Wastewater Treatment Using MBBR in Cold Climates

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  Proceedings of Mine Water Solutions in Extreme Environments, 2015 April 12-15, 2015, Vancouver, Canada Published by InfoMine © 2014 InfoMine, ISBN: 978-0-9917905-7-9 1 Wastewater treatment using MBBR in cold climates Caroline Dale , Veolia Water Technologies Inc, Cary, USA Marc Laliberte, Veolia Water Technologies Canada Inc, Montreal, Canada David Oliphant , Veolia Water Technologies Canada Inc , Mississauga, Canada Maria Ekenberg, Veolia Water Technologies, Lund, Sweden Abstract Biological wastewater treatment in cold climates can be a challenge due to the low reaction rates; however the alternative of heating large flows of wastewater results in significant operating costs. Using a fixed biomass instead of a suspended biomass allows the process to be operated at much lower temperature as sludge age is no longer a designing parameter. The MBBR (Moving Bed Biofilm Reactor) is a preferred example of such a fixed film (or “attached growth”) process. Biomass develops on the inner surface of a carrier which is in continuous movement in the reactor. Extensive laboratory trials have been undertaken using the MBBR process to demonstrate that long term nitrogen removal can be sustained down to 4°C and selenium removal can be sustained down to 7°C. Results from 2 separate studies treating mine effluents are presented. The first case study refers to a 2 stage MBBR system treating synthetic water to simulate a mine effluent containing 8 mg/l NH 4 -N and 18 mg/l N-NO 3  where complete nitrogen removal was sustained at 4°C for a period of 3 months. The second case study refers to a trial using a mine effluent containing 25 mg/l N-NO 3  and 45µg/l Total Se. A 2-stage MBBR was used to remove both nitrate and selenate at an operating temperature of 7°C. Introduction The storage of tailings from mineral processing operations is achieved in large impoundment areas (variously known as tailings ponds, tailings storage facilities or tailing impoundment areas). While specific regulations vary between sites, mines are not allowed to discharge water acutely toxic to specific species of fish such the rainbow trout (Oncorhynchus mykiss ) or invertebrates such as  Daphnia magna, and discharge limits are being increasingly tightened. Treatment for the removal of metals, nitrogen  M INE W ATER S OLUTIONS IN E XTREME E NVIRONMENTS ,   2014   ●   V ANCOUVER ,   C ANADA   2 (ammonia and/or nitrate and nitrite, principally contributed by ANFO explosive used in blasting) as well as chloride, sulphate and/or total dissolved solids (TDS) is increasingly required prior to discharge to meet regulatory standards. Biological treatment has been applied for many years to remove nitrogen from municipal effluent, even at cold temperatures. With a growing need for nitrogen removal from tailings ponds, laboratory studies have been undertaken to determine whether nitrification/ denitrification could be sustained at very low temperatures (<5°C) to reflect the conditions of Canadian mines. In recent years, selenium present in mine effluents as selenate and selenite has also become a concern. Selenium is highly toxic to aquatic life (Chapman et al , 2009) and the discharge limitations for total selenium are becoming increasingly stringent. Some discharge criteria for release of mine effluent into fresh water systems have been set to < 4.7 µg/l Total Se. Selenate is challenging and costly to remove with physico-chemical methods, but biological treatment has presented itself as a viable alternative over the past years. The process of biological reduction of selenate and selenite is very similar to denitrification. Experiments have shown that a denitrifying sludge can be acclimatized to reduce Se compounds (Takada et al  2008). During biological selenium reduction, microorganisms utilize selenate and selenite as electron acceptors. Selenate and selenite are reduced to particulate elemental selenium. Particulate elemental selenium can either be found as nanospheres inside the cell or external to the cell (Oremland et al , 2004) and thus can be separated from the wastewater by traditional liquid-solid separation methods. As with denitrification, biological treatment of selenate and selenite requires anoxic conditions and the presence of an electron donor, usually an organic carbon compound.   Laboratory studies have been undertaken to use the experience gained in operating MBBR for denitrification to develop a reliable process for selenate and selenite reduction. Operation at low temperature has also been studied. The MBBR Process The Moving Bed Biofilm Reactor (MBBR) was developed in Norway specifically to achieve nitrogen removal at cold temperatures. Since most Norwegian wastewater treatment plants are inside  buildings or underground caverns, a compact alternative to activated sludge was desirable when the requirement for nitrogen removal was implemented for larger wastewater treatment facilities in the 1990’s (Ødegaard et al  1999). The MBBR is a biofilm process which utilises a high density polyethylene carrier as biomass support. The carriers provide a high protected surface area for biofilm development. The srcinal AnoxKaldnes K1 carriers provide 500m 2 /m 3 . Today, the preferred carrier K5, shown in Figure 1, provides 800m 2 /m 3 .  M INE W ATER S OLUTIONS IN E XTREME E NVIRONMENTS ,   2014   ●   V ANCOUVER ,   C ANADA   3 Figure 1 : K5 media, 800m 2 /m 3   The carriers, which have a density of 0.95 to 0.98 kg/dm 3 , are maintained in continuous movement using air in aerobic systems and mechanical mixers in anoxic systems. The process has been described in  past literature (Ødegaard et al  1994, Ødegaard et al  1999). Since the introduction of MBBR technology in the 90’s, there have been over 100 plants built or retrofitted using this process in Scandinavia and over 750 installations worldwide. A simplified process schematic of the MBBR is shown in Figure 2. Figure 2 : Simplified schematic of aerobic MBBR (left) and anoxic MBBR (right)   Extensive pilot studies were under taken on municipal wastewater to demonstrate prolonged operation at low temperature (4 – 5°C for 3 months) prior to full scale installations (Rusten et al  2000). During these trials, it was demonstrated that the nitrification rate could be controlled by regulating the concentration of dissolved oxygen, such that increasing the DO concentration led to higher nitrification rates in a well established system, allowing process performance to be maintained under a higher load or at a lower temperature. Tailings Characteristics and Impact on Wastewater Treatment Plant Design As stated above, the chemical composition of tailings pond effluent will vary depending on the mining operations and the ore being processed. A full effluent characterisation will therefore be required prior to  M INE W ATER S OLUTIONS IN E XTREME E NVIRONMENTS ,   2014   ●   V ANCOUVER ,   C ANADA   4 the design of the wastewater treatment plant. Ammonium and nitrate will usually be found in mine water when explosives such as ammonium nitrate fuel oil (ANFO) are used for blasting. Further, the minimum operating temperature will be used in the design of the biological process, impacting the removal rates that can be achieved. Depending on their location, tailings ponds can experience wide temperature variation throughout the year. Figure 3 shows a typical temperature profile for a tailings pond in Ontario, Canada, where it can be seen that the water temperature falls and stays below 5°C for approximately 3 months of the year. In some cases, there will be sufficient holding capacity to allow the flow (and, therefore, the nitrogen daily load) to treatment to be decreased during the winter months, while still maintaining biological activity throughout the cold season to ensure that discharge guidelines are met. Figure 3: Temperature profile of water pumped from a tailings impoundment area The nitrogen removal rate in a biofilm system, typically expressed as g N removed   per m 2  of protected media surface area per day (gN/m 2 .d), has been shown to be highly dependent on operating temperature (Rusten et al  1995, Welander et al  2003). A study using fixed film biofilters to treat a mine effluent demonstrated that a nitrification rate of 0.33 g NH 4 -N/m 2 .d could be maintained at 5°C (Zaitsev et al  2008). Typically, the design of the MBBR will be done on the lowest operating temperature to ensure that there is sufficient capacity to achieve treatment under the worst conditions unless heating can be provided to maintain the design temperature. In Canadian mine operations, the minimum temperature at which nitrogen removal will need to be maintained will frequently be below 5°C. Although a long term study of MBBR operation downstream of municipal lagoons has shown that nitrification could be sustained at temperature as low as 1°C (Hoang et al  2014), little information was available on the removal rate achievable for mine effluents at temperatures below 5°C until the study described below.
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