Saman Nimali Gunasekara
Master’s Thesis: Improvements on Municipal WWT
This implies that the cuvette readings are very much sensitive, and that a very small amount of error during measuring could call for a considerable deviation. In addition, the measuring errors which could occur during making the dilutions could also have an intensified impact at cuvette reading stage. The following example calculation would be demonstrating such a situation: Considering actual total nitrogen concentrations, of 40 mg/l before treatment, and 38 mg/l TotN after treatment; If the dilution made for the before-treatment sample was actually 48.5% instead of 50%; the measurement would be (neglecting errors at cuvette test) = 40 × 0.485 = 19.4 mg/l (Eqn. 5.1) If the dilution made for the after-treatment sample was actually 51.5% instead of 50%; the measurement would be8 (neglecting errors at cuvette test) = 38 × 0.515 = 19.57 mg/l (Eqn. 5.2) The errors at dilution could be smaller, however to account for the errors occurring during the cuvette test procedure in this sample calculation it is supposed to be reasonable to assume such an error occurring solely at dilution and zero errors at cuvette tests. Therefore, through readings, it would be seen that the total nitrogen had increased, while in reality it would be a small amount of decrease. Another possible cause for this percentage increases of nitrogen contradicting the expectations on percentage reductions; could be that with the chemical reactions involved with both Ca 2+ and Mg2+, instead of precipitating nitrogen in the wastewater, it is accommodating reactions that release nitrogen, for instance through dissolution of NH4+ from suspended solids. However, the likelihood of such occurrence has not been much studied over and thus not much literature proof is obtainable on such. 5.1.4 pH The pH of the primary effluent had been on average 8.10 while that of wastewater samples immediately after the addition of the chemicals had been approximately within the range 8.1 – 9.2. The pH of the precipitation (read immediately after addition of chemicals) could be compared to several similar or relevant studies. A pilot plant scale evaluation of potential precipitants by Balmér and Frederiksen (1974) done on secondary precipitation of domestic wastewater (could be assumed the same as municipal wastewater) reveals that the effluent quality (i.e., treatment efficiency) was quite dependant on the pH in precipitation for all the commonest precipitants lime, alum and ferric iron. In addition, the study identifies the favourable pH ranges for such an enhanced effluent quality as; 5.0-6.0 for alum, 5.0-5.9 and above 8.0 for ferric iron and above 11.2 for lime. In the same study, in the lime precipitation process in particular, a strong tendency for total phosphorus removal had been identified with increasing pH (Balmér and Frederiksen, 1974) which is of more relevance to the thesis study focus. A more relevant and similar comparison would be to a study specified as a high-pH limemagnesium process by Dollof et al. (1972 cited in Semerjian and Ayoub, 2001, p. 392) which had yielded quite noteworthy cleaning results (discussed in the previous sections); where by the name itself implies the importance of having a higher pH value. This is more established by 8
In a volumetric flask of 50 ml, the difference of sample volume for 50% and 48.5% or 50% and 51.5% is just 0.75 ml; while when the size of the flask reduces; the amount reduces more, but the error gets magnified.
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