Impurities may be dissolved compounds as well as insoluble particles, and may be of organic or inorganic origin , Some of the more commonly found natural components containing organic material are, in decreasing size order, zooplankton, phytoplankton, bacteria, viruses, clay-humic acid complexes, humic acids, proteins, polysaccharides, fulvic acids, and very small species such as fatty acids, carbohydrates, amino acids and hydrocarbons. They are formed by the biological degradation of organic life substances , and include highly coloured compounds. Inorganic salts of natural origin are also present to some degree.
Dissolved compounds, defined as that which will pass through a membrane having pores of 0.45 fim size when measured as dissolved organic carbon (DOC), have levels in the range 0.1-115 mg L"1, with 5.75 mg L'1 being reported as a global average for streams . DOC poses a problem for the water treatment industry for a number of reasons. Apart from the aesthetic problems of colour, taste and odour, its presence poses a health hazard because of the formation of potentially carcinogenic compounds when the water is disinfected with chlorine or chloramine - the problem of disinfection by-products (DBPs). As well, DOC exacerbates the deterioration of the microbiological water quality in distribution systems, fouls membranes and ion-exchange resins, interferes with the oxidation of dissolved iron and manganese to insoluble easily removed forms, and encourages corrosion .
Although humic substances, encompassing fulvic and humic acids, constitute about 50% of the total DOC of a typical surface water, the proportion can be as high as 80% . Humic substances are unique and troublesome materials in that they have quite variable properties, in terms of acidity (pKa 3-5), molecular weight (several hundred to ten thousand) and molecular structure (mostly phenolic and carboxylic acid functionalities, but also alcohol, quinone, ether, ester and ketone groups). They behave as negatively charged colloids or anionic polyelectrolytes at natural pH levels and have surface-active properties, so that they can interact with particulate material and also, via their hydrophobic aromatic and aliphatic regions, with non-polar pollutants such as pesticides and polychlorinated biphenyls. Humic substances are often present as stable complexes with metal ions. The subtleties of their structure have been further detailed recently . These variable properties influence reactivity, which as mentioned changes spatially and temporally. If the smaller charged organic molecules are first removed from raw water by ion exchange, as carried out in one full-scale plant , a subsequent alum clarification stage is greatly facilitated: larger floes are formed that settle three times more rapidly, far less organics are left in the product water, and only 25% of the original alum dose is required in a conventional clarification process ,
The composition and effects of DOC on water treatment are covered more fully in the NOM reference chapter later in this book.
Suspended particulate matter and minute particles that cause turbidity are an important component of all natural waters and this is discussed in detail in a reference chapter. The charge on the particles is controlled by an adsorbed layer of natural organic matter, as well as by the salinity and the concentration of divalent cations in the water . Humic substances can adsorb onto the particles via surface metal cations. The surface charge potential of the particles is an important parameter influencing coagulation and adsorption behaviour. It can be monitored via particle microelectrophoresis, and in natural systems is invariably negative, irrespective of the nature of the primary particle . The coating of organics has a strong impact on the amount of coagulant required and the rate of coagulation, slowing the rate markedly at low salinities, but having less of an effect as the salinity increases ,
Was this article helpful?