Effectiveness of biopolymer coagulants in agricultural wastewater treatment at two contrasting levels of pollution

Agricultural soils erode by wind and rain and the sediments and nutrients end up by surface flow to watercourses. Agricultural pollution is a major environmental problem causing nutrient and organic pollution, increase in turbidity, excessive sedimentation, and biodiversity loss in aquatic ecosystems [1, 2, 3, 4]. Productivity in many aquatic ecosystems, especially in freshwaters, is often limited by phosphorus, although nitrogen limitation is not uncommon [5]. Thus, reducing phosphorus loading from agriculture is often the key to limit excessive eutrophication [6]. Despite the pervasiveness of the problem, there has been relatively few major advances in mitigation of agricultural pollution and agriculture continues to be one of the most significant polluters of waterbodies [7]. In addition, the degradation of agricultural soils and predicted global phosphorus crisis [8] places urgent needs to find ways to retain and recycle the nutrients lost from the soils.

Iron- and aluminum salts and synthetic polymers (e.g. polyacrylamides) have been traditionally applied for removal of phosphorus, suspended solids and other contaminants from wastewaters by coagulation/flocculation process [9, 10, 11]. However, the metal content and potential toxicity of these coagulants makes the recycling of the created sludge and retained phosphorus difficult in agriculture. Biopolymers (e.g. cellulose, chitin, tannin, starch) are polymeric biomolecules produced by living organisms. Several biopolymers can be chemically refined to work as cationic or anionic coagulation/flocculation agents for wastewater treatment [11, 12]. Compared to traditional synthetic polymers, the advantages of biopolymers are their lack of metals, biodegradability and non-toxicity, which makes the recycling of retained nutrients easier [10, 11, 13]. Several studies have tested the coagulation and flocculation properties of biopolymers to organic [14, 15, 16, 17, 18] and inorganic [19, 20, 21, 22, 23, 24] contaminants and at different levels of pollution, pH and dosage. Especially, the coagulation and flocculation properties of chitosan are rather extensively studied [13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26]. However, the vast majority of these experimental studies have simultaneously tested only one biopolymer and used synthetic and homogenous wastewater in the experiments [17, 19, 20, 21, 22, 27]. Only few studies exist on the effectiveness of biopolymer coagulants in treatment of polluted natural surface waters [28].

In this study, we used jar-testing to evaluate and compare the efficiency of tannin, starch and chitosan coagulants in phosphorus, turbidity and total organic carbon (TOC) removal from agricultural wastewaters. To our knowledge, this is the first study to test the applicability of biopolymer coagulants in treatment of natural agricultural wastewaters from diffuse agricultural pollution.

We collected water samples from two study sites that are influenced by agricultural land use and diffuse pollution. The sites are located in municipalities of Ruukki (64°38′, 25°1′) and Tarvaala (62°40, 25°19′) in Western and Central Finland, respectively. The agriculture in the area is mainly focused on dairy and meat production and the fields are used as a pasture or for growing feed to livestock. In Ruukki, we collected the water samples from drainage ditch (catchment area: 0.18 km²) that drains through pasture fields. In Tarvaala, we took the samples from man-made wetland (catchment area: 1.2 km²) that is used for treatment of agricultural runoff waters. The samples from Ruukki were collected in winter (February) and from Tarvaala in spring (April) of 2018.

We tested the effectiveness of the biopolymer coagulants in treating the agricultural waters by using jar-tester (Phipps & Bird, PB-900 series programmable JarTester, Fig. 1) with 1 L beakers. Each beaker contained 1 L of agricultural wastewater and the water temperature during the experiments was around 20 °C. The pH in Ruukki drainage ditch and Tarvaala wetland water samples was near-neutral (6.9 and 6.6, in drainage ditch and wetland, respectively). The performances of several biopolymer coagulants have been observed to be inert to variation in pH [22, 30] or have their best performance in slightly acidic [24, 25, 31] or in neutral conditions [14, 19, 20, 24, 32].

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The biopolymers were diluted to 1% active content solution. Tannin and starch were in liquid form and diluted with tap water. Chitosan powder (1 g) was dissolved with 10 mL 0.1 M HCL and heated and stirred with magnetic stirrer for 2 h. Finally, the dissolved chitosan was mixed with 90 mL of tap water to obtain 1% solution. We tested 7 different doses (0, 1, 2, 3, 5, 8, 10 mL/L) of each biopolymer coagulant and each dose was replicated twice. We measured residual total phosphorus concentration, water turbidity and total organic carbon (TOC) and mean value from the two replicates was calculated. After the addition of the biopolymer coagulants, the beakers were stirred at high velocity (200 rotations per minute) for 20 s and then stirring was decreased (40 rotations per minute) for 5 min. Finally, the treated water was allowed to settle for 15 min with no stirring. Water samples were collected with pipette from upper part of the beaker. The water quality variables were analyzed by using standard methods in FINAS accredited laboratory of SYKE (https://www.finas.fi/Documents/T003_M38_2018.pdf).

In the agricultural wastewater of Ruukki drainage ditch HTH, HTG (tannins) and chitosan performed very similarly (Fig. 2). For HTH the best reductions of total phosphorus (95%, from 320 to 22 µg/L) and turbidity (98%, from 145 to 2.8 FNU) were achieved at dose of 10 mL/L. However, at doses of 2–3 mL/L the phosphorus and turbidity reductions were already close to maximum reductions (86–90% for phosphorus and 95–96% for turbidity) and increasing the coagulant dose did not improve the residual water quality appreciably (Fig. 2). HTG had similar reductions at dose of 10 mL/L for phosphorus (90%, from 180 to 20 µg/L) and turbidity (97%, from 68 to 2.4 FNU) as HTH but almost equal residual water quality was achieved already at doses of 2–3 mL/L (81–87% for phosphorus and 93–96% for turbidity) (Fig. 2). Chitosan had the best reduction of phosphorus (89%, from 180 to 19 µg/L) and turbidity (97%, from 68 to 2.3 FNU) at doses of 8 and 5 mL/L, respectively. Similarly to tannins, the reductions were close to maximum values already at doses of 2–3 mL/L (83–88% for phosphorus and 93–96% for turbidity).

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Total organic carbon (TOC) did not reduce relatively as much as turbidity and total phosphorus. Maximum reductions were achieved at 3 mL/L for HTH (26%, from 33 to 24 mg/L) and HTG (15%, from 26 to 22 mg/L) and 5 mL/L for chitosan (27%, from 26 to 19 mg/L) (Fig. 2).

Starch coagulants had clearly different performance compared tannin and chitosan. In general, the best performance was achieved with 1 mL/L dose in Ruukki drainage ditch water, except for PP3435x10 that reached the best performance at dose of 2 mL/L (Fig. 2). The best reductions of phosphorus were 64% (from 230 to 84 µg/L), 80% (from 320 to 64 µg/L) and 67% (from 230 to 76 µg/L) for PP2545, PP3454x10 and CGKT, respectively. The best reductions of turbidity were 80% (from 148 to 31 FNU), 87% (from 145 to 19 FNU), 82% (from 149 to 28 FNU) and of TOC 14% (from 32 to 28 mg/L), 20% (from 33 to 26 mg/L), and 19% (from 32 to 26 mg/L) for PP2545, PP3454x10 and CGKT, respectively. At larger doses the performance was significantly poorer and TOC substantially increased (Fig. 2).

The starch (PP3545) coagulant and chitosan did not produce flocs in less polluted Tarvaala wetland water and the residual water quality was deteriorated, especially with starch (Fig. 3). Tannin coagulant (HTG) only formed flocs at doses of 5–8 mL/L and at 10 mL/L the solution restabilized and residual water quality decreased (Fig. 3). The best performance with HTG was achieved at dose of 5 mL/L, where total phosphorus concentration reduced 70% (from 71 to 22 µg/L), turbidity 82% (from 7.5 to 1.4 FNU) and TOC 22% (from 18 to 14 mg/L) (Fig. 3).

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Agricultural water pollution and loss of nutrients, especially phosphorus, is a major environmental problem that urgently needs sustainable solutions. Chemical treatment of agricultural effluents with traditional metal salts or polyacrylamids is a potential option to reduce nutrient and other pollutant loading. However, the metal residuals in treated water may adversely affect the receiving water bodies and recycling of the retained nutrients is difficult due to the metal content and non-biodegradability of the sludge. We found that metal-free and biodegradable biopolymer coagulants were very effective in reducing total phosphorus concentration and turbidity and to some extent the organic carbon concentration, but the effectiveness was dependent on optimal dose, the initial pollution level and type of biopolymer coagulant.

This study demonstrates that biopolymers are potential coagulants for treating agricultural water pollution. As many biopolymers, such as chitin, can be obtained from waste of industrial production, the use of biodegradable and metal-free biopolymers as coagulants is a promising management option for treatment of agricultural wastewater and in nutrient recycling. However, the performance of biopolymer coagulants in field conditions should be tested to address applicability in practical scale and in variable environmental conditions. The best potential of biopolymer coagulants in treatment of agricultural wastewaters is likely to be in treatment of point source effluents from farms that have high concentrations of nutrients and organic pollution or in treatment of diffuse loading that have a high concentration of pollutants. At low pollutant concentrations the biopolymer dosing needs to be very precise for optimal coagulation and flocculation or otherwise there is a danger of deteriorating the water quality. Thus, very accurate dose adjustment relative to wastewater volume needs to be implemented for treatment in applied settings. The substantial retention of phosphorus in biopolymer coagulants suggests high potential to recycle the retained phosphorus in agriculture. However, the remineralization of phosphorus and the response of crop plants to biopolymer sludge fertilization should be explored in further experiments.