Effects of treated wastewater on plants, soil chemical properties and CO2 emission

Author: Farah Jafarpisheh

  • Thesis download: available for open access on 4 Apr 2025.

Jafarpisheh, Farah, 2022 Effects of treated wastewater on plants, soil chemical properties and CO2 emission, Flinders University, College of Science and Engineering

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In this research, reusing the treated wastewater as sole source of nutrients, to produce biofuel or animal fodder, its environmental health risks and management are considered. The growth of Sorghum (with the potential for animal fodder or biofuel) and Eucalyptus (as fuel wood) irrigated with wastewater was a focus of this study.

Two varieties of Sorghum (SE1-fodder crop and SE2-biofuel crop) were successfully grown using treated wastewater from a waste stabilization pond. This is the first study investigating Sorghum varieties as an economic alternative to the current practice used in South Australia for final disposal of treated wastewater. Subsequently, Sorghum verities were grown irrigated treated wastewater from High Rate Algal Ponds (HRAPs). Uniquely, after harvesting the roots were left in the soil, allowing plants to regrow (ratoon crop), demonstrating the potential economic benefits of two harvests per year. The two Sorghum varieties selected were shown to produce green biomass (tillers, leaves, total top) and a high sugar content (Brix). The second Sorghum harvest resulted in the higher alcohol content, equal to 4.36 and 2.85 T ha-1 ethanol, equivalent by 116.94 and 76.43 MJ kg-1 ha-1, for total biomass of SE1 and SE2.

The relative growth of seven different species of Australian native Eucalyptus, irrigated with HRAPs treated wastewater at two application rates (0.8 and 1.6mm d-1) was evaluated. Eucalyptus camaldulensis was recommended as the species best suited to growth when irrigated with wastewater, surviving, and growing well even at the lower irrigation rate.

The effects of treated wastewater on soil and its environmental impacts were investigated. The HRAP treated wastewater increased both the cations and anions, sodium, potassium, magnesium, phosphate, sulfate, and chloride and decreased fluoride and calcium in the topsoil. The soil EC and pH increased at both irrigation rates. Reference to the Australian Guidelines for Water Recycling (NRMMC, 2006a), suggested that the irrigation wastewater presented a likely soil sodicity risk. Low chloride concentration in irrigation water (<350 mgL-1), indicated a low likelihood of increasing the cadmium concentration in crops. The results suggested that these risks could be managed by the recommended lower irrigation application rate (0.8 mm d-1), the selection of suitable plant species, site selection and addition of appropriate calcium amendments to the irrigated area.

The filtrate extracts from SoluSAMPLERS and effluent were analysed for nitrogen and phosphorus to investigate the potential effects of HRAPs on the groundwater system. The concentrations of nitrate and phosphate decreased, whereas that of nitrite increased with increasing soil profile depth from 330mm to the 930mm. Modelling using LEACHM suggested that if nitrogen and phosphorus could be utilized by plants prior to sporadic high rain events leading to deep leaching, HRAP effluents can be used for irrigation without leading to excessive nutrient accumulation in groundwater. Application of HRAP treated wastewater increased total organic carbon (TOC) and organic nitrogen in solid soil particles, and TOC in soil extracts compared with the original native soil (as defined as non-Irrigated soil, soils to which no effluent had been sprayed, and it is disturbed soil). Using treated wastewater improved the soil quality.

The effects of treated wastewater on soil CO2 flux, compared with native soil were determined using automated soil CO2 flux system (LI-COR 8100A). There was a high

correlation between the soil CO2 flux and mean seasonal temperature, where, in irrigated areas, the highest mean daily net flux was recorded in summer in both irrigated and native soil sites. Total annual net flux from the irrigated area (4.3 t CO2-C ha-1 year-1) was 22-fold more than that from the native soil (0.2 t CO2-C ha-1 year-1), which was caused by irrigation and the higher organic matter input to these soils.

Overall, this thesis presents a unique collection of work incorporating large scale field work and modelling. The results showed that using the nutrient rich treated wastewater from HRAPs for plant irrigation will increase the carbon sink in the soil and improve the soil quality and so, increase the natural plant biomass in the topsoil over the time. Sorghum plants produce 57-fold more biomass than Eucalyptus spp. This significant amount of biomass production can promote Sorghum as an additional carbon sink. Also, its potential for fuel and fodder potentially adds economic value in comparison with Eucalyptus utilisation as firewood.

Keywords: High Rate Algal Ponds, Wastewater irrigation, Soil, Environmental risk, Anion, Cation, CO2 flux, Soil respiration, Negative flux, Sorghum, ethanol, animal feed, energy, Eucalyptus spp., Firewood, LICOR

Subject: Environmental Health thesis

Thesis type: Doctor of Philosophy
Completed: 2022
School: College of Science and Engineering
Supervisor: Professor Howard Fallowfield