Risk based approach for managing salt accumulation in soil irrigated with recycled water

Recycling is one of the viable options to attain sustainable management of wastewater. The supply and reuse of recycled water may play an important role in enhancing urban water supplies in many water-scarce parts of industrialised countries because of its reduced treatment cost relative to seawater desalination and imported surface water. One such reuse option includes application of recycled water for irrigating urban open fields. Past literature suggests that the continuous use of recycled water over a long period of time may lead to the accumulation of salt in the root zone. Salt transport models to quantify salt accumulation in soil exist, but these do not consider the stochastic nature of the elements of salt accumulation process. Moreover, none of the past studies propose a framework to manage and control the salt accumulation process due to recycled water irrigation by considering stochastic nature of different components. The study described in the thesis details a novel methodology adopted for the development and implementation of an integrated risk based approach to control sources of salinity and the level of treatment required to use recycled water in irrigation in a sustainable manner. The study included laboratory and field work and involved thorough investigation of site specific soil, data analysis, development of relationships among elements of salt accumulation process, and incorporated long-term prediction modelling result and scientific knowledge into a framework. One of the key investigations conducted was to understand and monitor salt accumulation process in columns using sensors in terms of depth of soil, type of soil and type of irrigation water. Data generated from these experiments and output from simulation were used to develop the framework. Therefore, the overall aim of this study was to develop a framework with the help of a probabilistic method, namely, Bayesian belief network (BBN) to manage the salinity in the root zone due to recycled water irrigation. Results from the column study show that due to recycled water irrigation, soil water electrical conductivity (ECSW) was higher in the upper part of the column (0-0.2 m) than the lower part. This is because only applied irrigation water could not leach the salt from upper part to downward. When simulated rainfall was applied (once in a week) in a loamy sand column along with recycled water (twice in a week), the average ECSW showed a decreasing pattern with time. In another column study with silty loam soil, average sodium adsorption ratio due to recycled water (EC = 0.8 dS/m) irrigation was 3.6 times more than the tap water (EC = 0.2 dS/m) irrigation and 1.4 times less than the synthetic saline water (EC = 2.0 dS/m) irrigation. In the same column study, it was observed that the ratio of soluble cations (Na+: Mg2+: Ca2+: K+) in the soil sample changed than its initial ratio at the beginning of the study. The change in the ration occurred because of exchanging cations between soil and the water added for irrigation. A salt transport model HYDRUS 1D was validated with experimental results and used to predict risk of salt accumulation in field condition. The salt transport modelling carried out in this study shows that in drought condition, yearly averages ECSW exceeded the maximum salinity tolerance threshold of 5.0 dS/m for rye pasture due to recycled water irrigation in a loamy sand paddock. The ECSW exceeded 1, 59, 79, 87 and 90% for the years from 1 to 5, respectively. In another modelling with future climate condition between years 2021 and 2040 shows that ECSW was 24% higher in loamy soil paddock compared to loamy sand paddock. Amount of leachate in the loamy sand paddock was 27% more than the amount leached from loamy paddock, which may pose a salinity risk to the ground water if there is a perched aquifer in the field at a depth < 1 m. BBN framework analyses identified that for root zone ECSW of 2.25 dS/m, it is 92% probable that the Na+ concentration of the root zone soil water would be in the range of 5 – 15 mmol(c)/L; for ECSW of 16.5 dS/m, there is 86% probability that the Na+ concentration of root zone soil water would be in the range of 30 – 35 mmol(c)/L. Furthermore, over the study period of 2021 to 2040, it was found that the reduction of the posterior mean of recycled water EC by 13% (from μ=0.92 to μ=0.8 dS/m), brings the average root zone ECSW down from 6.5 dS/m to 4 dS/m, which is within the salinity threshold limit for rye pasture. The BBN framework also identified the most significant sources of salinity contributing to wastewater and proposed control strategy of those sources to minimise the salt accumulation in the soil for a sandy loam oval irrigated with recycled water. Results show that accumulation of salt in the root zone was largely due to the salt load in the wastewater stream from washing machines and the salt load in the wastewater from toilets was the second most influential source. It was found that by controlling multiple sources at the same time significantly reduces salt accumulation in the soil. It was observed that by using environmental friendly detergents reduce the TDS load in the laundry stream by 4 to 7 times and Na+ load by 2 times than popular brand detergents. Irrigation scheduling with recycled water is typically done while considering only the soil moisture levels. The study reported in this thesis proposes that besides considering the soil moisture levels, salt accumulation within the soil must be considered while irrigating open fields using recycled water. Proposed methods and outcome of this research would provide vital knowledge about the uncertainty associated with root zone salinisation of urban open fields, and better management and control of root zone salinity due to irrigation with recycled water. The study highlighted that any strategies that help in the reduction of salt in the recycled water will be beneficial in managing the soil salinity as a result of recycled water use for irrigating open fields. Hence, the proposed decision making tool for controlling the risk of soil salinisation can assist in developing recycled water irrigation schemes which are sustainable over the long-run