Effluent treatment precipitation

The principal calcium salt used as a flocculant is calcium hydroxide [1305-62-0] or lime. It has been used in water treatment for centuries (see Calcium compounds). Newer products are more effective, and its use in water and effluent treatment is declining (10). It is still used as a pH modifier and to precipitate metals as insoluble hydroxides. Lime is also sometimes used in combination with polymeric flocculants. 

Clearly, one option to reduce the add-on is to use high-efficiency size formulations. However, there is a limit to what can be achieved by this approach. Even if the add-on is reduced to only 5%, the pollution load is still substantial. The two main options to facilitate disposal are (a) recovery of size polymers and (b) biological effluent treatment. Recovery of size polymers, particularly from water-soluble synthetic sizes, is based on extraction washing using the minimum quantity of water. Recovery rates in the region of 50% have been quoted for polyfvinyl alcohol) and carboxymethylcellulose size formulations. It is necessary to apply one of three concentration techniques precipitation, condensation or ultrafiltration [205]. 

The European titanium dioxide producers have developed different effluent treatment processes to meet the environmental requirements [2.47]. The most important processes are the precipitation of gypsum (CaS04) from the weak acid [2.48] and the concentration and recovery of the free and bound acid. 

Most homogeneous catalytic processes involve transition metals. The permissible limits for heavy metals in the effluent stream are less than 2 ppm, which means that the effluent treatment plant has to remove the transition metal from the effluent very effectively. The metal is normally precipitated in the form of a salt from the liquid stream, and then filtered off, or allowed to settle in settling tanks. 

Jua Kali can bring in under-developed regions with possible long term environmental damage. E. Muleya (Midlands State University, Zimbabwe) has analysed the effluent from battery manufacturers, specifically the levels of zinc and cadmium. High levels of some metals were found in boreholes but, thankfully, not in tap water. Simple effluent treatment technology to remove heavy metals (for example via precipitation with lime) was proven to be effective and should improve the situation. 

Zinc salts may be used to inhibit corrosion of iron and steel due to the precipitation of Zn (OH) on the cathodic areas of the system. The barrier to oxygen transfer is not very durable and zinc is not generally used on its own. It is preferable to use zinc salts in conjunction with other additives, such as polyphosphates and phosphonate. Because of the toxicity problems the presence of Zri in plant effluents is generally not acceptable and if it is used, some form of effluent treatment is likely to be necessary before discharge. 

Part of the liquor removed by the filters is recycled and part is purged from the system via the effluent treatment plant. This uses lime to precipitate metals (extracted from the gases, the residual fly ash and the limestone), which are removed as a sludge. 

All the solubility products are low, certainly low enough to meet very strict emission legislation. Some salts, especially sulfides are especially low indeed, sulfide salt precipitation can lead to very effective effluent treatment. In practice however these limits are seldom achieved. The numbers in Table 14.5 are obtained in ideal laboratory conditions, for practical use they can only be used as a guide. Many other factors affect the efficiency of precipitation of these the most important is the presence of an excess of other ions or complexing agents. For example low levels of citrate can strongly affect iron precipitation. 

Extracellular precipitation can also be described as biomineralisation. The mechanism can range from being very simple to extremely sophisticated. The most complex systems involve cells which can control the crystallisation of common minerals (for example calcium carbonate) to such an extent that exoskeleton structure and shells can be constructed [27]. However, these complex mechanisms have little use in effluent treatment to date. 

Smith and co-workers [28] report on the effluent treatment for a zinc-lead smelter in the New South Wales, Australia. Treatment was by precipitation using a multi-stage addition of 15% w/v lime slurry to give pH 8.9 resulting in the typical final effluent concentrations shown in Table 6.8. It was necessary, however, to upgrade the system in order to meet stricter requirements. 

Considering the aforementioned reasons, the necessity of metal ion removal from liquid effluents becomes evident. The conventional technique applied for effluent treatment containing metal ions consists of increasing the pH by adding calcium oxide or another chemical. The metal ion is precipitated as an insoluble hydroxide and removed by flotation or sedimentation, concentrated, and disposed of in special class landfills, which represents an important cost for the treatment process. The main drawback of this technique is the high and irreversible consumption of chemicals and the fact that the problem is only transferred from the liquid to the solid phase [7]. 

The weak acid or scrubbing solution bleed represents a disposal issue and is usually sent to an effluent treatment facility where the acid is neutralised with lime to form gypsum and the dissolved metals are precipitated. The filter cake can be returned to the sinter plant feed where the CaO content will be useful as a flux addition. The filtrate solution containing chlorides can be discarded, provided... 

Liberated hydrogen sulphide can be used elsewhere in effluent treatment by precipitating insoluble metal sulphides. Because of its lower... 

Strict regulations already exist for chromate residues that require the use of expensive effluent treatments to achieve the desired residual concentrations by precipitating hexavalent chromium compounds. Despite their negative aspects, to date, no replacements exist in the market for carcinogenic chromates with the same efficiency for a range of aluminum alloys and steel, neither as pigment nor as a metal pretreatment."" ... 

Two main operational variables that differentiate the flotation of finely dispersed coUoids and precipitates in water treatment from the flotation of minerals is the need for quiescent pulp conditions (low turbulence) and the need for very fine bubble sizes in the former. This is accompHshed by the use of electroflotation and dissolved air flotation instead of mechanically generated bubbles which is common in mineral flotation practice. Electroflotation is a technique where fine gas bubbles (hydrogen and oxygen) are generated in the pulp by the appHcation of electricity to electrodes. These very fine bubbles are more suited to the flotation of very fine particles encountered in water treatment. Its industrial usage is not widespread. Dissolved air flotation is similar to vacuum flotation. Air-saturated slurries are subjected to vacuum for the generation of bubbles. The process finds limited appHcation in water treatment and in paper pulp effluent purification. The need to mn it batchwise renders it less versatile. 

Industrial Wastewater Treatment. Industrial wastewaters require different treatments depending on their sources. Plating waste contains toxic metals that are precipitated and insolubiHzed with lime (see Electroplating). Iron and other heavy metals are also precipitated from waste-pidde Hquor, which requires acid neutralization. Akin to pickle Hquor is the concentrated sulfuric acid waste, high in iron, that accumulates in smokeless powder ordinance and chemical plants. Lime is also useful in clarifying wastes from textile dyeworks and paper pulp mills and a wide variety of other wastes. Effluents from active and abandoned coal mines also have a high sulfuric acid and iron oxide content because of the presence of pyrite in coal. 

In secondary wastewater treatment plants receiving silver thiosulfate complexes, microorganisms convert this complex predominately to silver sulfide and some metallic silver (see Wastes, INDUSTRIAL). These silver species are substantially removed from the treatment plant effluent at the settling step (47,48). Any silver entering municipal secondary treatment plants tends to bind quickly to sulfide ions present in the system and precipitate into the treatment plant sludge (49). Thus, silver discharged to secondary wastewater treatment plants or into natural waters is not present as the free silver ion but rather as a complexed or insoluble species. 

---