Economic feasibility of wastewater reuse in agriculture: A case study

Proceedings of International Symposium on Environmental Pollution Control and Waste Management 7-10 January 2002, Tunis (EPCOWM’2002), p.598-607.

Economic feasibility of wastewater reuse in agriculture: A case study M. MASSOUD and M. EL-FADEL Department of Civil and Environmental Engineering, American University of Beirut, PO Box 11-0236, Bliss Street, Beirut, Lebanon Fax: 011-(961) 1 744 462. Email: [email protected] ABSTRACT Treated wastewater effluents may be reused for different purposes such as landscape irrigation (parks, green areas, golf courses, etc.), recreational activities, reclamation of urban streams and rivers, industrial uses, or groundwater recharge. Reusing effluent for irrigation can be the best way to utilize the wastewater since it removes many of the nutrients that are not removed with primary and secondary treatment processes, provides a renewable and highly reliable resource, and frees limited freshwater supplies for municipal purposes. The feasibility of wastewater reuse ultimately depends on the cost of reclaimed wastewater relative to alternative water supplies, and on public acceptance of the reclaimed wastewater. The present study assesses the viability of wastewater reuse for irrigation taking the Ghadir drainage area in Lebanon as a case study. The net present value and internal rate of return coupled with a cost-benefit analysis were used for this purpose. Scenarios were formulated with different restrictions and conditions to evaluate the feasibility of getting loans from international donors or private banks. A field survey was conducted to identify the social perception with respect to the reuse of treated wastewater and to determine the amount of water applied in irrigation with the corresponding cost. The study showed that the utilization of treated effluent in irrigation is profitable. Even with the worst-case scenario the benefit to cost ratio is greater than one. While, it is sometimes not feasible to manage the treatment plant without a certain tariff system due to the high investment, operation and maintenance costs, reusing the effluent for irrigation can reduce charges. Keywords Wastewater, treatment, agricultural reuse, cost benefit analysis.

1. Introduction Globally, freshwater water demands are increasing mainly due to population growth, increased per capita consumption, and the demands of the industrial and agricultural sectors. Greater water consumption is directly associated with increased wastewater generation that requires adequate treatment to prevent health risks and environmental degradation. Recently, the development of reclaimed domestic wastewater reuse projects has emerged as a potential non-convential resource to satisfy the continuously increasing demand for water (Shevah & Shwartz, 1996; Salgot and Pascual, 1996; Haruvy 1997; Angelakis et al., 1999; Bahri, 1999; Bonomo et al., 1999; Faby et al., 1999; Hamdy, 2001). Since several activities do not require water of potable quality, reuse of reclaimed water and nutrients recycling may be substituted for conventional resources. Treated wastewater effluents may be reused for different purposes such as landscape irrigation (parks, green areas, golf courses, etc), recreational activities, firefighting, reclamation of urban streams and rivers, industrial uses, and groundwater recharge (Ayers and Westcot, 1985; Pettygrove and Asano, 1985; Asano and Levine, 1996, Friedler, 2001). Reusing effluent for irrigation can be the best way to utilize the wastewater since it removes many of the nutrients that are not removed with primary and secondary treatment processes, provides a 598

Economic feasibility of wastewater reuse

renewable and highly reliable resource, reduces the need for expensive tertiary treatment, and frees limited freshwater supplies for municipal purposes. Considering that wastewater treatment systems are generally capital intensive and require expensive, specialized operators, reuse of secondary treated wastewater in irrigation provides revenues to offset the cost of treatment. The plant nutrients load in the effluent can also be an important factor in saving costs of fertilizers needed especially nitrogen and phosphorous (Hammer and Bastian, 1989). The reuse of wastewater has been successful for irrigation of a wide array of crops, and increases in crop yields from 10 to 30 percent have been reported (Asano, 1998). The feasibility of wastewater reuse ultimately depends on the cost of reclaimed wastewater relative to alternative supplies of water, and on public acceptance of the reclaimed wastewater. For social efficiency, every wastewater treatment decision must balance many variables and find the combination with greatest net benefit. This paper presents an assessment of the viability of wastewater reuse in agriculture taking the Ghadir drainage area in Lebanon as a case study. Following the definition of general guidelines for wastewater reuse for irrigation, the assessment examines the main challenges for developing water reuse, with emphasis on the choice of treatment method, economics, water quality issues, and social value of recycled water. The net present value coupled with a cost benefit analysis and internal rate of return were used for this purpose. Scenarios were formulated with different restrictions and conditions including a low interest rate (5 percent) and an on going interest rate (10 percent) to check the feasibility of getting loans from international donors or private banks. Moreover, a field survey was conducted to identify the farmers’ perception with respect to the reuse of treated wastewater in irrigation and to determine the amount of water applied in irrigation and the cost incurred. 2.Guidelines for Wastewater Reuse in Irrigation The major considerations of wastewater treatment are to reduce odor, protect public health, and allow for proper operation of the irrigation equipment. Generally, to reuse reclaimed wastewater in irrigation, it has to comply with agro-technical, environmental, and public health quality requirements. Table 1 describes the relative importance of these requirements. Table 1. Agro-technical, environmental, and public health quality requirements - relative importance of various parameters1 (Friedler, 2001) Parameter

Agrotechnical

Environmental

Public health

Salinity

3

0-3 (depending on the receiving water body)

0

Clogging potential

3 (Especially in drip irrigation)

0

0

Pathogens

2 (Farmers and consumers health)

1

3

Heavy metals

2 (Plant uptake)

3

1 (non-potable uses)

Xenobiotic compounds

2

3

0 (non-potable uses)

Nutrients

0 (Nutrients can replace costly fertilizers)

3 (Eutrophication)

0

Odours

1

3

0

1

0 = Not relevant; 1 = Low importance; 2 = Medium importance; 3 = High importance

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Massoud and El Fadel

Constituents of concern in wastewater treatment and reuse for irrigation purposes are presented in Table 2. It is assumed that the value of discharged wastewater is determined by the concentrations of five variables:

Total suspended solids (TSS) in mg/l

Biochemical oxygen demand (BOD5) in (mg/l)

Nutrient matter, measured by the concentrations of nitrogen and phosphates in mg/l

Escherichia coli, measured by the number of fecal coliform bacteria per milliliter

Water flow (Q), measured in cubic meters per day (m3/d)

TSS and BOD can be used as indicators for the turbidity and the amount of dissolved oxygen in the water. Clean water requires low turbidity and high dissolved oxygen, and, therefore, low levels of TSS and BOD. High nitrogen and phosphate levels can cause nutrient loading at the discharge site that may adversely affect flora and fauna of the aquatic ecosystem while elevated counts of Escherichia coli are commonly associated with health problems. Besides testing for fecal coliform bacteria, tests for residual chlorine can increase confidence in the disinfection system. Table 3 summarizes typical water quality, monitoring and operational guidelines for wastewater reuse in irrigation. Table 2. Constituents of concern in wastewater treatment and reuse in irrigation (adopted from Pettygrove and Asano, 1988) Constituent Suspended Solids

Measured Parameters Suspended solids, including volatile and fixed solids

Biodegardable Organics

Biochemical oxygen demand, Chemical oxygen demand

Pathogens

Indicator organisms, total and fecal coliforrm bacteria Nitrogen (N) Phosphorous (P) Potassium (K)

Nutrients

Stable Organics

Specific compounds (e.g. phenols, pesticides, chlorinated hydrocarbons)

Hydrogen ion Activity

PH

Heavy metals

Specific elements (e.g. cd, Zn, Ni, Hg)

Dissolved inorganics

Total dissolved solids, electrical conductivity, specific elements (e.g. Na, Ca, Mg, Cl,B) Free and combined chlorine

Residual chlorine

Reason for concern Suspended solids can lead to sludge deposits and anaerobic conditions when untreated wastewater is discharged in the aquatic environment. Suspended solids cause plugging in irrigation systems. If discharged to the environment, their biological decomposition can lead to the depletion of dissolved oxygen in receiving waters and to the development of septic conditions Communications diseases can be transmitted by the pathogens in wastewater: bacteria, viruses, parasites. Nutrients are essential for plant growth, and their presence normally enhances the value of the water for irrigation. When discharged to the aquatic environment, nitrogen and phosphorous can lead to the growth of undesirable aquatic life. When discharged in excessive amounts on land, nutrients can also lead to the pollution of ground water and inhibit plant growth. These organics tend to resist conventional methods of wastewater treatment. Some organic compounds are toxic in the environment, and their presence may limit the suitability of the wastewater for irrigation. The pH of wastewater affects metal solubility, as well as alkalinity of solids. Normal pHrange in municipal wastewater is 6.5-8.5, but industrial waste can alter pH significantly. Some heavy metals accumulate in the environment and are toxic to plants and animals. Their presence may limit the suitability of the wastewater for irrigation. Excessive salinity may damage some crops. Specific ions such as chloride, Sodium, Boron are toxic to some crops. Sodium may pose soil permeability problems. Excessive amount of free available chlorine (>0.50 mg/l Cl2 may cause leaf tip burn and damage some sensitive crops. However, most chlorine in reclaimed wastewater is in a combined form, which does not cause crop damage.

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Table 3. Guidelines for wastewater reuse in irrigation (USEPA/USAID, 1992) Type of reuse Food crops commercially processed

Treatment required Secondary Disinfection

Surface irrigation of orchards and vineyards Pasturage for milking animals

Secondary Disinfection

Pasturage for livestock Forestation Food crops not commercially processed Surface or spray irrigation of any crop, including crops consumed raw All types of landscape irrigation (e.g. golf courses, parks, cemeteries)

Secondary Filtration Disinfection

Reclaimed water quality pH = 6-9 BOD ≤ 30 mg/l SS = 30 mg/l FC ≤ 200/100 ml Cl2 residual = 1 mg/l min pH = 6-9 BOD ≤ 30 mg/l SS ≤ 30 mg/l FC ≤ 200/100 ml Cl2 residual = 1 mg/l min

Recommended monitoring Weekly Weekly Daily Daily Continuous

pH = 6-9 BOD = 10 mg/l Turbidity ≤ 1 NTU FC = 0/100 ml Cl2 residual = 1 mg/l min

Weekly Weekly Daily Daily Continuous

Weekly Weekly Daily Daily Continuous

Setback distances 300 ft from potable water supply wells 100 ft from areas accessible to public 300 ft from potable water supply wells 100 ft from areas accessible to public 50 ft from potable water supply wells

3. Ghadir Project Characterization Municipal wastewater management in Lebanon has been non-existent particularly during the many years of civil unrest during which the very few existing treatment plants were damaged and/or put out of operation. The general trend for wastewater management in urban areas along the seashores where the greater majority of the population resides, has been limited to a deteriorated wastewater collection system that typically discharges into the sea. In other urban as well as rural areas, septic systems are commonly used. Untreated wastewater is directly discharged into the sea, rivers, irrigation channels, valleys, and ravines. This method of disposal has been practiced for all types of wastewater including domestic, commercial, and industrial. Such practices created severe risk to public health and the environment. As a result, the government has adopted a country-wide plan to rehabilitate/upgrade existing plants and construct new ones to minimize such impacts. The largest of these plants is located south of the capital Beirut at the site of a preliminary treatment facility (Figure 1). The upgrading of the existing facility considers a secondary treatment process prior to sea disposal through a 2.5 Km outfall (CES/BTD, 2000).

601


Ghadir WWTP

Schematic view

Aerial view

Figure 1. Location of the Ghadir wastewater treatment plant

4. Feasibility of Wastewater Reuse in Agriculture In response to the need to reduce the cost of treated wastewater and corresponding user charges, a cost benefit analysis study was conducted to assess the feasibility of reusing treated wastewater in agriculture. Initially, a social survey was carried out using a questionnaire developed specifically for the purpose of evaluating the willingness of farmers to use the treated effluent generated from the wastewater treatment plant in irrigation. Farmers in the areas in the vicinity of the treatment plant were interviewed and the questionnaire was filled to determine the actual enterprise budget of crops cultivated in the study area with emphasis on the amount of water currently used and the cost incurred for irrigation purposes. Social Survey Results Interviews were conducted with a total of 36 farmers owning the great majority of land in the area. The questions addressed were related mainly to their age, educational level, crops cultivated, water sources for irrigation, willingness to utilize treated effluent in irrigation, and at what price. Figures 2 to 4 depict the main findings of the survey.

602

70 60 50 40 30 20 10 0

Percent respondents

Percent respondents


Artesian well

Sewer network

River

Yes

No

No response

Figure 3: Acceptance to utilize treated effluent for irrigation

Figure 2: Water resources used for irrigation

Percent respondents

70 60 50 40 30 20 10 0

70 60 50 40 30 20 10 0 Same

Less

More

None

Figure 4: Willingness to pay for treated effluent compared to freshwater

The age of interviewed farmers varied from 24 to 78 years old with an educational level distributed as follows: 12 percent have reached university level, 28 percent high school, 13.5 percent intermediate, 30 percent elementary, and 7.5 percent illiterate. The crops cultivated are mainly strawberry, banana, and vegetables. Private wells and river water are used for irrigation with more than 62 percent of farmers reporting problems in securing enough water to their land due to water scarcity and salt water intrusion. Most farmers (63 percent) showed willingness to use treated wastewater and pay the same amount they are currently paying for irrigation water. The survey showed that the farmers who are willing to utilize the treated effluent are aware of its potential advantages in terms of nutrients present in the effluent. The main reasons for not utilizing treated wastewater in irrigation include the availability of freshwater sources and the doubt that the quality of the treated effluent might be inconvenient for irrigation. Economic Assessment Identification and quantification of costs and benefits The total costs of a wastewater treatment plant can be divided into investment and operational and maintenance (O&M) costs. These include but not limited to:

The value of the existing assets of the pretreatment plant (including the existing sea outfall system). 603


The main conveyors required to connect the existing sewerage system to the proposed treatment plant.

The reinvestment cost for electrical and mechanical equipment (after 15 years).

The annual O&M cost directly related to the sewage treatment plant.

The cost of maintenance, electricity, chemicals, and personnel.

Moreover, since the use of reclaimed water for agriculture requires both seasonal and long-term storage, building reservoirs in which the effluent can be stored would allow matching daily variations in irrigation water demand, increase the reliability of water supply, and improve water quality to meet guidelines for unrestricted irrigation. On the other hand, as demand for irrigation water is mainly during the dry season, seasonal storage during the non-irrigation period would increase reclaimed water reuse and prevent coastal water contamination. As such, an additional investment (reservoirs, pipelines, pumping stations, etc…) and O&M cost is added to the wastewater treatment plant cost. Assessing the benefits of a wastewater reuse project is generally a critical problem due to the difficulties encountered in quantifying those benefits. The survey revealed that the farmers’ willingness to pay for the treated effluent is about 1.07/m2/yr US$. Thus, the total potential benefits will amount to about 29.6 million US$. A summary of the total costs and benefits is presented in Table 4. Table 4. Total costs and benefits of the wastewater reuse project Category Wastewater treatment plant Wastewater reuse project Total costs Total benefits (MUS$/yr)

Investment Cost (MUS$) 157.8 3.2 161.0 29.6

O&M cost (MUS$) 4.6 0.32 4.92

The cost benefit analysis was then conducted using the following specifications and assumptions:

The population connected is 1.3 million in 2005 and 1.8-2.0 million in 2040

The lifetime of the main civil works of the wastewater treatment plant is up to 40 years considering that local constraints might change during this period

Investment and O&M cost figures are primarily based on constant prices at 1997 price level; stated in US$, using a uniform exchange rate of 1=1500LL for all cost categories; and do not include duties or taxes

The total peak flow of the wastewater treatment plant is about 164,160 m3/d

The efficiency of the treatment plant is about 80 percent, thus the total effluent coming out of the plant will be about 131,328 m3/day

The elevation between the plant and the highest point in the study area is 200 m and the distance is 4,000 m

1 ha on the coastal area requires a continuous flow of 72 m3/d (open field in summer) and 50 m3/d (green house in winter)

The treatment plant operates 20 hrs/day (4 hrs down load)

The total agricultural are is 11.1 Km2

The total landscape area is 0.32 Km2 604


Total water requirement is 79,620 m3/day

The value of the existing assets in million US$ of the pretreatment plant and the sea outfall is of the order of 33.3. The value of the existing sewerage network is about 50 million US$.

Cost benefit analysis The standard criterion for deciding whether a project can be justified on economic principles is net present value (NPV) which is computed by assigning monetary values to benefits and costs, discounting future benefits and costs using an appropriate discount rate, and subtracting the sum total of discounted costs from the sum total of discounted benefits (Equation 1). Discounting benefits and costs transform gains and losses occurring in different time periods to a common unit of measurement. While projects with positive NPV increase social resources and are generally preferred, negative NPV projects should generally be avoided. The internal rate of return (IRR) implied by the stream of benefits and costs and the benefit to cost ratio (BCR) are other criteria that are appropriate for evaluating the economic feasibility of projects. While the IRR is the rate (r) that would make the NPV of the project equal to zero (Equation 2), the BCR is defined in terms of discounted values (Equation 3) (Perkins, 1994).

NPV =

C0 =

B 0 − Co B 1 − C 1 Bt − Ct Bn − Cn + + .... + + 0 (1 + d ) (1 + d ) (1 + d )t (1 + d )n

(1)

B1 − C 1 Bt − Ct Bn − Cn + .... + + .... + 1 t (1 + r ) (1 + r ) (1 + r )n

n B Bt =∑ C t =0 (1 + d )t

Where:

n

(2)

Ct

∑ (1 + d ) t =0

(3)

t

Ct = The dollar value of costs incurred at time t, Bt =

The dollar value of benefits incurred at time t,

d

=

The discount rate, and

n

=

The life of the project in years.

Three basic scenarios were considered. Scenario 1 represents the no project alternative i.e., without reusing treated effluent for irrigation. Scenario 2 with project alternative taking the total investment and O&M cost of the project and the net benefits that are gained due to reuse of wastewater in irrigation. Scenario 3 with project alternative but taking only 60 percent of the total investment and O&M cost considering that the total water demand for irrigation (79,620 m3/day) accounts for about 60 percent of the treated effluent (131,328 m3/day). A low interest rate (5 percent) and an on-going interest rate (10 percent) were used to check the feasibility of getting loans from international donors or private banks.

605


A summary of the NPV, BCR, and IRR for the three scenarios is presented in Table 5. The economic analysis shows that the utilization of treated effluent in irrigation can be profitable depending on the loan interest rate. While, it is sometimes not feasible to manage the treatment plant without a certain tariff system due to the high investment, operation and maintenance costs, reusing the effluent for irrigation can reduce charges. Evidently, extending the agricultural area will increase the benefits. Table 5. Summary of the cost benefit analysis Scenario 1 2 3

NPV 10% -132.7 -64.0 24.5

BCR 5% -164.2 21.0 130.5

10% 0 0.7 1.18

IRR 5% 0 1.1 1.8

7.8 6.2

5. Conclusion and Recommendations The effectiveness of reusing wastewater for irrigation, while difficult to quantify, is seen primarily in terms of the diminished demand for potable quality freshwater. The wastewater used for irrigation requires less treatment and generates less sludge. Communities have considered this alternative because of either very stringent requirements for wastewater discharge to the surrounding stream or the absence of a receiving stream. Reusing effluent for irrigation can be the best use of the wastewater in a water scarce area. Moreover, irrigating treated wastewater recycles nutrients and makes it even more beneficial as combined fertilizer and water for the crops. Although the NPV cannot be always estimated, efforts to measure it can produce useful insights even when the monetary values of some benefits or costs cannot be determined. Results of this study clearly encourage governments to pay serious attention to the reclamation of wastewater for irrigation purposes. Moreover, the cost benefit analysis revealed that public or private investment in this field could be both economically and socially justifiable. Generally, before designing a wastewater treatment plant, it is recommended to consider the wastewater reuse options first given that the treatment process depends on the ultimate use of the effluent. Moreover, the initial selection of the location of the treatment plant site should be based upon the location of future reuse sites. Finally, wastewater reuse demonstration projects for agricultural and other reuse alternatives should be conducted concurrently with public awareness campaigns to educate the users.

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