Nitrogen removal in lagoons

4.11.1 Introduction

The BOD and suspended solids removal capability of lagoon systems has been reasonably well-documented, and reliable designs are possible; however, the nitrogen removal capability of wastewater lagoons has been given little consideration in system designs until recently. Nitrogen removal can be critical in many situations because ammonia nitrogen in low concentrations can adversely affect some young fish in receiving waters, and the addition of nitrogen to surface waters

FIGURE 4.27 Praxair® In-Situ Oxygenation (I-SO™) system. (Courtesy of Praxair Technology, Inc., Danbury, CT.)

can cause eutrophication. In addition, nitrogen is often the controlling parameter for the design of land treatment systems. Any nitrogen removal in the preliminary lagoon units can result in very significant savings in land and costs for the final land treatment site. The following sections describe several conventional and commercial products that have been developed for nitrogen removal.

4.11.2 Facultative Systems

Nitrogen loss from streams, lakes, impoundments, and wastewater lagoons has been observed for many years. Extensive data on nitrogen losses in lagoon systems were insufficient for a comprehensive analysis of this issue until the early 1980s, and no agreement was reached on the removal mechanisms. Various investigators have suggested algae uptake, sludge deposition, adsorption by bottom soils, nitrification, denitrification, and loss of ammonia as a gas to the atmosphere (volatilization). Evaluations by Pano and Middlebrooks (1982), USEPA (1983), Reed (1984), and Reed et al. (1995) suggest that a combination of factors may be responsible, with the dominant mechanism under favorable conditions being volatilization losses to the atmosphere, as shown by the relative size of the arrows in Figure 4.28.

The U.S. Environmental Protection Agency sponsored comprehensive studies of facultative wastewater lagoon systems in the late 1970s (Bowen, 1977; Hill and Shindala, 1977; McKinney, 1977; Reynolds et al., 1977). These results

Lost to Atmosphere

Plants and Animals

Lost to Atmosphere

Plants and Animals

Effluent

Bottom Deposits

Bottom Deposits

FIGURE 4.28 Nitrogen pathways in wastewater lagoons under favorable conditions.

provided verification that significant nitrogen removal does occur in lagoon systems. Key findings from those studies are summarized in Table 4.17. These results verify the consensus of previous investigators that nitrogen removal was in some way related to pH, detention time, and temperature in the lagoon system. The pH fluctuates as a result of algae-carbonate interactions in the lagoon, so wastewater alkalinity is important. Under ideal conditions, up to 95% nitrogen removal can be achieved from facultative wastewater stabilization lagoons.

Several recent studies of nitrogen removal have been completed, but the quantity of data is limited. A study of 178 facultative lagoons in France showed an average nitrogen removal of 60 to 70%; however, only a limited quantity of data was available from each lagoon system (Racault et al., 1995). Wrigley and Toerien (1990) studied four small-scale facultative lagoons in series for 21 months and observed an 82% reduction in ammonia nitrogen, but an extensive sampling program similar to those conducted by the USEPA in the late 1970s was not carried out.

Shilton (1995) quantified the removal of ammonia nitrogen from a facultative lagoon treating piggery wastewater and found that the rate of volatilization varied from 0.07 to 0.314 lb/1000 ft2-d (355 to 1534 mg/m2-day). The rate of volatilization increased at higher concentrations of ammonia nitrogen and TKN.

Soares et al. (1995) monitored ammonia nitrogen removal in a wastewater stabilization lagoon complex of varying geometries and depths in Brazil. The ammonia nitrogen concentrations were lowered to 5 mg/L in the maturation lagoons, thus making the effluent satisfactory for discharge to surface waters. It was found that the ammonia removal in the facultative and maturation lagoons could be modeled by the equations based on the volatilization mechanism proposed by Pano and Middlebrooks (1982).

Commercial products, as mentioned in the introduction to this section, appear to offer improvements that may remove significant amounts of ammonia nitrogen and some total nitrogen. Some of the options are described below.

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