Descriptive Methods and Compromise Programming for Promoting Agricultural Reuse of Treated Wastewater

Introduction

Reusing Treated WasteWater (TWW) in an agricultural perimeter is a profitable activity not only for farmers but further for concerned water resource management agencies. Relevant is that this activity may help tackle the scarcity of conventional resources in arid and semi-arid countries. It can also contribute to lessen the negative effects of TWW loading on the environment. Reusing TWW may present a multitude of disadvantages, however. Hence, when the wastewater resource is treated on the secondary stage, its reuse in agriculture is severally controlled. To this effect, international organizations such as WHO and local agencies set many restrictions over the irrigation of market gardening. This measure can partly explain the reluctance of a large number of farmers to reuse TWW. Further, most of the interviewed farmers reusing this type of resource expressed their resentment about the bad quality of the TWW they have access to. Though being quite cheap, many farmers refuse to pay advances to reuse TWW.

The management of TWW in agricultural perimeter entails many disadvantages for decision makers. The agricultural TWW reuse scheme needs an appropriate structure (appropriate irrigation and drainage system, roads, control of reuse practices, etc). So, governmental agencies are the most appropriate institution to control, supervise and offer this non-conventional resource. The investment implementation and the maintenance as well as operating fees are very expensive. Moreover, after 15 to 20 years of exploitation, this kind of schemes needs to be rehabilitated. Costly investments are needed as a result. This rehabilitation can be joined to the decision of improving the resource quality and then moved to the third level of treatment so as to authorize gardening market production. All those structural decisions must be carefully considered, studied and evaluated.

As the Tunisian experience in reusing TWW is important and dates from 1965, guidelines for exploitation and implementation schemes are widely developed. The need now is to evaluate this experience and to learn lessons from success and above all the cases of failure. The decision can be radical, such as deciding to give up the agricultural irrigation with TWW, to continue that experience or to substitute the resource with a conventional one (if it is possible). There is another situation that needs to be studied in case of continuing this experience. Such a situation can be expressed in the following query: Shall we improve the quality of the resource and may be reach the third level of treatment?.

The classical methods used to evaluate this experience are generally based on descriptive techniques. They give an idea about positive and negative effects, tangible and intangible effects. The difficulty lies in measuring the difference between those effects, especially in the case of qualitative ones. Many parameters can be considered in a dashboard that evaluates the reuse activity. If the positive and negative differences effects sum is negative, the general economic equilibrium will be less valuable.. This tool gives some priority to some factors. It helps offset certain weakness in the activity, but structural decision such us giving up the reuse cannot be made from the analysis using this dashboard.

The present study reports on a practical case of TWW agricultural reuse in Tunisia, in North Africa. Given the importance of the Tunisian experience in this sector, our focus will be on how we evaluate a pilot experience, using descriptive methods and multicriteria decision making approach. We will present in a first part most necessarily keys to prepare a useful survey for reliable data. In a second part, we present a systemic method to make decision. This multicriteria technique, which is based on Lp metrics, is called Compromise Programming (CP).