Water quality modelling of a 128 km transmission main and farm networks, Mirfa–Liwa, Abu Dhabi

The challenge

Long residence times at 45 °C

The transmission main holds around 415,000 m³ and each storage tank holds a day of demand, so water can be in the system for days at low demand. The source water carries chlorine dioxide from the desalination plant and contains bromide, which raises a bromate risk if free chlorine is used too early. The 0.20–0.50 mg/L residual had to be achieved at every one of 8,011 farm connections.

1. Transmission: multi-species (MSX) model tracking chlorine dioxide, chlorite, chlorate, bromide, free chlorine, hypobromous acid, bromate and water age
2. Decay coefficients from bottle tests at the 45 °C design temperature, with an aged-pipe wall decay allowance
3. Four cases: full and half demand, each with 0.10 and 0.30 mg/L chlorine dioxide at the source
4. Distribution: free chlorine residual computed at all 8,011 farm connections at full and 50% demand, with no credit taken for incoming residual
5. Dosing strategy developed: primary dosing at the four distribution pumping stations, zone boosters at tank outlets and in-line branch boosters
6. Hypochlorination capacity sized at each dosing point