MULTI OBJECTIVES MODEL TO OPTIMISE THE TREATED WASTEWATER USES IN GAZA STRIP. O. OUDA (1), A. BARDOSSY (1) (1) Institut fuer Wasserbau, Universitaet Stuttgart Fax: +49-(0)711-685-4746/ e-mail: omar.ouda@iws.uni-stuttgart.de Key words: Wastewater reuse, Multi objectives model, and Gaza Strip. ============================================================================ Abstract The Gaza Strip faces a serious water shortage problem, with a present water shortage of about 61 Mm 3 /year. The problem is projected to become even larger in the future due to a high population growth of about 3.2%. The water deficit is presently covered by abstraction of the groundwater beyond the sustainable yield where, groundwater is the only natural source of water in the Gaza strip, and importing water from Israel. The reuse of treated wastewater is one of the non-conventional water resources, which can cover part of water shortage problem. The projected wastewater production is about 43 Mm 3 /year for the year 2010 and it is expected to increase rapidly up to 98 Mm 3 /year by the year 2025. Irrigated agriculture consumed about 60% (90 Mm 3 /year) of water in Gaza strip, from which 50% used for trees irrigation, which could be irrigated by treated wastewater based on WHO recommendation. Presently, there is not any reuse schemes in the Gaza District, only few illegal farmers practice. A Multi objectives optimisation model (MOM) based on linear programming techniques aimed to optimise the treated wastewater use under the consideration of biophysical and socio-economical conditions in Gaza strip was implemented for wet, dry and average combined rainfall years. The available agriculture area, available groundwater, crop pattern change, Available treated wastewater and Local agriculture products demand were considered as constrains. Irrigation water demand for each crop and Average product prices were considered as input. The cultivated area, treated wastewater demand, and water using efficiency were calculated for each crop in each zone. Three single objective models were implemented for the three meteorological conditions. The first to maximise the treated water reuse, second to minimise the treated water reuse and the third to maximise the treated water using efficiency US$/ m 3 . These models have been used to support the selection of initial objective values for the Multi objectives model. The MOM presents a good analytical basis for policy makers toward optimising the treated wastewater use through consideration of the socio-economical and environmental aspects of reuse. 1. Introduction Water shortage is increasingly becoming an urgent problem in arid and semi arid areas. The growing population and the expansion of irrigated agriculture are causing heavy water demand. The use of treated municipal wastewater is an attractive option to meet the irrigated agriculture water demand and to keep the good water quality for domestic purposes. The Gaza Strip depends in groundwater aquifer for water supply. The water demand already exceeds the natural recharge of the aquifer. The total annual consumption of water based on the year 1997 estimation was 131 Million cubic meter (Mm 3 ), of which 90 Mm 3 is used for irrigated agriculture, 39 Mm 3 for domestic purposes, and about 2 Mm 3 for industrial sector. The average annual recharge of groundwater aquifer is about 60 Mm 3 in addition to about 10 Mm 3 imported from Israel, so the annual water deficit is about 61 Mm 3 ( MOPIC 1998). There are many researches and projects have been studied the use of treated wastewater for agriculture purposes in Gaza strip, but most of previous works concentrate mainly in the technical evaluation of reuse.