Encuesta evaluación prueba piloto del sitio web de la lbea

In paper and pulp applications process water is subject to high temperatures, moderately acidic pH (often pH 4-6 for white paper) and high electrolyte concentrations that can aggravate the corrosion potential of the water on equipment. The pH is controlled by the addition of alum, sulphur dioxide, sulphate and sulphuric acid. The use of closed water circuits is increasing in the paper mill industry resulting in a high concentration of salts and organics in the recirculating water. The presence of chloride ions in process sections, such as the suction press roll and the headbox, can aggravate the severity of corrosion as chloride accumulates in crevices located in sites that are difficult to examine [180].

Sprays are used for application of water over the paper in the paper machine. The spray water needs to have low TSS and control of ionic species to avoid precipitation and scaling in pipes and nozzles. Suspended solids should not be an issue from tertiary treated recycled water and higher. But secondary treated recycled water may require some further treatment in the form of filtration to reduce the suspended solids level. Biological growth and deposits can also threaten to disrupt nozzles in wash systems and biocide treatment can be important to prevent biological fouling of the nozzles [181].

The main concerns of poor water quality in the pulp and paper industry are:

• Biological growth. This can lead to clogging of plant, odours, the texture and uniformity of the product paper and human health. Preventing this requires control of COD and nutrients and a biocide (3 mg.L-1 _residual

chlorine is generally best, although SAPPI, South Africa use 0.8 mg.L-1 _to

control microbial growth).

• Scaling and corrosion. This requires control of silica, aluminium (alumina), hardness and TDS levels. Use of scale and corrosion inhibitors is also useful.

• Final product quality. Problems with colour can occur with iron, manganese and microorganisms in the water, while brightness is compromised by suspended solids.

Table 3.17. Major water quality parameters of concern for paper manufacture at Mondi [32]

Parameter Significance

Conductivity Corrosion

Colour Product Colour

Residual Cl Control of microorganisms _{At > 1 mg.L}-1 _{interferes with dyes}

Chloride CorrosionAffects starch

Needs to be less than 80 mg.L-1

Iron Colour, bleaching

Sulphate Scale and deposits_{Needs to be less than 80 mg.L}-1

Manganese and copper Colour, bleaching

Oil/Soap/Grease Foaming

TOC < 5 mg.L-1 _{for fouling of resins}

E. coli Health

Total Aerobic Bacteria Filamentous slime _{(biofouling by Sphaerotilus natans)}

If phosphate is not removed, secondary treated recycled water represents a scaling threat where higher temperatures are encountered (this threat is lesser at lower temperature). Where a recirculated system is used it is strongly recommended that phosphate removal be performed. Biological nitrification of recycled water will generate nitric acid that will help control scale to a degree, but where the threat is high addition of antiscalant (a chelating agent and dispersant/surfactant) would be recommended. Tertiary treated recycled water and above may be suitable for these purposes.

The control of TDS and colour are two major considerations for recycled effluent according to experiences from paper plants such as Mondi Paper and SAPPI ENSTRA, South Africa, which use recycled water from STPs. The solids content of the water can be reduced by flocculation, sand filtration and colour by activated carbon towers to prevent staining of the final product. Chlorine addition to produce a chlorine residual of 0.8 mg.L-1 _{at the point of use minimises the attack}

of chlorine onto the paper, whilst controlling microbial growth. The major parameters of concern are shown in Table 3.17.

The general water quality requirements for the paper/pulp industry would mean a minimum of tertiary treated recycled water with colour removal. It would also be recommended to ensure the chlorine residual does not exceed 1 mg.L-1_{. If need}

be, this can be reduced using sodium bisulphite or a similar agent.

3.5.4.1 Example: Mondi Paper, Durban, South Africa

The Mondi Paper papermill in the Durban suburb of Merebank has been operating on recycled water since 1972 [111]. Since 2001 it has been receiving 47.5 ML.day-1 _{of water from the Durban Water Recycling Plant (DWRP). This water}

takes the form of tertiary treated recycled water that has undergone ozonation and passed through activated carbon to reduce the colour [97] (initially colour present in the recycled water from dyehouse effluent was an issue for Mondi

3.5.5 Textiles

Manufacture and preparation of textiles is generally performed using wet processing (Figure 3.4). The process operations consist of pre-treatment, dyeing, printing and finishing.

• Pre-treatment: singeing, carbonising, solvent wash, desizing, caustic kier boiling, bleaching, mercerising, gumming, washing and rinsing;

• Dyeing: water is used in washing and rinsing operations and also in the preparation of dye and treatment liquor;

• Printing: includes washing and rinsing operations;

In the dyeing process a batch of fabric is rolled between two rollers through a dye liquor, or looped over the winch reel, dropped in the dye liquor and repeatedly folded stirring the dye (for woollens and synthetics). Application of enzymes, soil repellents, surfactants and silicone compounds is often adopted as a finishing process.

Process water is required for the majority of the unit processes adopted as fabric and yarn undergo multiple cycles of washing and rinsing during production. The process water may need to be pre-treated for the removal of impurities present in surface, ground and recycled waters such as calcium, magnesium, iron and manganese carbonates, phosphates, nitrates and chlorides. The water is then mixed with chemicals suited for each process operation. For instance, acids are used in acidic dyeing processes for synthetic fibres and blends, for pre-treatments of cotton and as pH regulators in finishing processes (pH 4.5 to 6) [182]

Corrosion and scale control are also important considerations as process water is led through pipes or tubes to the machines and salts are used in reactive dyeing processes.

The water source’s turbidity, colour (in the form of iron and manganese salts and organic dyes) and high hardness should be avoided. In addition nitrates and nitrites can interfere with the dyeing process and should be considered for each dye formulation [182]. Addition of thiosulphate to neutralise free chlorine can also be usedul as it reduces the risk of fabric bleaching.

In general, for textile processing tertiary treated recycled water with colour removal would be the minimum. Chlorine should also be completely removed using sodium bisulphite or a similar agent. An important benefit to using tertiary treated recycled water is the greater buffering capacity it affords [183]. This in turn can lead to better control over the dyeing process. This is highlighted in the example below.

3.5.5.1 Example: Tuftex of California, Los Angeles, California

Tuftex of California is the largest carpet manufacturer on the west coast of the USA. They have been using a tertiary treated wastewater (3.8ML.day-1_{) in their}

dyeing process since about 1994 [183]. Both the dyebecks and continuous dyeing processes are used onsite. The source of recycled water was specifically chosen to be low in ammonia, iron and manganese. A small amount of treatment is performed onsite, primarily sodium thiosulphate addition to remove the residual chlorine prior to dyeing (there is a water provider operated chlorination station also on the property to reduce biological growth due to the system having a large residence time). While minor formulation changes have been needed, the overall chemical costs were reduced. This included a 10% reduction in buffering compounds due to the higher buffering capacity inherent in recycled water [183]. The redyeing rate has also dropped from 5% using potable water to 3% using recycled [183]. This is believed to be due to greater consistency in the quality of supply. There have not been any reports of loss in product quality from consumers.