Page 251 - Membranes for Industrial Wastewater Recovery and Re-Use

220 Membranes for lndustrial Wastewater Recovery and Re-use

The full details of the design may be viewed at any time from the File menu
(Preview All) or by the short-cut key command Ctrl V.
For the problem specified it must be assumed that the hardness is all as calcium
and the alkalinity as bicarbonate, so that these can be directly entered into the
Feed Analysis screen. Composition values must be entered in the appropriate
units (mg/l or mg/l as CaC03 in this case). An estimate of the sodium or chloride
concentration is needed to complete the analysis and achieve the target TDS of
1200 mg/l. The package automatically adjusts the specification with the
counter-ion (i.e. chloride or sodium ions) to obtain electroneutrality.
On the following pH Adjustment screen the required pH to which the water
feeding into the array must be entered. It is simpler to do this once the scaling
propensity (via the Langelier Scaling Index, LSI) has been calculated. On the
following Flow Rates and Recovery menu the product flow rate (3024 m3/day
and the recovery (75%) must be entered.
Next screen is the Membrane Array Configuration, which includes an
Array Wizard that allows an estimate of the flux to be entered. RO membranes
will normally operate at a flux of between 20 and 50 LMH, depending on
the degree of fouling. Membrane elements can be selected from the left-hand
drop-down menu and modules from the right hand one. For this large flow rate,
a large membrane module is appropriate (prefix “8” for 8-inch diameter
modules and MAG for the largest membrane element area). Having entered a
flux value, the preliminary design is complete and the GO button can be
pressed.
If a fairly conservative flux of 2 5 LMH is chosen for the array design, the only
error arising will be from the scaling propensity of the water. This can be
corrected by acid addition to being the LSI below zero. Again, trial and error is
required for this, and through iteration it can be established that adjusting the
pH to around 6.1-6.3 (depending on the membrane selected) by pre-dosing with
either sulphuric or hydrochloric acid is sufficient.
The product water quality must then be checked to establish if 98% rejection
has been achieved (i.e. the permeate product TDS is no more than 24 mg/l. 2% of
the feed TDS of 1200 mg/l) and that the pH is around 7. The permeate product
pH is always acidic due to the unrestricted permeation of carbon dioxide, which
can be removed by stripping and/or neutralised by lime or caustic dosing. In
reality, the controlling the product pH to 7 following C02 stripping would be
extremely difficult for such a low TDS water. The permeate TDS can also, of
course, be altered by membrane selection. Selection, once again, proceeds by
trial and error - and more than one option is possible. A possible design is
detailed below.

Pretreatment: Dosing to pH 6.2 with 93.2% sulphuric acid at 376.3 kg/day
Membrane: TFC 8832HR-575MAG
Array: 10:5 2-stage array, 4 elements/module (60 elements total)
Post-treatment: Degassing to 5 mg/l dissolved carbon dioxide (dosing to pH 7
with sodium hydroxide)
Spec. energy dem.: 215 kW for 126 m3/h flow, 1.71 kWh m-3.