**1. Introduction**

Human beings in the global ecosystem are no longer consumers, but wasters. To achieve "Transforming our World: the 2030 Agenda for Sustainable Development (2030 Agenda)" and set the Sustainable Development Goals (SDGs) consisting of 17 global goals in 2015 [1], human beings need to become former consumers and stay within the energy and matter cycles of the global ecosystems. One of the examples is the sixth goal of "Clean Water and Sanitation" in the 17 SDGs. The cause of anthropogenic eutrophication and subsequent organic pollution is the imbalance in the ecological matter cycle that occurs in human social life.

In Japan, the anthropogenic eutrophication and the organic pollution in 1960' had simultaneously been caused by the direct influx of wastewaters from industries and households (**Figure 1a**) [2]. On the other hand, most of such eutrophication in recent years is induced by the anthropogenic influx of nutrient salts, such as phosphate and nitrate salts, which are the essential nutrients of living organisms (**Figure 1b**) [3]. It is known that orthophosphate ion, that is, phosphate ion (Pi) is the most causative nutrient. These nutrient salts are contained in the effluent of sewage treatment plants or industrial wastewater treatment plants, or in leaching water from farms or live stocks [3, 4]. In the cases of these plants, only an ecological decomposer is employed for the biodegradation of organic matters contained in the wastewater (**Figure 1b**). This is the root cause of current eutrophication and subsequent organic pollution. Therefore, such anthropogenic influx of nutrient salts causes the water bloom by phytoplankton as an ecological producer, and the remains of phytoplankton cause organic pollution. To investigate the ecological phenomenon of such anthropogenic eutrophication and subsequent organic pollution, we observed the water ecosystem and water qualities of a eutrophied pond as a model water body [5]. The results led to a study on hydroponics [6] and water chemical remediation (WCR) that was developed for simultaneous removal of Pi and phytoplankton from the anthropogenically eutrophied pond [7]. A series of the studies will be described elsewhere.

In our studies, we have also developed measurement methods for these water pollutions. As one of the ways to measure the degrees of water pollution, several biosensors have been developed. The biosensor consists of a molecular recognition

#### **Figure 1.**

*(a) Schematic diagram in comparison on the causes of organic pollution in the 1960s and the recent past [2]. The schematic diagram of (b) was permitted from Springer Nature [2] and slightly modified for this chapter.*

*Developmental Studies on Practical Enzymatic Phosphate Ion Biosensors and Microbial BOD… DOI: http://dx.doi.org/10.5772/intechopen.104377*

#### **Figure 2.**

*Biosensor. (a) Principles and (b) automatic Pi biosensor instruments of a desktop-type [8] and (c) a submersible buoy-type [9], (i) a column compactly integrating a Pi biosensor system, (ii) a continuous-remote sensing system for total water-quality monitoring, and (iii) the continuous-remote sensing system is shipped by a boat and sunk vertically (a) and (c) were permitted from Springer Nature [10] and (b) was permitted from Taylor and Francis [11]).*

element and a transducer and has the features of a simple and rapid measurement device (**Figure 2a**). The first biosensor was studied for medical use by Updike and Hicks in 1967 and developed to determine glucose concentration employing an enzyme-catalyzed reaction (enzymatic reaction) by glucose oxidase (GOD) [12]. This enzymatic glucose biosensor used a Clark-type electrode [13] and measured dissolved oxygen (DO) consumption caused by the GOD reaction. Since then, biosensors as measurement devices have been developed not only for medical uses [14] but also for food or environmental uses [10]. Thus, many biosensors and their associated techniques have been studied and developed [10]. In particular, the biosensor for environmental use requires highly sensitive and wide-determination range (dynamic range) measurement of the analyte. In addition, depending on what is being analyzed, the environmental biosensors require the feature to perform either specific or nonspecific measurements. Furthermore, for practical use, it is also required the application style of the biosensor, such as on-site use, continuous use, or laboratory use.

To satisfy these requirements, many kinds of biosensors for environmental water pollution have been developed worldwide [3, 10, 15]. In the second section of this chapter, the development of enzymatic Pi biosensors for eutrophication and in the third section, the development of microbial biochemical oxygen demand (BOD) biosensors for eutrophication are described. In the final fourth section, the contents of this chapter are concluded.
