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Figure 6 kWh/me and % water recovery

The application of these conditions - especially the reduction of the number of acid washings - led to a considerable reduction in the consumption of additives, while keeping the abatement throughput.

4.1 The quest for compatibility of the concentrated discharge.

The tests aimed at obtaining a discharge compatible with the limits set out earlier were run working progressively on the reduction of the percentage of water recovery: practically speaking. It was decided to increase the quantity of water drawn in by-pass from the input and delivered to the brine tank to dilute it; this led to an increase of the quantity of discharged brine.

This operation was performed in stages, lowering the yield from 94% step by step to 77%.

Measurements of the concentration of N03 ion for discharge showed the following general trend:

• with recovery around 94%, there is a concentrate discharge between 500 and 600 mg/1.

• with recovery around 90%, there is a concentrate discharge between 300 and 330 mg/1.

• with recovery around 80%, there is a concentrate discharge between 200 and 250 mg/1. On the basis of incoming concentrations of untreated water of around 50 mg/1 of N03~

ion, one can make a rather accurate estimate on the basis of which one can say that in order to reach the limit set previously. It will be necessary to achieve recovery values around 70%.

The important factor which emerged from the tests that were made is that - despite the concentration of recirculating brine having been heavily reduced (from around 7000 (iS/cm to around 2500 (xS/cm), the percentage of ion nitrate abatement on the water produced is always near the previous levels, i.e. 70%. Therefore, the decision to preserve high percentages of water recovery was dictated by reasons of opportunity (especially in financial and management terms) and not by any technical limits of the EDR technology.

4.2 Assessing management costs

On the basis of the technical and management data gathered during the experimentation, we can draw up a sufficiently accurate estimate of management costs on the basis of the size of the plant. The following data cannot be applied directly to plants with a larger throughput, but it is quite probable that costs will diminish, since some will be spread out over larger production (additive consumption, labour, and analyses).

Total operating costs, on the basis of the experimental data, are lower than the 337 it£/m3(Italian Lire) of the data submitted by the Milan CAP which refer to a Reverse Osmosis treatment system, for a similar application3.

Pilot Plan Throughput m3/year

About 205,000 m3 (average N03~ ion concentration of less than 15 mg/1)

Electric Power it£/m3 (estimate 250 It/kWh)

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