Introductory Chapter: Ammonia Removal and Recovery

1. Introduction

With the application of nitrate-containing fertilizers, consumption of animal products, and industrial production activities, ever more ammonia and nitrate are being discharged into rivers and lakes, which may cause eutrophication and deterioration of aquatic environments. Traditionally, ammonia removal is achieved with biological processes such as nitrification, breakpoint chlorination, air stripping, reverse osmosis, zeolite adsorption, and so on. However, these processes either require high capital investment or are chemistry-intensive. Moreover, these processes focus on the removal instead of ammonia recovery. Ammonium nitrogen (N) is an important nutritional element in fertilizer besides phosphorus (P) and potassium (K). Recovering nutrients, instead of simply removing from wastewater, is drawing more attention to keep natural resources reliable and sustainable and minimize carbon footprint. Two processes focusing on nutrient removal and recovery have stood out in recent years and drawn attention from engineers, facility operators, and regulators.

2. Gas-permeable membrane for ammonia recovery

In the application of a gas-permeable microporous membrane, the wastewater stream is first adjusted to a pH value of at least 9.5 (Figure 1). And then, the stream passes through one side of the membrane and dissociates ammonium from water, and ammonia penetrates through the membrane; a dilute acid solution is circulated on the other side of the membrane, and sequesters ammonia to form ammonium sulfate (Figure 2). Ammonium sulfate solution can be used as a by-product for fertilizers or other purposes. The hydrophobic hollow fiber membrane can be used as a medium to separate aqueous phases because it is not inherently selective between permeating species. The driving force of mass transfer is the concentration difference between the two sides of the membrane. On the two sides of the membrane, pH values are distinctively different. On the wastewater side, the pH is at least 9.5 or higher, and on the dilute acid side, the pH is 2 or lower.

Figure 1.

Ammonium speciation in water at different pH [1].

Figure 2.

Schematic diagram for gas-permeable membrane for ammonia recovery (from [2]).

This process was applied in full-scale systems in manufacturing facilities [2, 3]. It was found that removal rate was achieved as high as 95 to 97%. The two important operating parameters are wastewater pH and temperature. With an increasing pH value of the wastewater stream, more ammonium species is converted from ammonium to ammonia; the mass transfer is thereby enhanced, and ammonia removal efficiencies are further improved. Since Henry’s law constant increases at an elevated temperature, and favors the gas phase concentrations (Figure 3), temperature will also affect the rate of transfer from the liquid to the gas phase, with faster rate at higher temperatures. Therefore, increased temperature will improve the ammonia removal in this application. On the dilute acid side, it is circulated counter current, and pH is maintained 2 or less by supplementing acid, until ammonia sulfate reaches a certain level. Typically, ammonium sulfate concentrations can reach to 25 to 30%. Because the acid stream is not in contact with the wastewater stream, high-quality ammonia sulfate solution can be obtained.

Figure 3.

Effect of temperature on Henry’s law constant for ammonia [4].

This process has significant advantages such as low capital investment, small footprint, lower energy cost, and recovery of a valuable by-product as ammonium sulfate.

Currently, 3 M and duPont are promoting their own Degasification Membrane Modules for the application of ammonia removal and recovery.

3. Struvite precipitation for nutrient recovery

Struvite is a crystal compound, consisting of magnesium, phosphorus, and nitrogen. It naturally forms in many parts of a wastewater treatment plant, such as anaerobic digesters, aerobic sludge digesters, digestate pipes and pumps and valves, plant feed pumps , sludge-holding tanks or thickeners, centrifuges, outfall pipes, centrate pipes, and so forth. It creates scales and may also cause process disruptions. Its chemical formula is NH₄MgPO₄.6H₂O. Stoichiometric reaction is shown below:

$$M{g}^{2+}+N{H}^{4+}+PO43^{-}+6H_{2}O=MgN{H}_{4}P{O}_4:6H_{2}O$$E1

This compound is slightly soluble in water. When spread in agriculture fields, it slowly releases nitrogen and phosphorus, which are important nutritional elements for plant growth.

Digested sludge usually consists of high concentration of ammonia and phosphate, because bacteria decompose and release ammonia nitrogen and phosphorus during aerobic or anaerobic digestion. Animal wastes, such as swine waste, also consists of high concentrations of ammonia and phosphorus. These wastewater streams may cause concerns of scale formation and process disruptions and yet present opportunity for nutrient removal and recovery if properly managed.

Under proper temperature and pH, struvite forms when the concentrations of ammonium, phosphate, and magnesium exceed the solubility product, and struvite precipitates in a molar ratio of 1:1:1 of ammonium, phosphate, and magnesium.

It was summarized that the preferred pH rage is 8 to 9 and a high Mg/P ratio drives a higher efficiency of struvite conversion [6]. The presence of calcium, however, negatively affects the formation of struvite. Figure 4 shows the influence of pH and Mg/P ratio on the struvite formation efficiency.

Figure 4.

Influence of pH and Mg/P molar ratio on P recovery [5].

Ostara’s Peral® process, which recovers struvite through controlled precipitation in a fluidized bed from digestate, was successfully applied in plants in North America and Europe. It removes and recovers nutrients (ammonia and phosphorus) from digestate and produces high premium-crystal-form struvite, which, in turn, is recycled back for agricultural use as a fertilizer [7]. Using a fluidized bed reactor for struvite precipitation, it was found the payback period is less than 10 years in Budds Farm wastewater treatment plant in England [8].