Shortages of irrigation water along with reinforced pressures on regional available water resources owing to population growth, urbanization, industrialization, and climate change [1
] are an important driver in the reuse of treated municipal wastewater (TWW) for agricultural activities, especially in the irrigation of rice (Oryza sativa
L.) paddy fields. The reuse of TWW has been demonstrated to diminish water scarcity, alleviate the degradation of wastewater-receiving environments, and conserve other freshwater sources [2
]. In addition, irrigation with TWW is advantageous to the improvement of soil fertility and rice productivity while reducing the use of commercial fertilizers [3
] as a result of the substantial amounts of plant nutrients such as N, P, K, and organic matter available in TWW. However, these benefits are likely countered by substantial concerns about human health risks, owing to potential pathogenic microorganisms, heavy metals, and other contaminants in wastewater [4
]. Thus, it is necessary to develop new irrigation management strategies and technologies to optimize the advantages while minimizing the aforementioned downsides.
We recently introduced an innovative rice cultivation system in which TWW was continuously supplied into paddy fields throughout the crop seasons in order to achieve very high yields and superior rice protein content of a forage rice without the supplementation of N fertilizers. However, P fertilizer was still applied at a high rate (160 kg P2
) to paddies in the system [6
]. As one of the most important nutrients for agricultural production, P demand is always increasing. This has caused considerable pressure on the mining of phosphate rock, which is projected to be significantly depleted by the end of this century [7
]. Furthermore, P utilization efficiency is particularly low, owing to significant losses [8
] that have led to serious environmental problems such as red tide and eutrophication [7
Reducing the use of P fertilizers and enhancing use efficiency are not only critical for alleviating the negative impacts of P on the natural environment but also probably conducive to farmers achieving a higher profit. On the other hand, at the end of the previous crop season [6
], a significant increase of P content in the paddy soils irrigated with TWW and simultaneously supplemented with P fertilizer raised a hypothesis that the high yields and superior rice protein content might be maintained during subsequent crop seasons without necessitating the supplementation of synthetic P fertilizers. However, the rate of soil P buildup may slow down and eventually reach steady-state after long-term TWW irrigation [9
] due to the downward movement of P binding minerals. In such case, there is the possibility that P input with TWW and steady-state soil P levels are insufficient to meet the P demand, so P fertilization might again become necessary [6
In addition to water scarcity, an increasing energy demand has been an important issue facing the world nowadays [10
]. This has triggered an ongoing motivation to find and develop renewable energy sources. Among possible alternatives for renewable bioenergy, the application of microbial fuel cells (MFCs) has been suggested as a promising approach to generate electricity directly by using bacteria to break down organic substrates under the anaerobic condition of paddy soil [11
]. As a result, considerable effort has been made to evaluate and advance the performance of MFCs associated with paddy fields [11
Apart from the essential plant nutrients (N, P, K) and micronutrients (Na, Ca, Mg, etc.), TWW also contains a significant amount of organic matter (OM), which might benefit the electricity generation of MFCs in paddy fields irrigated with TWW [17
]. Thus, we hypothesized that the performance of MFCs could be effectively elevated in the fields employing continuous irrigation with TWW by supplying substantial amounts of OM into the paddy soil. Though we attempted to investigate the performance of MFCs under such irrigation methods, the electricity generation was not assessed properly, owing to a number of unexpected failures in electrode connections and the operation of the MFCs during our previous study [6
]. In this follow-up experiment, we modified the MFC systems to avoid such failures and anticipated that the electricity generation could be thoroughly estimated in response to continuous irrigation with TWW.
Overall, the present study aims to test the aforementioned hypotheses. The specific objective of the study is to investigate (1) the effects of continuous irrigation with TWW on the performance of rice plants and changes in soil nutrition with and without the supplementation of P fertilizers, and (2) the performance of MFCs in paddy fields under different conditions of continuous irrigation with TWW.
The results obtained in this study show that the TWW used for irrigation satisfied the irrigation water quality criteria and that there were no adverse effects of TWW irrigation (Treatment A, B, C, E, and F) on the characteristics of rice growth, yield, and yield components, or on the heavy metal content in soils and brown rice, compared with those in the control (Treatment D). In addition, there were positive effects of TWW irrigation on rice growth and yield capacity compared with conventional irrigation. Overall, the safety and suitability of TWW irrigation in rice cultivation were demonstrated in this present study with the following highlights:
A high yield and nutritional quality of brown rice could be achieved by continuous TWW irrigation without any fertilizer application, especially in BI, which was superior to both TI and the conventional irrigation method. Nevertheless, monitoring P in the soil after each season is still recommended to determine whether the application of P fertilizers is required in subsequent growing seasons.
No hazardous metals accumulated in the soil or in the harvested rice when the soil was used repeatedly with continuous TWW irrigation. However, monitoring heavy metals in the soil and brown rice every season is highly recommended to fully characterize the effects of TWW irrigation.
The electric output from the MFC systems was still low compared with that reported from normal paddy fields, even when the connection was modified using graphite rods instead of copper cables. Further studies are necessary to optimize the potential of MFC systems in paddy fields.