In this paper, thermo-economic evaluation of hybrid solar-conventional energy supply in a zero liquid discharge plant has been conducted. The zero liquid discharge plant includes a brine concentrator and a crystallizer. Brine concentrator and crystallizer utilize falling film and forced circulation evaporators respectively. The plant was studied thermo-economically in capital of Iran, Tehran city weather conditions. The plant performance and economics were investigated in four design (winter, spring, summer and autumn design) conditions based on the seasonal weather data and compared with the conventional energy supply system. It was concluded that the design proposed based on average weather conditions shows no economic advantage over the conventional design; due to accounting for financial aids and subsides for the fuel cost at 0.05 $/m(3), affecting the steam and water costs. However, a parametric study demonstrated that if the financial aids associated with the fuel are disregarded, it will have a pronounced effect on the cost of steam and produced water.