7.3.3.1 Nitrogen Forms

Nitrogen in municipal wastewater originates from human urine and excreta. Of 100% influent nitrogen, 85% is in the treated wastewater, and 15% is in the digested sludge solids. About 40% is ammonia and 60% is bound in organic matter. The annual per capita nitrogen ranges from 4 to 6 kg/cap/year that results in 35 mg/l total nitrogen in wastewater stream (Figure 7.9).

Total nitrogen is the sum of organic, ammonia, nitrate, nitrite, and gaseous nitrogen. Decomposition of nitrogenous organic matter releases ammonia to solution:

(7.5)

Nitrifying bacteria will oxidize ammonia to nitrite and nitrate under aerobic conditions as shown in (7.6):

(7.6)

Denitrification will take place under anaerobic or anoxic conditions. Organic matter (AH₂) is oxidized by nitrate which serves as the hydrogen acceptor. The end product is nitrogen gas:

(7.7)

Primary sedimentation removes less than 15% of total nitrogen, while AS removes another 10%. As a result, only 4% BOD contributed by N could be removed. In anaerobic digester, 40% of the organic nitrogen in the sludge could be converted to ammonia, and 10% of the original 25% is recycled to the treatment plant and appears in the effluent, which then contains 85% of the influent nitrogen. Typically, nitrogen removal in conventional biological treatment systems is less than 40%. Therefore, tertiary treatment referred as nitrification/denitrification is required to achieve 1 mg/l discharge standard in most states of the United States.

7.3.3.2 Nitrification

Conventional activated sludge (CAS) is designed to remove carbonaceous BOD with minimal removal of ammonia nitrogen. The high ammonia content and low BOD provide greater growth potential for the nitrifiers relative to the heterotrophs. Therefore, the nitrification process at an increased sludge age compensates for lower operating temperature to ensure the growth rate of nitrifying bacteria. In effluent, some of these bacteria is lost and would be supplemented by newly produced bacteria. Temperature, pH, and dissolved oxygen concentration are the important parameters. The dissolved oxygen level should be greater than 1.0 mg/l at optimal pH near 8.4. Lime or soda ash may be needed to raise the pH to the optimum level in the nitrification tank to supplement the depleting alkalinity. Ammonia nitrogen loadings should vary from 160 to 320 g/m³/day at 10–20 °C, respectively, with aeration period from 4 to 6 h.

7.3.3.3 Denitrification

Nitrate can be reduced to nitrogen gas by facultative heterotrophic bacteria in an anoxic environment. An organic carbon source such as methanol is needed to act as a hydrogen donor and to supply carbon for biological synthesis. Methanol is commonly used due to its high biodegradability. Since methanol is expensive, the cost of methanol is the major limit of denitrification process. Biodegradable organic matter can be used as an oxygen acceptor (hydrogen donor) for the conversion of nitrate to nitrogen gas. Therefore, a portion of raw wastewater could supply the carbonaceous BOD required:

(7.8)

In the past engineering design, a biological nitrification/denitrification process requires the mixing of raw organic matter with nitrified wastewater. As a result, an aerobic zone is needed for nitrification and an anoxic zone for denitrification (Figure 7.10).

Methanol reacts with nitrate as follows:

(7.9)

Since the effluent from nitrification also contains dissolved oxygen and nitrite, the total methanol required as a hydrogen donor in denitrification can be estimated using Equation 7.10:

(7.10)

In addition, methanol is a carbon source in bacterial synthesis (Figure 7.11). Assuming that 30–50% excess methanol is needed for bacteria synthesis, the total methanol demand can be calculated using Equation 7.11:

(7.11)

where

• CH₃OH = methanol, mg/l
• DO = dissolved oxygen, mg/l
• NO₂–N = nitrite nitrogen, mg/l
• NO₃–N = nitrate nitrogen, mg/l

A plug‐flow tank with underwater mixers is usually followed by a clarifier for sludge separation and return in designing the denitrification system. The retention time from 2 to 4 h is required for denitrification of a domestic wastewater depending on nitrate loading and temperature. Since methanol is expensive, traditional denitrification following nitrification is gradually being replaced by anaerobic ammonium oxidation (Anammox).

7.3.3.4 Anammox

The conventional biological nitrification and denitrification requires costly methanol. Therefore, a huge energy unit consumption up to 4.0 kWh/kg‐N is required. As a result, this denitrification process is considered as unsustainable design in wastewater treatment. To reduce this energy consumption and to eliminate the addition of methanol, anaerobic ammonia oxidation (anammox) was discovered in 1990 (Van Loosdrecht and Salem, 2006). In anammox, ammonium is the electron donor, and nitrite is the electron acceptor that is converted anaerobically into mainly nitrogen gas and some nitrate by anammox bacteria (Strous et al., 1998). Figure 7.12 shows that anammox is the shortcut between ammonia and nitrate under biological degradation by planctomycetes. Since NO₂⁻ donates electron to ammonia, it reduces aeration and sludge production and eliminates methanol requirement as shown in Figure 7.12.

Table 7.13 shows the optimal parameters for the anammox process.

Quantitatively, anammox could reduce organic carbon (methanol), aeration, and sludge production by 100, 60, and 90%, respectively (Van Loosdrecht and Salem, 2006; Siegrist et al., 2008). The energy demand of the semibatch reactor (SBR) sidestream treatment systems ranged from as low as 0.8 kWh/kg‐N to around 2 kWh/kg‐N. For example, Wett et al. (2010) reported 1.2 kWh/kg‐N. Compared with a conventional nitrification and denitrification (N/DN) sidestream treatment with an energy demand of approximately 4.0 kWh/kg‐N (only accounting for electricity consumption in the sidestream), the savings of PN/A SBR systems are at least 50% depending on the oxygen transfer efficiency of aeration type. Therefore, more than 100 plants have anammox worldwide in 2014. SBR consists of 50% of all the plants, while granular systems include moving bed biofilm reactors (Rosenwinkel and Cornelius, 2005).

Nitrite is both an essential substrate and an inhibitor to the reaction. Strous et al. (1999) suggested that nitrite concentrations greater than 100 mg‐N/l will completely stop the reaction. Egli et al. (2001) showed that anammox was only inhibited at nitrite concentrations higher than 182 mg‐N/l. However, as high as 350 mg‐N/l barely caused a 50% inhibition of the anammox process (Dapena‐Mora et al. 2007). Inhibition frequency and time are all important for the performance of anammox. For these reasons, more than 50% of operation problems of anammox are due to the buildup of either nitrite or nitrate (Lackner et al., 2014).

Figure 7.13 illustrates the patented OLAND scheme and the mainstream line of WWTPs, respectively.

Table 7.14 shows that the chemical costs of nitrification/denitrification of $1.839 million/year due to addition of methanol could be completely eliminated with the anammox process. Indeed, anammox is the key technology in retrofitting traditional WWTP to energy‐positive WWTP in Austria.

7.5.3.1 Xiongan Project

1. On‐site WWTP could play a significant role since Xiongan has many villages or satellite cities. For on‐site WWT, what type of wastewater treatment reactor would you recommend?
2. As the Xiongan district expands with WW generated from one million people in 1 year and two million people in 5 years, would you still recommend on‐site WWTP only? What are the advantages of centralized WWTP?
3. How much sludge and methanol could be saved if your new WWTP adopted anammox annually when OLAND was used in the side and mainstream line, respectively?
4. What would be the O&M cost saving over a life cycle of 50 years when OLAND is used in the side and mainstream line, respectively?
5. What would be the O&M cost saving over a life cycle of 50 years when UV disinfection is to replace chlorination?

7.5.3.2 Proposal Project

1. Make an appointment with the WWTP manager of your local town, and get data on methanol consumption and sludge produced annually. After you have analyzed the WWTP processes, please do the following:
   1. Develop an alternative using anammox and quantify monetary saving by eliminating methanol for nitrification/denitrification for over 50 years. Assuming flow rate is 120 MGD average daily flow; inflow total is 40 mg/l as nitrogen. Forty percent of the load would be removed after AS process. The side stream anammox is being proposed to retrofit the plant. Quantify the monetary saving by eliminating methanol for nitrification/denitrification by using side stream anammox over 50 years. Assume that 0.49 gal of methanol is needed for every pound of nitrogen removed in the conventional nitrification/denitrification process.
   2. Design an alternative using UV disinfection to replace chlorination for this 120 MGD. Following the examples used in the chapter, estimate the capital and O&M costs for both UV disinfection and chlorination over 50 year design life expectancy.