Use of Sensors in Wastewater Quality Monitoring—A Review of Available Technologies

Abstract

Wastewater treatment involves removing nutrients from wastewater before discharging it to water courses. Many technologies have been developed for this purpose, such as the activated sludge process and others. This process is a biological method performed by a mixed community of microorganisms in an aerobic aquatic environment. These microorganisms derive energy from carbonaceous organic matter in aerated wastewater for the production of new cells in a process known as synthesis, while simultaneously releasing energy through the conversion of this organic matter into compounds that contain lower energy, such as carbon dioxide and water, in a process called respiration. As well, a variable number of microorganisms in the system obtain energy by converting ammonia nitrogen to nitrate nitrogen in a process termed nitrification. This consortium of microorganisms, the biological component of the process, is known collectively as activated sludge. EBPR (Enhanced biological phosphorus removal) is a wastewater treatment configuration applied to activated sludge systems for the removal of phosphate. The common element in EBPR implementations is the presence of an anaerobic stage (nitrate and oxygen are absent) prior to the aeration stage. Under these conditions a group of heterotrophic bacteria, called polyphosphate-accumulating organisms (PAO) are selectively enriched in the bacterial community within the activated sludge. These bacteria accumulate large quantities of polyphosphate within their cells and the removal of phosphorus is said to be enhanced (R. G. Benedict et al. 1971). SBR (Sequencing batch reactors) is a fill-and-draw activated sludge system which is used in most lab-scale systems in order to enrich the sludge with PAO. The management of these reactors is mostly based on off-line measurements such as volatile fatty acids and phosphorus. However, off-line monitoring of the SBR cycle implies low frequency data sampling and delay between sampling and availability of the results. This is an obstacle for a proper process monitoring and makes difficult the application of control strategies to the process. For this reason, the on-line monitoring of the SBR cycle would improve the daily process management, as well as facilitate the on-line detection of abnormal situations and the implementation of new control strategies. Moreover the control of these processes has become more complex and the demand on continuous monitoring has increased. This may cause difficulties if this is handled manually and an online control system should make the process more controllable and less complex. In recent years there has been a lack of a proper sensor which can be used for on-line real time monitoring. This paper will review the available technologies for online monitoring outlining their advantages and disadvantages. The paper will also present current developments at Liverpool John Moores University with regards to the development of such a sensor and investigate the possibility of applying it to in-situ applications. In addition, the use of microwave technology is investigated for enhancing sensor performance.