Performance of a Constructed Wetland and Pretreatment System Receiving Potato Farm Wash Water

Vera Bosak, Andrew VanderZaag, Anna Crolla, Christopher Kinsley, Robert Gordon
2016 Water  
Many potato processors require on-farm washing of potatoes, creating large quantities of wastewater that requires treatment, starting in the fall until the end of the potato storage period in mid-summer. We studied the treatment of wastewater from a potato farm in Ontario, Canada, using a system of pretreatment (sedimentation, aeration) followed by a surface-flow wetland with a dense growth of cattails (Typha sp.). The raw wastewater had high average concentrations of 5-day biochemical oxygen
more » ... iochemical oxygen demand (BOD 5 ; 1113 mg¨L´1), total suspended solids (TSS; 4338 mg¨L´1), total nitrogen (TN; 311 mg¨L´1) and total phosphorus (TP; 42.5 mg¨L´1). Due to high influent loads, the pretreatment was enlarged during annual sediment cleaning at the end of Year 1 (Y1), which increased the hydraulic retention time and delayed the seasonal onset of wetland loading from winter in Y1 to spring in Year 2 (Y2). Total concentration reduction for the treatment system (pretreatment + wetland) in Y2 was 96% BOD 5 , 99% TSS, 86% TN and 90% TP; and in Y1 was 79% BOD 5 , 97% TSS, 62% TN and 54% TP. Overall, the best treatment in both the pretreatment and the wetland was seen in spring months. The enlarged pretreatment system enabled seasonal loading of the wetland during the spring and summer, which facilitated improved treatment performance. large potato processing plant, which was treated by primary solids removal in an industrial clarifier and then by large integrated wetland systems [3, 4] . Although they did not report raw wastewater strength, concentrations after the clarifier were: TSS 300 to 600 mg¨L´1, chemical oxygen demand 1000 to 3000 mg¨L´1, total Kjeldalh nitrogen (TKN) 90 to 170 mg¨L´1 and ammonium-N (NH 4 -N) 70 to 115 mg¨L´1 [3, 4] . Processing water from three root vegetable farms in Finland that washed and peeled their vegetables contained 820 to 3700 mg 7-day biochemical oxygen demand L´1, 150 to 2600 mg TS L´1, 28 to 320 mg¨L´1 total nitrogen (TN) and 4.0 to 34 mg¨L´1 total phosphorus (TP). Although there has been no published research on farm-based potato wash water and its treatment, high concentrations of nutrients and organic matter are expected, as potatoes contain 2% protein, 17% carbohydrates and 0.6 mg¨g´1 of phosphorus [5] . Land-based treatment systems are well suited to agricultural applications given their low cost and farm land availability [3] . These systems include sedimentation basins, aerated basins and treatment wetlands. Constructed treatment wetlands have been commonly used in other types of farming (e.g., wastewater from on-farm slaughterhouses, dairy milk houses, piggeries) [6-10]. Constructed wetlands utilize natural processes taking place in wetlands for biological treatment of wastewater [9] . The combination of shallow water and abundant macrophytes creates an environment advantageous for microbial growth. This removes carbon and nutrients from the wastewater by uptake into plant material or conversion to gaseous by-products [9, 11] . Kadlec et al. (1997) reported on the performance of two pilot-scale treatment wetland systems (20 m 2 and 2685 m 2 ), and Burgoon et al. (1999) reported on a full-size treatment system (16 ha), all of which treated potato processing factory wastewater after primary sedimentation. The initial sedimentation step was achieved in an industrial clarifier treating ca. 5000 m 3¨d ay´1. This was followed by a 'sedimentation' wetland (that required periodic sediment removal), then a mineralization wetland, followed by a nitrification step in a vertical flow subsurface wetland (intermittent sand filter) and, lastly, a denitrification wetland [3, 4] . In the full-scale system, effluent from the final wetland was stored and then irrigated on forage crops. Overall concentration reductions between the wastewater entering the first treatment cell (effluent from the clarifier) and the final wetland cell ranged from 33% for NH 4 -N to 95% for COD [3, 4] . Potato farms are much smaller than industrial potato processing plants and, thus, cannot utilize large-scale clarifiers. Furthermore, unlike many other facilities, potato shipping and wash water production is not year-round, rather occurring from October to July and, therefore, in large part during the colder months of the year. As a result, there are several cold climate considerations including plant senescence and the release of nutrients in wetland cells, snow and ice cover, reduced biological activity and preferential flow due to freezing [10] . Although constructed wetlands have been shown to be effective in cold climates [12, 13] , treatment is wastewater-specific and, therefore, needs to be examined specifically with potato wash water. This research examined the performance of a constructed wetland and pretreatment system over two years. The goal was to inform the design of an affordable, cold climate, surface flow system that can aid farmers by consistently treating wash water. Materials and Methods Farm Description The research took place at Sunrise Potato Storage LTD in Alliston, Ontario, Canada [14] . After harvest, potatoes were shipped out of the storage of over 10 months (October to July). Wash water was created from the transport (fluming), washing and sorting of potatoes prior to being loaded onto trucks. Wash water quantity and quality varied from month-to-month, based on processor demand for potatoes, shipment size, occurrence of rot and presence of soil. Preparing potatoes for shipment lasted 3 to 8 h¨day´1 with the flume water continuously re-circulated throughout the day. At the end of each day, all wash water was pumped out of the storage facility and loaded into the outdoor treatment Water 2016, 8, 183 3 of 14 system. During the second year of the study (Y2), actions were taken to reduce contaminants entering the wash water and the volume of water used [15]. Treatment System Description The treatment system was constructed adjacent to the storage facility on a low permeability, clay soil in the summer of 2008 (Figure 1) . System size was determined using the first-order, plug flow, design equation to treat estimated loads of total suspended solids (TSS) and 5-day biochemical oxygen demand (BOD 5 ) [11, 12] ; however, at that time, the quantity of wash water produced and the level of BOD 5 and TSS had not been characterized (and no literature values were available). Water 2016, 8, 183 3 of 15 Wash water quantity and quality varied from month-to-month, based on processor demand for potatoes, shipment size, occurrence of rot and presence of soil. Preparing potatoes for shipment lasted 3 to 8 h day −1 with the flume water continuously re-circulated throughout the day. At the end of each day, all wash water was pumped out of the storage facility and loaded into the outdoor treatment system. During the second year of the study (Y2), actions were taken to reduce contaminants entering the wash water and the volume of water used [15] . Treatment System Description The treatment system was constructed adjacent to the storage facility on a low permeability, clay soil in the summer of 2008 (Figure 1) . System size was determined using the first-order, plug flow, design equation to treat estimated loads of total suspended solids (TSS) and 5-day biochemical oxygen demand (BOD5) [11, 12] ; however, at that time, the quantity of wash water produced and the level of BOD5 and TSS had not been characterized (and no literature values were available).
doi:10.3390/w8050183 fatcat:wlclpsiz5rbbfouowgjzd4spdi