Treating highly loaded industrial effluents are challenging when treatment wetlands (TWs) are the choice selected for treatment. If TWs are selected as the solution, passive systems demand relatively large areas and may generate operational limitations and are not flexible in case of new treatment demands since no process adjustments can be made once the system is built. New types of intensified TW, such as aerated systems, have opened new possibilities in the field of the Wetland Technology and have shown capacity to treat several types of wastewater (WW). Aerated wetlands have been built across the USA and several European countries, but not a single system has been built in Denmark as to date. Thanks to an EU funded project and the cooperation of several partners under a consortium with the acronym HIGHWET, a new system is being built at the premises of a food processing factory in the vicinity of Faarup Denmark. The system will treat the WW generated at the plant and consists of a pretreatment system in the form of an anaerobic reactor, followed by two 1 m deep vertical flow beds, one of them aerated while the other bed is not aerated. Following these beds, the plant has two horizontal sub-surface flow beds fitted with aeration. For improving the P removal, media with high P binding capacity is are being used as filling material. In additiuon and for research purposes, the influent pump well plant is fitted with a dosing system in order to obtain pollutant loads at will so performance limits and kinetic constants can be established from the performance of the plant.

INTRODUCTION

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 characterization

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).

Table 1

Results of analysis of the characterization of the raw WW before the design

 Time Env. Temp oWater temp oC. Ce (μS/cm) pH O2 conc. (mg/L) O2 Satur. (%) TSS (mg/L) BOD5 (mg/L) COD (mg/L) BOD5/COD NH4-N(mg/L) NO3-N (mg/L) TN (mg/L) TP (mg/L) 
Sample 1 14:00   822 5.4 0.1 88 360 431 0.84 19.1 0.8 22 7.7 
Sample 2 8:30 −2 8.8 925 4.5 0.1 139 582 677 0.86 93.2 1.2 95 12.4 
Sample 3 13:00 11.9 690 4.8 0.1 61 430 485 0.89 28.5 1.0 30 9.5 
Sample 4 14:30 12.1 781 0.1 110 641 721 0.89 35.7 1.1 38 11.2 
Average  1.0 10.9 805 4.9 0.1 1.0 99.5 503 579 0.9 44 1.0 46 10.2 
Stdv  2.6 1.9 97 0.4 0.0 0.0 33 130 142 0.0 33 0.2 33 2.0 
 Time Env. Temp oWater temp oC. Ce (μS/cm) pH O2 conc. (mg/L) O2 Satur. (%) TSS (mg/L) BOD5 (mg/L) COD (mg/L) BOD5/COD NH4-N(mg/L) NO3-N (mg/L) TN (mg/L) TP (mg/L) 
Sample 1 14:00   822 5.4 0.1 88 360 431 0.84 19.1 0.8 22 7.7 
Sample 2 8:30 −2 8.8 925 4.5 0.1 139 582 677 0.86 93.2 1.2 95 12.4 
Sample 3 13:00 11.9 690 4.8 0.1 61 430 485 0.89 28.5 1.0 30 9.5 
Sample 4 14:30 12.1 781 0.1 110 641 721 0.89 35.7 1.1 38 11.2 
Average  1.0 10.9 805 4.9 0.1 1.0 99.5 503 579 0.9 44 1.0 46 10.2 
Stdv  2.6 1.9 97 0.4 0.0 0.0 33 130 142 0.0 33 0.2 33 2.0 

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.

Operating conditions

Even though relative loading is low, the purpose of the HIGHWET project is to obtain information and treatment capacity of highly loaded WW using intensified constructed wetlands that implies that actions must be taken to increase the organic and nutrient load. During the initial period, the system will be loaded using the discharged WW from the plant which will benefit the establishment of the plants, the development of the bacteria and biofilm and the growth of the specific anaerobic bacteria in the primary treatment. Since the treatment plant is a natural system, the initial loading should not stress the plants and must guarantee good development of bacteria and therefore very high loadings are not to be used during the establishment period. According to the results obtained from the characterization campaigns, the concentrations in the discharged WW are within the limits of what can be considered as high strength WW (Metcalf & Eddy 2004). Although highly loaded, the system designed and constructed at KT Food premises is expected to have a capacity to treat WW with at least 10 times higher concentrations, which serves well since the company is planning to increase its production. The KT Food treatment plant plan and exploitation was approved by the municipal environmental office in July 2014 and allowed the construction of the plant (Figure 1).
Figure 1

Geographical location and disposition of the two treatment trains. WW are collected from the factory as well as from the house (modified from google maps).

Figure 1

Geographical location and disposition of the two treatment trains. WW are collected from the factory as well as from the house (modified from google maps).

KT Food treatment plant design

The system built at the KT Food premises comprises a primary treatment by means of a hydrolytic anaerobic digester, followed by a pumping system that divides the flow in two treatment trains. The first treatment train is fitted with an aerated vertical flow bed followed by an aerated horizontal sub-surface flow bed and finishes with a P-removal unit. Along all the points of the water line wells permit the recirculation and the control of the flow in between and to the different structures. The second treatment train is fitted with a non-aerated vertical flow constructed wetland, followed by an aerated horizontal subsurface flow wetland and a final well where P removal media are being tested to assess the P removal capacity. This treatment train is also fitted with wells, pumps and hydraulic controls to optimize the performance of the system. The media to be tested are commercially available at the market and preliminary analyses have shown P removal potential. Figure 2 presents a detailed blue print of the design showing the two treatment trains, the pumping and aeration system as well as all the wells and the recirculation possibilities of the system.
Figure 2

Blue print of the plant designed and constructed to treat the WW produced by the KT Food factory.

Figure 2

Blue print of the plant designed and constructed to treat the WW produced by the KT Food factory.

Water generated from the food production factory and the family residence is collected and transported in the same pipe by gravity to a pumping well where a fixed daily flow can be selected at will. Following the first pump well, water is conducted to a second pump well where the influent flow can be regulated in small hourly doses towards the anaerobic reactor. Additionally and since the purpose of HIGHWET is to test highly loaded WW, a dosing system consisting of a 1 m3 tank in which a spiking solution can be dosed to obtain the desired organic and nutrient loading to determine operational limits throughout the testing period. Following the second well the WW is pumped towards the primary treatment which is achieved by means of the anaerobic reactor. After this reactor, water drains to a pumping well where water is divided and pumped to each of the two treatment trains. It is planned that the eastern treatment train will treat 80% of the flow while the western most train will treat 20% of the flow. As water goes through the treatment trains there are several wells and flow regulating weirs are installed to control water volumes to the treatment structures (Figure 3).
Figure 3

Flow diagram of the treatment plant installed at KT food in Denmark (not to scale).

Figure 3

Flow diagram of the treatment plant installed at KT food in Denmark (not to scale).

Treatment trains

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 western treatment train follows the same treatment steps but there are some variations in the treatment approach. The first bed is a 16 m2, 1 m deep vertical flow planted bed, filled with sand. It is not aerated and will operate as an unsaturated bed. The only oxygen transferred to the bed will occur by passive air diffusion, both, at the bed surface and by using pipes that connect the atmosphere to the drainage system at the bottom of the bed. Water follows the same path as in the previous treatment train to a recirculating well and to a 3 m2 aerated horizontal sub-surface flow wetland cell and finally to a P removal filter (Figure 4).
Figure 4

Image of the two planted vertical flow beds once the construction was finished.

Figure 4

Image of the two planted vertical flow beds once the construction was finished.

Aeration system

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.

Wells, measuring devices and recirculation structures

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 60o 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.

CONCLUSIONS

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.

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