Sewage and TSE quantity characteristics
The WWT facilities have been implemented for Zabol with a capacity of 39,000 m3/day. Table 1 shows the volume of water consumption and sewage production based on the sewage coefficient in urban communities of the study area.
As shown in Table 1, the total water consumption in the study area is 22.538 mcm/year while based on the development conditions. Afterward, the sewage volume was calculated to 16.194 mcm/year, considering the sewage coefficient and water consumption.
Continuously, the sewage data obtained from the Water and Wastewater Organization of Zabol city, Iran, showed that the sewage entrance to the treatment plants of the study area is about 19,000 m3/day and 137 working days. Therefore, the TSE volume of the WWT plant was calculated based on the following scenarios of (1) data obtained from the Water and Wastewater Organization, Iran, and (2) based on the capacity of WWT plant. Note that the working days for both scenarios will be 137. The calculation is based on Eq. (1). The total TSE volume for scenarios 1 and 2 is 2.8 and 5.1 mcm/year, respectively.
The difference between the calculation based on capacity and the existing data is due to the removal of raw sewage before entering the treatment plant, which has caused health and environmental problems in the region. Data obtained from Iran Department of Environment34 showed that 1.68 mcm/y of sewage were extracted for the farms. Previous studies in the same study area also reported the significant (P < 0.05) value of heavy metals in the soil of agricultural lands due to the application of sewage instead of TSE38. The results also revealed the efficiency of the WWT facilities based on scenarios 1 and 2, so far, very little of the potential of TSE resources in the study area was executed and a total of -90 percent of the treatment is underused. However, it is necessary to mention that more working days of WWT plant, could not increase the TSE volume, as the sewage volume based on return percentage is still low and around 30%. Thus, it is recommended that the government or private sector invested on the raising the coefficient of return.
Sewage and TSE quality characteristics
Water quality is the key factor in TSE reuse. Currently, Iran’s urban sewage treatment process mainly involves conventional methods such as coagulation, sedimentation, filtration, and disinfection. The methods of sewage treatment in Zabol and Zahak, in the study area, are stabilization pond and oxidation pond, respectively. In other urban areas, the produced sewage enters directly into the absorption wells of sewage disposal or inappropriately enters the environment. The stabilization pond is practical in hot and sunny areas because sunlight causes the photosynthetic action of oxygen to provide the oxidation required by sewage pollution. In addition, in the treatment method of stabilization ponds, no wastes that need special treatment are produced, and the quality after that is among the latest treatment methods of sewage quality and used in irrigation of agricultural lands, parks, and surface waters.
The chemical characteristics of sewage and TSE was examined on a seasonally and yearly scale. The average values of physicochemical factors in sewage and TSE of the Zabol sewage treatment plant in three months of summer are presented in Table 2.
As shown in Fig. 5, the efficiency of average seasonally values of input and output of the WWT facilities in the study area indicated that the highest removal efficiency is achieved for nitrate equal to 75% while the lowest was for TH with 3%. On the other hand, among the heavy metals and trace elements, the highest and lowest removal efficiency is for Cr and Li, respectively.
The efficiency of average seasonally values of WWT facilities in the study area.
Continuously, bacterial indicators of fecal contamination and enteric viruses are present in high concentrations in raw sewage. Hence, the values of TSE in the period 2017–2019 of the Zabol WWT plant showed in Fig. 6. The results of the fecal coliform (FC) removal efficiency showed that removal of culturable FC reduced by order to > 5. Note that typical abundance of total and fecal coliforms (FC) in raw sewage are 107–109 and 106–108 100/mL, respectively, and were reduced by 1–5 orders of magnitude in treated TSE, depending on the type of treatment39,40. Classical treatments, which do not include any specific disinfection step, reduce fecal micro-organisms densities by 1–3 orders of magnitude40, but because of their high abundance in raw sewage, they are still discharged in large numbers with treated TSEs in the environment.
The results of the abundance of total coliforms (TC) and fecal coliforms (FC).
Additionally, the results of yearly values of physicochemical factors of Zabol TSE (mg/L) including BOD5, COD, TDS, TH, and EC in the period of 2017–2019, showed in Fig. 7. The yearly results suggested that the values through the years of investigation did not show significant changes. In the following parts, the possibility of TSE evaluated considering various standards.
The results of yearly values of physicochemical factors of Zabol TSE.
Potential application of TSE
Comparing the quality of the TSE and sewage are based on various regulations showed in Table 3. It includes the food and agriculture organization (FAO), US environmental protection agency (USEPA), the Canadian water quality index (CWQI), and Iran’s national standards (INS), considering the irrigation and recreational application.
According to the FAO Guide41 for Classifying Agricultural Water Quality, as shown in Table 3, the most crucial parameters for the application of TSE in irrigation include electrical conductivity (EC), sodium uptake ratio (SAR), chlorine, BOD, COD, and FC. However, three out of seven parameters namely BOD, COD, and FC in the TSE are largely erratic with the limits recommended in the standards.
Based on USEPA42, the value of total suspended solids in TSE of Zabol WWT plant largely inconsistent with the limits recommended in the standards for TSE reuse. However, TDS, EC, and pH, met the criteria. Moreover, except TSS and pH, the other chemical parameters of sewage also meet the criteria. It is worth mentioning that EPA does not require or restrict any types of water reuse. Generally, states maintain primary regulatory authority (i.e., primacy) in allocating and developing water resources. Some US states have established programs to specifically address reuse, and some have incorporated water reuse into their existing programs. EPA, states, tribes, and local governments implement programs under the Safe Drinking Water Act and the Clean Water Act to protect the quality of drinking water source waters, community drinking water, and waterbodies like rivers and lakes.
According to INS regulations for irrigation and recreation reuse of TSE33, the value parameters tested for the TSE of the Zabol WWT plant are following the limits recommended in the standards for consumption as irrigation (except chlorine) and recreation projects.
Finally, the CWQI is a means to provide consistent procedures for Canadian jurisdictions to report water quality information to both management and the public. The CWQI value ranges between 1 and 100, and the result is further simplified by assigning it to a descriptive category in Table 4.
The results of CWQI software for analyzing the TSE of the WWT plant in the study area, as shown in Table 5 and Fig. 8, indicated its poor quality for drinking, and aquatic. While it is fair for livestock and marginal for irrigation. However, considering the purpose of this study for irrigation of the native plants, it met the criteria. Note that the input data set is based on the period of 2017–2019.
CWQI tets results for TSE of WWT plant in the study area.
The results of this section indicated the consideration of various parameters due to various regulations and demonstrated that the treatment technology upgrade was significantly better than those of urban miscellaneous water and agriculture water standards, indicating this system can be widely used for urban landscape hydration. Moreover, squeezing the sewage treatment process for being cost effective could be recommended considering the measurements of FC, BOD, and COD.
Optimal area suggestion for project execution
Considering three steps of wind erosion which are detachment, transportation, and deposition, the sand fixation methods have to be done in the detachment area to be more effective. Hence, the most advantageous regions for project execution were selected based on the factors of (a) discovering the dust origins, and (b) vegetation cover. Regarding the first concern, it was shown that the dry sediments of the Farah river43, and the presence of dunes between the two sand movements corridors in Sistan, namely Jazinak (near Zabol city) and Tasuki corridors (shown in Fig. 9), was increased the dust concentration in Zabol city37,44 while the agricultural lands, and other infrastructures such as roads, and irrigation canals developed in the area between Zahedan and Zabol city.
Locations and names of Hamuns lake and sand movement corridors in the study area © 2022 by Springer Nature Limited is licensed under Attribution 4.0 International (created by ArcMap 10.5).
Subsequently, based on a guide that 30% of vegetation cover has a significant effect on the process of soil detachment45,46, and soil protection in the desert areas47, the regions with less than 30% vegetation cover in the study area based on field observation was investigated and showed in Fig. 10. Field observation demonstrated that most areas along with the Jazinak sand corridor and Zabol city have 1–15% and 15–30%36, which are in the priority for stabilization.
The critical dust hotspot and dust origins in the study area © 2022 by Springer Nature Limited is licensed under Attribution 4.0 International (created by ArcMap 10.5).
The results are consistent with Abbasi et al.37, reported that the Hamun Baringak Lake plays a crucial role in the aeolian mobilization of sediments in the Sistan region because of the hydrological droughts that led to the gradual decline of the wetland vegetation cover. Notably, Jahantigh48, in the same study area, reported that the average forage yield of Aeluropus lagopoides in Hamun Hirmand lake in the condition of the water inflow and during drought, was estimated to be 8869 and 173 kg/ha, respectively. It can be explained by the effect of water presence on plant production and cover. However, the average of bare soil of Hamun lake was estimated to be 7.5% and 84.2% in the two periods of water inflow and drought, respectively48. It indicated the impact of dusty days. Therefore, the mentioned areas with the vegetation cover below 30% prioritized for stabilization techniques to dust reduction or mitigation.
The detailed field investigation of the land use and vegetation cover, as shown in Fig. 12, indicated the presence of native plants such as A. lagopoides and Tamarix spp. Based on Fig. 11, among the Tamarix genus, the three species of T. aphylla, T. stricta, and T.hispida were observed in the study area. T. stricta is a native species to Iran with benefits including, traditional therapeutic uses in Persian Medicine49,50. Also, the soil EC in the habitat of T. aphylla (15.70 mhos/cm) is almost the same as the control area (15.80 mhos/cm) in the depth of 0–30 cm; while the available potassium in T. aphylla habitat (460 mg/l) was also more than the control area (180 mg/l)51. Hence, the afforestation of Tamarix spp. has caused the addition of soil amendments and increased the clods.
The most land use/cover in the study area.
Consequently, the water requirement of the plants in the desert area consisting of T.aphylla, is reported in Table 6. The water requirement of T. stricta was estimated based on Table 6 to be 580 m3/ha for 500 plants no./ha with a vegetation cover of 10–30%.
Moreover, Fig. 12 shows the vast (50% more) soil coverage of T. stricta in the collar area compared to T. aphylla. Therefore, it is more appropriate to cultivate T. stricta than T. aphylla for the biological restoration of the region. Note that the introduced dust mitigation technique using TSE of Zabol WWT can play a specific role in the rehabilitation of soil cover in the mentioned area due to the low water need of native plants. Consequently, it has a significant impact on dust reduction in Zabol city.
The picture of (a) T. stricta and (b) T. aphylla in the study area.
Hence, based on the hotspots of dust origins in the study area, the most appropriate sites for the project executions of TSE were selected, as shown in Fig. 13. Investigations indicated that a total of 27,500 ha are suitable for the project excision. Hence, considering the water requirement of 500 m3/ha/year, TSE volume of 5.1 mcm/year, vegetation cover of below 30%, and other observations such as the soil coverage in the collar area, the native plant of T. stricta selected for the afforestation of 10,000 ha on the west part of Zabol. This region has the priority in stabilization due to companionship to the corridors with a vegetation cover of 16–30%.
Area suggested for the dust mitigation project execution by the application of TSE © 2022 by Springer Nature Limited is licensed under Attribution 4.0 International (created by ArcMap 10.5).
Cost analysis
Finally, due to the vast area of TSE application, the total of 27,500 ha, with the puprose of dust mitigation, the project execution costs must have been addressed. Hence, Fig. 13 shows the distance of Zabol city to Hamun Hirmand and Baringak lake for transportation calculation. Accordingly, the distance from Zabol to Hamun Hirmand and Baringak lake is 14 and 33 km, respectively. The whole area around Zabol city to Hammon Hirmand lake is cultivated lands; hence, the existing roads reduced construction costs.
The two main modes of transportation are trucks and pipelines. There are various pros and cons to both methods. Truck transportation is favored for low volume and short distances, while its costs rapidly increase for large-scale transportation. On the other hand, pipeline transportation is appropriate for large volumes, and long travel distances as it has a positive impact on reducing greenhouse gas emissions. Using pipelines also reduces noise, reduces highway traffic, and improves highway safety.
Based on the literature, the variable and fixed transportation cost components depend on the type of product shipped, design requirements, and other decisions related to facility planning. For the sewage sludge with a pH level of 7.0 ± 0.1; hence, a low-cost PVC pipe suggested. Moreover, for cost optimization, as the WWT facilities in the study area do not generate enough volume daily, it makes economical sense to store sewage for a few days to increase the shipped volume. However, reducing the storage to a single day condenses these investment costs drastically52.
It was estimated that the total costs for a facility-owned and rented single trailer truck with a capacity of 30 m3 to be $5.6/m3 and 7.4/m3/km, respectively53. Hence, the variable unit transportation cost along a pipeline with a capacity of 480 m3/day is estimated to be $0.144/m3/km. In despite of previous studies mentioning that it is more economical to use a pipeline rather than a rented single trailer truck if the volume shipped is greater than 700 m3/day, in the study area, it is more economical to use a facility-owned single trailer truck, while the shipped volume is 1200 m3/day due to the low cost of petroleum and very close distance of the suggested area.
Source: Ecology - nature.com
