## 4 Discussion  
The unionized form of ammonium--free ammonia (NH\({}_{3}\) or FA) and free nitrous acid (HNO\({}_{2}\) or FNA) in the wastewater [7] are directly influenced by wastewater pH. An increase in pH leads to an increase in FA concentration. Under alkaline conditions (pH \(>\) 7), the concentration of FNA will increase as the pH decreases. The principle is characterized by ionized ammonium (NH\({}_{4}\)+) and free ammonia (NH\({}_{3}\)) equilibrium (1) and the nitrous acid equilibrium (2)  
\[\text{NH}_{3}+\text{H}_{2}\text{O}\leftrightarrow\text{NH}_{4}{}^{+}+\text{OH} ^{-} \tag{1}\]  
\[\text{H}^{+}+\text{NO}_{2}{}^{-}\leftrightarrow\text{HNO}_{2} \tag{2}\]  
The concentrations of NH\({}_{3}\) and HNO\({}_{2}\) are a function of total ammoniacal nitrogen (TAN = NH\({}_{4}\)\({}^{+}\) + NH\({}_{3}\)) and total nitrite concentrations (NO\({}_{2}\)\({}^{-}\) + HNO\({}_{2}\)), pH, and temperature [4]. Inorganic carbon source is necessary for the alkalinity requirements to buffer the acidification of the sludge due to the nitritation process [2]. To evaluate the effect of rapid pH variation on the activated sludge process, two critical pH levels, 6.5 and 8.5, were selected for lab-scale tests under the study. Calculated concentration of FNA in acidic reactor was 0.228 mg/L (NH\({}_{4}\)-N = 53.0 \(\pm\) 7.0 mg/L, T = 20 \({}^{\circ}\)C, pH 6.5) and concentration of FA in alkaline reactor was 8.82 mg/L (NH\({}_{4}\)-N = 53.0 \(\pm\) 7.0 mg/L, \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multicolumn{2}{c}{**Average (n = 3)**} & \multirow{2}{*}{**Acclimatization**} & \multicolumn{6}{c}{**Acidic Reactor, ind/mL**} & \multicolumn{6}{c}{**Alkaline Reactor, ind/mL**} \\ \hline
**Microfauna Group** & **Genus/Species** & **Cycle** & **0 h** & **24 h** & **48 h** & **72 h** & **0 h** & **24 h** & **48 h** & **72 h** \\ \hline \multirow{3}{*}{gymnamoebeae} & _Amoeba limax_ & 23 & 33 & 90 & 80 & 123 & 37 & 37 & 57 & 53 \\ \cline{2-10}  & _Mayorella_ & 103 & 160 & 310 & 330 & 343 & 197 & 303 & 173 & 267 \\ \hline \multirow{3}{*}{testate amoebae} & _Arcella_ & 117 & 103 & 90 & 90 & 57 & 157 & 90 & 73 & 60 \\ \hline \multirow{3}{*}{crawling ciliates} & _Chilodonella_ & 27 & 43 & 40 & 57 & 187 & 50 & 40 & 123 & 33 \\ \cline{2-10}  & _Aspidisca_ & 140 & 120 & 63 & 93 & 47 & 140 & 57 & 37 & 37 \\ \hline \multirow{3}{*}{free swimming ciliates} & _Litonotus_ & 330 & 583 & 250 & 60 & 37 & 493 & 247 & 73 & 77 \\ \cline{2-10}  & _Proordon_ & 537 & 567 & 550 & 420 & 217 & 583 & 500 & 287 & 220 \\ \cline{2-10}  & _Hologhraa_ & 60 & 30 & 57 & 57 & 23 & 23 & 47 & 60 & 40 \\ \cline{2-10}  & _Glaucoma_ & 50 & 27 & 33 & 77 & 87 & 40 & 43 & 77 & 60 \\ \cline{2-10}  & _Spirostomum_ & 20 & 37 & 13 & 17 & 3 & 17 & 33 & 17 & 13 \\ \hline \multirow{3}{*}{stalked ciliates} & _Epistylis_ & 436 & 465 & 348 & 156 & 440 & 239 & 396 & 251 & 436 \\ \cline{2-10}  & _Carchesimum_ & 77 & 30 & 50 & 40 & 20 & 13 & 33 & 47 & 23 \\ \cline{2-10}  & _V. corullaria_ & 93 & 80 & 60 & 73 & 73 & 67 & 133 & 50 & 50 \\ \cline{2-10}  & _V. microstoma_ & 347 & 150 & 193 & 140 & 127 & 187 & 163 & 57 & 30 \\ \hline \multirow{3}{*}{carnivorous ciliates} & _Acineta_ & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 3 & 0 \\ \cline{2-10}  & _Tokophraa_ & 3 & 0 & 0 & 3 & 3 & 3 & 3 & 10 & 17 \\ \hline \multirow{3}{*}{rotifers} & _Rotaria_ & 47 & 40 & 40 & 37 & 37 & 63 & 60 & 50 & 60 \\ \cline{2-10}  & _Cephalodella_ & 20 & 30 & 27 & 17 & 10 & 30 & 27 & 20 & 27 \\ \hline \multirow{3}{*}{worms} & _Nematoda_ & 23 & 20 & 17 & 20 & 7 & 7 & 3 & 10 & 3 \\ \cline{2-10}  & _Aeolosoma_ & 3 & 3 & 10 & 13 & 13 & 7 & 13 & 17 & 10 \\ \hline \multirow{3}{*}{zoogloea} & _Z. ramifera_ & 60 & 70 & 163 & 293 & 383 & 73 & 100 & 150 & 130 \\ \cline{2-10}  & _Z. uvu_ & 50 & 47 & 87 & 120 & 190 & 43 & 67 & 93 & 70 \\ \hline \multicolumn{10}{c}{Total abundance, ind/mL} & 2566 & 2639 & 2492 & 2193 & 2427 & 2469 & 2396 & 1734 & 1716 \\ \hline \multicolumn{10}{c}{Shannon–Weaver index} & 2.44 & 2.33 & 2.50 & 2.60 & 2.48 & 2.40 & 2.50 & 2.71 & 2.49 \\ \hline \hline \end{tabular}
\end{table}