Design and performance evaluation of a highly loaded aerated treatment wetland managing effluents from a food processing industry in Denmark

KT Food is company located in Faarup in the municipality of Mariagerfjord in the Danish Northern Region. KT Food was founded in 2003 and its main purpose is to produce manufactured food that is distributed in Denmark and sold across the country. According to the municipal and national discharge regulations the company must treat their own wastewater (WW) before discharge since it is not feasible to connect the plant to a centralized WW treatment plant. Several technologies were suggested and the selected solution was an intensified constructed wetland (Wallace 2001, Murphy & Cooper 2011). An intensified constructed wetland implies the use of devices that can enhance the performance of the system. The most common approach is to use air blowers and an aeration grid installed at the bottom of the wetland, to increase the amount of dissolved oxygen available in the bed and thus supply the needed O₂ to deplete organic matter measured as Chemical Oxygen Demand (COD), Biological Oxygen Demand (BOD₅), to nitrify or oxidize pollutants present in the WW (Ouellet-Plamondon et al. 2006, Wallace et al. 2007, Van Oirschot et al., 2013, Redmond et al. 2014). The airflow and air volumes in the system can be adjusted according to the pollutant applied loading rate by regulating the air blower power or the aeration times in the bed schemes. The treatment system was selected and the design prepared by the coordinated work of the SMEs (Kilian Water Aps, KT Food, Rietland Ltd, and Sedaqua) and the research institutions (Aarhus University and AIMEN) under a European FP7 funded project, called HIGHWET(FP7/SME-2013, Grant agreement N° 605445; for more information http:www.highwet.eu).

Raw WW at the site is produced by two food factories and a residence, discharged to a collection well where water was pumped to a septic tank septic and subsequently to the environment with no further treatment. Before tackling the treatment plant design, a WW characterization campaign was performed where samples were taken at different times of the day to assess the possible loading variation along the day. Grab samples were taken at the point of discharge to the environment and analyzed in situ using calibrated electrodes for temperature, electric conductivity, pH, dissolved oxygen concentration and oxygen saturation. Total suspended solids, COD, BOD₅, nitrogen species and phosphorus concentrations of the samples, were measured at the Aarhus University laboratory following standard methods (APHA 2005) (Table 1).

The results show that compared to the concentrations expected from WW discharged from a food production factory, the water quality measured during the campaigns are relatively low concentrations of organic matter as well as nutrients. Additional work involved a survey at the factory to evaluate their production processes and the potential production of substances that could affect the performance of the system. The survey showed that the factory management is aware of waste management and same environmental practices that reduce the use of water and the production of waste. Oil, grease and solid waste are properly managed, separated and are not disposed with the discharged WW.

The two treatment trains have similar structures as follows: 16 m² vertical flow bed, followed by 3 m² horizontal flow beds and a 1.5 m³ wells to test phosphorus removal capacity of different materials (Figure 2).

The eastern train first structure is a vertical flow reedbed of 16 m², 1 m deep fitted with an aeration system and planted with Phragmites australis (common reed) and Iris spp. The bed was filled with gravel and operates under saturated conditions. Following the vertical flow bed the water is conducted to a distribution well where flow can be controlled in direction (recirculation) and in volume by means of a calibrated weir. After the well, the water goes through a 3 m² planted horizontal sub-surface flow bed that is also fitted with an aeration system. After this bed, water drains to a well and like the previous well; water can be re-directed at will to other structures or recirculated or else can continue along the eastern train to a P removal well. This well is filled with P removal media that is to be tested in the field. Once water goes through the P removal well water goes to a final well and from there discharge to the environment.

The HIGHWET project deals with the treatment of high loaded WW generated by the WW produced by different industries. Constructed wetland technology is able to treat different types of WW but as the loads increase the area demand to achieve proper pollutant removal also increases since oxygen availability is limited by the physical characteristics of the beds. To supply the extra oxygen needed to warrant the removal efficiency; while maintaining the advantages provided by constructed wetland technology, an external supply of air can be provided. (Wallace et al. 2007, Nivala et al. 2007, Murphy et al. 2012, Nivala et al. 2013.) The aeration system designed and installed at the KT Food plant consists of a series of pipes installed at the bottom of the beds that will provide the necessary oxygen to the WW to maintain the proper concentration of dissolved oxygen while water is being treated. As mentioned in previous sections, aeration systems were installed in one of the vertical flow beds (eastern treatment train) and both of the horizontal flow beds. The aeration lines are kept pressurized by air pumps that provide uniform distribution of air throughout the bed surface. Since the two treatment trains have different loading rates the aeration strategy differs in between trains and the amount of aeration can be adjusted by increasing the aeration time, switching the blowers on and off as the load is increased in the course of the development of the research project.

A complex and highly technical system like the one built for the treatment of WW at KT Food requires the installation of wells, hydraulic control systems and pumps. According to the design and the research goals set in the project the treatment plant must have accurate measurement of water flow, the possibility of hydraulic control of the recirculating structures and wells and to test the P removal capacity of two materials that were selected from a list of media with the capacity to remove P from waters.

Flow measurement will be done using an electromagnetic Siemens mag 3100 flow sensor installed in a 0.60 m well placed after the first well. The flow meter can be read through a digital display and can also be connected to a data logger that can register water flow in function of time.

The HIGHWET system is also fitted with the possibility of recirculating and directing water to the different beds. For controlling the direction of the flow and the volumes, the treatment plant is fitted with six wells that collect water and from which water can be distributed at will. The design of the control of each system is specific to each well, depending on the hydraulic head available and the direction where water is to be recirculated to. For controlling the water volumes the plant is fitted with 5 calibrated 60^o V notch weirs where water can be accurately measured and water volumes and direction can be controlled.

In addition to the recirculation wells the treatment plant is fitted with 1.5 m³ wells where P removal material is to be tested. The wells were installed following each of the treatment trains and involved the installation of Ø 1 m concrete wells, followed by smaller Ø 0.6 m PE wells to control the flow.

In addition to the structures, the system at KT Foods has been fitted with the necessary electrical controls to permit the operation of pumps, feeding systems, air blowers as well as the measuring devices and alarms.

Now that the system is constructed, the establishment process begins. Initially, the system will be loaded with the water produced form the industry and the house. Once the system is performing well and stable, higher loads will be tested until reaching treatment limits that will help to determine design parameters.

The testing will go on for at least a year which gives the possibility of testing the performance during the year round and under different seasons. Further research will have to be agreed between KT Food and the partners.