Wastewater reuse—regional and economic considerations

doi:10.1016/S0921-3449(98)00010-X

Resources, Conservation and Recycling

Volume 23, Issues 1–2, July 1998, Pages 57-66

https://doi.org/10.1016/S0921-3449(98)00010-X Get rights and content

Abstract

Wastewater treatment and disposal has become a major component of policy-making subject, because of growing awareness of environmental quality issues. Agricultural use of treated effluents helps to maintain environmental quality, and simultaneously furthers other national goals such as providing sustainable agriculture while preserving scarce water sources. Another advantage of irrigation with wastewater is the possibility of decreasing the purification level and the derived treatment costs, thanks to the role of soil and crops in acting as a bio-filter. Also, using the nutrients available in wastewater may diminish fertilization costs. The considerations for policy decision making include many factors such as the level of purlfication, the location of agriculture and the types of crops. The regional dimension is reflected in the way the decision on where to convey the wastewater influences the choice of agricultural crops, the fate of aquifers and the related costs. For example, in Israel, wastewater treated in the center of the country may pollute the coastal aquifer, whereas conveying this wastewater to the south, where there is no active aquifer gives rise to increased conveyance costs and loss of agriculture in the center. In this paper a general methodology for environmental policy decision making in relation to wastewater is developed and applied in an Israeli case study.

Introduction

Israel is characterized by scarce water resources which limit agricultural production possibilities. The annual amount of water consumption is 1900 million m³, allocated among urban, industrial and agricultural uses, with approximately 1200 million m³ (63%) for current agricultural consumption [1]. Good-quality water resources available for agricultural use tend to decrease with population growth, which enhances domestic water use, and will need to be 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.

Wastewater is a preferred unconventional water source, since the supply is increasing because of population growth, there is enhanced awareness of environmental quality, and its costs are relatively low. Urban sewage must be treated to adapt it to agricultural uses, but treatment is also essential for safe environmental disposal, therefore, the relevant costs of wastewater for agricultural reuse are just the additional costs needed for adaptation to agriculture 2, 21. Wastewater can serve as a source of both water and nutrients, thus also reducing fertilization costs.

Benefits of agricultural reuse of wastewater are expressed when agricultural production is maintained while water sources and environmental quality are preserved. At the same time, wastewater irrigation may be hazardous to the environment, since the influent and hence the effluent contain pollutants such as macro- and micro-organic and inorganic matter. Macro-organic matter including biochemical oxygen demand (BOD), chemical oxygen demand (COD) and total suspended solids (TSS), micro-organic pollutants, trace elements, pathogenic microorganisms, macro-nutrients (nitrogen, phosphorus) and salinity. These constituents may harm the environment, health, the soil, aquifers and crops 3, 4.

Any decision making related to wastewater reuse should consider both aspects: benefits and hazards. Hazards can be decreased by improving effluent quality and–or conveying effluents to distant locations, both of which involve increased costs. An optimal decision-making procedure aims at maximization of net national benefits, i.e., benefits minus costs and minus the value of environmental damage [5]. The complex considerations involved in wastewater agricultural reuse are described by [6].

The present paper focuses on regional considerations of whether and whither to transfer effluents. Treated effluents need not be discarded or used for irrigation near their point of origin; they may be conveyed to other locations because of local surplus and–or environmental impacts. The supply of wastewater is determined by the density of population in a specified region, and this density determines the prices of land in general and–or the availability of agricultural land in particular. Hence, metropolitan areas produce wastewater surpluses, while peripheral areas may exhibit deficits. To preserve environmental quality, surplus wastewater should be disposed of to surface water bodies, or conveyed to other locations for agricultural use.

Conveyance to other locations is also justified by environmental considerations, since health hazards decrease in sparsely than in densely populated areas. Also, damage to aquifers is reduced when wastewater is transferred far away from an active aquifer. Such conveyance involves increased costs that should be balanced against the resulting decrease in environmental hazards.

In Israel, the central area is both very densely populated and located over the main coastal aquifer, whereas the southern area is sparsely populated and has no active aquifer. Hence, wastewater surpluses are expected to characterize the center, and their environmental threats can be decreased by conveyance to the southern areas. Such conveyance involves high investment and operating costs, which will affect the future of agriculture in that region as the supply of fresh water for agriculture diminishes and therefore the regional decisions are important.

The present paper determines regional allocations of wastewater on the basis of comparisons of net benefits of irrigation in southern with those in central Israel. Also, for various levels of environmental hazards, a linear programming optimization model is applied, which simultaneously determines the mix of agricultural branches, the mix of water sources, and the locations of water use and agriculture.

Section snippets

Comparison of alternatives

Wastewater irrigation in Israel is compared between two regions located in central and southern Israel. Wastewater produced in the center can be used in this region with two treatment levels—secondary as based on activated sludge with low land use and high capital and energy inputs [7]and tertiary. Additional tertiary stages to improve wastewater quality include for example, nitrification–denitrification to reduce the nitrogen level, and soil and aquifer treatment (SAT). These local irrigation

Optimization model

The model is oriented to maximize net benefit The target function is $Maximize Σ_{i} Σ_{j} Σ_{k} Σ_{l} (W_{i,j,k,l} *(U_{i,j,k,l} −C_{i,j,k,l} −D_{i,j,k,l})$ where: i=type of crop (citrus, cotton, wheat, peanuts, potatoes, and other vegetables); j=type of water (good quality, secondary, tertiary); k=season (k=1, 2); l=location (l=1, 2); W_i,j,k,l=quantity of water for crop i type j season k in location l (m³); U_ij,k,l, C_i,j,k,l and D_i,j,k,l=utility, cost and damage, respectively ($/m³).

Maximization is subject to restrictions of

Description of case-study

The cultivated area of chosen crops included in model is 7527 ha out of 12 800 ha (%) in Besor and 11 054 ha out of 16 200 ha in Ra'anana (%) (Table 2). Quantity of good-quality water for irrigation is 12.5 million m³ in Besor and 62 million m³ in Ra'anana (total quantity multiplied by proportion of cultivated area for chosen crops). Amount of wastewater is 31.5 million m³ to be allocated for both regions.

Damage to aquifer is caused by leaching of nitrates, and leached nitrogen restriction

Results

Results of the optimization model are presented in Table 3Table 4, each referring to increasing levels of environmental damage in central region, as expressed by stricter restrictions of permitted nitrogen leaching levels. According to current planning, citrus is the main crop to be irrigated by wastewater conveyed to south. Hence, we referred to two cases regarding its extent in the south: (i) limiting citrus area in the south to its current cultivated area (Table 3), and (ii) limiting citrus

Discussion and conclusions

Irrigating with effluent in the central area could maintain agriculture and reduce costs if it were demanded by the agriculture in this region. By transferring wastewater to the south some benefits are lost. Nevertheless, if such a transfer were needed, because of scarcity of agricultural land in the center and–or an environmental hazard that seems critical in a specific location, the farmers using the conveyed treated effluent in the south would thereby enable the costs of river disposal to be

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Wastewater reuse—regional and economic considerations

Abstract

Introduction

Section snippets

Comparison of alternatives

Optimization model

Description of case-study

Results

Discussion and conclusions

Agricultural Water Management

Subsidy by irrigation water—20 years backwards and 20 years forwards

Water and Irrigation

Cost benefit analysis of wastewater treatment in the water scarce economy of Israel: a case study

J Financial Management Anal