Introduction

Agriculture drainage plays an important role due to its large volume and decentralized nature. Agricultural drains are one of the most significant sources of return water because of their large volume. Drains will have different characteristics depending on the location and the activity. The quality of water is a relative term that represents its physical, chemical, and biological characteristics and affects its use for a specific purpose1. Accordingly, the important qualitative parameters of agricultural drains include electrical conductivity, special ions (sodium, chlorine, and boron), nutrients (phosphorus and nitrogen), as well as toxins and pesticides. Various physical and chemical processes exist to remove these metals, including sedimentation, filtration, evaporation, ion exchange, membrane separation, solvent extraction, etc. However, these methods are not entirely effective for low metal concentrations are expensive, and cannot completely separate metals. In addition, some of these methods require expensive equipment. Recently, bio sorption has been promoted as an efficient and cheap process. Heavy metals can be removed from wastewater with inexpensive materials such as algae, wheat straw, and crab skin2,3. Inexpensive materials, especially agricultural waste, can create favorable environmental conditions during the treatment process. However, plants that are compatible with drainage water quality are more suitable.

Considering the presence of nutrients in agricultural drainage water for irrigation of crops, one of the factors that reduce their quality is the presence of various ions, which, if they accumulate in the soil, make it difficult for the roots to absorb nutrients. In recent years, a lot of research has been done in the field of using natural materials with the ability to absorb pollutants. Robertson and Merkel4 treated drainage effluent in a pond containing sawdust to the dimensions of 1*2.5*20 meters and found after analyzing the water that the concentration of nitrates increased from 4.8 to 1.04 mg. The use of wood sawdust bioreactors is recommended for reducing nitrate levels in drainage systems5. The research conducted at the University of Waterloo was conducted on the efficiency of the sawdust filter in removing suspended phosphorus (insoluble) in agricultural runoff derived from surface drainage systems; As a result of its high porosity, permeability, surface roughness and plate-like particle structure, sawdust is observed to be very efficient in physical filtration of phosphorus particles6. In a study, treated wastewater using a three-layer combined biological filter (charcoal, ion exchange, coarse sand mixture) for agriculture in Baghdad was investigated. The results showed that BOD and COD decreased by more than 60%7.

Using a multilayer filter with artificial straws for gray water treatment, research was conducted to evaluate the performance of the combined system. It was found that artificial reeds account for more than 60% of the efficiency of the system8. In a study, the effectiveness of the combination of microfiltration and ion exchange was investigated in the treatment of wastewater from dairy factories that contained high amounts of organic matter and salinity, and it was concluded that organic matter and salinity were reduced by 95% and 83%, respectively9.

A study conducted on a wetland with a subsurface flow filled with a bed of sawdust to treat agricultural runoff has found that the efficiency of this system in removing total nitrogen is approximately 53%10.

In a study, the drainage channel of agricultural irrigation water was treated by absorption methods using sawdust. The results showed that the use of sawdust-activated carbon led to the removal of unwanted odors from the KSD water channel and increased salt in the channel water, which was increased by EC, TDS, heavy metals (Na, Mg, Ca, Cl, So4), and SAR or pH decreased in irrigation standards11. The possibility of water purification for agricultural irrigation drainage channels including the main agricultural irrigation water channel and the main KSD agricultural irrigation water channel contaminated with sewage using multimedia filtration technology with (three layers) (commercial coal, coarse sand, and mixed ion exchange) in Three flow rates (0.24, 0.32, 0.48) m2/hr were investigated for contact time (two hours). A comparison of the results showed that the best removal for both channels was obtained after contact time 2 h12. A constructed wetland with a non-permeable surface flow (SFCW), which treats agricultural drainage water in northern Italy, was investigated to obtain information on the potential for effective pesticide reduction. A mixture of imidacloprid, fungicide dimethomorph, and herbicide glyphosate herbicide was used, simulating a rain event13. Studies show that the biological adsorption process can be used as an efficient filter along with other conventional purification processes to remove pesticides in urban water treatment plants14.

In the direction of sustainable development and preservation of the environment, especially in rivers, the control of runoff coming from agricultural lands into rivers is of particular importance. Runoff from irrigation and drainage networks should be efficiently managed so that it is disposed of or optimally removed from them. Considering the extensive extent of lands covered by drainage networks and the necessity for their development, as well as their growing trend, it is expected that the amount of drainage will grow, and planning for their utilization is necessary. Records and studies available indicate that comprehensive and documented research in the field of methods of purification and recycling of agricultural drains has not been conducted, and only a few studies have been conducted in the laboratory. The conduct of applied research in field pilot conditions to test the feasibility of treating and reusing drain water is an important research priority. Considering that the present study was conducted to evaluate the efficacy of biological filters with different substrate types in the treatment of agricultural wastewater in Khuzestan province, located in the south of Iran. Conducting practical research on the efficiency of biological filters in the purification of pollutants caused by agricultural drainage water, in particular residues of chemical fertilizers (nutrients) from drains, and engineering its design and operation can be a significant achievement in wastewater treatment.

The contributions of this article are as follows:

  • Investigating the effectiveness of biological filters with various substrates for treating agricultural wastewater and reusing them in agriculture.

  • Evaluating the efficiency of four types of biological filters for the treatment of agricultural drainage water with different retention times.

  • Investigating the relationship between retention time and removal efficiency of nutrients and agricultural toxins.

  • Introducing the best treatment to remove agricultural toxins.

Materials and methods

Experimental site

This study was carried out in Khuzestan province, located in the south of Iran using the drainage water from Amirkabir and Mirza Kochak Khan’s sugarcane agro-industries (March to September 2020) (Fig. 1). Amirkabir and Mirza Kochak Khan’s sugarcane agro-industries is one of seven companies to develop sugarcane agro-industries in the southern region of Khuzestan. Sugarcane farms discharge 25 m3/s of drainage water into Naseri Wetlands. It is located approximately 17 Km north of Khorramshahr city at latitude 303805.94′ N and longitude 480759.86′ E. The source of water used in the sugarcane fields is the Karun River. In terms of salinity, this drainage water is in the range of 6 dS/m, but it has limitations due to residues of toxins and chemical fertilizers as well as heavy metals15. Due to the limited quality of the effluent, it is currently discharged into surface water sources, causing pollution and nutrient contamination. In Khuzestan province, the climate is classified as semi-desert in low-latitude areas, with an average annual temperature exceeding 18 degrees Celsius, dry summers, and fog in winter. According to a statistical analysis of the existing meteorological stations, Khuzestan and its adjacent coasts are among the warmest regions in the world. Since the average annual temperature ranges between 24 and 30 °C, there is no equivalent in terms of heat in its latitudes. The average annual rainfall is 358 mm.

Fig. 1
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Geographical location of Mirza Kochak Khan’s agriculture and industry [https://earth.gosur.com/?gad_source=1&gclid=Cj0KCQjwsaqzBhDdARIsAK2gqnd03_pn48acVn6ZbveuftHA7j1ZerZlUtps300416zbuK5zklZ3BQAaAthaEALw_wcB].

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Construction of a drainage water treatment system (bio filters)

This project was implemented on a site measuring 60*12 square meters and included four pools (bed treatment) measuring 1*5*10 meters in parallel. The implementation map of the plan is shown in Fig. 2. Each pool was insulated using a geomembrane cover. Before building a field pilot, synthetic adsorbents were pre-tested according to the concentration of harmful substances in the used drain, and the amount and speed of absorption of the filters were determined in the laboratory. For this purpose, four parallel ponds were constructed near the drain. According to the research records and the abundance of materials in the area, substrate treatments included straw and wheat stubble, sawdust, rice husks, and cotton stalks (Fig. 3). Table 1 presents the physical characteristics of the investigated filters.

Fig. 2
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The implementation map of the plan.

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Fig. 3
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The materials used as biological filters.

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Table 1 A description of the filters used in the research.
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About 80% of the pool space is occupied by filters. Over the filters, a layer of permeable sand and gravel with an average diameter of one centimeter was placed to a depth of 20 cm. After determining the characteristics of the bio-filters, including the hydraulic conductivity coefficient and preliminary field testing, the inlet flow rate of the filters was determined. Bio-filters are a biological method for water and wastewater treatment with different types. The common feature of all of them is the presence of a substrate for the stabilization and growth of microorganisms, which adheres to the substrate in the form of a biological layer (biofilm) and plays the main role of biological purification. These filters are divided into different groups according to the type of material and the way they are placed. The types and materials of the substrates can be different (various materials such as plastic and PVC, wood, wheat straw, sand, etc.), and four different types of substrates were used in this study. The mechanism of pollutant purification in this system is as follows:

  • Physical processes, including sedimentation, and filtration.

  • Biological processes: including consumption of pollutants by microorganisms fixed on the substrate (biofilm).

  • Chemical processes, including absorption, oxidation, and reduction reactions that contaminated water or sewage, upon entering the system, passes through a substrate covered with microorganisms (biofilm), during which the pollutants in the water include physical, chemical, organic substances, nutrients, etc. They are refined through three types of processes.

Through a drainage pipe installed in the bottom of the pool, purified water was drained. A flexible pipe was connected to the drainage pipe at the outlet of the bio-filter. Due to the flexibility of the bio-filter output, it was possible to control the flow rate and change the load drop by changing the output height. It was tried to consider the design and construction of the investigated system as economical in terms of its potential for implementation by farmers in the future. From the beginning of spring 2020, after the completion of the executive operation of the pilot construction, drainage water entered the pilot construction and data was collected.

The selection of materials used as biological filters

A major factor in the selection of materials used as biological filters is the type of material, which has been investigated and found to be capable of absorbing materials. Next, the type of material was examined based on its abundance in the region so that it could be easily prepared and utilized. In choosing the type of material, the porosity of the material was also taken into consideration, and only materials with high porosity were used to create a larger surface area for water contact. The polluting material in the drainage water has also been considered in the selection of the type of material. The polluting material is selected based on its type. The majority of the research conducted in this field has been conducted in a laboratory where the incoming water for purification was artificially prepared and contained high levels of pollutants. However, the real conditions on the farm resulted in different results and the amount of substances in the incoming water was much lower, which made controlling the situation much easier. In this project, different types of purifying materials were selected and agricultural wastes such as rice paddy husk, straw, and wheat stubble, cotton stalks, and sawdust were used as filters. A major objective of this research was to assess the possibility of reusing purified drain water in agriculture according to its quantitative-qualitative condition and its qualitative fluctuations.

A description of the mechanism of action of the proposed bio-filters

In the beginning of the system, the sand and gravel layer acts as a filter, removing garbage and suspended substances from incoming drainage. It also prevents suspended particles from entering the bio-filter and disrupting the purification process (biological absorption and decomposition). In a biological filter system, physical, chemical, and biological processes are used to purify water. Sedimentation and filtration are physical processes. During a six-month period, weekly samples were collected from the inlet and outlet of the pilot (after the purification process) to evaluate the effectiveness of the research system. Qualitative factors under investigation include agricultural toxins (Atrazine, Roundup, Paraquat and 2, 4-D) and nutrient elements and compounds (nitrate, nitrogen, phosphate, and total phosphorus), which are provided in Table 2 for assessing the quality of the incoming effluent.

Table 2 The concentrations of qualitative factors in the incoming drainage water of the study area.
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In general, the reasons for choosing the investigated factors were as follows:

  • The presence of the desired pollutants in the researched drainage: based on the investigations, the residues of agricultural toxins and nutrients are considered the most important pollutants of the researched drainage.

  • The extent of their negative effects on the quality of the receiving water resources of these drains: considering that the drains are discharged into surface water sources (rivers), agricultural toxins and nutrients have destructive effects on the quality of these receiving sources. In particular, agricultural pesticides cause the death of aquatic life and nutrient elements increase the growth of algae.

  • Agricultural poisons: selected poisons have been selected from among the most used poisons in the study area.

  • The cost of conducting experiments: due to the financial limitation of the research, an attempt has been made to select the most important factors and manage costs.

Initially, the system was launched with a delay of two days, and a month was spent testing its compatibility and stability. In biological purification systems, it takes some time for the system to reach a stable state. Essentially, the microorganisms responsible for wastewater treatment become accustomed to the quality conditions and hydraulic load (adapt) and the population of microorganisms, as well as the biological layer, are formed and reach a stable level of efficiency and performance in the treatment process. Upon reaching stability in the output results, the duration was gradually increased to 5 and then to 10 days. Weekly sampling and testing were performed on the input and output of the treatments. The input and output of treatments were sampled and tested at weekly intervals throughout any retention period, and the results of both sampling and testing were analysed. The present research was conducted in the form of a split plot in the form of a completely random design and with SPSS software, and the comparison of means was done by Duncan’s multi-range test, and the appropriate statistical combination was obtained by using tables and graphs. During this study, four different types of bio-filters, including sawdust, cotton stalks, wheat straw and stubble, and rice husks, were investigated as main factors and three retention times, 2, 5, and 10 days, as sub-main factors.

Results and discussion

Effects of biological filters and retention time on nutrient composition

The variance analysis of the obtained information indicated that the main factor of filter type and the sub factor of retention time had a significant effect at the probability level of 1% on the amount of nitrate, total nitrogen, phosphate, and total phosphorus. Furthermore, the interaction between filter type and retention time was significant for nitrate, total nitrogen, phosphate, and total phosphorus (Table 3). Examining the present results with similar study30 implemented with a similar system for the purification of Moghan agricultural sewage shows that the system used in Khuzestan is more effective in removing total nitrogen and total phosphorus, 5% and 8%, respectively. The reason can be the relatively warm temperature of the Khuzestan region compared to Moghan30. Yargholi and Kanani15 achieved an efficiency equal to 80% in removing nitrogen compounds, 83% in removing phosphorus compounds and an efficiency of more than 90% for removing phosphorus toxins, which is more satisfactory compared to the present results. But, considering that artificial reed requires a large area compared to bio-filter, the stated efficiency needs more investigation.

Table 3 Variance analysis of nutrient compounds under the influence of different treatments.
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Examining the results shows that in the biological filter, which is a type of attached biological process (formation of biofilm on the substrate), the performance for removing agricultural toxins and nutrients (nitrogen and phosphorus compounds) is much higher than the biological processes of suspended growth (such as Activated sludge) in which, unlike the attached growth methods, the microorganisms responsible for the treatment are suspended in the liquid (sewage)31,32.

Microorganisms responsible for removing nitrogen and phosphorus compounds have a slow growth rate and it takes more time for their population to increase effectively33. In attached growth processes, because the microorganisms are connected to the surface of the substrate, they are not separated from the system along with the outflow, and this causes the gradual increase of bacteria responsible for removing nitrogen and phosphorus and their species dominate34. In suspended growth systems, these conditions are not provided, microorganisms leave the system along with the outflow, and there is not enough opportunity for nitrogen and phosphorus-purifying bacteria to multiply and dominate. The substrate used in biological filters, as an absorbent material, has a significant contribution to the removal of nitrogen compounds, phosphorus, and agricultural toxins. In addition, as a source of carbon, it is available to purify bacteria, and providing the required carbon causes the continuation of the purification process and increases the efficiency35,42.

Influence of biological filters on nutrient removal efficiency

Due to the use of chemical fertilizers upstream, the incoming water contains a high level of nitrates, thus making it necessary to treat all water drains. Based on the results of the study, it was found that nitrate levels were high in the incoming water, and that all biological filters were effective in reducing nitrate levels. According to the results, the treatment of the studied filters was relatively effective. In examining the effect of filter type on filtration efficiency for nitrate, total nitrogen, phosphate, and total phosphorus in incoming effluent, it was determined that the average quality improvement (for three periods of 2, 5 and 10 days) for rice husk, straw and stubble, cotton stems and sawdust for nitrate was 44.31, 42.69, 49.73 and 52.68%, respectively (Table 4).

Table 4 The independent effects of biological filters and retention times on nutrient compounds (%).
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Based on the results of the study, the straw and stubble treatment was slightly less effective for reducing nitrate than the other three treatments, and of the current treatments, sawdust was the most effective at reducing nitrate by 52.68%. In terms of total nitrogen, this performance equates to 40.77%, 39.02%, 45.16% and 49.60%, respectively, for rice husk, wheat straw and stubble, cotton stalk and sawdust. Similar to the nitrate factor, for the total nitrogen factor, straw and stubble have the lowest yield with the slightest difference, whereas sawdust has the best yield with a relatively significant difference and equals 49.60. According to the results for the phosphate factor, rice husk, wheat straw, cotton stalk, and sawdust achieved 39.16%, 36.37%, 42.54%, and 44.57% efficiency, respectively. The sawdust treatment with 44.57% has the highest efficiency as compared to straw and stubble with 36.37%. The performance pattern for total phosphorus is similar to that for total nitrogen, and straw and stubble have the lowest efficiency (34.68%) and sawdust has the highest efficiency (46.50%). This factor showed that rice husk and cotton stalk treatments had 35.53% and 40.90% efficiency removal, respectively. Based on the results of the filter material treatments, it can be concluded that sawdust is more efficient and performs better than the other three filters16,17,18. Bio sorbents are effective in absorbing nitrate from aqueous solutions in the laboratory environment19,20. The results of determining the appropriate amount of adsorbent to have maximum efficiency showed that the most appropriate amount of adsorbent is 1 gr/40 ml of solution. As the amount of adsorbent decreases, the efficiency of the adsorbent decreases. Nitrate uptake by sawdust decreased from 4.8 to 1.04 mg/L when sawdust was used as a medium for nitrate uptake21,22.

Effects of different retention times on nutrient removal efficiency

Results of the analysis of effluent from biological filters with a residence time of 2, 5 and 10 days are presented in Table 4. Nitrate, total nitrogen, phosphate, and total phosphorus levels decrease as residence time increases. Different retention times have different effects on the removal efficiency of nitrate, total nitrogen, phosphate, and total phosphorus. It was observed that the retention time has a direct relationship with the amount of nutrients by examining the increase in the retention time and the removal percentage that corresponds to it. In other words, in the retention time used in this research, with the increase in retention time, the average nitrate, total nitrogen, phosphate and total phosphorus in different filters decreases and the percentage of removal efficiency of nutrient compounds increases (Table 4). So that the highest removal percentages of nitrate, total nitrogen, phosphate and total phosphorus were 69.37, 66.51, 54.64 and 53.39%, respectively, for the 10-day period.

The difference in removal efficiency between the remaining times was significantly different from one another. There was an increase in removal efficiency of nitrate, total nitrogen, phosphate, and total phosphorus under the retention time of 10-days compared to the retention time of 2 days by 52.18, 69.61, 59.63 and 61.05, respectively. Overall, the results indicate an increase in removal efficiency compared to the retention time in the system. Results indicate that by doubling the retention time for each of the investigated factors, the removal efficiency did not double, but increased by 40, 50, 17.58 and 21.87% compared to a retention time of 5-days (Table 4). The results of this study indicate that removal efficiency and absorption capacity are directly related to retention time. The goal of this research and similar studies is to identify the optimal retention time, as a short retention time will not result in the appropriate performance, and a long retention time may result in ammonium accumulation23,24. Since increasing the retention time allows for better performance of the reactors, it is expected that increasing the retention time will improve the reactors’ performance in removing nitrate.

Rivas et al.25 showed that the efficiency of nitrate removal by biological reactors increases with increasing retention time. In addition, the rate of nitrate removal increases with increasing retention time. Because more time allows the bacteria to decompose the organic substrate, leading to higher nitrate removal efficiency, which is consistent with the results of the present study. Increasing the retention time provides a longer period of time for microbes to interact with each other26,27.

Interaction effects between biological filter type and retention time in nutrient removal efficiency

Table 5 illustrates the interaction effects of filter types (rice husk, straw and stubble, cotton stalk and sawdust) in removing nitrates, total nitrogen, phosphates, and total phosphorus pollutants over the retention time of 2, 5 and 10 days. In the evaluation of the filter’s ability to remove nitrate from the incoming wastewater, it has been found that the nitrate removal efficiency varied from a minimum of 17.94% during the retention time of 2 days with straw and stubble treatment to a maximum of 74.03% during the retention time of 10 days with sawdust treatment. After 10 days, straw and stubble filters were able to remove 74.03% of nitrate, while cotton stem filters lowered their performance to 71.23% and ranked second. The results indicate that the sawdust filter had a higher removal percentage compared to the other treatments at a retention time of ten days28,29,43, whereas the cotton stem treatment had a removal percentage of 68.31 after the sawdust treatment with a retention time of 10-days and was ranked highest in terms of nitrogen removal.

Table 5 The interaction effect of different filters at different retention times on nutrient compounds (%).
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Based on the results, the phosphate removal efficiency ranged from 18.34% in the straw and stubble treatment for a retention time of 2 days to 58.31% in the cotton stem treatment for a retention time of 10 days. With a removal efficiency of 57.97% during retention time of 10 days, the cotton stalk filter ranked first in phosphate removal, and the sawdust filter ranked second after the cotton stalk filter. A removal efficiency of total phosphorus was observed from a minimum value of 16.84 in two days in the straw and stubble treatment to a maximum value of 61.85 in ten days in the sawdust filter. Based on the results of this study, the sawdust filter showed a higher removal rate than the other treatments over a period of retention time of 10 days. Cotton stem treatment ranked second in total phosphorus removal after sawdust treatment after 10 days’ retention period with 56.91% removal, which is consistent with similar research results36,37,44.

In similar studies, the effectiveness of the system in removing total nitrogen from agricultural runoff was investigated38,39,40,45. In the natural state, the efficiency with a retention time of 5 days is equivalent to 65%, and if organic materials are added and the retention time is increased to 15 days, the performance of the system can be increased to 100% (32, 46). The studies showed that according to the 75% efficiency of removing nitrogen compounds during the ten-day retention time and without adding organic substances, the researched system has an acceptable efficiency, which is in agreement with the results of the present study36.

The effects of biological filters and retention times on agricultural pesticides

The results indicate that the effects of retention time on the levels of Atrazine, Tofordi, Paraquat, and Rundap at the level of probability of 1% are significant. According to the results, the main factor of filter type had a significant effect on Atrazine poison at a 5% probability level, while their influence on Tofordi, Paraquat, and Roundup was significant at a 1% probability level. Based on the results of the statistical analysis, the interaction effect of filter type and retention time on the amount of Atrazine was significant at 5% probability, which is in agreement with similar studies40,41. Alternatively, the amount of Tofordi, Paraquat, and Roundup was significant at the 1% level (Table 6).

Table 6 Variance analysis of agricultural pesticides under the influence of different treatments.
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The effect of biological filters on the removal efficiency of agricultural runoff toxins

Each of the biological filters was effective in removing the agricultural toxins Atrazine, Tofordi, Paraquat, and Rundap from the agricultural drainage (Table 7). Based on the comparison of the averages, the results indicated that various types of biological filters reduce the levels of Artesine toxin. Rice paddy biological filters reduced Artesine poison in drain water by 61.76%, compared to other biological adsorbents, and were recognized as the most suitable biological adsorbent for Artesine poison. In comparison with straw and stubble, cotton stem sawdust biological filters, this biological filter reduced the amount of Artesine toxin in drainage water by 2.28, 2.62 and 1.74, respectively. Following rice paddy, sawdust filter with 60.70% reduction and straw with 60.38% reduction showed the highest reduction. Based on the positive effects of rice paddy biological filters in reducing Artesine toxins, especially in large quantities, it is feasible to use these biological filters to purify drain water of Artesine toxins (Table 7).

Table 7 The independent effects of biological filters and retention times on agricultural pesticides (%).
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The results of the comparison of the average independent effects of biological filters on the removal efficiency of Tofordi indicated that the sawdust filter reduced Tofordi poison with the highest removal efficiency of 56.20% compared to other biological filters. The effectiveness of this biological filter was significantly greater than the performance of the cotton stem and rice paddy biological filters, respectively, in reducing the amount of Tofordi toxin in drain water by 3.04, 9.43, and 6.99%, respectively. Following sawdust, cotton stalk filter reduced by 55.51%, rice straw and paddy reduced by 53.46%. The sawdust filter has had the greatest effect on the reduction of the Tofordi poison parameter (Table 7). Compared with other biological adsorbents, cotton stem and sawdust biological filters both reduced Paraquat poison levels in drain water with target efficiencies of 55.56 and 55.16%, respectively. These biological adsorbents have been proven to be the most effective in purifying Paraquat poison from drain water. In comparison to cotton stems and sawdust, rice paddy, straw and stubble showed a reduction of 53.39%, followed by rice straw and paddy at 51.61%. With the acceptable efficiency of the cotton stem filter and sawdust in reducing Paraquat poison, especially in large amounts, it is possible to apply these biological filters to the treatment of Paraquat poison in drains. According to the study, sawdust biological filters reduced 55.46% of Roundup poison in effluents more than other biological filters, while cotton stalks, rice paddy, straw, and wheat stubble decreased 54.05, 51.69 and 49.76% of Roundup poison, respectively. In general, sawdust biological filters were effective in reducing Roundup poison, while straw and stubble biological filters were less effective in removing Roundup poison.

Effects of different retention times on the removal efficiency of agricultural pesticides

The retention time is an important parameter for the proper use of an adsorbent in practical applications and it represents the speed at which the adsorbent absorbs among the various parameters that should be considered. The trend of the changes in the removal efficiency of Atrazine, 2, 4-D, Paraquat, and Roundup for 2, 5 and 10 days is illustrated in Table 7. It is evident that different time delays had different effects on the removal efficiency of Atrazine, 2, 4-D, Paraquat and Roundup toxins. Therefore, since increasing the retention time creates enough opportunity for the reactor to perform better, it is expected that increasing the retention time will increase the efficiency of the reactors.

It was observed that the highest removal efficiency was obtained with each of 2, 5 and 10-day retention times. There was a significant difference between the removal efficiency for each of the remaining times. So that the highest removal efficiency of Atrazine, 2, 4-D, Paraquat and Roundup toxins was observed with the amount of 90.60, 80.84, 87.37 and 86.92% in the 10 days’ retention period and then in the 5 days’ retention with acceptable removal efficiency equal to 70/32, 63/11, 55/75 and 53/87 had a better performance in removing Atrazine, Topodi, Paraquat and Roundup toxins. The removal efficiency of Atrazine, 2, 4-D, Paraquat, and Randap toxins were 76.44, 75.4, 80, and 79.9%, respectively, under a 10-day retention time, compared to a 2-day retention time. A removal efficiency of 22.38, 21.9, 36.9 and 38% was observed after 10-day retention time compared to 5-day retention time. According to the results, the removal rate of Atrazine, 2, 4-D, Paraquat, and Randap toxins increased with an increase in retention time from 2 days to 10 days (Table 7).

Interaction effect of biological filter type and retention time on nutrient removal efficiency

The Table 8 presents the comparisons of the effectiveness of different filter types (rice husks, straw and stubble, cotton stalks and sawdust) in removing the agricultural toxins Atrazine, Tofordi, Paraquat, and Roundup from agricultural drains at retention times of 2, 5 and 10 days. Using the sawdust filter for purifying Atrazine from incoming wastewater shows that the amount of Atrazine decreases between the minimum value of 20.84% in the two-day period and the maximum value of 91.73% in the 10-day period. This filter removed the greatest amount of Atrazine in retention times of 10 days as compared to the other three filters. The removal rate of Tofordi ranged from 18.66% in the retention times of 2 days in rice paddy to 84.27% in sawdust after a retention times of 10 days. Based on the results, the sawdust filter had a higher removal percentage than the other treatments during the retention time of 10 days. Regarding the removal efficiency of Tofordi, the cotton stem treatment in the retention time of 10 days with an 81.14% removal rate ranked second after the rice paddy treatment.

Table 8 The interaction effect of different filters at different retention times on agricultural pesticides (%).
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The trend of changes in the removal efficiency of Paraquat was similar to that of Atrazine and Tofordi. Thus, the highest removal efficiency of Paraquat (89.81%) was observed in the sawdust filter in the retention time of 10 days, whereas straw and stubble treatment with 2-days of retention time with 16.16% had the lowest removal efficiency of Paraquat from the incoming effluent. Roundup removal efficiency ranged from a minimum value of 15.56% in the retention time of 2-day and in the straw and stubble treatment to a maximum value of 88.46% in the retention time of10-day in the sawdust treatment. Rice paddy filter with a removal efficiency of 86.41% took second place following sawdust treatment for its effectiveness in removing Paraquat poison. The sawdust filter performed the best in removing Roundup from incoming drainage water in in the retention time of 10 days when compared to other filters and in different retention periods. According to the results of filter material treatments at different retention times, sawdust performed better and was more efficient than rice husk, straw, stubble, and cotton stalk for each of the retention times of 2, 5, and 10 days. It is important to consider this point when choosing and proposing a filter for each region. A suitable filter must be selected for each region in every respect.

Compared to other filters and retention times, the sawdust filter demonstrated a good ability to remove agricultural pesticides and nutrient compounds in a retention time of 10 days, and significantly reduced the amount of these parameters in the incoming drainage water. In comparison to other types of filters, this type proved to be much more effective in reducing the levels of agricultural pesticides and nutrient compounds. Due to the availability of sawdust throughout the country, the results of this research are important, and as a general result, this filter can be cited and used as a reliable substrate with satisfactory efficiency compared to other filters. In light of the type of filters used, it is important to note that despite sawdust’s relative superiority, the other filters also possess acceptable efficiency with some slight differences. According to the geographical location and availability of the desired materials, they can also be used as substrates in biological filters for drain purification.

Conclusion

The results of the present research demonstrated that the use of biological filters with different retention times may be a suitable solution for removing nutrients and agricultural toxins from agricultural drainage water. In accordance with the results, biological filters were capable of removing nutrient compounds and agricultural toxins at different retention times. By the retention time increases, the average of nitrate, total nitrogen, phosphate, and total phosphorus decreases. Different retention times result in different removal efficiency of nitrate, total nitrogen, phosphate, and total phosphorus. Based on an analysis of retention time and removed efficiency of nutrients, retention time is directly related to nutrient removal. In other words, in this research has been found to increase with increasing retention time, decreasing average nitrate levels, total nitrogen levels, phosphate levels, and total phosphorus levels, and resulting in increased removal efficiency of nutrient compounds. In comparison with other biological filters used to reduce nitrates in drain water, straw and wheat stubble had the highest removal efficiency. In comparison to the initial amount of nutrient compounds in the inlet drainage water, the removal efficiency for nutrient compounds in biological filters decreased dramatically after the filters were utilized, and this decrease increased with the increasing retention time. Therefore, compared to other biological filters and retention times, the sawdust biological filter showed the highest removal efficiency with a retention time of 10 days. The results indicated that the sawdust biological filter had a significant effect on reducing nutrients and agricultural toxins in comparison to other filters. According to the results of filter material treatments at different retention times, sawdust performed better at retention times of 2, 5, and 10 days than rice husk, straw, stubble, and cotton stalk. In selecting and proposing the type of effective filter for each region, this point should be noted. The filter selected must be suitable in all respects for implementation in each region. The present study demonstrated that the sawdust filter had a good performance in removing agricultural toxins and nutrient compounds in a retention time of 10 days as compared to other filters and retention times, and significantly reduced the amount of these parameters in the incoming drainage water and this type of filter was much more effective in reducing the parameters of agricultural toxins and nutrient compounds. For future directions, long-term retention periods, different seasons, and biological filters and their comparison in the form of biochar should be investigated to get a better view.