a b s t r a c t
Sulfate is a common contaminant in industrial wastewaters, Sulfate causes corrosion in
sewers المواسيرduring wastewater discharge, and leads to serious difficulties during on-site anaerobic لاهواءيwastewater treatment, due to sulfate reduction to sulfide, as described below:
1. The production of sulfide is an occupational health and safety issueمساله and a maintenance problem as hydrogen sulfide is a poisonous gas and, when being oxidized to SO2 and then sulfuric acid, corrodes concrete and metal structures and gas engines.
Two major possible drawbacks عوائقwith precipitation of jarosite and ettringite are that
a.Aignificant crystallisation rates may only be realised at elevated temperatures (60–90 C; see
Elgersma et al., 1993; Alvarez-Ayuso and Nugteren, 2005; Christoe, 1976)
b.And the minerals are usually only stable at extreme pH conditions (as indicated above).
These crystallisation conditions are unfavorable due to substantial energy costs for heating, and chemical costs for the adjustment of pH, as well as re-adjustment prior to subsequent anaerobic treatment.
The application of gypsum and ettringite crystallisation to treat sulfate rich acid mine waters is well documented.
Gel-denhuys et al. (2003) developed a gypsum crystallisation process for treatment of acid mine waters where limestone is used to neutralize acidic feed water (pH 2–3).
Lime is used to raise the pH to 12 in order that most of the metals in the water.
In this research we obtain the removal of sulfate by two methods
1.Two composite sample method.
2.three composite sample method.
2. Sulfide is a wastewater contaminant, contributes to BOD and COD, and produces a high odour wastewater product.
3. For every mole of sulfate reduced, one mole of potential methane is lost, so the methane yield may be reduced by a considerable amount.
An alternative is to remove sulfate from the wastewater prior to biological reduction to sulfide. Available methods include ion exchange, ion selective membranes and crystallization of sulfate minerals. Of these methods, crystallization is likely the most economical
Metal cations which precipitate with sulfate, and are also available in the form of low-cost additives, are limited.
The choice is likely restricted to calcium, to form calcium sulfate dihydrate (gypsum), a marginally soluble mineral.
As stated above, calcium is often present in the wastewater, and if added, lime is one of the lowest cost commodity chemicals available.
In addition, co-precipitation to form minerals which are less soluble than gypsum may be available to provide enhanced sulfate removal.
Examples include:
1. The calcium-aluminium-sulfate mineral, ettringite, precipitated by the presence or the addition of aluminium( for example as a calcium aluminate) and calcium at high pH (12).
With this mineral the sulfate levels can drop to less than 200 mgS L-1 with reasonable amounts of aluminium and calcium (Geldenhuys et al., 2003; Smit and Sibilski, 2003).
2. The mineral jarosite, which forms by the combination of iron, sulfate and another cation which may be ammonium, potassium or sodium.
This mineral is commonly precipitated at acidic pH (2.
in the metallurgical recovery of
zinc, where iron(III) must first be removed from solution.
Although conceptually discussed in the paper of (Wang et al. 2006), no references were found in the literature to large scale application of jarosite precipitation for sulfate
removal from industrial wastewaters