Effect of DO, salinity and NH 4 -N concentration on Nitrification with

The composition of the feed permeates to the nitrification bioreactor were analyzed before each of the evaluated batches. The average pH and T of the feed permeates were equal to 6,5±0,3 and 20±1 °C respectively and during nitrification treatment the pH was adjusted and kept at an approximate value of 7,5 with the help of the pH controllers. The results obtained in the chemical analysis for the reverse osmosis (ROP), landfill leachate (LLP) and mixed-permeates are given in table 60.

Table 60.Composition analysis in the feed permeates to the bioreactor during the effect of DO, salinity and NH4-N conc. on Nitrification with LLP and ROP

Substance/ As illustrated in table 60, the concentration of NH4-N in the different feed permeates ranged approximately in between 70 to 2000 mg/L where in average the NH4-N to TN ratio was approximately equal to 0,91±0,02, which indicated that the organic-N content in the different permeates was about 9% considering that the concentration of the other inorganic N species including NO2 and NO3 were very low. Also the COD concentration range was very wide from about 130 to 6900 mg/L with an average TOC to COD ratio of 0,23±0,01. Furthermore, the LLP and ROP used during the biological treatment were collected at different times of the year; hence some of the concentrations and/or parameters especially for the case of TOC and COD do not correlate exactly with the volumetric relations. However, most of the obtained values are within the estimated 10% errors.

Furthermore, the operating conditions measured during the nitrification treatment of the permeates and mixed-permeates at the different volumetric ratios are described in table 61.

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Table 61. Operating conditions measured during effect of DO, salinity and NH4-N conc. on Nitrification with LLP and ROP As indicated in table 61, in order to properly correlate the effect of DO in the treated permeate by substrate limitation, only the ROP and the 10% LLP mixed-permeates were analyzed at the low DO concentration range considering that the NH4-N concentration and salinity were relatively low ranging approximately in between 70 to 250 mg/L and 2,5 to 13,0 dS/m respectively. Thus, minimizing potential inhibitions during nitrification due to the presence of FA, FNA and organic and/or inorganic toxic substances. For instance, for FA based on the NH4-N concentration of 250 mg/L and figure 12 at pH and T values of 7,5 and 20°C the concentrations of FA during nitrification in the ROP and 10% LLP mixed-permeates were below 10 mg/L, which as indicated in section 2.5, around this value FA might have inhibition effects in AOB.

Furthermore, as indicated in table 15 the concentration of Ni, Cr and Cu in the LLP were about 0,68 mg/L and smaller than 0,2 and 0,08 mg/L respectively, which also as discussed in section 2.5, these concentration values were higher or close to the reported concentration levels of 0,25, 0,25 and 0,1 mg/L for Ni, Cr and Cu respectively where inhibition of ammonia oxidation might take place. Moreover, the pH, T and liquid superficial velocity in the evaluated batches were relatively close to each other during the nitrification treatment.

The results obtained in nitrification during the treatment of the permeates for each of the evaluated conditions of DO and different NH4-N and salinity concentrations are given in figures 110 through 115.

ROP _90%

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Figure 110. Nitrification in ROP; pH: 7,5±0,3; Salinity: 2,8±0,3 dS/m; DO: < 1 mg/L; vLiq: 12,7±1,3 m/h; T: 19±1°C

Figure 111. Nitrification in 10% LLP; pH: 7,5±0,3; Salinity:

13,1±0,3 dS/m; DO: < 1 mg/L; vLiq: 12,7±1,3 m/h; T: 20±2°C

Figure 112. Nitrification in 20% LLP; pH: 7,5±0,3; Salinity:

22,5±0,3 dS/m; DO: 2 – 3 mg/L; vLiq: 12,7±1,3 m/h; T: 24±1°C

Figure 113. Nitrification in 50% LLP; pH: 7,5±0,3; Salinity:

49,0±0,3 dS/m; DO: 2 – 3 mg/L; vLiq: 12,7±1,3 m/h; T: 22±1°C

Figure 114. Nitrification in 80% LLP; pH: 7,5±0,3; Salinity:

78,1±0,4 dS/m; DO: > 5 mg/L; vLiq: 12,7±1,3 m/h; T: 20±1°C

Figure 115. Nitrification in LLP; pH: 7,5±0,3; Salinity: 90,5±0,4 dS/m; DO: > 5 mg/L; vLiq: 12,7±1,3 m/h; T: 22±1°C

As illustrated in figures 110 through 113 at the relatively similar operating conditions the AORs were independent of the initial NH4-N concentrations in the permeates, which based on the linear regressions with R² values of about 0,99 obtained from the concentration data as a function of time indicated linearity, which is characteristic of a

y = -27.95x + 68.533

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zero-order reaction. Also, as discussed in section 2.5 the obtained results correlated with the Monod model, which indicates low specific grow rates of microorganisms at low substrate concentration; hence the AOR estimated for the low DO concentration batches corresponding to ROP and 10% LLP decreased in average 79% with respect to the AOR obtained in the 20% LLP mixed-permeates batch. Furthermore, the AOR in the 20% LLP was about 10% higher than the AOR in the 50% LLP, this increase in the AOR might be correlated not only with the lower salinity but also with the slightly higher temperature condition in the 20% LLP considering that as discussed in section 2.5 for the case of AOB about 10% increase in the specific grow rate could be expected for an increase in 1°C. Moreover, based on salinity, no considerable inhibition was observed for the AOB up to the value of about 50 dS/m. But, inhibition by toxicity was observed in the AOB with salinity values of about 78 and 90 dS/m where even with DO concentrations above 5 mg/L their estimated AOR with respect to the 20% LLP batch decreased to 64 and 95% respectively. Furthermore, during the treatment of the 80% LLP batch with salinity of 78 dS/m severe nitrite accumulation was observed, which indicated very low activity of NOB and during the treatment of the LLP with salinity of about 90 dS/m the activities of both AOB and NOB were severely affected. Hence, complete nitrification or conversion from NH4-N to NO3-N was only possible at the different salinity conditions up to the 50% LLP batch with salinity value of about 49 dS/m where in average about 76±2% of the initial NH4-N was converted to NO3-N. Additionally, for the batches up to the 50%

LLP the average TOC percentage removed during nitrification treatment was equal to 50±4%. The effect of salinity, DO and initial NH4-N concentration on Nitrification are summarized in figure 116, the percentage decreased in the estimated AORs was calculated with respect to the AOR value of about 130 mg/L^.d obtained in the mixed-permeate with salinity of 22,5±0,3 dS/m corresponding to the 20% in volume LLP.

Figure 116. AOR in FBB with LL mixed-permeates during effect of DO, salinity and NH4-N conc. on Nitrification at pH: 7,5±0,3; vLiq.: 12,7±1,3 m/h; T: 21±2°C

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Furthermore, based on salinity and considering the complex composition of the LLP and the potential inhibition effect of other substances, for the case of 80% LLP and LLP it was unclear to determine how large was the effect of salt toxicity in the nitrifying microorganisms but it did provide a reference point with respect to the AOB tolerance for the landfill leachate permeate. The AOR results obtained for the mixed-permeates and LLP with salinity values in between 20 to 90 dS/m are illustrated in figure 117.

Figure 117. AOB tolerance with respect to salinity in LLP

As indicated in figure 117, based on the linear trend and with respect to the mixed-permeate with salinity of about 22 dS/m less than 10% of the AOB activity or AOR was affected up to salinity values of about 50 dS/m but more than 50% of their activity was lost at salinity conditions of 78 dS/m and reaching a 95% decrease in the AOR with the LLP. Hence, a decrease of less than 50% in AOB activity or AOR might be expected with landfill leachate permeates at salinity conditions ranging in between 50 to 70 dS/m.

3.5.2.2. Effect of pH on Nitrification and Denitrification