Nitrate (NO₃) remains a major concern for surface waters. Nitrate (and phosphate) from fertilizers, wastewater treatment facilities and other sources can lead to hypertrophication and hypoxia in waters, resulting in fish kills and dead zones. 

Biological nitrogen removal by suspended growth has limited ability to remove nitrate through denitrification. Some systems can achieve a total nitrogen (TN) of less than 3 mg/L, most being nitrate-nitrogen (NO₃). However, the average TN effluent concentration for suspended growth systems designed for nitrification and denitrification is 5 to 8 mg/L. As total nitrogen limits become more stringent, processes must achieve much lower nitrogen concentrations. Deep bed, denitrification biofilters can achieve NO₃ levels of less than 1 mg/L. 

Today, treatment plants are looking to sand filtration for added removal after clarification. This tertiary filtration may be the final treatment before disinfection or pretreatment for an advanced process such as microfiltration. In either case, with proper design, monitoring, and control, sand filtration can also be used to reduce nitrate to well below 1 mg/L. 

Process requirements 

Physically, the deep bed biofilter for nitrogen removal is a monomedia filter, essentially the same in structure as a filter used for drinking water or tertiary wastewater treatment. The biofilter uses siliceous sand between 1.5 and 3.0 mm in diameter. The sand grains can be rounded, though some recent studies indicate that rough, angular sand may be superior in retaining the biomass responsible for denitrification within the voids. To achieve total nitrogen reduction through denitrification: 

• Nitrate must be the predominant form of nitrogen entering the filter. Essentially all the organic nitrogen and ammonia must be converted to nitrate through oxidation in the preceding processes. To get this near-total conversion of organic nitrogen and ammonia to nitrate, almost all carbonaceous material will be oxidized.
• Dissolved oxygen must be as close to zero as possible. Oxygen inhibits the activity of the denitrifying enzymes.
• A supplemental electron donor must be provided. For most denitrification systems, methanol provides needed electrons. Methanol has many benefits: It is readily available at low cost, produces very little sludge, releases low amounts of volatile organic compounds, and does not add nitrogen or phosphorus to the process. 

Process operation 

The biofilter operates like a regular drinking water filtration unit, but with a few critical differences. Just as in a water filtration system, the water flows into the filter through the washwater troughs, over the trough weirs, and down through the media (both sand and support gravel) to the underdrains. From the underdrains, it is collected to a flume and flows to the clearwell. In the clearwell, which also holds the backwash water, the effluent accumulates until it overflows that channel weir and proceeds to disinfection and discharge. 

And, as in drinking water filtration, once the pressure drop through the filter reaches a certain level, indicating solids accumulation is high, a full backwash sequence begins. Water and air are sent up through the filter at a rate required to remove the solids, which are carried over the weirs of the washwater troughs. From the troughs, the backwash water with solids flows into a mudwell. 

However, in the wastewater application, nitrogen gas accumulates within the filter voids, causing the pressure drop through the filter bed to increase more rapidly than it would from solids accumulation alone. At this point, the filter does not need to be thoroughly cleaned – and in fact this is not desirable, because the biomass solids are needed to remove the nitrate through denitrification. Instead, the nitrogen gas has to be released to free up the void spaces. 

So for these intermediate pressure increases, a short backwash of water is used for about five minutes – this is called “bumping” the filter. This short, water-only backwash is enough to drive the nitrogen gas out of the voids and restore the filter’s treatment efficiency. When bumping the filter no longer restores the pressure drop to acceptable levels, a full backwash with air scour is initiated to remove solids. 

After an individual filter cell receives a complete water and air scour backwash, it may have reduced denitrification capacity until the biomass becomes reestablished. For this reason, these systems are designed to use several small filters rather than a few large ones. This enables the blended effluent to remain within very tight treatment standards. An angular media may retain the biological solids more effectively than rounded media, allowing the filter to recover faster. 

The deep bed provides the oxygen-free environment needed for the denitrification biomass. The volume of the basin is also designed to achieve the hydraulic retention time required for the biological reaction – about 30 minutes (empty bed contact time). Hydraulic loadings are lower than for standard drinking water systems and typically are 1 to 2 gpm/sf of filter area. 

Process control 

In addition to controlling the backwash cycles, control of the methanol feed is critical to denitrification. Too much and methanol will leave with the effluent, increasing effluent BOD. Too little and denitrification will decrease due to lack of adequate electron donors. 

The development of new, reliable real-time nitrate analyzers enables accurate methanol feed control in two basic ways: 

• By measuring nitrate in the influent (feed forward control loop).
• By measuring nitrate in the effluent (feed back control loop). 

A more complex compound loop involves analysis of the influent and effluent nitrate levels. The gross methanol feed signal is determined based upon influent concentration of nitrate, and the signal is then trimmed based upon the effluent nitrate concentration. 

Putting it all together for a better environment 

Sand media filtration, besides providing tertiary treatment, can deliver highly cost-effective control of nitrate to meet increasingly demanding standards. All it takes is a little biology. 

About the Author
Janet McSwain is a licensed Professional Engineer and freelance writer, specializing in the areas of water and wastewater treatment. Contact her at j.mcswain@watersolutionsb2b.com or www.watersolutionsb2b.com.