Design

2.1 TRO/SRO Design

2.1.1 Pre-treatment section. A block flow diagram of the TRO/SRO plant is shown in figure 1. As pre-treatment, sulphuric acid is added to the ash water to control the pH between 3,0 and 6,5. This is followed by sand filtration to reduce the suspended solids concentration to ca 40 mg/i. A scale inhibitor is added and the ash water is heated to 27°C. In order to limit biological fouling, chlorination is done twice a week during which a free chlorine level of ca 0,5 mgU is maintained for thirty minutes.

Permeate to boilers

Brine to evaporators

Figure 1 Blockflow diagram of the TRO plant

Permeate to boilers

Brine to evaporators

Figure 1 Blockflow diagram of the TRO plant

2.1.2 Membrane section. The TRO section consists of eleven units. Each unit consists of 80 parallel branches each with 10 modules in series, giving a total of 8 800 modules. The plant is also equipped with a flow reversal mechanism and a sponge ball is shuttled through the membranes at thirty-minute intervals for in line cleaning.

The plant is operated at a constant water recovery of ca 40 %. The production capacity of each unit is 23 m3/h of permeate and the total capacity of the TRO plant is 250 m3/h. The waste stream is treated by means of three evaporators employing falling film and mechanical vapour recompression technology.

After TRO treatment the permeate is upgraded in two spiral reverse osmosis (SRO) units. Each unit has a design production capacity of 128 m3/h and water recovery is controlled at ca 90%. Standard high rejection, thin film composite polyamide membranes in a 10:5:3 configuration is used. Pre-treatment consists only of sodium-meta-bisulphite (SMBS) dosing to protect the polyamide membranes whenever chlorination is done at the TRO plant.

2.2 EDR/SRO Design

2.2.1 Pre-treatment section. A block flow diagram of the EDR/SRO plant is shown in figure 2. Pre-treatment in this case consists of clarification, sand filtration, cartridge filtration, anti sealant dosing, pH control using hydrochloric acid and heating by steam.

In the clarification step, FeC^ as well as cationic and anionic coagulation/flocculation agents are added to ensure proper clarification. As part of the pre-treatment step, potassium permanganate and sodium hypochlorite are added to oxidise manganese and iron to the insoluble oxide form.

Clarification is followed by gravity sand filtration and cartridge filtration (10 jam) to ensure that the silt density index is less than 5. The backwash from the sand filters is collected in a backwash recovery tank from which it is blended into the feed to the clarifier.

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