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.
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.