sample. We can use TOC to determine the type of technology we require, and monitor the effectiveness of that equipment once the plant is built. We cari relate TOC analyses to the potential for RO membrane and ion-exchange resin fouling, and design accordingly. Some organics pose unique health risks and process challenges, and do require specific analytical work. 3. Common Pitfalls To Avoid With Water Treatment Equipment O Activated carbon beds are generally very poor choices in most water treatment plant layouts and should be avoided al all costs. Carbon elevates pH, adds particulate matter (carbon fines), and leaches hardness. Ahead of membrane-based technologies, they pose a real fouling threat. They are a well-known breeding ground for bacteria that can contaminate downstream processes. For chlorine removal, the most elegant, cost effective solution is to use bisulfite. it is easy to use, and the reaction is almost instantaneous. Ahead of membrane technology, uncatylised sulfite should be used. O Engineering firms and water treatment equipment vendors are keen to specify ion-exchange (zeolite) softeners ahead of reverse osmosis systems to prevent scale of the membranes. in most applications, this is totallyunnecessary, and will add to the capital and operating cost, and is an environmentally unsound solution. Equipment vendors stili work with very conservative and outdated scaling indices that do not reflect technology advances in chemical additives to combat scaling and fouling of membrane systems. For example, it is common to see vendors work with a Langelier Scaling lndex (LSI) of 1.5 as a maximum, with the addition of a generic anti-scalant additive. They also often limit silica concentrations to 100 mg/L for fear of silica fouling. However, products exist today that allow operation at LSls in excess of 3.0 and silica concentrations close to 300 mg/L. One of the added justifications equipment vendors use for installing zeolite softeners is that they provide a good filtration barrier for the RO plant. True, ion-exchange resin beds can be good particulate filters, yet not cost-effective at doing it. Let true filters do the filtration. There are certain instances where the use of zeolite softening ahead of a RO plant is justified. With the advent of lectrodionisation (EDI) as a post-RO polishing step, the permeate feed to the EDI needs to have a hardness co·ncentration of less than 1 mg/L. lf the raw water hardness exceeds 100 mg/L, the EDI will likely receive undesirable hardness leakage. A softener upstream of the RO unit will solve that problem. But even under these circumstances, it often makes more sense to install only partial flow softening and avoid higher capital equipment and operating costs, large equipment footprints, as well as unnecessary saline softener regeneration effluent. Resin softeners are also well-know bacteria breeding grounds that can cause fouling in the RO system. MAKALE / ARTICLE Table 2 - Recommended essential analyses for the evaluation ofa raw water supply Commonly Parameter Typically used symbol Notes/concerns reported as or abbreviation PH - Useful as a general indicator Calcium mg/L as CaH Scale forming Hardness CaCO3 Magnesium mg/L as MgH Scale forming Hardness CaCO3 mg/L as Foulant. Sum of Total silica SiO2 reactive and the ion colloidal silica Reactive mg/L as SiO2 Foulant silica the ion Total iron mg/L as Fe Foulant at low the ion concentrations Manganese mg/L as Mn Foulant at low the ion concentrations Total mg/L as Al Foulant at low aluminum the ion concentrations mg/L as Scale forming at Barium Ba very low the ion concentrations Strontium mg/L as Sr Scale forming the ion Total Catch-all. Salinity dissolved mg/L TDS solids indicator Conductivity Micro COND Same as Siemens/cm TDS above Suspended mg/L TSS Foulant solids Sodium mg/L as Na Generally not an the ion issue Scale forming. "P" mg/L as M-Alk orTotal alkalinity should be Bicarbonate tested for in some CaCO3 alkalinity waters and/or certain applications Chloride mg/L as cı Corrosion the ion precursor Total sulphur, mg/L as SO4 Scale forming as sulfate the ion mg/L as Very problematic Fluoride F yet rare in most the ion waters Nitrate mg/L as N03 Origin: agricultural the ion runoff Total organic mg/L as TOC Useful general carbon C parameter Total mg/L as P04 Eutrophication; phosphate the ion foulant APHAor Foulant. Colour Pt-Co units APHA, TCU Very important in surface waters Note: Tlıe above coııstitutes a we/1-detailed represeııtaıioıı ofmosı ıııater soıırces tlıat are ıısed in Poıııer planı projecıs. Otlıer species slıoııld be ıested (e.g. aıııınonia, p!ıeııols, 1 oil & grease, coppeı; sıılphide, mercwy) as speciflc ııeeds dictate. ENERJi DÜNYASI EYLÜL 2006 79
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