Article Request Page ASABE Journal Article
Treatment of Potato FarmWastewater with Coagulation
V. K. Bosak, A. C. VanderZaag, A. Crolla, C. Kinsley, S. S. Miller, D. Chabot, R. J. Gordon
Published in Applied Engineering in Agriculture 33(1): 95-101 (doi: 10.13031/aea.11609). Copyright 2017 American Society of Agricultural and Biological Engineers.
The authors have paid for open access for this article. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License https://creativecommons.org/licenses/by-nc-nd/4.0/
Submitted for review in October 2015 as manuscript number NRES 11609; approved for publication by the Natural Resources & Environmental SystemsCommunityof ASABE in November 2016.
The authors are Vera K. Bosak, Graduate Student,Ottawa Research and Development Centre, Agriculture and Agri-Food Canada, Ottawa, Ontario, Canada, and School of Environmental Sciences, University of Guelph, Guelph, Ontario, Canada; Andrew C. VanderZaag, Research Scientist,Ottawa Research and Development Centre, Agriculture and Agri-Food Canada, Ottawa, Ontario, Canada; Anna Crolla, Senior Researcher,Ontario Rural Wastewater Centre, University of Guelph, Alfred, Ontario, Canada; Christopher Kinsley, ASABE Member, Senior Researcher,Ontario Rural Wastewater Centre, University of Guelph, Alfred, Ontario, Canada; S. Shea Miller, Research Scientist, Denise Chabot, Research Technician,Ottawa Research and Development Centre, Agriculture and Agri-Food Canada, Ottawa, Ontario, Canada; Robert J. Gordon, Professor,School of Environmental Sciences, University of Guelph, Guelph, Ontario, Canada. Corresponding author: Andrew C. VanderZaag, Ottawa Research and Development Centre, Agriculture and Agri-Food Canada, 960 Carling Ave., Ottawa, ON, K1A 0C6 Canada; phone: 613-759-1254; e-mail: Andy.VanderZaag@agr.gc.ca.
Abstract. The processing wastewater from an on-farm potato storage facility contains substantial concentrations of colloidal particles that are hard to remove through sedimentation alone. This study evaluated coagulation as a potential approach for reducing total suspended solid levels. Wastewater was coagulated with two proprietary Nalco polymers, as well as aluminum sulfate (alum) and ferric chloride (FeCl3). One of the Nalco polymers required the smallest volume to achieve 50%, 75%, or 90% total suspended solids removal. However, alum was consistently the least expensive product, despite the larger volume required. Although cost is an important factor for farmers, the convenience of using a smaller volume and the effects of coagulation on pH are also important factors to consider. Both polymers had minimal effect on pH, whereas alum and FeCl3 resulted in a pH below 6 at high concentrations. In consequence, alum and FeCl3 require additional chemicals to maintain a biologically neutral pH, thus also requiring extra work and expense. Future research should focus on on-farm coagulant trials to verify laboratory results and optimize protocols for on-farm use.
Keywords.Agriculture, Aluminum sulfate, Coagulation, Ferric chloride, Polymer, Potato wastewater.
The management of agricultural crop wastewater is a growing concern for farmers, particularly potato producers, given that the processing industry increasingly requires more on-farm crop washing and pre-processing (sorting, dirt removal, peeling, and cutting). Because of its organic source, potato wastewater is high in nutrients, such as nitrogen and phosphorus, and carbon (Lehto et al., 2009; Bosak Zurowsky, 2015). Our recent studies identified that potato wastewater contains a substantial concentration of colloidal particles that are difficult to remove through natural sedimentation (Bosak et al., 2015, 2016b). In Canada, potatoes are harvested in the fall, and growers store their potatoes for shipping or processing throughout the fall, winter, and spring. This means that most of the wastewater is created during cold months when air temperatures are often below 0°C. During this time, biological treatment in outdoor systems is least effective and sedimentation is hindered by ice formation. As a result, chemical and physical treatments, including coagulation, may be better suited for treating this wastewater.
Coagulation is the destabilization of colloidal particles that are smaller than 1 µm (Tchobanoglous and Schroeder, 1985). Aluminum sulfate (alum) and ferric chloride (FeCl3) are multivalent cation metals, which coagulate by precipitation and enmeshment. The bonding of OH- (hydroxide) from water to the metal creates a precipitant, which forms a sweeping floc that gathers colloidal particles (Tchobanoglous and Schroeder, 1985). In contrast, polymers and polymer blends have high molecular weights and offer several advantages: they require lower concentrations, create less sludge, do not need pH adjustment, and produce flocs that are more resistant to breakage (Tyagi et al., 2010). Polymer-blend coagulants are available from chemical companies in the form of convenient, proprietary products for industrial use. Although polymer blends may be more efficient at solids removal and easier to use than traditional coagulants (e.g., alum and FeCl3), it is unclear whether polymer blends are more cost-effective (Tyagi et al., 2010). Additionally, traditional coagulants require specific pH ranges to work optimally, which may require additional chemicals. Song et al. (2004) found that suspended solids removal from tannery wastewater was optimal at pH 6 to 8 when using alum, and at pH 7 to 8 when FeCl3 was used. Owing to the ionic bonding by metal coagulants, the supernatant may subsequently have an altered pH (Tchobanoglous and Schroeder, 1985). This may cause problems with further wastewater treatment, where specific ranges are also required by discharge regulations, and are needed for biological treatment, such as nitrification/denitrification (Tchobanoglous and Schroeder, 1985).
Research on the coagulation of agricultural wastewaters has covered a broad range of agricultural sources (e.g., abattoirs, pulp mills, and wineries) but has been limited to larger-scale operations with plentiful resources (Amuda and Alade, 2006; Braz et al., 2010; Wang et al., 2011). Little or no information is available on potato wastewater coagulation in the context of decentralized, on-farm treatment, which differs from large-scale operations in many ways. The currently available options for farmers are costly and inconvenient, are focused on infrastructure (e.g., septic systems and large-scale wastewater treatment systems), and often require large start-up costs and considerable labor and maintenance. Thus, in addition to the effectiveness of coagulation in potato wastewater, it is important to study other aspects of coagulation practices, such as comparative cost-effectiveness and changes in wastewater pH, within the context of on-farm treatment.
Given that potato wastewater contains persistent colloidal particles that do not settle, the purpose of this study was to evaluate coagulation as a potential treatment for potato wastewater. This includes a comparison of the removal efficiency of traditional coagulants (alum and FeCl3) and polymers, evaluated in terms of cost-effectiveness, as well as a comparison of pH change. Both are discussed in terms of suitability for on-farm use in conjunction with other treatments.
Materials and Methods
Potato Farm Wastewater
Potato wastewater was from a potato storage facility in Alliston, Ontario, Canada, that stores 14,000,000 kg of potatoes and creates 3,000,000 L of wastewater per year. Before being shipped to processing factories, potatoes were scrubbed of dirt, sorted, and washed with clean water from a groundwater well. Following each potato shipping event, the wastewater was immediately pumped out into an existing on-farm land-based treatment system (Bosak Zurowsky, 2015; Bosak et al., 2016a). The entire system had a retention time of six months, with aerobic, anaerobic, and wetland cells for the treatment of organic matter and nutrients as well as solids (Bosak Zurowsky, 2015; Bosak et al., 2016b). Total suspended solids (TSS) were chosen as the main contaminant of focus for the present study because persistent colloidal particles were not removed from the wastewater even after six months retention time in the on-farm treatment system. As part of a larger study, wastewater on the farm had been analyzed for total nitrogen, total phosphorus, TSS, 5-d biochemical oxygen demand (BOD5), and pH, over the course of two shipping seasons (Bosak Zurowsky, 2015; Bosak et al., 2016b). The entire treatment system was not able to reduce the TSS in the outflow to an acceptable level (<25 mg L-1 TSS) (Ontario Ministry of Environment and Energy, 1994); TSS after the settling basin was 375 mg L-1 on average, and TSS in the final outflow of the final cell ranged from 32.2 to 316 mg L-1 (Bosak et al., 2016b). Moreover, a related study at same site (Bosak et al., 2015), observed positive correlations between the removal of TSS and the removal of BOD5 (r2 = 0.60), total phosphorus removal (r2 = 0.69), and removal of total Kjeldahl nitrogen (r2 = 0.51). The water quality criteria for these contaminants is <25 mg L-1 BOD5, <0.3 mg L-1 TP, and <1.0 mg L-1 NH3 (Ontario Ministry of Environment and Energy, 1994). These positive correlations indicate that removal of TSS also removed associated nutrients and organic matter.
Collection of Wastewater
Wastewater collection and coagulant testing were performed in the fall of 2013. Wastewater was collected near the outlet of the primary sedimentation basin, in order to ensure that large particles had settled out (this basin removed more than 90% of TSS from the raw wastewater; Bosak et al., 2016b). A water sampling pole was used to fill 1-L sample bottles with wastewater from the basin. Sample bottles (33 in total) were packed into a cooler and shipped to the Agriculture and Agri-Food Canada Research Branch at the Central Experimental Farm in Ottawa, Ontario, Canada, where coagulation was tested. The wastewater was kept refrigerated at 4°C.
Coagulants and Flocculation Procedure
The coagulation of potato wastewater was tested with four coagulants. Alum (aluminum sulfate octadecahydrate extra pure; Acros Organics) and FeCl3 (ferric chloride hexahydrate; MP Biomedicals), were obtained from Fisher Scientific (Cat. Nos. AC400590025 and 15350080; Fisher Scientific, Waltham, Mass.). Two proprietary polymer blends (Ultrion 8186 and Ultrion 8185) were obtained from the manufacturer based on their recommendation (Nalco Holding Co., Naperville, Ill.).
Preliminary tests were performed to find the best range of concentrations and stirring speeds for testing, and the appropriate settling time to account for natural settling in the control. Immediately before each experiment, the coagulants were made into solutions, following manufacturer recommendations. Polymers were made into 1% (v/v) solutions, alum was made into 2% w/v solutions, and the FeCl3 was made into 3% w/v solutions. The solutions were added to the wastewater to achieve the seven different concentrations of coagulant: 0.01 to 0.5 mL L-1 for each polymer, 0.02 to 1 g L-1 for alum, and 0.03 to 1 g L-1 for FeCl3 (table 1). To account for natural settling, each trial included a control, comprised of wastewater without coagulant, and coagulant performance was calculated relative to the control. All trials were conducted in triplicate.
After coagulant solutions were added to 300-mL samples of wastewater, the samples underwent rapid stirring (180 rpm), followed by slow mixing (80 rpm) with a glass stirring rod for 30 s, which was sufficient to flocculate. This method was similar to Wang et al. (2011), who used 2 mins of 200 rpm fast mixing and 10 min of 40 rpm slow mixing. Our mixing times were decreased due to the smaller volume of wastewater coagulated. Tests were done using 300 mL (rather than 1 L) of wastewater per jar due to the difficulty of transporting wastewater from the farm to our lab. To account for sedimentation in the control samples, the supernatant was sampled with a pipette after 24 ± 3 h. Uncoagulated wastewater and supernatant samples were analyzed for pH immediately after sampling with a calibrated YSI 556 multimeter (YSI Inc., Yellow Spring, Ohio). Samples were refrigerated until TSS analysis within 5 d by standard methods (Eaton et al., 2005).
Table 1. Concentration of coagulantsin potato wastewater (mL L-1 or g L-1 of wastewater), using polymers 8185 and 8186, aluminum sulfate (alum), and ferric chloride (FeCl3). Concentration of Coagulant[a] Polymers (mL L-1) FeCl3 (g L-1) Alum (g L-1) 0.01 0.03 0.02 0.02 0.05 0.03 0.03 0.10 0.07 0.07 0.20 0.13 0.17 0.50 0.33 0.33 0.80 0.67 0.50 1.00 1.00
[a]All coagulants were tested in triplicate over seven concentrations.
Concentration reductions were calculated with reference to the control, as:
Ccoagulant = concentration in the supernatant of the
Ccontrol = concentration in the supernatant of the control,
%CR = concentration reduction, of the coagulant
relative to the control.
Concentration-response curves were created for each coagulant by means of regression analysis in SigmaPlot 12.0 software (Systat Software Inc., San Jose, Calif.). Best fits were selected from several options (linear, linear-log, and polynomial) on the basis of r2, p-value, and residuals. The curves had the following equation:
b0 = natural log-transformed intercept,
b1 = slope of the logarithmic equation,
x = coagulant concentration,
y = removal efficiency.
Using the regression equation, we estimated the concentrations required for 50%, 70%, and 90% TSS removal (EC50, EC75, and EC90, respectively). The effective concentration, EC50, was defined as the point where 50% removal of TSS was achieved. From these values, the total amount of coagulant and price per year to treat 3,000,000 L of wastewater were calculated. Prices for the polymers were obtained from Nalco, and prices for the alum and FeCl3 were obtained from Alphachem, a local distributor of chemicals.
One-way ANOVA and Holm-idįk analysis were used to find the difference between coagulants with different TSS concentrations in wastewater.
Light microscopy of the floc formations was performed on wastewater samples coagulated to reach EC50, EC75, and EC90 with polymer 8185 and on a control. Samples were taken immediately following coagulant addition and stirring, before flocs were able to settle. The samples were collected with pipette tips cut to create a wider opening and prevent floc damage. The samples were transferred to a glass depression slide. Cover slips were not used, because condensation would form on the inside of the cover slip. Images were obtained with an Axioplan 2 imaging light microscope and Axiovision software (Carl Zeiss AG, Jena, Germany). For each sample, a series of images along the Z-axis were obtained at 5× magnification and merged into a composite image; no more than 10 images per stack were collected to create each Z-stack image.
Results and Discussion
Characteristics of Potato Wastewater
The raw wastewater, measured at the potato farm over two years, had TSS concentrations of 5,400 ± 3,800 mg L-1 in year 1 and 3,400±1,900 mg L-1 in year 2 (Bosak Zurowsky, 2015; Bosak et al., 2016b). The other contaminant concentrations were 1,100 mg L-1 BOD5, 300 mg L-1 total nitrogen, and 40 mg L-1 total phosphorus over the course of the two years of monitoring (Bosak et al., 2016b). Raw wastewater composition varied during a shipping season, or even from day to day, as a result of a combination of factors, including potato type, soil moisture at harvest, occurrence of rot, and quantity of potatoes shipped. The volume of wastewater used also varied, depending on the demand for potatoes from processing plants (Bosak et al., 2016a). After the on-farm sedimentation basin, the wastewater concentration was on average 375 mg L-1 TSS, 800 mg L-1 BOD5, 135 mg L-1 total nitrogen, and 20 mg L-1 total phosphorus, over the course of the two years (Bosak et al., 2016b). The control wastewater (uncoagulated sample with sedimentation alone) from this experiment contained on average 470 mg L-1 TSS and had an average pH of 7.8.
Total Suspended Solids Removal
The removal of TSS from all concentrations and coagulants ranged from 0% to 99.8% with increasing coagulant concentration. Removal behavior was similar in all coagulants: TSS removal was dependent on concentration and coagulant type, and concentration-responses were all found to be linear-log. At the highest concentrations (0.5 mL L-1 for polymers and 1.0 g L-1 for alum and FeCl3; table 1), the resulting average TSS concentrations in the supernatant of each coagulant type were 60.8, 130, 17.7, and 6.08 mg L-1 with polymer 8185, polymer 8186, alum, and FeCl3, respectively.
Both polymers had similar removal. Polymer 8185 had slightly better removal than 8186, with a b0 of 98.7 (% TSS removal) and a b1 of 21.6 (fig. 1). Since the x-axis is natural log-transformed, when the concentration is approximately 0.01 mL L-1 or g mL-1, then the percent TSS reduction (x) equals 0. Therefore, the b0 is the percentage of TSS removal at a concentration of 0.01 mL L-1. The b1 indicates the percentage of TSS reduction achieved for a given coagulant concentration increase. Polymer 8185 had slightly higher maximum b0 and b1 than polymer 8186, indicating that polymer 8185 had a greater increase in percentage of TSS removal for a given concentration increase (fig. 1). Polymer 8186 had a b0 of 90.1 (% TSS removal) and a b1 of 19.7 (fig. 1).
Figure 1. Reduction of total suspended solids (TSS) in on-farm potato wastewater with various concentrations of polymers 8186 (a) and 8185 (b) (mL L-1), and aluminum sulfate (alum) (c) and ferric chloride (FeCl3) (d) (g L-1), and the corresponding pH changes with alum (e) and FeCl3 (f).
Of the traditional coagulants, alum performed slightly better than FeCl3. Alum had a b0 of 106 (% TSS removal) and a b1 of 26.3 (fig. 1). Ferric chloride behaved similarly, with a b0 of 99.2 (% TSS removal) and a b1 of 29.0 (fig. 1).
Other agricultural studies reported removals of TSS similar to the results of our study (table 2). Braz et al. (2010) found 83% removal of TSS from winery wastewater by means of alum (20 mL of 5% [w/v] solution) and 76.8% removal by means of FeCl3 (10 mL of 5% [w/v] solution). Karim and Sistrunk (1985) reported 91% removal of TSS from abrasive-peeled potato processing water when it was treated with FeCl3 at a pH of 7. The optimal treatment in that study used a mixture of FeCl3 and a polymer, which increased TSS removal to 99%. Other studies also suggested that mixtures of coagulants and polymers resulted in superior removal (table 2). Although results specific to Nalco polymers were not found in the literature, we can compare our results with those obtained with mixed polymer coagulants. Tyagi et al. (2010) found that the addition of a polymer with a high molecular weight produced consistently better TSS removal over all concentrations tested, owing to particle bridging between polymer chains. Those authors observed an additional 26% TSS removal when a polymer was added to alum and an additional 31% TSS removal when a polymer was added to FeCl3. The results of our study are consistent with those of the above studies, with similar removal for the polymers, alum, and FeCl3. However, the polymers were at least 50% more diluted.
Table 2. Literature values for removal efficiency of total suspended solids (TSS). Study Wastewater Coagulant[a] Concentration (g L-1) % TSS removal Karim and Sistrunk, 1985 Potato processing FeCl3 0.150 91 Karim and Sistrunk, 1985 Potato processing FeCl3 + polymer 0.15 + 0.02 99 Tyagi et al., 2010 Municipal FeCl3 0.300 80 Tyagi et al., 2010 Municipal FeCl3 + polymer 0.30 + 0.002 99 Tyagi et al., 2010 Municipal alum 0.30 78 Tyagi et al., 2010 Municipal alum + polymer 0.30 + 0.002 98 Amuda and Alade, 2006 Abattoir alum 2.00 70 Amuda and Alade, 2006 Abattoir alum + polymer 0.75 + 0.01 95 Guida et al., 2007 Municipal alum 0.150 >75 Al-Mutairi et al., 2004 Abattoir alum 0.101.00 98-99 Al-Mutairi et al., 2004 Abattoir Praestol polymer 0.060.09 95-96
[a]FeCl3, ferric chloride; alum, aluminum sulfate.
Figure 2. Micrographs at 5× magnification of flocs created with the polymer coagulant Ultrion 8185: (a) control; (b) 50% total soluble solids (TSS) removal; (c) 75% TSS removal; and (d) 90% TSS removal.
Micrographs (fig. 2) of wastewater coagulated with polymer 8185 show that although all samples had sedimentation in the center of the depression, there was a marked difference in appearance between the coagulated solids and the settled solids (no coagulant). The coagulated flocs appeared less densely compacted in comparison with the more densely settled solids in the control. Additionally, with increasing concentration, which achieved 50%, 75%, and 90% TSS removal, the images show fewer but larger flocs (>100 µm) as well as fewer residual suspended solids (<100 µm) surrounding the center floc. Larger flocs allow faster settling, and the reduction of residual particles directly shows the improved flocculation of the solids.
Effects on pH
The pH was found to be a significant variable, dependent upon coagulant type and concentration (p < 0.0001). The pH of the supernatant was not significantly altered by the polymers. It increased from 7.8 (control) to 8.0 in the highest concentrations. This minor increase occurred most likely because the polymers were non-ionic and used adsorption bridging instead of ionic bonding to form particle bonds. Alum and FeCl3 had an effect on the pH of the wastewater because of the ionic bonding of particles (fig. 1). The correlation of TSS removal and change in pH for both alum and FeCl3 had a moderately strong negative linear relationship (r2 = 0.74 in both; fig. 1). Lowering the pH below biologically neutral levels (6-8) may hinder biological treatment of the wastewater at later stages of the treatment process.
Comparative cost-effectiveness was calculated by using TSS removal efficiency and prices supplied by local chemical distributors. All monetary amounts are expressed in Canadian dollars. The polymer prices were $15.70 and $14.20 L-1 for polymers 8185 and 8186, respectively. The company Alphachem quoted prices of $2.86 L-1 for a 50% w/v alum solution and $4.62 L-1 for a 40% w/v FeCl3 solution (table 3).
The polymers were more expensive per liter than the traditional coagulants (alum and FeCl3). To achieve EC50, the prices of the polymers would be $4,900 and $5,600 a-1 for polymers 8185 and 8186, respectively. The prices for the corresponding removal by means of alum and FeCl3 would be $2,100 and $6,300 a-1, respectively. The volumes of product needed to achieve EC50 would be 313 and 393 L for polymers 8185 and 8186, respectively, compared to 714 and 1,375 L for alum and FeCl3 solutions, respectively. Achieving EC75 would require 999 L a-1 of polymer 8185, at a cost of $15,700 a-1, and 1,395 L a-1 of polymer 8186, at a cost of $19,800 a-1. Compared with the polymers, alum solution would require a larger volume (1,846 L a-1), but the price would be much lower ($5,400 a-1), and FeCl3 would require 3,256 L a-1, at a price of $15,000 a-1. This shows that although the polymers are more expensive for a given treatment, a smaller quantity of them is required. For comparison purposes, we also determined the mass of TSS removed per liter or kilogram of coagulant for EC50, EC75, and EC90, as shown in table 3. Due to the logarithmic nature of the TSS removal, the additional coagulant is marginally less effective at increasing % TSS removal from the wastewater. Thus, EC50, EC 75, and EC90 in table 3 illustrate the diminishing effectiveness as higher removals are achieved.
Table 3. Quantity and costs (CAD) per year to remove 50% (EC50), 75% (EC75), and 90% (EC90) of the total suspended solids (TSS) in potato wastewater with the coagulants polymer 8185, polymer 8186, aluminum sulfate, and ferric chloride, and the mass of TSS removed per liter of coagulant used.[a] EC50 EC75 EC90 Coagulant Price Quantity Cost TSS Removal Quantity Cost TSS Removal Quantity Cost TSS Removal Unit (L or kg a-1) ($ a-1) (kg L-1 or kg kg-1) (L or kg a-1) ($ a-1) (kg L-1 or kg kg-1) (L or kg a-1) ($ a-1) (kg L- or kg kg-1) Polymer 8185 15.70 $ L-1 313 $4,900 6.05 999 $15,700 1.90 2,004 $31,460 0.95 Polymer 8186 14.20 $ L-1 393 $5,600 4.83 1,395 $19,800 1.36 2,985 $42,385 0.63 Alum 5.80[b] $ kg-1 357 $2,100 5.31 923 $5,400 2.05 1,633 $9,470 1.16 FeCl3 11.50[b] $ kg-1 550 $6,300 3.45 1,302 $15,000 1.46 2,184 $25,121 0.87
[a] All calculations were based on treating 3,000,000 L of wastewater annually.
[b] Price based on $2.90 L-1 for a 50% w/v alum solution, and $4.60 L-1 for a 40% w/v FeCl3 solution. Quantity of Alum and FeCl3 refer only to the mass of coagulant. The quantity of 50% w/v alum solution is 714, 1846, and 3265 L a-1 for EC50, EC75, and EC90, respectively. The quantity of 40% FeCl3 solution is 1375, 3256, and 5461 L a-1 for EC50, EC75, and EC90, respectively.
Although price is often the most important factor for farmers, other factors must also be considered when choosing a coagulant. The quantity of coagulant is important, because smaller quantities make storage and dosing easier. Large quantities of chemicals require a lot of space, which may be inconvenient in the storage facility where potato processing occurs. In addition, large volumes make coagulants more difficult to use, especially because they must be diluted in water first to make coagulation properly effective. Large quantities may also create more sludge following coagulation, which could increase the rate of sediment accumulation and thus the need for cleaning. For polymers, their suitability (e.g., biodegradability and residues) should be discussed with the manufacturers prior to on-farm use.
Another important factor is pH, which was discussed earlier. The pH of the wastewater was decreased when alum and FeCl3 were used but was essentially unchanged when polymers were used. In consequence, when alum and FeCl3 are used, additional chemicals are necessary to bring the pH back to neutral, so that optimal biological treatment can be carried out farther along in the treatment system. This necessity creates additional work, adds complexity, and increases the price. Adjusting the pH of the wastewater would require measuring the existing pH and calculating at each event the quantity of chemical required to return the pH to neutral. Moreover, although no values are mentioned here, there would be an extra cost for the additional chemical. It is important for farmers to be aware of all the factors involved when deciding on coagulation practices.
Because wastewater on the farm changed throughout the shipping season as a result of many factors (such as crop type and the presence of rot or soil), coagulation practices need to be tested on the farm throughout the year to see how the TSS in the wastewater changes and what actual quantities of coagulant are required. Future research should also evaluate combinations of polymers, alum, and FeCl3 treat wastewater, as well as, natural coagulants (Bodlund et al., 2014; Teh et al., 2014).
All coagulants were effective at removing TSS, with good correlation between removal and application rate. The Nalco polymer 8185 had the best treatment efficiency, requiring the least amount of product for a given percentage of TSS concentration reduction (for 75% TSS removal, 999 L a-1 for polymer 8185 vs. 1,395 L a-1 for polymer 8186, 1,846 L a-1 for alum, and 3,256 L a-1 for FeCl3). Ferric chloride was always the least efficient, requiring more than twice the product volume for a given percentage of TSS concentration reduction in comparison with the other coagulants. In terms of cost-effectiveness, alum was consistently the lowest-cost coagulant for a given percentage of TSS concentration reduction. Depending on the percentage of TSS removal, alum was between two and three times cheaper than the next cheapest coagulant was. The price differences between the other coagulants were much smaller than the differences between alum and the other coagulants. Ferric chloride was cheaper at EC75 and EC90 than the polymers were, although at EC50, polymer 8185 was more cost-efficient. It is, however, important to examine all factors, which include practicality for on-farm use and hidden costs. Because FeCl3 required larger volumes (more than double), it would be harder to store and handle on a farm. Also, alum and FeCl3 caused a drop in pH (down to 4.3 and 5.3, respectively) in the wastewater, and thus additional chemicals would be needed to bring the pH back to a neutral level for continued biological treatment (e.g., in a treatment wetland) or discharge.
This research was funded by the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), with support from Agriculture and Agri-Food Canada (AAFC) and the Ontario Agricultural College (OAC) of the University of Guelph. No endorsement is implied by the use of specific coagulation products in this research.
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