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2.2 The organic high polymer flocculant
Comparing with the inorganic flocculants, the synthetic organic flocculant features the less dosage and the faster flocculating speed, they are less affected by the coexisting salts, by the PH value of the medium and the surrounding temperature, they also generate the less sludge. Besides, the organic high polymer flocculants can molecule with the electrophilic groups such as –COO, -NH, SO3, -OH etc, with the chain structure and the ring structure which is conducive to get the pollutants into the flocs, therefore achieve the good decolorization effect. In general, the chroma removal rate of the organic high polymer flocculants is about 20% higher than the inorganic flocculants. At the present, the most widely used organic high polymer flocculant is the polyacrylamide. It can adapt to a variety of the flocculating objects, with the less dosage, the high efficiency, the less sludge left and the easy post treatment advantage. It is always used with other inorganic flocculants, such as the aluminum chloride.
By the synthetic high polymer flocculants and its monomer or its hydrolyzed or degraded products are often toxic, such as the polyacrylamide (PAM) monomer has the neurotoxic and teratogenic, the carcinogenic and the mutagenic effects.
2.3 The microbial flocculant
The microbial flocculant is kind of new water treatment agent which is safe and efficient with the natural degradation effect. It uses the method of the biotechnology to extract and purify the microorganisms or theirs secretions and obtain the microbial blocculating agent. Till now, it has been found more than 17 species of the microorganisms with the flocculating function, indluding the fungi, the bacteria, the actinomycetes and the yeast bacteria etc. This kind of microbial flocculant is divided into:
(1) The flocculant with the direct use of the microbial cells, such as some bacteria fungi, the actinomyucetes and the yeast, they are existing widely in the soil, the activated sludge and the sediments etc.
(2) The flocculant of the extractions from the microbial cell wall,, such as the ingredients of the glucan, the mannan, the protein and the N-acetyl glucosamine from the yeast cell wall.
(3) The flocculant of the cellular metabolism from the microorganisms. It is the capsule and the mucus material which is the metabolites of the microbial cells secreted into the extracellular cell, besides the water, the main components of this kind of flocculant is the polysaccharide, a small amount of the polypeptide, the protein, the lipid and its complexes. In some degree, the polysaccharide can be used as the flocculant.
So far, the microbial flocculant found to have the best flocculating effect is the red flat Nocardia sp. NOC-1. It could be used for the treatment of the livestock wastewater, for the settlement of the expanding sludge and the decoloring of the pulp waste water (the black liquid), the colored dyeing waste water etc, the effect is remarkable.
Although the study of the microbial flocculant is repeatedly reported, most of the study is still in the stage of the laboratory research and not in the industrial application. In our country, the research on this flocculant is later, the present study is limited to the screening stage.
At present, the cheap and practical, the nontoxic and efficient flocculant is the direction of the development. The organic high polymer flocculant will gradually replace the inorganic flocculants which are widely used at present. ON the other hand, the use of the microbial flocculant has the advantages like the stability, the safety, the high efficiency and the low consumption; it is one of the most promising flocculant in the world now. Therefore, the future development on the microbial flocculant does not only the development of the cheap and efficient new type microbial flocculant, but also the study on the use of the microbial flocculant with other flocculants. The practical test showed that the use of the two kinds of flocculants can coordinate each other and can be complementary, which can not only improve the flocculantion efficiency, but also can reduce the dosage of the flocculants.