The costs and benefits of process measurement instruments in sewage treatment plants were the subject of intensive debate in the 1990s. Cost-benefit studies, comparing the potential savings in energy, consumables and personnel with the additional costs of using the instruments, resulted in recommendations specifying the minimum sewage treatment plant size at which the use of such process optimisation and monitoring instruments would be economically worthwhile. In view of the subsequent advances in measurement technology, the increased costs of energy and sludge disposal, and the stricter demands on the operation of sewage treatment plants, it is time to take another look at these studies and adjust them to the changed situation.
Many plant operators are under growing pressure to make their wastewater treatment processes safer and more effective. The most critical processes, in terms of cost and the quality of treatment, are nitrification, denitrification and the chemical elimination of phosphorus. Process analysers for NH4-N (indophenol-blue method or gas-sensitive electrodes) and PO4-P (vanadate-molybdate or molybdenum-blue) are used to optimise and monitor operations in large plants (>25,000 PE). NO3-N is still measured by process probes based on UV absorption. The simple handling and extremely short reaction time of these probes make them ideal sensors (“dip and measure”).
Process measurement instruments for nutrient parameters are rarely employed in smaller plants (5000–20,000 PE). In general, the only instruments used are oxygen sensors, whose cost-efficiency is obvious, for regulating the oxygen concentration, or probes for measuring outflow turbidity as a key parameter for monitoring plant efficiency. However, small and medium sized sewage treatment plants have to handle greater fluctuations in influent concentrations and hydraulic loading than larger ones (e.g. distinctive diurnal variations, storm surges, illegal agricultural discharges). Since small plants have to function with ever fewer personnel – even now, many plants are not permanently staffed – process measurement instruments can help, in combination with modern data transfer technology and, possibly, automation technology to ensure reliable and cost-efficient plant operation that can cope with variable inflow conditions.
One of the main reasons for the still cautious application of process measurement instruments is the cost-benefit analyses carried out in the mid 1990s, when investment and operating costs were still very high. Even today, the results of these analyses colour the attitudes of many operators, engineers, regulatory bodies, etc. towards process measurement technology. In view of the ever more stringent environmental demands and the increasing costs of energy and sludge disposal on the one hand, and the steadily decreasing operating and investment costs of measurement instruments on the other, it is time to take another look at the costs and benefits of online measurement technology.