Many types of fixed media filters are available and have been used for on- site treatment systems. The basic model of such filters is the sand or gravel filter, with or without vegetation. Many possible variations in configuration and operation are possible to achieve particular treatment goals. The goal that will be discussed in this presentation is nutrient reduction. The basic types of fixed media filters will be discussed in terms of the removal mechanisms for nitrogen and phosphorus. Modifications that enhance nutrient removal will be discussed, as will as treatment performance.

Although nitrogen (N) and phosphorus (P) are both important plant nutrients that can stimulate excess growth of plants and algae in receiving waters, they behave very differently and thus require different mechanisms for removal. Removal of N and P are not entirely independent of removal of BOD and suspended solids, the more traditional concerns for on-site systems, so these parameters will be discussed to some extent as well. Removal of suspended solids, in particular, will affect removal of N and P because some of these nutrients will be contained in the particulate form. Nitrogen removal shares many traits with BOD removal, and the biological processes involved are affected by the presence or absence of BOD. Phosphorus removal is usually not dominated by biological processes, however, and is much less affected by BOD. Because N and P are removed by different mechanisms, they will be discussed separately.

Nitrogen removal is primarily accomplished using bacterial reactions. Nitrogen in wastewater is mostly in the form of ammonia (NH₄⁺ or NH₃) or organic compounds which break down to release ammonia. Historically, concerns with ammonia have included the oxygen demand associated with ammonia and toxicity to aquatic organisms of the NH₃ form of ammonia. A special type of bacteria, called nitrifying bacteria, convert ammonia to nitrate:

This conversion of ammonia to nitrate addresses the historical concerns with ammonia. However, concern with nitrogen as a nutrient is not addressed by this reaction alone, as all the N is still in the water in the form of nitrate. Nitrate can also be transformed by bacterial reactions. Denitrifying bacteria convert nitrate to N2 gas:

There are several types of commonly used fixed media filters. One common type is frequently called the intermittent sand filter. These are operated as once-through, vertical flow packed beds, with the packing media usually being sand. The key to good performance is to not allow full saturation of the bed so that oxygen can transfer into the liquid from the air. If BOD and/or suspended solids loadings are too high, clogging of the top of the media can occur. Nitrification usually occurs deeper in the bed, but denitrification may be more difficult to achieve due to BOD removal in the upper part of the filter.

Another common type of system is generally called a recirculating sand filter. In these systems, part of the effluent is recycled back to combine with the influent. This dilution of the influent helps to prevent over-loading the top of the filter. The media used may be sand or gravel, and is frequently coarser than in an intermittent sand filter. Loading rates can also be higher with these types of systems because more removal occurs throughout the depth of the filter. These types of systems achieve better total nitrogen removal because the nitrified effluent combines with the influent which contains BOD, thus allowing greater opportunity for denitrification (NO₃ to N₂). These systems are also operated with intermittent or unsaturated flow to allow oxygen transfer to the liquid. Even with oxygen transfer to the liquid, anaerobic conditions will occur within the bacterial slime layers that form around the media particles.

Sand/gravel filters are also designed and operated as vegetated systems. Such systems go by a number of different names, including subsurface flow constructed wetlands, horizontal flow constructed wetlands, vertical flow constructed wetlands, rock-reed filters, and root-zone method. The vertical flow versions of these systems operate very much like sand filters with various types of plants growing in the surface. Plants can add to the removal efficiency of a system by keeping the surface from developing a crust, keeping channels of flow open to help prevent saturated conditions, providing various types of environmental microzones around the roots, and possibly contributing oxygen and/or organic compounds through the root hairs. Plants will also directly take up nutrients, thus removing them from the liquid. However, this is usually not a major removal mechanism for nutrients in these types of systems.

The horizontal flow versions of vegetated fixed media filters are operated with saturated flow below the surface. These types of systems are characterized by low oxygen concentrations. They can be used to follow a vertical flow filter to provide denitrification for total nitrogen removal. Plants may contribute organics for this process, either by transporting organic compounds through the roots or by enhancing leaching of organics from debris on the surface. These systems can also be operated with intermittent flow to allow altering aerobic and anaerobic environments.

Factors that affect the performance will be discussed, including hydraulic loading rate, mass loading rate, dosing cycle, size and uniformity of media, and BOD (too much and not enough). Modifications of the basic types of systems for enhancement of nitrogen removal will also be discussed.

The second major nutrient of concern in wastewater treatment is phosphorus. Phosphorus is not transformed by biological reactions (as is nitrogen). However, it does adsorb to some substances and forms insoluble compounds that precipitate and can be removed by filtration. For fixed media filters, the method most often tried for P removal is addition of an adsorptive substance to the media, such as iron or aluminum. The adsorptive capacity will become saturated over time. So, if P removal is important, the system should be designed to allow for replacement of the filter media.

Limited performance data for nutrient removal in these types of systems is available in the literature. Results from monitoring studies will be discussed. Results will also be discussed from operation of a system in rural Chatham County, North Carolina, using simple, aesthetically pleasing treatment components constructed in outdoor and indoor environments to reclaim domestic sewage for water reuse (by Halford House of Aqua Neoterics). A courtyard containing constructed wetlands and a solarium with soil filter components were designed to treat sewage from a small business facility and to provide recreational space for its 60 employees. The system is designed to treat and reuse 4500 L/day (1200 gal/day) of domestic sewage from the business. After treatment within a septic tank, the partially treated water is pressure dosed into vegetated sand filters stacked upon a subsurface horizontal flow constructed wetland cell at a hydraulic loading rate of 40-120 L/m²/day (1-3 gal/ft²/day). Dosing is controlled by a time switch to insure 6-to 8-hour intervals between cycles to maintain an aerobic environment within the upper 1 ft of the substrate within the cell. After moving vertically to the cell bottom, the partially treated water flows by gravity into another horizontal flow cell. The water is then pumped into 6 soil filter boxes in the solarium. The boxes contain different filter materials to test their effectiveness and are planted with tropical vegetation. Samples were taken monthly for three years from inlet and outlet locations of the systems components.

Average concentrations (and removals) in the effluent over the three years of monitoring were total nitrogen, 11.3 mg/L (79.5%); ammonium nitrogen, 0.06 mg/L (99.7%); total phosphorus, 2.8 mg/L (57.1%); total suspended solids, 1.5 mg/L (96.9%); and COD, 6.3 mg/L (96.0%). Each of the components will be discussed individually as to effectiveness of each type of component for on-site reuse treatment systems.

Please address any questions to Dr. David Lindbo.

This page created by
Roland O. Coburn, Research Technician I on 2/17/03.