Denitrification filtering

As the Environment Agency begins to impose total nitrogen limits in the UK, Frank Rogalla teams up with Chris de Barbadillo to discuss denitrification filters


In the new cycle of AMPs in the UK, the Environment Agency (EA) is starting to impose total nitrogen limits in accordance with the European Urban Wastewater Directive for nutrient sensitive zones. Ammonia oxidation in activated sludge processes was optimised in the UK by Downing and Painter in the 1960s, and then implemented in many plants on a large scale to reduce oxygen demand in receiving waters. Nitrate, on the other hand, was of concern mainly in relation to drinking water supplies until the programme to protect the North Sea and Baltic Sea from eutrophication came into force in the Nordic countries in the 1990s. These new requirements often asked for very low total nitrogen residuals, which are not easy to reach by using activated sludge, making an additional removal step necessary.

The technology to retrofit activated sludge with denitrification and solids removal in a downflow deep-bed filter was first patented in 1973. This was followed by a series of patents for other components, including the use of backwash water to release nitrogen gas produced during the denitrification process. In the US, especially in Florida, where inland watercourses were very sensitive to eutrophication, a number of large municipal facilities began to implement downflow denitrification filters in the late 1970s. For example, in Gainesville and Tampa, full-scale operating experience showed that nitrate-nitrogen and nitrite-nitrogen (NOx-N) could reliably be reduced to concentrations of less than 1mg/L.
With the expiration of the original process patents by Severn Trent Services (Tetra), several other filter suppliers included denitrification options, such as FB Leopold and US Filter. In addition, a number of utilities have retrofit existing filters for tertiary denitrification, including Munich in Germany and Raleigh, North Carolina in the US.

Filter configurations: up or down
There are two main denitrification filter configurations: conventional downflow filters and upflow continuous backwash filters. Downflow filters operate in a conventional filtration mode, and consist of underdrain supported gravel and sand (Figure 1).

Wastewater enters the filter over influent weirs located along the length of the filter bed. Filter effluent is conveyed from the bottom of the filter over a control weir into a clearwell. Filters must be briefly taken out of service at regular intervals for a short backwashing cycle consisting of both air scouring and air-water backwashing. Nitrogen release cycles are needed periodically to prevent nitrogen gas bubbles from accumulating in the media. A mudwell is normally provided for the equalisation of backwash water waste in order to avoid sending it to the plant headworks in slugs. The filter influent and backwash piping are similar to that of conventional filters and can be housed in an indoor pipe gallery or installed outdoors.

Upflow continuous backwash filters are installed in modular steel tanks or in multiple concrete cells, and are supplied by Parkson (DynaSand) or Paques (Astrasand). The influent flows upward through the filter, and effluent is removed at the top of the bed. The countercurrent sand bed is slowly drawn downward into an airlift system, where compressed air rises, draws the sand upward and scours it. At the top of the airlift, the sand is returned to the sand bed through a washer and separator. Filtered water rises through the separator, washing away the lighter dirt particles, and allowing the large heavy sand grains, now cleaned, to re-enter the filter at the top of the bed (Figure 2). Because the backwash water continuously exits near the top of the filter, individual cells do not need to be taken out of service. The reject water weir is set at a lower elevation than the effluent weir to allow clean water to be continuously introduced to the washer/separator by differential head, thus eliminating the need for typical backwash supply pumps, and allowing for a relatively simple piping and valve arrangement.

Comparison of filter equipment
When designing a denitrification filter, it is important to examine differences in equipment and experience offered by the manufacturers. Downflow units are generally similar in layout to conventional filters, whereas requirements for upflow operations are different. The major design considerations include process performance, loading rates, influent weir, media, underdrain, process control and methanol feed control, as discussed in the following sections.
  • Experience and process performance - Severn Trent Services has over 25 years of successful operating experience in denitrification and offers performance guarantees for meeting an effluent NOx-N of 1mg/L. Full-scale documentation from multiple facilities include recent data that show effluent NO3-N concentrations of less than 0.5mg/L under cold weather conditions.

  • US Filter has offered denitrification filters for about 15 years, mostly at small installations of a few Ml/d, some of which have operated to meet the Florida advanced wastewater treatment standard of 3mg/L TN.
    Leopold has designed conventional water and wastewater filters for decades and recently has offered their filter for tertiary denitrification applications. Two installations in North Carolina show that less than 1mg/L NO3-N can be met at low loading rates.
    A number of Dynasand installations capable of denitrification have been constructed in the US and Puerto Rico over the past 15 years, and show that the filters can achieve less than 1mg/L NOx-N. In addition, pilot testing was conducted in 1989 by the University of Florida and by Black & Veatch in 2005 at the Hagerstown WwTW, Maryland.
    There are four Astrasand installations in Europe that are currently operating for denitrification, with the first one commissioned in 1999. Full-scale data from the De Groot Lucht plant in the Netherlands show that during 2000-2002, the Astrasand® filters reduced average influent NO3-N concentrations of 18mg/L to about 2mg/L at cold wastewater temperatures.
    All of the filters are capable of achieving an effluent solids concentration of 5mg/L or lower, and some are approved for Title 22 reuse requirements in California.
  • Design loading rates - design procedures for denitrification filters as reported in the literature have mainly focused on hydraulic and mass loading guidelines, suggesting 2.5-5m/h and nitrate loading rates up to 4kg NOx-N/m3/day. Using early data, design curves comparing nitrate removal efficiency with empty bed detention time for downflow denitrification filters were developed (Savage, 1983) and have been included in subsequent textbook publications.
  • Filter influent weir - downflow denitrification filters are operated at a variable level and generally have a significant drop over the influent weir. This simplifies nitrogen release, but may result in entrainment of dissolved oxygen (DO) and a related additional need for carbon, as well as reaction volume to consume the oxygen - and reduction of the nitrate removal efficiency if either of these is limiting. To mitigate this problem, curvilinear weirs are offered to encourage laminar flow down the wall in an attempt to minimise DO entrainment. Alternatively, operation at constant-level would reduce the elevation drop from the influent weir, decreasing the level of DO entrainment.
  • Media - a denitrification filter normally consists of approximately 2m of media over several layers of support gravel - for example, high grade 6x9 mesh silica sand with a 2-3mm effective size. In addition, a uniformity coefficient of approximately 1.35 and a minimum sphericity of 0.8 are required, reportedly in order to intensify backwash and nitrogen release cycles and lower backwash water volume. Upflow continuous filters are provided with a finer sand media (1.2-1.65mm effective size, as compared to a minimum 2mm effective size for the down flow filters), allowing 1.35-1.45mm effective size sub-round media or 1.55-1.65mm sub-angular media, both with a uniformity coefficient range of 1.3-1.6.
  • Underdrain - while upflow continuous backwash filters do not need underdrains, early experience with downflow denitrification filters suggested that nozzle filter bottoms were prone to fouling and failure (Pickard, 1985). To avoid this issue, all filter suppliers have developed block underdrains, for example the Tetra T-block, which is specifically designed for bioreactor service and consists of HDPE-jacketed, concrete-filled blocks. Alternatively, there is more limited experience with HDPE blocks and it is unclear as to whether fouling will be a significant issue over the long term when operating in the denitrification mode at moderate to high loading rates.
  • Nitrogen release cycle - a downflow wastewater flow is countercurrent to the path of the nitrogen gas produced in the denitrification process, which can be trapped and increase headloss. A periodic nitrogen release cycle is then needed, consisting of pumping backwash water up through the filter for 30-120s. The influent valve to the filter can remain open, speeding up the cycle and minimising filter downtime. Upflow continuous operation is co-current with the escape direction of the nitrogen gas, which is also drawn into the airlift, making a separate degassing cycle unnecessary.
  • Backwashing and filter controls - during operation of the denitrification filter, solids are filtered from the wastewater and accumulate in the media. Additional solids are produced through bacterial growth of the denitrification process. Backwashing is required to remove both filtered and excess biological solids, usually initiated on the basis of increased headloss through the filter or on a timed basis. Managed by integrated control packages, typical water and air scour rates during backwash are 15-25m3/m2/h and 90-150m3/m2/h respectively. As much as 10kg of solids/m2 can be reliably captured between backwashes, although normal design backwash frequency accounts for about half that value. Upflow filters operate with a small continuous backwash stream, with sand bed turnover rates ranging from 30-50cm/h or 4-6 bed turnovers per day. Recent pilot testing indicated that bed turnover rates of half those values were effective for denitrification. Although the airlift compressors are relatively small, it is estimated that the equivalent air scour is 2,700m3/m2/h and the equivalent washwater rate is 125-250m3/m2/h.
  • Methanol control system - methanol is normally dosed to the denitrification filter influent prior to the flow split between filter cells. In cases of more lenient effluent BOD and NO3-N levels, methanol control is mainly related to optimising chemical usage. Simple flow paced or feed-forward control using plant flow and filter influent nitrate concentration can be used, reasonably matching methanol dosing to actual requirements. To avoid overdosing and increased BOD concentrations in the filter effluent, methanol concentrations can be more precisely controlled with the influent flow rate, and influent and effluent nitrate concentrations can be measured simultaneously using an online nutrient analyser. This guarantees no net increase in total organic carbon across the filter. The advantages of tighter methanol control can be significant if the plant has a stringent BOD limit of 5mg/L or lower, in combination with a requirement for very low NOx-N concentration (less than 1mg/L). Under these conditions, the tighter control and reduced risk can be a critical component of ensuring the plant meets limits reliably.

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