ ha−1 )
57
54
57
76
–193
–19
62
–34
5
Saving in water costs ($ ha−1 )
56
50
50
52
81
69
73
6
Total saving ($ ha−1 )
113
104
107
128
–112
50
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WASTEWATER IRRIGATION-ECONOMIC CONCERNS
Table III. Fertilization saving by irrigating with secondary treated effluents
73
39
Source: Model’s computations. The crops named corn 1, corn 2 and corn 3 differ by growing months.
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310
Table IV. Treatment levels (mg L−1 ): Regional view-point (including energy costs) Month
Cotton
Corn 1
2
Avocado
Mango
Orange
Grapefruit
Region
3 3 60 51 29 3 3 3
3 56 60 60 3 38 3 3
50 60 60 58 3 5 3 4
50 60 60 58 3 5 3 4
50 60 60 60 50 40 5 9
3
60 60 60 60 5 7
60 60 60
Area (1000 ha)
0.432
0.432
0.209
0.432
0.115
0.044
0.000
0.090
1.756
Added value (base) ($ ha−1 )
1083
673
642
669
2528
6160
–319
6340
Total $2.76 million
Added value (optimal) ($ ha−1 )
1494
1027
992
1022
3030
6656
202
6860
Total $3.00 million
Growth (%)
37%
52%
54%
52%
19%
8%
positive
8%
9%
60 60 60 60 60 60
Source: Model’s computations. The crops named corn 1, corn 2 and corn 3 differ by growing months.
NAVA HARUVY ET AL.
April May June July August September October November
Month
Cotton
Corn 1 60 60 60
2
Avocado
Mango
Orange
Grapefruit
Region
2 2 32 51 41 2 2 2
26 60 60 60 2 38 2 3
50 60 60 60 2 2 2 2
50 60 60 60 2 2 2 2
9 60 60 60 48 2 2 2
3
April May June July August September October November
60 60 2 2
Area (1000ha)
0.432
0.432
0.209
0.432
0.115
0.044
0.000
0.090
1.756
Added value (base) ($ ha−1 )
1275
847
812
847
2806
6433
–68
6657
Total $3.06 million
Added value (optimal) ($ ha−1 )
1650
1147
1118
1154
3246
6857
419
7080
Total $3.23 million
Growth (%)
29%
35%
38%
36%
16%
7%
positive
6%
5%
60 60 60 60 60 60
WASTEWATER IRRIGATION-ECONOMIC CONCERNS
Table V. Treatment levels (mg L−1 ): Farmers’ view-point (without energy costs)
Source: Model’s computations. The crops named corn 1, corn 2 and corn 3 differ by growing months.
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Table VI. Total value added ($ million) (cultivated areas of 1760 ha)
Tertiary wastewater (base) Secondary wastewater Optimal treatment level Growth (relative to base) (%)
Regional level (considering treatment cost)
Farmers’ level (considering only production costs)
2.76 2.95 3.00 8.7%
3.06 3.19 3.23 5.5%
price decreases due to changes in fruit markets among export, industry and local markets. On the basis of these assumptions, we estimated accumulated saving through irrigation with wastewater as composed of fertilization saving minus the estimated damage. These savings are relatively high for cotton, corn and orange growing (Table III, row 4), and could be raised further by adapting treatment levels to crops needs. Another factor affecting profitability of irrigation with wastewater is the water price. If treatment costs are reflected in water prices (Equation (4)), there is additional motivation to use wastewater with lower treatment levels (and, therefore, with higher nitrogen levels). Allowing the amount of remaining nitrogen to increase in wastewater from 0 to 40 mg L−1 , decreases energy costs by 0.011$ m−3 . When the saving in water costs (Table III, line 5) is added to the other accumulated saving, farmers benefit by irrigating most crops with secondary wastewater (Table III, row 6). Profits on specific crops can be increased further by adapting wastewater treatment levels to crop needs. For each crop, we estimated for each month a vector of treatment levels, which would maximize its net profit (Equation (1)). This vector is affected by the saving in fertilization costs, so that recommended wastewater nitrogen levels increase in months of heavy fertilization; the levels are influenced by water costs (if these are reflected in water prices); with higher nitrogen levels in months of intensive irrigation. Treatment levels are also affected by expected damage, with nitrogen levels being reduced in months when crops are highly liable to damage. In Table IV, we see optimal treatment levels for various crops, months and agricultural parameters. For example, in cotton, low treatment levels (high nitrogen amounts) occur during April–July, being irrigation and fertilization months, and, high treatment levels take place through August–September, being damage months. These optimal treatment levels increase profits for cotton by 37%, and for other crops by 8–54% (Table IV, last row). Hence, agricultural profitability increases by adapting treatment levels to the needs of each crop.
WASTEWATER IRRIGATION-ECONOMIC CONCERNS
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Treatment levels for the whole region were estimated by means of an optimal planning model, which maximizes profits for the whole region (Equation (1)) as restricted by existing area and water amounts. Nitrogen levels (Table IV, last column) were high in May–July (60 mg L−1 ), medium in April, August and September (40– 50 mg L−1 ), and low in October and November (5–9 mg L−1 ). The total profits for the whole region increased by 9%. Another point of view is, that the farmers facing a constant water price, which does not reflect energy costs (Table V). Optimal nitrogen levels are high in fertilization months, low in damage months, and medium where fertilization application and damage sensitivity occur simultaneously, and are not influenced by irrigation periods. Optimal regional nitrogen levels are high in May–July, medium in August and low in April and September–November. Profits for individual crops, increase by 6–38% (Table VI, last row), while the total profit for the whole region, on an optimal mix of crops, increases by 5%. We compare total profits for irrigation with secondary treated effluents (40 mg L−1 ) with those when tertiary effluents (with no nitrogen left) are used (Table VI). One can see that, at the regional level, the profit of $2.76 million increases by 7% for irrigation with secondary effluents, and 8.7% for irrigation with optimal treatment level (Table VI, last row). For the farmers, who consider only production costs (their water price is constant and does not reflect treatment costs), the total profit of $3.06 million increases by 4% for irrigation with secondary effluents, and by 5.5% for irrigation with optimal treatment level (Table VI, last row). 4. Summary We have described the agricultural effects of irrigation with secondary wastewater, placing emphasis on the concentration of nitrates remaining in effluents. Added value increases with increasing nitrates concentration, since costs of fertilization and irrigation decrease, but it is affected by possible hazards to crops. Adaptation of wastewater treatment levels to the regional mix of crops and to crop fertilization demands enhances agricultural incomes. Analysis from the national point of view, which also takes account of environmental effects, such as nitrates leaching or salinity accumulation, may lead to different results from that of the farmers’ viewpoint and will appear in another article. References Berkovitz, S.: 1985, Corn Growing, Israel Ministry of Agriculture (in Hebrew). Feigin, A., Ravina, I. and Shalhevet, J.: 1990, Irrigation with Treated Sewage Effluent, Ecological Series, Springer Verlag, New York. Haruvy, N.: 1994, Recycled water utilization in Israel: focus on wastewater pricing versus national and farmers objectives, J. Financ. Manage. Anal. 7(2), 39–49. Haruvy, N. and Sadan, E.: 1994, Cost benefit analysis of wastewater treatment in the water scarce economy of Israel: a case study, J. Financ. Manage. Anal. 7(1), 44–51.
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Haruvy, N., Hadas, A. and Hadas, A.: 1997, Cost assessment of various means of averting environmental damage and ground water contamination from nitrate seepage, Agricultural Water Management 32, 307–320. Haruvy, N., Hadas, A., Ravina, I. and Shalhevet, S.: 1999, Cost Assessment of Averting Groundwater Pollution, The 7th International conference of the Israel society for ecology and environmental quality sciences on ‘Environmental Challenges for the Next Millennium’, Jerusalem, Israel. Israel Ministry of Agriculture: 1994, Agricultural Branches in the Years 1992–1993, Ministry of Agriculture, Tel-Aviv (in Hebrew). Israel Water Commission: 1995, Report, Tel-Aviv (in Hebrew). Maas, E. V.: 1986, Salt tolerance of plants, Appl. Agricult. Res. 1, 12–26. Nielsen, D. R., Van Genuchten, M. Th. and Biggar, J. W.: 1986, Water flow and transport processes in the unsaturated zone, Water Resour. Res. 22, 895–1085. Pettygrove, G. S. and Asano, T. (eds): 1985, Irrigation With Reclaimed Municipal Wastewater – Guidance Manual, Lewis Publishers Inc., Chelsea, Michigan. Phillips, F. M.: 1994, Environmental tracers for water movement in the desert soils of the American South West, Soil Sci. Soc. Am. J. 58, 11–24. Sadan, E. and Haruvy, N.: 1994, Subsidy by irrigation water – 20 years backward and 20 years forwards, Water and Irrigation 335, 7–9 (in Hebrew). Shelef, G.: 1991, Using water of marginal quality for crop production: Major issues, Agricult. Water Manage. 25, 233–269. Steinhardt, R., Kalmor, D, Miari, A. and Lahav, E.: 1996, Yield Loss of Avocado Trees Due to Salinity as Affected by Root Stocks and Management, In: E. Rosenberg and S. Sarig (eds), 7th International Conference on Water and Irrigation Proceedings, Tel Aviv, Israel. U.S. Environmental Protection Agency, 1992: Guidelines for Water Reuse, U.S., EPA, Washington DC.
