Advances in Water Treatment and Enviromental Management

192 WATER TREATMENTProduct quality may be restored with a bicarbonate conditioning following brine regeneration butthis adds greatly to the cost and complication of the process.c) Due to chromatographic banding in the resin column, the production run at first yields a very highchlorideproduct, followed by a bicarbonate breakthrough. Such quality variation is undesirable in amains supply and can only be overcome by several parallel units running out of phase with oneanother, or very large mixing storage.d) When the conventional resin’s nitrate capacity is exhausted, the column continues to take up sulphatewhich displaces a high concentration of nitrate into the product. It is therefore dangerous to continuethe run beyond the nitrate breakthrough point.e) The spent regenerant contains the nitrate and other ions removed from the water, plus excessregenerant. Disposal of this solution presents environmental problems whose gravity depends on thelocation.AH these problems are aggravated by high sulphate in the raw water. Nitrate selective resins have recentlybecome available, with a higher cost and a lower working capacity and regeneration efficiency, but whoseselectivity means that little of this capacity is “waste” in removing sulphate, so that the capacity andefficiency with respect to nitrate are effectively higher when denitrifying high sulphate waters.The normal practice is to treat only a part of the water, removing nitrate to a very low level, and to blendback with untreated water for a product whose nitrate content is within the EEC directive. This reducescosts and eases the problems in b) and c) above. Counterflow regeneration is normally necessary to achievethe low nitrate level required to make this practical.3. INVESTIGATIONS INTO ION EXCHANGEExperimentation on ion exchange columns is slow with a column normally taking three cycles of run andregeneration to reach an operating equilibrium. Typically a laboratory test column will produce little morethan one experimental result per week’s experimentation, whose results are in the form of breakthroughcurves. These reveal nothing directly about events within the column.In the past ion exchange users have been content with the results obtained from this form of directexperimentation, because in the common industrial softening or deionisation applications, which aim atnear-total removal of a class of ions, chromatographic banding within the column appears relativelyunimportant. Denitrification, by contrast, is a complex chromatographic process, and needs a fresh approach.4. COMPUTER MODELLINGCranfield Institute of technology started work on anion exchange denitrification in 1984, supported at firstby the Science & Engineering Research Council, and currently by the South Staffordshire Waterworks Co.It was identified early on that for reasonable progress a computer model was needed to simulate the process.The Cranfield model is based on the concept of the column as a series of Theoretical Plates (TPs) in whichthe liquid comes to equilibrium with the resin in each plate before passing on to the next plate.The equilibrium state in each TP is assumed to have been reached when the four ions under considerationare all in binary equilibrium—i.e. the equilibrium constants for the exchange reactions Cl-NO 3 , C1-SO 4 , andC1-HCO 3 are all satisfied. Curiously, no resin manufacturer investigates these fundamental properties oftheir products, and we therefore had to determine them ourselves—a laborious task, made difficult byanalytical problems of determining small concentrations of one ion in the presence of large interferingconcentrations of another.Once the model was working, we estimated the height of column equivalent to a theoretical plate (HETP) bytrial and error, matching computer runs with actual experimental data, to give us the number of plates(NTP).After continuing improvement, the simulation can be operated with regeneration in co-or counterflow, withadditional regeneration stages if required. The NTP can be changed for different stages of the cycle, tosimulate differences in kinetic conditions e.g. between regeneration and run. A leakage factor, by which a