Combined ion exchange / biological denitrification for nitrate removal from ground water

Abstract

This thesis deals with the development of a new process for nitrate removal from ground water. High nitrate concentrations in ground water are a result of fertilization in agriculture. According to a directive of the European Community the maximum admissible concentration of nitrate in drinking water is 11.3 mg NO 3--N/l and the guide level is 5.6 mg NO 3--N/l. To supply water that meets this standard several water supply companies will have to remove nitrate from ground water. Two existing techniques, viz ion exchange and biological denitrification, have serious disadvantages when used separately. Therefore, a new process has been developed that consists of a combination of ion exchange and biological denitrification. In this process nitrate is removed from the ground water by ion exchange. The ion exchange resins are regenerated in a closed circuit through an upflow sludge blanket (USB) denitrification reactor. In this reactor denitrifying bacteria remove nitrate from the regenerant, so that it can be used again and has not to be disposed of. As compared with conventional regeneration of anion exchange resins regeneration salt requirement and brine production are minimized. Further, in contrast with traditional single denitrification procedures, there is no direct contact between ground water and denitrifying bacteria.The first part of this thesis deals with the effect of high salt concentrations, as present in the closed regeneration system, on biological denitrification. Concentrations up to 30 g NaHCO 3 /l or 30 g NaCl/l have only little effect on the activity of denitrifying sludge. With high NaHCO 3 concentrations the sludge yield coefficient decreases and nitrite accumulation is suppressed. High sulfate concentrations (5.5 9 SO 42-/l) do not result in sulfide production in an USB denitrification reactor fed with methanol, when methanol is added in an appropriate ratio to the amount of nitrate to be denitrified.The second part of this thesis deals with the ion exchange part of the combined process. Regeneration of anion exchange resins can be achieved with a solution containing 30 g NaHCO 3 /l provided that a larger flow rate and a longer regeneration time are used as compared with conventional regeneration procedures. With nitrate selective resins it is possible to remove nitrate from ground water that contains high sulfate concentrations, while the nitrate capacity of these resins is not affected by high sulfate concentrations in the regenerant. To safeguard the bacteriological drinking water quality the resins have to be disinfected after each regeneration cycle by rinsing with 0.075% peracetic acid for 15 minutes or by rinsing with 0.20% hydrogen peroxide for 45 minutes. Since the first possibility results in an important loss of resin capacity on the long term, only the latter can be applied in practice.The third part of this thesis deals with the operation of a lab-scale pilot plant. The most important process variables studied were the regenerant composition (NaCl or NaHCO 3 ), the ion exchange resin type (sulfate selective or nitrate selective) and the ground water composition (low sulfate concentration or high sulfate concentration). To explain some phenomena that were observed during this research a computer model has been developed. With this model the regeneration of anion exchange resins in a closed circuit can be optimized