Better control of wastewater treatment requires the development of pertinent sensors. For Alternating Aerobic-Anoxic Activated Sludge, the relatively simple sensors measuring ORP, DO and pH appeared to be useful for managing the aeration. Bending-points on the ORP and pH curves are linked to the major biological activities playing a role in nitrification and denitrification. In this work, it is shown that the appearance or the absence of these bending-points and the pH variation slope allow the operating state of the biological system to be identified. Case studies of over- and under-loaded conditions and over- and under-aerated systems are examined. The diagnosis procedure can be used to regulate the aeration flow rate, to optimize the ORP thresholds and the timers, and to evaluate the organic load and the potentials of the bacterial populations involved in nitrogen elimination.
The necessity to achieve nitrogen and phosphorus removal in wastewater, according to the European Directive (EEC 1991), leads to the conception of new methods to control the aeration of low-loaded activated sludge plants. The behaviour of N.NOx concentrations and of ORP during a complete nitrification-denitrification cycle is described by a typical profile with 3 bending-points: α, β and χ. The goal of this study is to get more insights into the biological and chemical signification of the bending-points. This leads to the conception of new real-time control systems able to be adaptive to the influent load variations and free from ORP drift problems. The results obtained on pilot-scale plant using a three bending-points based control strategy show a real advantage through a decrease of the global aeration duration.
This article has the objective of showing the cause effect relationship between the dynamics of growth of autotrophic populations involved in nitrification and the uncontrolled accumulation of nitrite ions. This accumulation results in a disequilibrium in number or viability between the genera Nitrosomonas and Nicrobacter. This disequilibrium can be imposed, for example, by an inhibition of the activity of the genus Nitrobacter linked to the presence of free ammonia in the environment. The threshold of inhibition and the resultant degree of accumulation of nitrite depend both on the history of the sludge utilised as inoculum (mixed autotrophic population or enriched in one of two sources) and on the hydraulic regime of the reactor (completely mixed reactor for the activated sludges and piston reactor for the fixed cultures). These results enable us to better understand the behaviour of a nitrification reactor and to propose solutions either to avoid the accumulation of nitrites or to intensify this accumulation with the goal of proposing a new process of nitritation-denitritation via the nitrites route (nitrates shunt).
The simultaneous removal of carbonaceous and nitrogenous pollution by the activated sludge process is becoming common in industrial and municipal wastewater treatment plants. An oxygenation monitoring process has been developed, which is based on the dynamic analysis of ORP and DO signals and allows the detection of specific characteristic points at the end of the biological nitrification and denitrification. The aim of this study is to validate this process in a food-processing industry WWTP (slaughterhouse) having large variations of carbonaceous and nitrogenous loads. In order to treat during the peak period, pure oxygen is used. The first part of the study provides a precise diagnosis of the WWTP operation by the analysis of the ORP and DO signals. It is particularly easy to estimate the level of nitrogen treatment actually achieved and the oxygen requirements, and to detect the over- or under-oxygenated phases. Thanks to the monitoring process, the aerobic period of each cycle is reduced to the optimal duration, providing a reduction of 30% on the energy consumption compared to a traditional schedule. We have demonstrated that the use of pure oxygen associated with the existing air system is particularly relevant for the peak period. The revamping of an existing plant to simultaneously treat the carbon and the ammonia in the same basin is now technically feasible.
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