Nitrogen Reduction in the AdvanTex^® Textile Treatment Plant 

• Design Criteria
• Process Overview
• Nitrification
• Denitrification

Design Criteria

Of specific importance for many projects in environmentally sensitive areas is the ability of the treatment process to remove nitrogen from the effluent. NSF testing shows that treated effluent levels of TN = 10 mg/L can be achieved when the packed bed reactor is loaded at no more than 0.0087 lb/ft²/day.  Data suggest a linear relationship between nitrogen load and nitrogen removal. Therefore to achieve TN levels of less than 20 mg/L (at peak flows), the applied load may increase to 0.0174 lb/ft²/day.Based on these conservative design criteria it can confidently be expected that the proposed treatment plants will achieve the following effluent standards.

Table 1:  Effluent Quality Criteria – AdvanTex^®

Process Overview

Most nitrogen excreted by humans is in the form of organic nitrogen (dead cell material, proteins, amino acids) and urea. After entering the septic tank, this organic nitrogen is broken down fairly rapidly and completely to ammonia, NH₃, by microorganisms in the septic tank. Thus, ammonia is the primary form of nitrogen leaving the septic tank. To remove total nitrogen from wastewater the ammonia must be converted to gaseous nitrogen, N₂. In this form nitrogen is inert and can be released to the air.

Biological conversion of ammonia to nitrogen gas is a two step process. Ammonia must first be oxidized to nitrate; nitrate is then reduced to nitrogen gas (Crites and Tchobanoglous, 1998; Henze et al. 1995). These reactions require different environments and are carried out in separate areas of the treatment system.

Each person in a household generates wastes containing ~4 (range 3-7) kg of nitrogen per year (Gold and Sims, 2001;Henze et al., 1995; Crites and Tchobanoglous, 1998), commonly resulting in daily N loads of 10-13 g N per day (Crites and Tchobanoglous, 1998; NZLTC, 2000).

Nitrification

The first step in the process, conversion of ammonia to nitrite and then to nitrate, is called nitrification
(NH₃  → NO₂^- → NO₃^-). The process is summarised in the following equations:

NH₄⁺ +  ³/₂ O₂ → NO₂^- + 2H⁺ + H₂O

NO₂^- + ¹/₂ O₂ → NO₃^-

The process is mediated by the bacteria Nitrosomonas and Nitrobacter which require an aerobic environment for growth and metabolism of nitrogen. Thus, the nitrification process must proceed under aerobic conditions. In the case of the AdvanTex^® filter in mode 3 (see figure 3), the nitrification process takes place in the aerobic conditions prevalent in the textile material. The nitrifiers are among the naturally occurring organisms that populate the surface of the textile fibres.

During nitrification around 9 parts of oxygen are consumed in converting 2 parts of ammonia to nitrate.  Reduction of BOD also competes for free oxygen, with more aggressive (heterotrophic) organisms. This means that the nitrification process tends to follow the depletion of organic content.  The venting of the textile pods (forced in the case of commercial systems) increases available oxygen and encourages aerobic conditions. The growth of (autotrophic) nitrifying microbes typically takes 1-3 months (depending on available oxygen and temperature).

For domestic strength wastewater the influent TKN (organic plus ammonia nitrogen) is typically 60-65 mg/L, with a typical TN (TKN plus NO₂^- + NO₃^-) levels of 65 mg/L and ammonia levels of 60 mg/L.

Typical nitrification processes in the AdvanTex^® provide 98-99% reduction to < 5 mg/L of ammonia in the final effluent.

Denitrification

The second step of the process, the conversion of nitrate to nitrogen gas, is referred to as denitrification. This process can be summarised as:

NO₃^- +  ⁵/₆ CH₃OH → ¹/₂ N₂ + ⁵/₆ CO₂ + ⁷/₆ H₂O + OH^-

This reduction of nitrate to nitrogen gas is also mediated by bacteria and requires low oxygen conditions, with dissolved oxygen levels near zero. The bacteria also require a carbon food source for cell synthesis and nitrogen reduction. The bacteria metabolise the carbonaceous material or BOD in the wastewater as a food source, metabolising it to carbon dioxide. The denitrification process is enhanced by returning part of the nitrified effluent stream to the carbon-rich (high BOD), low oxygen environment in the anoxic blend tank prior to the recirculation