Results of removal efficiency

The removal efficiencies of COD and phosphorous in the pilot plant with the initial operating conditions were well in line within the requirements of removal. The removal efficiencies were consequently above 80% and 90% of COD and total phosphorous respectively. The concentrations of COD in the influent and effluent are shown in Figure 3.13 and had an average removal of 88% during initial testing. All the measurements of effluent COD can also be observed to be below 125 mg/L. Secondly the concentrations of total phosphorous in the influent and effluent are shown in Figure 3.14. The effluent concentration is most frequently close to the limit of detection, and the average removal rate of phosphorous in the period was found to be therefore > 98%. These satisfactory results supported the decision about reducing the chemical dosage.

Figure 3.13 COD removal with initial chemical conditions.

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Figure 3.14 Tot P removal with initial chemical conditions.

Despite of that the DMF pilot plant was not operated for long, there were testing of influent and effluent values of COD and P for the optimised operation conditions I (Table 3.2). Regardless of the small amount of data, the results are presented graphically, to provide a similar picture as in the previous section. Influent and effluent values of COD and total phosphorous are shown in Figure 3.15 and Figure 3.16, respectively. The removal of both COD and phosphorus was found to be satisfactory during the short operation time, with average removal efficiency of 87% and 97% respectively, meaning there was no doubt of fulfilling the removal requirements.

With the small number of attained data due to the operation condition running for such a short time, it would be wrong to claim the used chemical dosage always would give satisfactory removal. It would, therefore, be necessary to run the operation condition longer. However, this was not possible due to too rapid development of TMP.

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Figure 3.15 COD removal with the first change of chemical dosage.

Figure 3.16 Tot P removal with the first change of chemical dosage.

One noteworthy observation is the removal of phosphorous still being adequate with influent concentrations of ca. 6 mg P/L, which is higher influent values than what observed during the previous operation condition described in the preceding section.

Having the same dosage of the precipitating agent PACl, as for the first set of operating conditions, the removal was expected to be similar in terms of phosphorous removal for the optimised operation conditions II (Table 3.2). This turned out to be the case, and yet

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again, the removal was beyond satisfaction, as can be seen in Figure 3.17 for COD and in Figure 3.18 for phosphorous.

Figure 3.17 COD removal with the second change of chemical dosage.

Figure 3.18 Tot P removal with the second chemical dosage.

During the entire period, the treatment requirements of COD and phosphorous were met, with both the coagulant dosages. Perhaps could an even lower concentration of PACl be used, but the time span in which the dosage was tested was not long enough to test a further reduction and see if it would meet the requirements. However, the results show potential for further reduction, considering the removal was well within the requirements for both constituents overall.

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Using the chosen dosage of 9 mg Al/L, and considering an influent concentration of phosphorous of 2.9 mg/L (based on Table 4.1), a molar ratio of 3.6(Al:P) was found. This is, for example, lower than what found by (Wang et al., 2005) who found the optimum molar ratio of aluminium to phosphorous of 4.37, while Cai et al. (2020) reported success with a ratio of 2.5 in MBR effluents. The latter supports the potential for further

reduction.

The chemical dosages were based on a flow of ca. 3.1 m³/h; however, by taking a closer look at the inflow, it was observed some slight fluctuation around this value. These fluctuations are generally minimal; as can be seen in Appendix C. With this slight instability, there will be some uncertainties about the chemical concentration being precisely as stated at some points. Sometimes the desired concentration of PACl and polymer is exceeded, and sometimes it is not met. This will have an impact on the removal efficiency to some extent. The measured inflow can be found in Appendix C.

It might seem like the incoming phosphorous concentration had seasonal variations, as it can be observed that the influent concentration in was consequently below ca. 4 mg P/L between November and March (Figure 3.14Figure 3.13), while in Figure 3.18 was

consequently above ca. 4 mg P/L. The same can be observed for the incoming COD in Figure 3.13 and Figure 3.17, with the incoming COD concentrations in May, generally being higher than those between November and February. This may be coincidental, or it may be the case that there are seasonal variations. Either way, the pilot plant showed sufficient removal of both constituents independent of this variation.

An accredited laboratory, Eurofins in Moss, has also been analysing samples from the pilot plant. These analyses are not as frequent as the COD and phosphorous analyses described in the preceding sections. The results are presented in Table 3.10 and confirm the high removal efficiency shown in the regular day to day analyses performed in the internal lab of the wastewater treatment plant, as well as including some additional constituents.

Table 3.10 Removal rates from accredited lab analyses.

Date

The pilot process was not designed with nitrogen removal in mind, which is reflected by the low nitrogen removal. As today, there is no knowledge about nitrogen removal reequirements in the new treatment plant either.

Weekly composite sample with the removal of heavy metals was only analysed once, at the same time as the last analysis in Table 3.10, and the results can be found in Table 3.11. Comparing these to the average removal of the main wastewater treatment plant, the pilot plant has slightly higher removal for all constituents; the main treatment plant experiences an increase in the concentration of nickel.

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Table 3.11 Removal of heavy metals from accredited lab analyses.

Parameter Inlet [µg/L] Outlet [µg/L] Removal [%]

aHalf of the lowest detection value.