Management of emissions to water

Waste water discharge treatment is the environmental operation in which significant differences arise from one production plant to another [33, CEFIC-ESAPA, 2004], [41, Solvay S.A., 2003], [45, UBA - Germany, 2001], [53, EIPPCB, 2004], [39, S. Leszczynski et al, 1978].

Apart from cooling water, waste waters from the soda ash production plants are characterised by a high concentration of suspended solids and dissolved salts, as well as a high temperature and a high alkalinity. These solids and salts are unreacted limestones and salts of natural origin as well as the ashes of the coke – refer to Section 2.4.7 and Figure 2.4 below.

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Anorganische Grundchemikalien – Feststoffe und andere - August 2007 75 Suspended solids and dissolved salts originate from three different steps of the process (names in brackets make reference to the name of the corresponding streams in Figure 2.2):

brine purification (L01) ammonia recovery (L02)

cleaning of CO2 gas originating from the calcination of limestone (L03), which is a minor contribution in the balance of suspended solids.

In most production plants, the effluent from brine purification is discharged jointly with the effluent originating from the distillation unit. The possibilities of using products from brine purification are mentioned in Section 2.3.10.4.3. The typical composition varies according to the quality of the raw materials. Different treatment schemes have been developed according to the geographical location of the production plants and the requirements of the local regulatory authorities.

The following options are available for the treatment of liquid effluents:

direct discharge of raw effluent, with or without partial removal of some fraction of the solids, and with or without preliminary pH adjustment

indirect discharge of waste water after removal of suspended solids (for possible re-use of this material see Section 2.3.10.4.2), and with or without preliminary pH adjustment

further treatment to produce by-products, such as CaCl2, etc. (see Section 2.3.10.4.1).

Depending on the location of a soda ash plant and raw materials deposits, two basic lines are established for the treatment of suspended solids: total dispersion, and/or deposition/dispersion (separation of the suspended solids and liquid dispersion).

2.3.10.1 Total dispersion

Total dispersion, with or without the previous separation of coarse suspended solids [85, EIPPCB, 2004-2005], is employed when a production plant is close to the sea or high flow rivers [41, Solvay S.A., 2003], [53, EIPPCB, 2004].

This technique ensures that the solid material is assimilated with the natural sediments of similar composition. Chlorides and other soluble salts present in the liquid fraction are dispersed in a medium which, in the sea case, already contains them in large quantities.

2.3.10.1.1 Marine outfalls

Waste waters from the production of soda ash containing suspended solids, in some cases is discharged directly to the sea or into an estuary under tidal influence by means of an open channel or underwater outfalls, designed with the necessary environmental and technical studies. For more detailed information on this technique, refer to Section 2.4.7.2.

2.3.10.1.2 Lake and river discharge

The direct discharge of waste water to fresh water environment is practised in one case without any treatment and in one case with some preliminary removal of coarse solids. For more detailed information on this technique, refer to Section 2.4.7.3.

In other cases across the EU, waste waters discharged to fresh water, are subjected to solids removal treatment prior to discharge of the soluble fraction – see Sections 2.3.10.2, 2.3.10.2.1, and 2.3.10.2.2 below.

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76 August 2007 Anorganische Grundchemikalien – Feststoffe und andere - 2.3.10.2 Deposition/dispersion

Deposition/dispersion has generally been used where there is no suitable environmental medium to allow for total dispersion. This method involves the physical separation of liquid and solid phases. The liquid phase is then discharged to a local watercourse with or without pH adjustment as appropriate, and solids are used to build up the settling basin itself.

The underground deposition of the solids is carried out when salt deposits are found near production plants and when deposit characteristics and the salt extraction system enables it.

Where market conditions permit, it is also possible to use the separated solids after further treatment as a lime fertiliser.

2.3.10.2.1 Settling ponds

The clarification by decanting large quantities of suspended solids from aqueous effluents is usually achieved in settling ponds. The location and operating area of settling ponds depend on several factors. For a more detailed description of this technique, including: purpose and principles of liquid/solid separation, operation, monitoring, hydraulic confinement, coverage and final closure of settling ponds, refer to Section 2.4.7.4 below.

2.3.10.2.2 Underground disposal

Methods for the underground disposal of the distillation solids in salt cavities have been developed. Firstly, the suspended insoluble material is separated from the clear liquor phase and is re-suspended in saturated crude brine. The resulting slurry is then pumped to dedicated

„disused‟ salt cavities where the solids settle out within the large volume of the cavity.

Displaced brine from the cavity is recycled for the repeated transport of the solids. As with basin settling, the clear liquor phase is discharged to the local watercourse. For a more detailed description of this technique, refer to Section 2.4.7.5 below.

2.3.10.3 Liquid effluent discharge options

The impact of direct discharge of the liquid phase containing soluble salts into rivers is linked to the flowrate of the receiving rivers, flowrate fluctuations and the inherent qualities of the water, including its natural salinity.

2.3.10.3.1 Equalisation in modulation basins

In some cases of small or medium size receiving waters or low seasonal flows, a buffer equalisation storage basin may be used to store the settled liquid effluent prior to discharge. The holding capacity of such a buffer basin is determined by the average and peak load to be discharged and the available diluting flowrate of the receiving water. The basin may be as large as the equivalent of 6 to 9 months of production (typically of 3 million m³ capacity).

The optimal use of equalisation basins allows to minimise the impact of chloride on the receiving water and to avoid high peak concentrations during low river flow conditions.

In determining appropriate control levels, it is critical that the local conditions and the quality of the receiving waters are taken into account.

Flow equalisation storage basins can be built as aboveground basins with earth or stone walls or in excavated areas. The walls and the bottom of such basins need to be impervious (usually made in concrete or polyethylene liners) or need to have a recovery pumping of the leakage flow.

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Anorganische Grundchemikalien – Feststoffe und andere - August 2007 77 The management of buffer equalisation basins can be optimised by continuous monitoring of the flowrate and chloride concentration in the receiving water, after complete mixing, thus controlling the daily discharge which is allowed.

2.3.10.3.2 Adjustment of pH

The typical pH value of the raw effluent is higher than 11.5 due to the alkalinity of OH^- ions contained in Ca(OH)2. Theoretically, the pH adjustment of such an effluent can be achieved either by mixing, in open channels or basins, with natural or raw waters containing dissolved calcium bicarbonate, by reacting with gas containing CO2 (for example, flue-gas from power units), in pH adjustment columns or by other pH adjustment mechanisms, if acid solutions or acids waste waters are available.

In practice, the pH adjustment of soda ash waste water is usually achieved by mixing it with natural water according to the following mechanism:

Ca(OH)2 + Ca(HCO3)2 2 CaCO3 + 2 H2O

Waste water is mixed with available natural water (either cooling waters after use or surface waters: river, channel, lake, sea or undergroundwater, etc.) in a typical ratio natural water/waste water at 5:1 to 10:1. The CaCO3 particles formed are discharged or settled in ponds, in natural or artificial lakes or in a dedicated channel of the waterway or estuary.

Appropriate hydraulic retention time for settling in quiescent waters is usually 6 to 8 hours.

Periodic removal of settled particles is achieved by dredging where the speed of the existing stream is not sufficient to keep the particles in suspension up to settling zones (e.g. the sea).

This method offers numerous advantages: the pH adjustment mechanism is efficient and reliable, no consumption of supplementary reactants is needed, the settled particles are inert, and finally neither complex mixing and decanting equipment nor instrumentation and monitoring are needed.

2.3.10.4 By-products recovery and re-use 2.3.10.4.1 Dissolved CaCl2 in distillation waste water

The recovery of CaCl2 dissolved in waste water from distillation requires a large amount of energy mainly in the form of steam to concentrate the diluted solution to solid CaCl2 (see Section 2.3.6.1). Moreover, the market for CaCl2 is limited. For these reasons, the number of CaCl2 recovery units operating in soda ash plants has progressively decreased (refer also to Section 7.11).

2.3.10.4.2 Suspended solids in distillation waste water

Numerous studies have been carried out in order to find ways to recover and re-use the suspended material coming out of the distillation unit.

The major difficulty to overcome is the removal of the salt content of the material by several washing steps. These operations can consume very large quantities of water, depending on the required chlorides content, which is to be further discharged as a salt solution.

The second difficulty is to dry the material to an acceptable level of residual humidity for transportation and re-use. This can be achieved by gravity settling and dehydration, but this takes a long time, or by drying in a rotary kiln, which requires a large amount of energy.

The costs associated with the complete removal and treatment (decanting, washing and dehydration) for the re-use of suspended solids in the waste water effluent of a soda ash factory are high. In most cases, they are now prohibitive for full scale implementation.

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78 August 2007 Anorganische Grundchemikalien – Feststoffe und andere - Attempts have been made to recover the coarse solid fraction suspended in the distillation waste water. The efficiency of clarifying in hydrocyclones is limited due to operational constraints (scaling, plugging, erosion) and require frequent maintenance interventions and cleaning.

Due to the higher grain size, the dewaterability of the fraction obtained is easier but, because the remaining chloride content is around 15 %, a preliminary washing is necessary in order to enable the re-use or recovery.

Other obstacles to overcome are to find a commercial or useful application for the residual material. In general, it exhibits unfavorable properties for civil engineering works or construction material, due to the presence of residual chlorides, the low size of particles, the thixotropic characteristics of the material and variability of its composition.

Some practical experience has been gained with certain industrial recovery options, such as soil amendment, cement manufacture, gypsum raw material, concrete filler, but none – apart from soil modifiers – have been implemented to a full industrial scale. In this last case, final dewatering is achieved using press filters, and the product can be used without further drying.

2.3.10.4.3 Product from brine purification

Several attempts have been made to recover the CaCO3 fraction from the brine purification effluent, but they inevitably faced the problem of the chloride content in the effluent needing to be treated and of the impurities remaining after the treatment. The final product could never compete with purer products available on the market.

Some experience of agricultural applications for acidic soils with a high clay content is available, but the use of the recovered CaCO3 is restricted to the soils exhibiting the corresponding characteristics.

It should be noted that heavy metals originating from the brine are passed to the solids removed from the brine during its purification. Therefore, where there are no viable end applications for the solids from brine purification, they are normally handled with the other solids arising from the process or, where geological conditions are favourable, are separately re-injected into the salt strata from which the salt was originally extracted.

2.3.11 Solid materials management