Verification of an alternative sludge treatment process for pathogen reduction at two wastewater treatment plants in Victoria, Australia

Integrating renewable energy in sewage sludge treatment through greenhouse solar drying: A review

Sewage sludge is the main by-product of wastewater treatment plants, requiring significant environmental and economic burdens for its management and disposal. Recently, solar drying processes, often performed through solar greenhouses, received interest due to their limited energy requirement and renewable energy exploitation. The dried sludge shows significant volume and mass reductions, reducing transportation and disposal costs. However, its physicochemical and microbiological characteristics must be properly assessed, especially if agricultural reuse is the final sludge destination, due to the possible accumulation of (micro)pollutants in the soil. This review depicts the state-of-the-art solar drying processes of sewage sludge, with a focus on the technological aspects and the sludge quality. The review discusses greenhouse-specific features, sludge composition (organic matter, pathogens, heavy metals and emerging pollutants) and drying conditions (seasonality, ventilation, sludge mixing and thickness, and drying speed). The economic aspects connected to sludge solar drying are presented. The limitations of this technology are discussed as well, including the limited applicability to wet sludge and the environmental issues connected to greenhouse structure degradation. A wider application of sludge solar drying is recommended to increase the sustainability of small and medium wastewater treatment plants, especially in areas with high amounts of solar radiation and dry weather conditions, while thermal drying still appears preferable for large plants. More agronomic studies must be conducted to assess the possible pollutant accumulation in crops, and alternative uses of the dried sludge (e.g., energy recovery through incineration, pyrolysis or gasification; utilization in construction materials) should be explored, also using life cycle assessment.

Quality and risk management frameworks for biosolids: An assessment of current international practice

Biosolids, a product of wastewater treatment, provide a valuable resource, but to optimize the use of this resource it is necessary to manage risks posed to public health and the environment. Key requirements include identifying contaminant sources and providing barriers to ensure containment and treatment while maintaining the viability and value of biosolids products. Responsibility for managing biosolids is the remit of many stakeholders but primarily it rests with private and public wastewater facilities. The global variabilities in the way biosolids resources are acknowledged, applied, and managed are substantial. For example, some countries are increasing incineration because of their ability to remove contaminants while others have experienced a proportional decrease in incineration dependent on industrial resources or regarding resource recovery costs and needs. Some jurisdictions focus on energy recovery and others on land application. A risk management framework is a tool that may provide a suitable holistic approach to biosolids management. With this focus, current instruments in practice globally to manage biosolids were assessed for the degree to which they have adopted a risk management framework. To form a basis for this assessment a set of criteria was established by concept mapping several internationally recognized standards. Guidelines for a range of developed and developing countries were then assessed against these criteria. That process enabled the identification of which current practices were holistic in terms of applying biosolids risk management principles from production to end-use. Through this process, risk management gaps and vulnerabilities were identified. The results reveal that the incorporation of risk standards into risk management frameworks around the world is variable for the presence of risk criteria and the scale of detail provided. Contaminant concentrations need perspective within the changing risk landscape for stakeholders and the environment while jointly the opportunities and contaminant challenges require solutions that balance risks.

Combining environmental isotopes with Contaminants of Emerging Concern (CECs) to characterise wastewater derived impacts on groundwater quality

The potential for Wastewater Treatment Plants (WWTPs) to cause adverse impacts to groundwater quality is a major global environmental challenge. Robust and sensitive techniques are required to characterise these impacts, particularly in settings with multiple potential contaminant sources (e.g. agricultural vs. site-derived). Stable (δ²H_H2O, δ¹⁸O_H2O, δ¹⁵N_NO3, δ¹⁸O_NO3 and δ¹³C_DIC) and radioactive (³H and ¹⁴C) isotopes were used in conjunction with three Contaminants of Emerging Concern (CECs) - carbamazepine, simazine and sulfamethoxazole - to discriminate between multiple potential contamination sources at an Australian WWTP. The radioactive isotope tritium provided a sensitive indicator of recent (post-1990s) leakage, with groundwater activities between 0.68 and 1.83 TU, suggesting WWTP infrastructure (activities between 1.65 and 2.41) acted as a recharge 'window', inputting treated or partially treated effluent to the underlying groundwater system. This was corroborated by water stable isotopes, which showed clear demarcation between δ¹⁸O_H2O and δ²H_H2O in background groundwater (δ¹⁸O_H2O and δ²H_H2O values of approximately -5 and -28‰, respectively) and those associated with on-site wastewater (median δ¹⁸O_H2O and δ²H_H2O values of -1.2 and -7.6‰, respectively), with groundwater down-gradient of the plant plotting on a mixing line between these values. The CECs, particularly the carbamazepine:simazine ratio, provided a means to further distinguish wastewater impacts from other sources, with groundwater down-gradient of the plant reporting elevated ratios (median of 0.98) compared to those up-gradient (median of 0.11). Distinctive CEC ratios in impacted groundwater close to the WWTP (∼3.0) and further down-gradient (2.7-9.3) are interpreted to represent a change in composition over time (i.e., recent vs. legacy contamination), consistent with the site development timeline and possible changes in effluent composition resulting from infrastructure upgrades over time. The data indicate a complex set of co-mingled plumes, reflecting different inputs (in terms of both quantity and concentration) over time. Our approach provides a means to better characterise the nature and timing of wastewater derived impacts on groundwater systems, with significant global implications for site management, potentially allowing more targeted monitoring, management and remedial actions to be undertaken.

Delineation of contaminant sources and denitrification using isotopes of nitrate near a wastewater treatment plant in peri-urban settings

Distinguishing sources of groundwater contamination in regions with multiple potential sources can be challenging using conventional markers. In this study, isotopes of nitrate (δ¹⁵N_NO3 and δ¹⁸O_NO3) were examined in conjunction with other hydrochemical parameters to better distinguish sources of groundwater contamination, where intensive agriculture occurs adjacent to a wastewater treatment plant (WWTP). High nitrate concentrations were found in groundwater both within the WWTP site and surrounding market garden farms (maximum of 99 mg/L and 78 mg/L nitrate as N, respectively). Ranges and median δ¹⁵N_NO3 values showed clear differences between sample groups. In groundwater close to the WWTP, δ¹⁵N_NO3 and δ¹⁸O_NO3 values ranged from 10.4 to 41.2‰ and -0.5to 21.3‰, respectively, indicating predominantly sewage-sourced nitrate, while samples within market gardens showed evidence of mixed fertilizer (manure and synthetic) sourced nitrate, with δ¹⁵N_NO3 and δ¹⁸O_NO3 values between 7.2 and 29.8‰ and 0.4 to 15.1‰, respectively. Nitrate interpreted to be derived from the WWTP was also typically associated with elevated ammonia as N (median concentration of 17 mg/L) and SO₄ (median concentration of 350 mg/L). These distinctive signatures allowed for clearer delineation of the extent and overlap between different contaminant plumes than otherwise possible. Geochemical conditions in groundwater surrounding the WWTP appear to promote denitrification, evident through enrichment in δ¹⁵N_NO3 and δ¹⁸O_NO3 and reduced nitrate concentrations between sampling rounds (locally). However, isotopic signatures in market garden areas showed no evidence of denitrification, and groundwater exhibited conditions likely to preserve nitrate (e.g. dissolved oxygen levels >2 mg/L). There is limited evidence of nitrate contamination currently impacting a nearby groundwater dependent ecosystem (Tootgarook Swamp), located down-gradient from the WWTP. This research demonstrates that a combination of hydrochemical and isotope data can help resolve sources of groundwater contamination and characterise nutrient degradation behaviour in settings with multiple inputs.

Physicochemical Properties of Biochars Produced from Biosolids in Victoria, Australia

Some of the barriers associated with the land application of biosolids generated in wastewater treatment plants can be eliminated simply by converting the biosolids into biochar using a thermal conversion process called “pyrolysis”. In the current work, eight biosolids from four different wastewater treatment plants in southeast Melbourne, Victoria, Australia were collected and pyrolysed to produce biochars at two different temperatures (500 and 700 °C). In addition, characterisation studies were carried out on the biochars to obtain their physicochemical properties, which were subsequently compared with the properties of the parent biosolids. The major findings of the work demonstrated that biochars exhibited large decreases in DTPA-extractable metals such as Cd, Cu, and Zn, and also led to favorable changes in several chemical and physical characteristics (i.e., pH, Olsen P, electrical conductivity, and surface area) for agricultural land application compared to their original form (i.e., biosolids). Overall, the study suggests that there is great potential for converting biosolids to biochar using pyrolysis. This may not only improve the properties of biosolids for land application, but also has potential to reduce the risk to receiving environments and, furthermore, eliminate many of the costly elements associated with biosolids stockpiling and management.

Helminth log reduction values for recycling water from sewage for the protection of human and stock health

The LRVs required to decrease HE concentrations in raw sewage to an acceptable level to manage the risk to human and livestock health were determined. An LRV of 3.0 was required to meet the HBT of 1 μDALY pppy in SE Australia where human helminth infections are not endemic. In comparison, a similar exposure volume and LRV in endemic regions would result in a HBT of 100 μDALY pppy. The risks posed by cattle- and pig-related helminths were also managed acceptably with the treatment of sewage providing an LRV of 3.0. New design equations were derived to determine LRVs based on hydraulic residence times (HRTs) in an activated sludge plant (ASP) and lagoons. The new equation for lagoons indicated that an LRV of 3.0 could be achieved with a HRT of 18 days or less.

Occurrence and fate of Ascaris lumbricoides ova in biosolids in Victoria, Australia: a human health risk assessment of biosolids storage periods

Reuse of sewage biosolids in Victoria, Australia, typically involves mesophilic anaerobic digestion followed by air-drying and long-term storage to ensure removal of ova of soil-transmitted helminths (STH) such as Ascaris lumbricoides. Long-term storage degrades the biosolids' agronomic quality due to the loss of key plant nutrients and takes up large areas of storage space. The impact of varying biosolids holding times and other processes on STH using Ascaris as the reference STH pathogen was examined in this study using a quantitative risk analysis approach. Risk modelling of the potential human health impacts from the presence of Ascaris ova in biosolids was undertaken for discrete holding periods of 1, 2 and 3 years. Modelling showed that to meet the WHO 1 μDALY·person^-1·year^-1 disease burdens guideline for limiting exposure category, a biosolids storage period of 1.24 years or 2.1 years would be required, depending on the data source of ova shedding rates per worm (Bangladesh or Nigeria, respectively). The soil exposure and salad/root vegetable consumption models included a number of variables with moderate to high degrees of uncertainty. Monte Carlo simulation was used to assess the effect of uncertainty in model input variables and to assist in highlighting areas for further research.